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diff --git a/41192.txt b/41192.txt deleted file mode 100644 index 6a6d056..0000000 --- a/41192.txt +++ /dev/null @@ -1,10783 +0,0 @@ -The Project Gutenberg EBook of Animal Proteins, by Hugh Garner Bennett - -This eBook is for the use of anyone anywhere at no cost and with -almost no restrictions whatsoever. You may copy it, give it away or -re-use it under the terms of the Project Gutenberg License included -with this eBook or online at www.gutenberg.org - - -Title: Animal Proteins - -Author: Hugh Garner Bennett - -Release Date: October 26, 2012 [EBook #41192] - -Language: English - -Character set encoding: ASCII - -*** START OF THIS PROJECT GUTENBERG EBOOK ANIMAL PROTEINS *** - - - - -Produced by Juliet Sutherland, Joanna Johnston and the -Online Distributed Proofreading Team at http://www.pgdp.net -(This file was produced from images generously made -available by The Internet Archive/Million Book Project) - - - - - -Transcriber's Note: - -Minor typographical errors and inconsistencies have been corrected. Some -words had inconsistent hyphenation throughout the book; these have been -made consistent. - -In the mathematical and chemical expressions, the caret character -represents "to the power of", e.g. e^2 means e-squared. Curly braces are -used to represent underscores, e.g. n{1} means n with a subscript of 1. -Equal signs are used to represent =bold words=, and underscores are used -to represent italics. - -On page 152 NaCO{23} has been corrected to Na{2}CO{3}. On page 212, -the variable n has been replaced with the correctly subscripted forms -n{1} and n{2}. - -The index entry for Hemlock bark had no page number in the original -text, so the correct page number, 34, has been supplied. - -Ions are shown as Fe+++, instead of using superscripts. - -There is some inconsistency in the notation used in the original text -for chemical formulae such as Na{2}Cr{2}O{7}(2H{2}O). These have -been regularized to use the modern mid-dot, for example, -Na{2}Cr{2}O{7}.2H{2}O. - -Greek letters in the original have been represented in this version by -the name of the letter enclosed in square brackets, e.g. [alpha]. - - - - - ANIMAL PROTEINS - - - BY - - HUGH GARNER BENNETT, M.Sc. (LEEDS) - - MEMBER OF THE SOCIETY OF LEATHER TRADES' CHEMISTS; FORMERLY ASSISTANT - LECTURER AND DEMONSTRATOR AT THE LEATHER INDUSTRIES DEPARTMENT OF THE - UNIVERSITY OF LEEDS - - AUTHOR OF "THE MANUFACTURE OF LEATHER" - - - LONDON - - BAILLIERE, TINDALL AND COX 8, HENRIETTA STREET, COVENT GARDEN - 1921 - - (_All rights reserved_) - - - - - GENERAL PREFACE - - -The rapid development of Applied Chemistry in recent years has brought -about a revolution in all branches of technology. This growth has been -accelerated during the war, and the British Empire has now an -opportunity of increasing its industrial output by the application of -this knowledge to the raw materials available in the different parts of -the world. The subject in this series of handbooks will be treated from -the chemical rather than the engineering standpoint. The industrial -aspect will also be more prominent than that of the laboratory. Each -volume will be complete in itself, and will give a general survey of the -industry, showing how chemical principles have been applied and have -affected manufacture. The influence of new inventions on the development -of the industry will be shown, as also the effect of industrial -requirements in stimulating invention. Historical notes will be a -feature in dealing with the different branches of the subject, but they -will be kept within moderate limits. Present tendencies and possible -future developments will have attention, and some space will be devoted -to a comparison of industrial methods and progress in the chief -producing countries. There will be a general bibliography, and also a -select bibliography to follow each section. Statistical information will -only be introduced in so far as it serves to illustrate the line of -argument. - -Each book will be divided into sections instead of chapters, and the -sections will deal with separate branches of the subject in the manner -of a special article or monograph. An attempt will, in fact, be made to -get away from the orthodox textbook manner, not only to make the -treatment original, but also to appeal to the very large class of -readers already possessing good textbooks, of which there are quite -sufficient. The books should also be found useful by men of affairs -having no special technical knowledge, but who may require from time -to time to refer to technical matters in a book of moderate compass, -with references to the large standard works for fuller details on -special points if required. - -To the advanced student the books should be especially valuable. His -mind is often crammed with the hard facts and details of his subject -which crowd out the power of realizing the industry as a whole. These -books are intended to remedy such a state of affairs. While -recapitulating the essential basic facts, they will aim at presenting -the reality of the living industry. It has long been a drawback of our -technical education that the college graduate, on commencing his -industrial career, is positively handicapped by his academic knowledge -because of his lack of information on current industrial conditions. A -book giving a comprehensive survey of the industry can be of very -material assistance to the student as an adjunct to his ordinary -textbooks, and this is one of the chief objects of the present series. -Those actually engaged in the industry who have specialized in rather -narrow limits will probably find these books more readable than the -larger textbooks when they wish to refresh their memories in regard to -branches of the subject with which they are not immediately concerned. - -The volume will also serve as a guide to the standard literature of the -subject, and prove of value to the consultant, so that, having obtained -a comprehensive view of the whole industry, he can go at once to the -proper authorities for more elaborate information on special points, and -thus save a couple of days spent in hunting through the libraries of -scientific societies. - -As far as this country is concerned, it is believed that the general -scheme of this series of handbooks is unique, and it is confidently -hoped that it will supply mental munitions for the coming industrial -war. I have been fortunate in securing writers for the different -volumes who are specially connected with the several departments of -Industrial Chemistry, and trust that the whole series will contribute -to the further development of applied chemistry throughout the Empire. - - SAMUEL RIDEAL. - - - - - AUTHOR'S PREFACE - - -It has been the author's chief concern that this volume should fulfil -its own part in the programme set forth in Dr. Rideal's General Preface. - -The leather, glue, and kindred trades have been for many years -recognized as chemical industries, but the great development of colloid -chemistry in the last few years has given these trades a more definite -status as such, and they can now be placed in the category of applied -physical chemistry. The time is probably not far distant when some -knowledge of pure physical chemistry will be a first essential to -students, chemists, chemical engineers, and to all engaged in these -industries in supervision, administration, or control. It is hoped that -this volume will stimulate the study of these industries from that -standpoint. - -As the author has previously written upon one of the industries involved -herein ("The Manufacture of Leather": Constable & Co.), he has, rather -inevitably, found it difficult to avoid altogether his own phraseology. -The changes of a decade, however, together with the wider field and -newer view-point, have made possible a radical difference of treatment. - -The author desires to acknowledge the help he has received from the many -books, essays, and researches which are mentioned in the references at -the end of each section, especially to Procter's "Principles of Leather -Manufacture," and also to thank Dr. Rideal for many useful suggestions. -The author would like also to acknowledge here his indebtedness (as well -as that of the trade generally) to the work of Dr. J. Gordon Parker, who, -through his researches, lectures, and teaching work, has done more than -any other man to disseminate a knowledge of practical methods of -tanning. - -The author's thanks are also due to his brother, Mr. W. Gordon Bennett, -M.Sc., A.I.C., M.C., for assistance in proof revision, and to his -father, Rev. John Bennett, for some literary criticism. - - H. GARNER BENNETT. -BEVERLY, _June_, 1921. - - - - - CONTENTS - - - - - GENERAL PREFACE - - AUTHOR'S PREFACE - - CONTENTS - - INTRODUCTION - - - - - * * * * * - - PART I. - - HIDES FOR HEAVY LEATHERS. - - SECTION - - 1. THE RAW MATERIAL OF HEAVY LEATHERS - - 2. THE PREPARATION OF PELT - - 3. VEGETABLE TANNAGE - - 4. FINISHING PROCESS - - 5. SOLE LEATHER - - 6. BELTING LEATHER - - 7. HARNESS LEATHER - - 8. UPPER LEATHERS - - 9. BAG LEATHER - - 10. PICKING BAND BUTTS - - - - - * * * * * - - PART II. - - SKINS FOR LIGHT LEATHERS. - - - 1. PRINCIPLES AND GENERAL METHODS OF - LIGHT LEATHER MANUFACTURE - - 2. GOATSKINS - - 3. SEALSKINS - - 4. SHEEPSKINS - - 5. CALFSKINS - - 6. JAPANNED AND ENAMELLED LEATHERS - - - - * * * * * - - PART III. - - CHROME LEATHERS. - - 1. THE NATURE OF CHROME LEATHERS - - 2. GENERAL METHODS OF CHROME LEATHER - MANUFACTURE - - 3. CHROME CALF - - 4. CHROME GOAT AND SHEEP - - 5. HEAVY CHROME LEATHERS - - - - - * * * * * - - PART IV. - - MISCELLANEOUS TANNAGES. - - 1. ALUM TANNAGES - - 2. FAT TANNAGES - - 3. OIL TANNAGES - - 4. FORMALDEHYDE TANNAGE - - 5. SYNTHETIC TANNING MATERIALS - - 6. COMBINATION TANNAGES - - 7. THE EVOLUTION OF THE LEATHER INDUSTRY - - - - - * * * * * - - PART V. - - GELATINE AND GLUE. - - - 1. PROPERTIES OF GELATINE AND GLUE - - 2. RAW MATERIALS AND PRELIMINARY - TREATMENT - - 3. EXTRACTION - - 4. CLARIFICATION AND DECOLORIZATION - - 5. BLEACHING - - 6. EVAPORATION - - 7. COOLING AND DRYING - - 8. USES OF GELATINE AND GLUE - - 9. THE EVOLUTION OF THE GELATINE AND - GLUE INDUSTRY - - - - - * * * * * - - PART VI. - - MISCELLANEOUS PROTEINS AND BYE-PRODUCTS. - - 1. BYE-PRODUCTS OF THE LEATHER TRADES - - 2. BYE-PRODUCTS OF THE GELATINE AND - GLUE TRADES - - 3. FOOD PROTEINS - - 4. MISCELLANEOUS ANIMAL PROTEINS - - - INDEX - - - - - ANIMAL PROTEINS - - - INTRODUCTION - - -Proteins are organic compounds of natural origin, being found in plants -and in animals, though much more plentifully in the latter. They are -compounds of great complexity of composition, and of very high molecular -weight. The constitution of none of them is fully understood, but -although there are a great number of different individual proteids, they -present typical resemblances and divergences which serve to -differentiate them from other groups of organic bodies, and also from -one another. - -Proteins resemble one another in both proximate and ultimate analysis. -They contain the usual elements in organic compounds, but in proportions -which do not vary over very wide limits. This range of variation is -given approximately below:-- - - - Element. Per cent. - - Carbon 49 to 55 - Hydrogen 6.4 to 7.3 - Oxygen 17 to 26 - Nitrogen 13 to 19 - Sulphur 0.3 to 3.0 - - -The most characteristic feature of the protein group is the amount of -nitrogen usually present. This is generally nearer the higher limit, -seldom falling below 15 per cent. This range for the nitrogen content is -determined largely by the nature of constituent groups which go to form -the proteid molecule. Roughly speaking, proteins consist of chains of -amido-acids and acid amides with smaller proportions of aromatic groups, -carbohydrate groups and thio compounds attached. In these chains an acid -radical may combine with the amido group of another amido acid, the acid -group of the latter combining with an amido group of another amido acid, -and so on. Hydrogen may be substituted in these chains by alkyl or -aromatic groups. There is obviously infinite possibility of variation in -constitution for compounds of this character, the general nature of -which varies very little. Practically all of the proteins are found in -the colloid state, and this makes them very difficult to purify and -renders the ultimate analysis in many cases doubtful. It is, for -example, often difficult to ascertain their moisture content, for many -are easily hydrolyzed with water only, and many part easily with the -elements of water, whilst on the other hand many are lyophile colloids -and practically cannot be dehydrated or dried. A few, such as gelatin -and some albumins, have been crystallized. - -The constituent groups have been investigated chiefly by hydrolytic -methods. The chains of amido acids are split up during hydrolysis, and -individual amido acids may thus be separated. The hydrolysis may be -assisted either by acids, alkalies or ferments, but follows a different -course according to the nature of the assistant. Under approximately -constant conditions of hydrolysis, the products obtained are in -approximately constant proportions, and this fact has been utilized by -Van Slyke in devising a method of proximate analysis. It is not possible -in this volume to enter deeply into the constitution of the different -proteids. Reference must be made to works on pure chemistry, especially -to those on advanced organic chemistry. It will be interesting, however, -to mention some of the amido acids and groups commonly occurring in -proteids. These comprise ornithine (1:4 diamido valeric acid), lysine -(1:5 diamido-caproic acid), arginine (1 amido, 4 guanidine valeric -acid), histidine, glycine (amidoacetic acid), alanine (amido propionic -acid), amido-valeric acid (amido-iso-caproic acid), liacine, pyrollidine -carboxylic acid, aspartic acid, glutamic acid (amido-glutaric acid), -phenyl-alanine, serine (hydroxy-amido propionic acid), purine -derivatives (_e.g._ guanine), indol derivatives (_e.g._ tryptophane -and skatol acetic acid), cystine (a thioserine anhydride), -glucosamine, and urea. - -There are a few general reactions which are typical of all proteins, and -which can usually be traced to definite groupings in the molecule. -Amongst these is the biuret reaction: a pink colour obtained by adding a -trace of copper sulphate and an excess of caustic soda. This is caused -by the biuret, NH(CONH{2}){2} radical or by similar diacidamide -groups, _e.g._ malonamide, oxamide, glycine amide. Another general -reaction is with "Millon's reagent," a solution of mercuric nitrate -containing nitrous fumes. On warming the proteid with this reagent, a -curdy pink precipitate or a red colour is obtained. This reaction is -caused by the tyrosine group (p. oxy [alpha] amido phenyl-propionic -acid). Another general reaction is to boil the protein with 1:2 nitric -acid for some days. A yellow flocculent precipitate of "xanthoproteic -acid" is obtained, and this dissolves in ammonia and caustic alkalies -with a brown or orange-red colour. Another characteristic of proteins is -that on dry distillation they yield mixtures of pyridine C{5}H{5}N, -pyrrol C{4}H{5}N, and their derivatives. - -On the subdivision, classification and nomenclature of the proteins much -ink has been spilled, and it is impossible in this volume to go into the -various systems which have been suggested. It should be noted, however, -that some writers habitually use the terms "proteid" or "albuminoid" as -synonyms for protein. The classification of proteins adopted in this -work is used because it is the most suitable for a volume on industrial -chemistry and has the additional merits that it is simple and is already -used in several standard works on industrial chemistry. It is based upon -the behaviour of the proteins towards water, a matter of obvious moment -in manufacturing processes. On this basis proteins may be divided into -albumins, keratins and gelatins. - -Cold water dissolves the albumins, does not affect the keratins, and -only swells the gelatins. The behaviour in hot water confirms and -elaborates the classification. When heated in water, the albumins -coagulate at temperatures of 70 deg.-75 deg. C., the gelatins (if swollen) -dissolve readily, whilst the keratins only dissolve at temperatures -above 100 deg. C. Albumins and keratins may be distinguished also from -gelatins by adding acetic acid and potassium ferrocyanide to their -aqueous solutions. Albumins and keratins give a precipitate, gelatins do -not. Another distinguishing reaction is to boil with alcohol, wash with -ether, and heat with hydrochloric acid (S.G. 1.2). Albumins give a -violet colour, keratins and gelatins do not. - -=Albumins= may be first discussed. They are typified by the casein of -milk and by white of egg. Their solutions in water are faintly -alkaline, optically active, and laevorotatory. They are coagulated by -heat and also by mineral acids, alcohol, and by many poisons. The -temperature of coagulation (usually about 72 deg. C.) is affected by -mineral salts, the effect being in lyotrope order (see Part V., -Section I.). The coagulated albumin behaves in most respects like a -keratin. Some of the albumins (globulins) are, strictly speaking, not -soluble in cold water, but readily dissolve in weak solutions of salt. -The albumins are coagulated from these solutions, as usual, when -heated. Into this special class fall myosin (of the muscles), -fibrinogen (of the blood) and vitellin (of egg yolk). By a gentle or -limited hydrolysis of the albumins with dilute acids in the cold, a -group of compounds called albuminates are obtained. They dissolve in -either acids or alkalies, and are precipitated by exact -neutralization. They may also be "salted" out by adding sodium -chloride or magnesium sulphate. They are not coagulated by heat. After -further hydrolysis with either acids, alkalies or ferments, very -soluble compounds are obtained called albumin peptones or albumoses. -These are soluble in alkalies, acids and water, and are readily -hydrolyzed further into amido acids and acid amides. They are very -similar to the peptones obtained from keratins and gelatins. They are -not coagulated by heat. - -=Keratins= are typified by the hair of animals. They soften somewhat -in cold water and even more in hot water, but are not dissolved until -digested for some time at temperatures exceeding 100 deg. C. With some -keratins, however, the cystine group is to some extent easily split -off by warm water, and on boiling with water hydrogen sulphide is -evolved. The sulphur content of keratins is often greater than the -average for proteids. All keratins are dissolved with great readiness -by solutions containing sulphydrates and hydrates, _e.g._ a solution -of sodium sulphide. In solutions of the hydrates of the alkali and -alkaline earth metals, keratins behave differently. Some dissolve with -great ease, some with difficulty, some only on heating and some not -even if digested with hot caustic soda. They are dissolved (with -hydrolysis) by heating with mineral acids, yielding peptones and -eventually amido acids, acid amides, etc. Many keratins have a -comparatively low content of nitrogen. - -=Gelatins= are very difficult to distinguish from one another, their -behaviour being closely similar to reagents. They are also very -readily hydrolyzed even with water, and the products of hydrolysis are -even more similar. The gelatins are known together, commercially, -under the general name of gelatine. Gelatins of different origin, -however, have undoubtedly a different composition, the nitrogen -content being variable. If the gelatins are not bleached whilst they -are being manufactured into commercial gelatine, they are called -"glue." Gelatine is colourless, transparent, devoid of taste and -smell. It is usually brittle. Its S.G. is about 1.42, and it melts at -140 deg. C. and decomposes. It is insoluble in organic solvents. When -swelling in cold water it may absorb up to 12 times its own weight of -water. The swollen product is called a "jelly." Jellies easily melt on -heating and a colloidal solution of gelatine is obtained. This "sets" -again to a jelly on cooling, even if only 1 per cent. gelatin (or -less) be present. The solution is optically active and laevorotatory, -but with very variable specific rotation. Some observers have thought -that the different gelatins have different specific rotations and may -so be distinguished. Gelatins are precipitated from solutions by many -reagents, such as alcohol, formalin, quinone, metaphosphoric acid, -tannins, and many salt solutions, _e.g._ those of aluminium, chromium -and iron, and of mercuric chloride, zinc sulphate, ammonium sulphate, -potassium carbonate, acidified brine. Many of these precipitations -have analogies in leather manufacture (see Parts I. to IV.). The -gelatin peptones or gelatoses are formed by hydrolysis with acids, -alkalies, ferment or even by digestion with hot water only. A more -detailed description of the properties of gelatine is given in Part -V., Section I. Gelatine is sometimes called "glutin" and "ossein." - -Animals are much the most important source of proteins, especially of -those which are of importance in industrial chemistry. Proteins occur -in nearly every part of all animals, and the "protoplasm" of the -living cell is itself a protein. The keratins include the horny -tissues of animals: the epidermis proper, the hair, horns, hoofs, -nails, claws, the sebaceous and sudoriferous glands and ducts, and -also the elastic fibres. The gelatins are obtained from the collagen -of the skin fibres, the bones, tendons, ligaments, cartilages, etc. -Fish bladders yield a strong gelatin. The albumins are obtained from -the ova, blood, lymph, muscles and other internal organs of animals. - -The classification of proteins herein adopted fits in well with the -scope and purpose of this volume. The keratins are of little -importance in chemical industry, but are of immense importance in -mechanical industry, _e.g._ the woollen trade, which is based upon the -keratin comprised by sheep wool. The collagen of the hide and skin -fibres is of vast importance to chemical industry, and is the basis of -the extensive leather trades discussed in Parts I. to IV. The waste -pieces of these trades, together with bones, form the raw material of -the manufacture of gelatin and glue, as discussed in Part V. The -proteids of animals' flesh and blood, milk and eggs form the source of -the food proteins discussed in Part VI. The food proteins embrace -chiefly albumins, but gelatins and even keratins are involved to some -extent. - - - - - PART I.--HIDES FOR HEAVY LEATHERS - - - Section I.--THE RAW MATERIAL OF HEAVY LEATHERS - - -The term "hide" possesses several shades of meaning. In its widest sense -it applies to the external covering of all animals, and is sometimes -used derogatively for human skin. In this wide sense, it is almost -synonymous with the term "skin." The term "hide," however, has a -narrower meaning, in which it applies only to the outer covering of the -larger animals, and in this sense is used rather in contrast with the -term "skin." Thus we speak of horse hides, cow hides, camel hides, and -buffalo hides. It is used in this sense in the title of Part I. of this -volume. As such hides are from large animals, the leather which is -manufactured therefrom is thick and in large pieces, and is therefore -commercially designated as "heavy leather." From the standpoint of -chemical industry hides are amongst the most important of animal -proteins, and their transformation into leather for boots, shoes, -belting, straps, harness, and bags comprises the "heavy leather trade," -which is one of the largest and most vital industries of the country. -The heavy leather trade predominates over other branches of leather -manufacture, not only because of the comparatively large weight and -value of the material handled, but also because the resulting products -have a more essential utility. There is also a still narrower use of the -term "hide," in which it applies only to the domesticated cattle--the -ox, heifer, bull and cow--which use arises from the fact that the hides -of these are both the largest and most valuable portion of the raw -material of the heavy leather industries. In a very narrow sense the -term is also sometimes applied only to ox hides, which for most heavy -leathers are the ideal raw material. - -=The Home Supply= of hides forms a large important proportion of -the total raw material. Its importance, moreover, is rapidly increasing, -for the excellence and abundance of the home supply determines the -extent to which it is necessary for the industry to purchase its raw -material abroad. The position of our national finances makes this an -increasingly serious matter, for hides are comparatively a very -expensive material. - -The quality of our home supply of hides is very valuable, being -determined by the conditions of the animal's life, its precise breed, -and by other factors such as age and sex. The best hides are usually -obtained from animals which have been most exposed to extremes of wind -and cold, as such conditions tend naturally to develop a thicker and -more compact covering. Broadly speaking, these include the hides from -cattle of the northern and hilly districts. The age of the animal when -killed is also a dominating factor. Calf skins are very soft, fine -grained and compact, the state of rapid growth favouring the existence -of much interfibrillar substance. The youngest animals supply suitable -raw material for various light leathers (see Part II., Section V.), and -are also very suitable for chrome work (see Part III., Section III.). -Bull and cow hides, on the other hand, are from animals whose growth is -complete, and show in consequence a lack of interfibrillar substance, -coarse fibres and a rough and often wrinkled grain. The resulting -leather tends consequently to be spongy, thin, empty and non-waterproof. -Intermediate between these extremes are the hides of the ox and heifer, -large, yet of good texture, and well supplied with interfibrillar -substance. These hides are much the best for sole leather, a firm, -smooth-grained and well-filled leather being needed. The term "kip" is -often applied to small hides and to hides from large calves. In the -trade, however, "kip" is sometimes used also for larger hides, as a -verbal enhancement of value; just as a man with a few old fowls is said -to keep "chickens." Cow hides tend to be "spready," _i.e._ to have a -large area per unit weight, and are therefore more suitable for dressing -leather. Bull hides are thicker in the neck and belly, and thinner in -the back, which characteristics reduce their commercial value. - -Market hides are sold by weight, and are therefore classified chiefly by -their weight, which is marked on near the tail by a system of -knife-cuts. The animals are flayed after cutting the hide down the belly -and on the inside of the legs. - -Of the various breeds, "Shorthorns" yield a large supply of useful -hides. The name, however, covers a variety of similar breeds, and the -hides therefrom are rather variable in texture and quality. They tend to -be greasy owing to high feeding. The "Herefords," obtained from Midland -markets, are generally excellent hides for sole and harness leathers. -They give a good yield of butt pelt, a stout and smooth shoulder, and -are not often greasy. "Devons" yield a good-textured and well-grown -hide, but are often badly warbled. The "Sussex" cross breeds yield -somewhat larger hides. "Suffolk Red Polls," common in East Anglia, yield -a good butt, and the cow hides make good dressing leather. "Channel -Island" cattle yield very thin hides, but with a fine undamaged grain. -Scotch hides possess deservedly the very highest reputation. The -climatic conditions favour the production of a hardy race of cattle with -thick well-grown hides, yielding a large proportion of butt. These hides -are amongst the best obtainable for heavy leather, and particularly for -sole leather. "Highlanders," "Aberdeen Angus," "Galloways" are typical -breeds, with short neck, legs and straight backs. Cross breeds are also -excellent (_e.g._ "Scotch Shorthorns"). The natural value of these hides -is further enhanced by the usual care in flaying. "Ayrshires" yield good -milch cows and consequently yield often a more spready hide. The Welsh -breeds for rather similar reasons also yield valuable hides. The Irish -"Kerrys" are small but stout, and yield hides suitable for light sole -leather. Irish cross-breeds, Shorthorns, have a rather bad reputation, -and are often ill flayed. - -All the varieties of the home supply are subject to various defects, -which influence seriously their commercial value. One of these defects -is warble holes or marks, caused by the Ox Warble fly (_Hypoderma -bovis_). This is a two-winged fly about half an inch long. The larva of -this fly, the "Warble maggot," lives and thrives in the skin of cattle, -and causes a sore and swelling. The life-history of this insect is still -in dispute, but it is generally thought that the eggs are laid in the -hair on the animal's back, and the young larva eats its way through the -hide until just below the dermis, and there feeds until mature. It then -creeps out of this "warble hole," falls to the ground, pupates for a -month, after which the imago or perfect insect emerges from the -chrysalis. Hides which have been thus infected have, in consequence, -often quite a number of holes through the most valuable part of the -hide, thereby rendering it unsuitable for many kinds of leather. Even -old "warbles" which have more or less healed up are a weakness, and -warbled hides and leather fetch a decidedly lower price than undamaged. -Another of these defects is bad flaying. Clearly the hide should be as -little cut as possible, but many of our market hides are abominably -gashed and often cut right through. This, of course, often reduces -seriously the commercial value of the hide. Careless treatment after -flaying also results in another common defect, viz. taint. As the term -implies, the hide is partly putrefied, sometimes only in patches, but -sometimes also so extensively as to render the hide quite rotten and -quite incapable of being made into leather at all. Hides are of course -putrescible, and dirt, blood, dung and warm weather encourage rapid -putrefaction. As market hides are usually uncured, this defect is -constantly appearing, and is a cause of considerable loss. Other defects -are due to injuries to the animal before it is killed, _e.g._ brands, -scratches due to hedges and barbed wire, old scabs, goad and tar marks. -All these reduce the value of the hide. - -All the defects in hides involve a very serious loss to the community, -and the time is rapidly approaching when their continuance is -insufferable. The loss is not usually very considerable to any -individual, though very large in the aggregate. The hide is a minor part -of the beast's value, and a somewhat damaged hide does not involve a -very serious loss to the farmer. Some with typical stupidity regard a -few warbles as "the sign of a healthy beast." These defects involve -practically no loss to the hide merchant, tanner or currier, as each -pays less for damaged material. The loss falls upon the community, and -the time is ripe for the community to insist upon the elimination of -these defects. The national resources will be for some years strained to -their uttermost, and preventable damage must be considered intolerable. -The principal defects in hides are preventable, and ought to be -prevented. The warble fly could, by a united effort, be rendered before -long practically extinct, a task which is facilitated by the fact that -it is not migrative. Bad flaying and careless treatment of hides -resulting in putrefaction are still more easily remedied. The communal -slaughter-house is long overdue from the standpoint of public health, -and would, under conditions of cleanliness and skilled workmanship and -oversight, also solve the problem of ill-flayed and tainted hides. - -The question of the raw material is of first importance to the leather -trades. There was, before the commencement of the European War, a -steadily increasing scarcity of hides, causing a constant increase in -their price. This was due partly to the fact that cattle were increasing -at a less rate than the population, partly to the growth of -civilization, and more extensive use of leather in proportion to the -world's population, and partly to the constant discovery of new uses for -leather, _e.g._ for motor cars, aeronautics, etc. The question of raw -material was under these conditions serious enough. The terrific -slaughter, necessary at the same time to provide the belligerents with -food and the army with leather, is bound to result in a serious crisis -for the leather industries; and in conjunction with the country's -financial condition, will make it absolutely necessary that all care -should be taken with the raw material of one of our most important -industries. The farmer who pays no heed to the warble fly, the man who -gashes the hide in flaying and who allows the hide to putrefy, are -equally criminal with the man who throws bread crusts into the dustbin. - -It is impossible to foresee, as yet, anything in the nature of a -satisfactory solution to the problem of raw material, especially in -respect to heavy leather production, for the food question will rank -first in the popular mind, and the earlier slaughter enjoined for the -more economical production of meat will scarcely tend to increase the -proportion of heavy hides. - -=The Foreign Supply= of hides is also of great importance and value. In -the case of imported hides precautions to prevent putrefaction are -essential, and some method of "curing" is always used. - -=Salting= the hides is one of the most satisfactory methods for -temporary preservation. The action of salt is hygroscopic, and mildly -antiseptic. Moisture is withdrawn from the hides, which are then under -conditions no longer favouring the growth of bacteria. Well-salted -hides will keep for years, especially if quite clean. A light salting -is also useful for a short preservation, and is becoming common in -hide markets and tanneries during the summer and autumn months. -Salting is a method used extensively in the United States. The "packer -hides" of the stockyards are carefully and systematically salted with -about 25 per cent. of salt and stored in cool cellars. The hides are -so piled up in heaps, that brine easily drains away. The great -disadvantage of salting is the so-called "salt stains." These stains -have been ascribed to the iron in the salt, to the iron in the blood, -to calcium sulphate in the salt, and also to chromogenic bacteria, -whose development is favoured by salting. The relative importance of -these factors is not yet satisfactorily determined, but cleanliness -and pure salt tend to eliminate the trouble. - -=Drying= the hides is a less satisfactory cure. The principle is -similar, viz. removal of moisture. Dried hides are, however, much -drier than salted, and are quite hard and horny, hence the name "flint -hides." The hides also lose much weight, a considerable advantage in -reducing freight. Tropical hides are often flint-dry, and where -preservatives are expensive or unprocurable, it is often the only -practicable method of cure. Nevertheless, the method has many serious -disadvantages, and is difficult to execute. If dried too slowly the -hides putrefy partially; if too quickly they dry on the outside, and -the interior is left to putrefy. The fact that hides are of uneven -thickness, and the climate often hot, increases the difficulty, and -often results in partial destruction of the fibrous structure of the -hide. When dried, moreover, the hides are still subject to the attacks -of insect larvae, for the prevention of which the usual sprinkling of -naphthalene or arsenic is only an imperfect remedy. This method of -cure is also a nuisance to the tanner, who has to employ labour, pits -and time in attempting to restore the hides to their original -condition, and often loses up to ten per cent. of the goods in so -doing. Dried hides are also subject to the presence of anthrax. - -=Dry Salting= the hides is an excellent method of curing. As the name -implies, it combines methods of drying and salting which are used -alternatively. The method is used extensively in South America. A -modified form of it is also used for preserving the "E.I. kips," which -are cured, however, not with common salt, but with earth containing up -to 70 per cent. of sodium sulphate. Dry-salted hides are largely free -from the defects of dried hides, but of course are more trouble to the -tanner in the process of soaking (see Section II.) than the wet-salted -goods. - -=Freezing= the hides is now a commercial process. On the whole the -process is satisfactory, but the expansion of water after freezing may -tend to damage the hide fibres. - -Sterilizing the hides has been frequently suggested, but no method has -yet been advocated which does not interfere either with the tanning -processes or with the quality of the finished leather. - -Hides from the European Continent, usually wet salted and well flayed, -exhibit much the same variable quality as the home supply, those from -highland districts tending to be thick, yet even, well grown, tight -textured and smooth grained, whilst those from lowland regions are -less satisfactory. Thus hides from the Swiss Alps and Scandinavia have -ranked high, whilst the spready Dutch cows are typical of a lowland -hide. In the hides which once came from Germany the same features -appear. Bavarian highland hides had an excellent reputation, whilst -those from Berlin, Cologne, etc., tended to be long in shank and not -well grown. French hides are often ill flayed, and Spanish and -Portuguese are often subject to scratches. Italian hides have a very -good name, being small but stout in butt. - -The American supply is important. South America yields an excellent -class of hide, salted or dry-salted. They are from an excellent breed -of animals, slaughtered and flayed with every care, and efficiently -cured. A most serious defect in this class of hide is the "brand," -which is both deep and large and in the most valuable part of the -hide. One side, however, is usually unbranded, so that each hide -yields one good "bend." These hides, _e.g._ "Frigorifics," have -recently been much more extensively tanned in Britain because of the -shortage in the home supply of market hides caused by the European -War. South America also yields good horse hides. North American hides -are usually wet-salted (_e.g._ packer hides). They are usually good. -Central America yields mostly dried hides exhibiting usual defects. - -The Asiatic supply comprises the frozen China hides, which are clean -but small, with flaying of uncertain quality. There are the buffalo -hides from Asia and East Europe, which are suitable for cheap and sole -and strap leather, and also the dry-salted "E.I. kips," obtained from -a small breed of Indian cattle, and extensively made into upper -leather. The Asiatic humped cattle also provide a limited supply. The -African supply is of increasing importance. The tropical parts yield -dried hides of uncertain quality, but the more temperate parts of -South Africa yield a growing supply of good quality. - - -REFERENCES. - - "The Manufacture of Leather" (Bennett), pp. 27-37. - - "Principles of Leather Manufacture" (Procter), pp. 33-56. - - "The Ox Warble or Bot Fly" (E. Ormerod). - - "The Making of Leather" (Procter), pp. 2-22. - - - - - Section II.--THE PREPARATION OF PELT - - -Before hides are tanned it is necessary for them to pass through a -series of preparatory processes. The object of these processes is to -obtain from the hide the true hide substance in a pure and suitable -condition. Each class of leather has its own appropriate processes, the -adjustment of which largely determines the quality of the finished -article. So prominent is the influence of these preparatory methods that -the paradox "good leather is made before tanning" is in trade circles -almost a platitude. These processes, sometimes lumped together under the -general name of "Wetwork," comprise soaking, liming, beam house work and -deliming. These will be discussed in turn. - -The term applied to the hide after these processes, but before tannage, -is "pelt." - -=Soaking= has for its object the cleansing and softening of the hides, -chiefly by means of water. It aims at the removal of dirt, blood, dung, -and curing materials by washing. The process is usually simple, and is -much the same for all classes of leather. The ideal to be aimed at is to -restore the hide to its condition when it left the animal's back. -Cleanliness in leather manufacture is as essential at the commencement -as anywhere, for the hide is in its most putrescible state. The soluble -proteids (blood, lymph, part of dung, etc.) which always adhere to hides -encourage the rapid growth of putrefactive bacteria, and cannot be -washed away too soon. Dung is often difficult to remove, being caked on -the butt end amongst the hair. Soaking only softens it, and mechanical -removal is usually necessary. If such substances are not removed, they -go forward with the goods into the lime liquors, causing stains, -loss of hide substance, and counteracting plumping. - -The detailed method and time of soaking are determined mainly by the -nature of the cure. One of the purposes of the soak liquors is to -dissolve the salt used in curing hides and to rehydrate the hide and -make it again soft and pliable. As a 10-per-cent. salt solution exerts a -solvent effect on hide substance, it is necessary soon to change the -first soak liquor of salted goods. - -Market hides, which are uncured, require the least soaking, the -cleansing effect being most needed. The hides are inserted into pits -("water dykes") of water for a few hours, and the water changed once or -twice. The soaking should not be prolonged as the hides are so -putrescible, and where it is customary to leave the goods in a soak -liquor overnight, it is advantageous to add a little slaked lime to the -water before inserting the goods. This not only softens hard water, but -is mildly antiseptic and plumping, and forms a suitable introduction to -the liming proper. Each pit contains a "pack" of 30-50 hides, according -to its capacity, which varies in different tanneries from 1000 to 2000 -gallons. Tainted goods, which are indicated by a characteristic white -colour on the flesh side and by loose hair, need a preliminary washing -either in a "drum," "tumbler" or in a "paddle." This ensures a rapid -change of liquor and the removal of most of the putrefactive agencies. -Bad cases may need the application of antiseptics, such as immersion in -0.1 per cent. carbolic acid; but if possible these should be avoided, as -they lengthen the time required for liming. After drumming or paddling, -tainted goods should be placed directly into a lime liquor. - -Salted hides need very similar treatment to uncured hides, but the -soaking is longer, because of the dehydration caused by salting. Hence -they receive also a greater number of changes of water, three or four -usually, but often more. As much loose salt as possible should be shaken -from the hides before insertion into any liquor. The employment of drum -or paddle before pit soaking is extremely useful to effect the rapid -removal of superficial salt, and is also useful after pit soaking to -remove the last traces. - -Dried and dry-salted goods need a soaking still more prolonged, up to -one week if water alone be used. With the assistance of caustic soda, -however, the process can be shortened to about two days. The first soak -liquor should consist of a 0.1 per cent. solution of caustic soda, and -after the goods have been inserted twenty-four hours, they will be -materially improved by a few hours' drumming or paddling. Another -caustic soda soak will complete the process. Sodium sulphide crystals -may replace caustic soda, but about three times the weight will be -needed. Carbonate of soda and caustic lime also are a convenient -commercial substitute for caustic soda. For 10 lbs. caustic soda, use 36 -lbs. carbonate and 7 lbs. lime. Extra lime should be added in all cases -when the water is hard. Acid liquors will also soften dried and -dry-salted goods, but such processes do not fit in so well with the -subsequent liming. The use of putrid soaks and stocks may be now -considered out of date. - -=Liming= follows soaking, and consists essentially in immersing the -hides for 7-10 days in milk of lime. The chief object in view is to -loosen the hair and prepare for its mechanical removal. Liming takes -place in pits, the tops of which are level with the limeyard floor. -The lime is slaked completely and mixed well with water in the pit, -being particularly well plunged just before the insertion of a pack of -goods. Saturated limewater is only a 0.13-per-cent. solution. The -goods are occasionally "handled" _i.e._ hauled out of the pit and -reinserted after plunging ("hauling" and "setting"). This is necessary -to keep the liquor saturated with lime. The hides are inserted one by -one, each being "poked down" to ensure its contact with the liquor. -The goods are invariably immersed first in a previously used lime -liquor. Most tanneries now carry this out in a systematic way, so as -to ensure regularity in the process. As the goods are large and heavy -it is less laborious to carry out the whole process in one pit. In -this "one-pit system" the goods are inserted for (say) four days in an -old used lime liquor, with occasional handling; this liquor is then -run to the drain and a new liquor made up in the same pit, into which -the goods are inserted for (say) five days. They are then hauled and -sent to the unhairers. Each pack thus gets two liquors, old and new. - -A better method is the "three-pit system." In this case each pack -receives three liquors and has (say) three days in each, first an "old -lime," then a "medium lime," and finally a "new lime." This system -ensures a greater regularity of treatment, and is deservedly the most -popular method for liming hides for sole leather. After being used once -as a "new lime," a liquor then becomes a "medium lime," and after being -thus used becomes the "old lime" which receives the green hides from the -soaks. The system involves the goods being shifted twice to another pit, -which is more laborious than reinsertion into the old pit, but if the -limeyard be arranged in "sets" or "rounds" of three pits, the shift is -usually only to the adjacent pit. One special advantage of this system -is that the top hides in one pit become the bottom hides in the next -pit, and _vice versa_. Rounds of more than three pits are sometimes -used. - -Many factories have now adopted systems in which there is no handling at -all. The hides are suspended in lime liquors which are agitated by -mechanical contrivances (_e.g._ Tilston-Melbourne process), or by jets -of compressed air (_e.g._ Forsare process). The goods are soaked and -limed "mellow to fresh" by changing the liquors by means of pumps, air -ejectors, etc. Thus the hides need no labour from first being inserted -until drawn for depilation. - -In liming, the whole of the epidermis as well as the hair is loosened, -and is subsequently removed in depilation. The corium or true hide -substance becomes much more swollen by imbibation of water, and when -taken out of the new lime is "plumped" to very firm jelly. This plumping -is a matter of prime importance to the tanner. The coarser fibres are -thereby split up into the finer constituent fibrils, which fact assists -very materially in obtaining a quick and complete tannage, good weight, -and a firm leather. During the liming, the natural grease of the hide is -saponified or emulsified, which prepares for its removal in scudding. -Liming is thus a complex process: the hair is loosened, the hide is -plumped, and the grease is "killed." All these results may be hastened -by the use of other alkalies in addition, and most heavy leather yards -assist the liming by adding also sodium sulphide or caustic soda or -both. Sodium sulphide is a powerful depilatant, and will alone unhair -hides easily in strong solutions even in a few hours. As in solution it -forms caustic soda by hydrolysis, it possesses also the powerful -plumping and saponifying powers characteristic of the latter. The -addition of arsenic sulphide (As{2}S{2}) (realgar) to the lime when -slaking causes the presence of calcium sulphydrate in the lime liquors -thus made. This is also a powerful depilatant, but not much used for -heavy leather. - -The function of the lime in depilating is complex and has occasioned -much discussion. Its main purpose, however, is that of a partial -antiseptic. When hides putrefy, one of the first results is that the -hair is loosened. In America depilation by "sweating" is carried out -commercially by such a mild putrefaction, the lime liquor permits a -similar fermentation at a slower rate, and all tannery lime liquors are -swarming with putrefactive bacteria. Liming is thus a safer method than -sweating, which may be easily carried too far. Various workers have -isolated specific organisms--Wood a _bacillus_, Schmitz-Dumont a -_streptococcus_--but it seems highly probable that the limeyard bacteria -are just the common organisms of putrefaction sorted out or selected by -the exact nature of the liquor and the method of working the limes. Many -putrefactive bacteria are very adaptable and could easily accommodate -themselves in this way. It is known that the exact nature of the -culture medium has a great influence on the rate of development of such -organisms, and which particular species thrive and obtain predominance -in any limeyard will depend upon the amount and nature of the dissolved -organic matter available as food, and upon the exact alkalinity and the -concentration of other apparently inert substances, such as common salt -and sodium, calcium and arsenic salts. Hence no two lime liquors operate -alike, and approximate regularity is only assured by systematic method. -In handling and shifting, the organisms are subjected to further -selection, and the most adaptable survive. It is probable that different -species may act symbiotically. The depilating organisms of lime liquors -are probably mostly anaerobes, but some may be anaerobic by adaptation. It -is probable that aerobic ferments commence the depilation, but this will -be done before the goods are put into work, or at any rate before they -reach the limes. More strictly, it is the enzymes secreted by bacteria -which are directly responsible for the hydrolytic work; these enzymes -are chiefly proteolytic (proteid splitting), but the lipolytic (fat -splitting) enzymes have also a place. - -The lime, however, not only limits and selects the course of the -putrefaction, but also affords more positive assistance. Lime plays its -own hydrolytic part and assists the depilation by purely chemical -action. Lime will unhair without the assistance of bacteria, but its -action is slow and forms a minor part of the operation in the average -limeyard. This action is due chiefly to its progressive formation of -calcium sulphydrate from the cystine group of the softer keratins. Lime -also plays an essential part in assisting the putrefactive fermentation. -It softens the keratins and thus assists the bacterial attack, it -hydrolyzes other proteids and provides the bacteria with food in -solution, the calcium ion increases the proteolytic action of certain -enzymes, and finally the apparently inert excess of undissolved lime has -an accelerating effect on the bacterial activity. - -In the average limeyard these various functions are inextricably mixed -up, and it is impossible to assign any definite proportion of the total -depilatory effect to any of the factors at work. Lime alone will unhair, -bacteria alone will unhair, and sulphides will also unhair without lime -or bacteria, but in the limeyard all three agencies are at work. -Putrefactive fermentation, however, obtains a good start. AErobic -fermentation commences with the slaughter of the animal, and the -anaerobic organisms soon commence their part, and are at work in the hide -house and soaks. On entering the limes, the purely chemical hydrolytic -action of lime is added to that of the bacterial enzymes as well as the -action of lime as bacterial assistant, and the three continue to operate -side by side. Each gives rise to the formation of calcium sulphydrate, -whose own special solvent effect is superadded. If sulphydrates be -deliberately added to the liquors there is yet another factor assisting. -Speaking broadly, the bacterial enzymes have their maximum activity in -the old limes, and the chemical action of sulphydrate formed from the -keratin cystine is also at a maximum in these liquors. The chemical -action of added sulphide, and the simple hydrolytic action of calcium -hydrate have their maximum activity in the new limes. Most observers -would agree that in practice the bacteria shoulder the greater part of -the work. - -From the limeyard is taken about the only waste bye-products of the -tannery, viz. the residues from the soak and lime pits. These consist -mainly of lime and chalk, with some hair and dung, and possibly a little -sulphide. The sludge possesses some value as a manure, especially if -from the soak pits on account of the greater nitrogen content. (Part -VI., Section I.) - -=The Beam House Work= consists in the mechanical removal of those -parts of the hide not wanted for leather manufacture. _Unhairing_ -removes the hair and the epidermis made loose in liming. The hides are -placed over a sloping "beam" with a convex surface, and the hair scraped -off with a blunt concave and double-handled knife. The hides are then -thrown into a pit of water. The hair is carefully collected, washed well -with water, preferably centrifuged, and then dried out by a current of -warm air. It forms a valuable bye-product. White hair is usually kept -separate and fetches a higher price. _Fleshing_ is the next process. The -hides are again placed over a beam, with the flesh side (_i.e._ the side -nearest the flesh) uppermost. Skilled workmen then cut off, with a sharp -convex knife, the fat, flesh and connective tissue left in flaying. -_Rounding_ is usually the next process. The unhaired and fleshed hide is -spread out flat and cut up into butt, shoulder and a pair of bellies. -These parts have different commercial values, and may afterwards be -tanned by different methods for very different purposes--for dressing -leather, and sometimes even for sole leather. _Scudding_ is the last -piece of beam work. The fleshed hides (whether rounded or not) are -washed, or at least rinsed, with water, and again placed on the beam -grain side up. They are then scraped with a rather sharp concave knife, -to remove "scud," which consists of hair roots and sheaths, lime soaps, -fat, pigment and other dirt. Short hair is shaved off by a very sharp -hand knife. - -The beam work demands a certain amount of skill from the workmen, -especially from the flesher, whose sharp knife may prove very wasteful -in incompetent hands. Hand labour was slowly but surely being replaced -by machinery before the war, and war-time conditions have greatly -accelerated the rate of transition. Beam house machinery is rapidly -becoming universal. The machines are cumbrous and expensive in cost and -in power, but machine work is quicker, less laborious, and needs much -fewer workmen. Many types of machine have been suggested, but the most -useful are those in which the hides pass over rollers and are -simultaneously acted upon by a rapidly revolving cylindrical knife with -spiral blades, one half being a left-handed and the other a right-handed -spiral, so that the hide is scraped outwards as well as in the direction -of motion. The part of the hide being acted upon rests on a pneumatic -roller. By changing the type of spiral knife cylinder the machine will -unhair, flesh or scud. - -=Deliming= is a general name covering a number of similar operations -whose primary object is the neutralization and removal of the caustic -lime and soda in the plumped pelt, or at any rate on the surface of the -hide. This is a preparation for the tan liquors. All the tannins and -many associated substances darken rapidly with oxidation when in -alkaline solution, so that to place the fully limed hide in a tan liquor -would give a dark-coloured leather. A short insertion in a bath of weak -acid would secure the elimination of surface lime and the disappearance -of this difficulty, but there are other purposes in deliming. The more -completely lime is removed the more the plumped pelt "falls" into a -soft, pliable, unswollen and relaxed condition, and this change assists -very materially in the production of a soft dressing leather, suitable -for boot uppers, bags, etc. For such leathers, therefore, the deliming -must be much more complete than for sole leather, in which the object is -to obtain a firm and plump leather. - -In the case of the softer dressing leathers, experience indicates the -advisability of allowing some further bacterial action on the -interfibrillar substance in order to produce the requisite pliability -and softness. This is secured by "bating" the hides. This process -consists in immersing the goods into a cold fermenting infusion of hen -or pigeon dung. The infusion is made in a special tub or pit with warm -water and allowed to stand for a day or two until the fermentation has -commenced, and then run into the bating pit through a coarse filter such -as sacking. The hides are immersed for some days, but are handled -frequently to ensure an even effect. The bate is always slightly -alkaline. The caustic alkalinity increases rapidly at first owing to the -diffusion of caustic lime, then at a slower rate, afterwards slowly -declining. This is explained by the production of organic acids, and -their salts with weak bases from the dung infusion by the action of -bacteria. The total alkalinity of the bate liquor increases rapidly at -first owing to the diffusion of lime and its liberation of organic -bases, then very slowly, but towards the end of the operation the total -alkalinity increases very rapidly indeed, owing probably to the -commencement of a violent anaerobic fermentation which produces ammonia -and other organic bases, and which heralds the approach of a -putrefactive action, which if allowed to continue for even a short time -will ruin the hides. Bating is consequently a risky process, and needs -experienced oversight. For goods which need only a mild bating, there is -the alternative of giving a longer liming in older limes. This of course -involves more bacterial hydrolysis, and perhaps does it in a safer, more -economical and certainly in a less offensive manner. Bating is often -followed by a further deliming by acids. Boric, lactic, acetic, formic -and butyric acids are all used, and with care even hydrochloric and -sulphuric acids may be employed. Innumerable "artificial" bates have -been put on the market, but most are merely weak acids, acid salts or -salts of strong acids with weak bases. An American "bacterial bate" -consists of a lactic fermentation of glucose in the presence of glue. - -Closely similar to bating is "puering," investigated by Wood. - -Drenching is another fermentive deliming process. In this the goods are -inserted into an infusion of bran. This is made by scalding the bran -with hot water, and allowing it to stand until it is about 70 deg.-90 deg. F. -The infusion is then "inoculated" with a few gallons of old drench -liquor, and the goods are immersed. This fermentation has been examined -carefully by J. T. Wood. First the enzyme cerealin converts bran starch -into glucose, which is then fermented by the drench bacteria with the -production of lactic acid, some acetic acid and small amounts of formic -and butyric acids. The butyric fermentation is liable to become too -violent. These acids, as they are formed, neutralize the lime in the -hides and plump the pelt slightly. - -Various gases (carbon dioxide, hydrogen, nitrogen, methane and -sulphuretted hydrogen) are involved, and the proportion produced in the -pelt itself has a peculiar opening effect on the hide fibres. The -activity of the drench can be decreased by dilution and by using a less -starchy bran, and can be increased by adding pea meal or rye meal. - -Drenching usually follows bating. Scudding sometimes follows deliming. - -The theory of the volume and elasticity changes of pelt during -preparation will be better understood after considering the behaviour of -gelatine gels. The determining factors are the _nett_ -charge of hydroxyl ions on the disperse phase, resulting from ionic -adsorptions, and the lyotrope influence of dissolved substances on the -continuous phase. - -In softening dried hides the swelling may be due to either influence, -but the latter tends to loss of hide substance and the production of -soft leather. - -In liming, the nett adsorption of hydroxyl ions is the principal factor, -but the lyotrope influence of the alkali cations and of the impurities -is important. Plump pelts are those in which the contained water is in a -relatively greater average state of compression. Few substances can -assist plumping, but many can hinder it. In plumping all lyotrope -influence is objectionable, and "sharp" (pure) alkali solutions are -required. Mellow limes reduce elasticity and plumpness by lyotrope -influence. - -In bating and puering the essential change is that before the process -the swelling is due chiefly to adsorption of hydroxyl ions, whereas -afterwards it is due chiefly to a composite lyotrope influence. - - -REFERENCES. - - "Principles of Leather Manufacture," Procter, pp. 108-184. - - "The Manufacture of Leather," Bennett, pp. 49-113. - - "Lyotrope Influence and Adsorption in the Theory of Wetwork," - Bennett, _J.S.L.T.C._, 1920, pp. 75-86. - - "Analytical Examination of Bating," Bennett, _Leather Trades - Review_, 1911, p. 972, and 1912, p. 28. - - "The Bating, Puering and Drenching of Skins," by J. T. Wood. - - - - - SECTION III.--VEGETABLE TANNAGE - - -All tannages have for their object the conversion of the readily -putrescible hide tissue of the corium (the pelt) into an -imputrescible, insoluble and permanent material called "leather" -which, possessing considerable strength and pliability, is capable of -application to a variety of useful purposes. The conditions necessary -for this transformation have been clearly stated by Procter.[1] For -the production of leather from pelt "it is not only necessary to dry -the fibres in a separate and non-adherent condition, but so to coat -them or alter their chemical character that they are no longer capable -of being swelled or rendered sticky by water." Whatever substance will -secure this permanent dehydration of the hide fibres in a separate -condition is called a "tanning material." The change from pelt to -leather is known as "tannage," the process is termed "tanning," and -those who undertake it are "tanners." - -[Footnote 1: "Principles of Leather Manufacture," p. 184.] - -In "vegetable tannage" the tanning materials are of vegetable origin, -and contain a group of organic compounds called "tannins" which are -extracted by the infusion of these materials with water. Pelt, when -immersed in these infusions, is converted into leather, rather slowly; -but a gelatin solution gives an immediate precipitate of "amorphous -leather," even if the tannin infusion be exceedingly dilute. The -tannins are aromatic compounds of phenolic character, and contain -carbon, hydrogen and oxygen only, but our knowledge of their chemical -constitution is exceedingly small owing to their instability and -colloid nature, which make impossible their preparation in a pure -state. They are all, however, derived from either catechol or -pyrogallol, and yield these substances if carefully heated to about -200 deg. C. The tannins are soluble in water, alcohol, acetone, ethyl -acetate and acetic acid, but insoluble in benzene, chloroform, carbon -disulphide, petroleum ether, -dilute sulphuric acid and _pure_ ethyl ether. The aqueous infusions of -the tannins are in reality colloidal solutions; _i.e._ heterogeneous -systems of two phases. The systems are lyophile, or, more particularly, -hydrophile, _i.e._ there is an affinity between the two phases. As usual -with lyophile systems the two phases may be considered as both liquid, -and an aqueous infusion of tannin forms an emulsoid sol, which therefore -is subject to the phenomenon of adsorption. The tannins are all -precipitated by solutions of basic lead acetate and copper acetate, and -many of them with varying completeness by solutions of many other -metallic salts and hydroxides, of basic dyestuffs and of alkaloids. They -give dark colorations with ferric salts. - -The tannins are widely distributed in plant-life, but only in a limited -number of cases do the plants contain sufficient tannin to render them -of commercial importance. Tannin is found in all parts of plants, but -usually in greatest amount in the bark or fruit. The tannins are -classified into "pyrogallol tans" and "catechol tans," according to the -parent phenol. This classification is confirmed by their chemical, -analytical and practical behaviour, and the vegetable tanning materials -may be classified into the same two groups, for, although even the same -plant contains both pyrogallol and catechol tans, it is usual to find in -any one part of the plant that one group is predominant. - -=Pyrogallol tans=, which are oftenest obtained from fruit or leaves, -contain usually about 52 per cent. of carbon. Used alone they produce a -rather soft and porous leather. Associated with them--in many cases -probably as decomposition products--are certain other substances of -well-known properties and constitution. These substances are not only -typical of the group, but also form the most valuable clue to the -chemical constitution of the group and the key to their chemical -behaviour. One of these substances is gallic acid (3:4:5 -trihydroxy-benzoic acid C{6}H{2}(OH){3}COOH), which possesses -properties very similar to the tannins, but does not precipitate gelatin -and will not itself make leather. Another of these substances is ellagic -acid C{14}H{6}O{8}, a double lactone of a -hexa-hydroxy-diphenyldicarboxylic acid. This is deposited as an -insoluble yellow powder from infusions of many pyrogallol tans, by -boiling with dilute acids only, allowing them to stand for a few days. -In practice the deposit is found as mud at the bottom of the tan pits, -and also upon the leather, to which it strongly adheres. It is -technically known as "bloom." It is insoluble in acids and cold alcohol, -but soluble in alkalies. It is a feeble dye-stuff. The pyrogallol tans -yield very different amounts of bloom. Other associated substances are -the sugars. In practice these sugars ferment to lactic, acetic, and -other acids which cause "sour" liquors. Such liquors plump the hides and -tend to give firm, thick leather. These acids also probably cause -increase of adsorption of tannin by the hide and therefore assist in -giving "good weight." Solutions of pyrogallol tans all give a blue-black -colour with a dilute solution of ferric alum. If a solution of sodium -arsenate be added to an infusion of pyrogallol tan diluted until no -longer distinctly coloured, and the mixture allowed to stand for about -two hours, a green colour develops at the surface of the liquid. The -reaction is due to gallic acid or a similar grouping, and is, in the -author's experience, the most satisfactory qualitative test for the -group. Another test is to mix equal volumes of a 0.4 per cent. infusion -of tan and a 10 per cent. solution of sodium bisulphite; a few drops of -10 per cent. potassium chromate are added, and either a transient -blood-red colour or a more permanent deep purple is obtained. The former -colour is due to gallic acid. If a tannin infusion be largely diluted -with hard water and a little iodine solution added, the pyrogallol tans -yield either a purple-red or a dark blue colour, the former being a -reaction of gallic acid. Pyrogallol tans yield no precipitate with -bromine water. They yield a yellow or brown colour when one drop of -infusion is added to concentrated sulphuric acid. - -Myrabolans is one of the most important of the pyrogallol tanning -materials. It is a name given to the dried fruit of _Terminalia chebula_ -and other species of Indian trees. The nuts resemble an elongated -walnut. They are dried and exported from many parts of India to all -parts of the world, but largely to this country. The varieties of -commerce are named according to origin and quality: thus we have "J1's," -_i.e._ Jubbelpore, No. 1 quality, "R1's" (Rajpore, No. 1), -"B1's" (Bhimley, No. 1), etc. The little difference in tannin strength -(about 32 per cent.) in these varieties is usually compensated by -corresponding differences in price. The quality of myrabolans cannot be -safely judged by appearance. Much bloom is deposited by myrabolans -liquors, especially by "J's." Myrabolans are amongst the most sugary of -tanning materials, containing up to 5-1/2 per cent. It is therefore one -of the best materials for giving a plump leather. Broadly speaking, -those varieties which yield most sugar yield least bloom, and _vice -versa_. Myrabolans tannin has a small affinity for hide substance and -penetrates the hide very slowly. It gives a "mellow" tannage, but a -bright, good colour, which characteristics are imparted to the leather -when the material is blended with other materials containing dark or -astringent tannins. When used alone it yields a rather spongy leather, -and it is not considered a good weight-giving material, though its -acid-producing powers are very helpful to other more astringent tannins. - -Valonia has been the other staple tanning material of the heavy leather -trade. It is the acorn cup of oaks common in Asia Minor and Greece, -chiefly the Turkish oak (_Quercus aegilops_). The fruit is gathered when -ripe and dried in layers of about one foot deep until the acorn drops -out, Smyrna is the great export centre. Greek valonia is obtained from -many parts of the Archipelago and mainland. It is gathered in a more -immature condition and includes the acorn. It is considered slightly -inferior in strength and colour to the Smyrna valonia. The exterior of -the acorn cup is covered with rather scaly protuberances known as -"beard," which contains usually about 40 per cent. of tannin. The cup -alone contains usually about 25 per cent. tannin, and the whole about 30 -per cent. The valonia tannin has been thought to contain two chemical -individuals, only one of which produces bloom. Parker and Leach[2] found -that the tannin of the cup produces more bloom than that of the beard, -and that Smyrna valonia yields more bloom than Greek. The more bloom is -deposited, the less acid will be produced. Under all conditions the -yield of bloom is large, and its deposition in and on the leather -assists materially in giving the weight and water-resisting powers -associated with sole leather which has been largely tanned with valonia. -The valonia tannins have only a moderate affinity for hide, which, like -myrabolans, they penetrate very slowly. When used alone the leather is -less yellow than that from myrabolans, and is also duller. After most of -its bloom has been deposited valonia makes a very suitable tannage for -dressing leather, and in conjunction with gambier has been largely thus -used. Since the outbreak of war the Turkish product has, of course, not -been available for importation. - -[Footnote 2: _J.S.S.I._, 1903, 1184.] - -Sumach[3] is the other pyrogallol tan of commercial importance. It -consists of the leaves and small twigs of the Sicilian sumach (_Rhus -coriaria_) cultivated in Italy extensively for export. The leaves are -hand picked, dried and often ground to powder. It contains 26-28 per -cent. of a tannin which yields little or no bloom, but much gallic acid. -It is an unstable tannin, and its infusion rapidly ferments. Sumach is a -very valuable tanning material, and when used alone gives an exceedingly -durable leather of excellent light colour. It gives a soft mellow -tannage, and is therefore most suitable for light leather tanning, and -is extensively used for this purpose. It is used, nevertheless, in large -quantities by the heavy leather tanners for finishing purposes, for it -contains some organic reducing agent which exerts a powerful bleaching -action on other tannages, and which assists to brighten as well as -lighten the rather dull appearance of leathers largely tanned with -valonia. It is rather an expensive tannin, but most manufacturers find -that its results are worth its cost. - -[Footnote 3: Also spelt Sumac and Shumac, and always pronounced like the -latter.] - -Other pyrogallol tans are also used to a limited extent. Algarobilla and -divi-divi are the fruit pods of several species of American _caesalpina_. -They are strong in tan (45 per cent.) and yield a light-coloured and -bright leather, but are unstable tans, yielding much bloom. Babla is a -small pod yielding a mellow tannage and much gallic acid. Celavinia is -another pod containing no colouring matter and giving an almost white -leather. The tannin is closely similar to that of oak galls. These last -were once extensively used for tanning in Austria. Willow bark is used -for tanning in Russia and Denmark. Valuable pyrogallol tannins are -obtained from oak wood and chestnut wood, but the woods are not used in -tanning as the percentage of tan is so small. - -=Catechol tans=, often obtained from barks, contain usually about 60 per -cent. of carbon. They are seldom used alone, for they usually have -little or no sugar associated, and hence their liquors do not either -"sour" or "plump." They can be used alone if artificially acidified, but -without acidifying or blending would give a rather flat leather, though -possibly firm. They yield no bloom or gallic acid, but have associated -with this other characteristic substances. Of these the catechins are -the most typical, and have been considered as the parent substances of -the catechol tans. - -The catechins are white crystalline substances, apparently isomers with -the general formula C{15}H{14}O{6}. They have different -melting-points, and varying amounts of water of crystallization, but are -otherwise exceedingly similar in properties. They are sparingly soluble -in cold water, but freely in hot, and in alcohol and ether. They are -precipitated by lead acetate, mercuric chloride and albumin, but not by -gelatin, tartar emetic or alkaloids. In gambier liquors they are -especially strong, and sometimes crystallize on the side of the pits, -being thus known as "whites." The phlobaphenes or "reds" are also -typical of catechol tans from which grow catechins; they can be formed -by boiling with dilute mineral acids. They are considered to be -anhydrides of the catechol tans. They are difficultly soluble in cold -water, but freely in hot, and in cold alcohol and dilute alkalies. They -are true tannins and alone are capable of making a red leather, but in -practice are often found as mud in the tan liquors owing to their -limited solubility. They naturally influence the colour of leather made -with catechol tans, which is usually distinctly redder than the leather -made from pyrogallol tans. Infusions of catechol (_cp._ catechin) give a -green-black colour with iron alum. The sodium arsenate test gives a red -colour due to catechin. The chromate and iodine tests mentioned for -pyrogallol tans give negative results with the catechol tans, but -bromine water gives a precipitate, and sulphuric acid a crimson colour. - -Mimosa bark is one of the most important catechol tans. It is usually -obtained in this country from Natal ("Natal bark"); but the tree (Sydney -green wattle, _Acacia mollissima_) is a native of Australia. It is being -cultivated now extensively in South Africa, and forms a most valuable -portion of the Empire's stock of tanning material. Its more extensive -use has been long recommended by the author,[4] but its gradually -increasing employment in British tanneries has been greatly accelerated -by the war, which has prevented its delivery in Germany and has cut off -Turkish valonia from Britain. It yields about 30 per cent. of a stable -and excellent tannin, and will produce a firm, durable leather, with a -colour much less red than that obtained from many other catechol tans. -It is an astringent tan, and if carelessly used yields a harsh or even -"drawn" grain. Most of the tannin is easily extracted, yielding a clear -infusion which penetrates fairly quickly and gives good weight. It -contains less than 1 per cent. of sugar, which unfortunately rapidly -ferments to carbonic acid, so that it is not a good plumping material. -It makes in all respects an excellent blend with myrabolans. Like all -catechol tans, the resulting leather darkens on exposure to sunlight. - -[Footnote 4: J.S.C.I., 1908, 1193.] - -Oak bark, from _Quercus robur_, is the ancient tanning material of -Britain, and is still used to a limited extent. It contains about 13 per -cent. of tannin and is mainly a catechol tan, but also contains a -pyrogallol derivative. It yields catechin, and gives a red colour with -the sodium arsenate test, but also will yield some bloom and gallic -acid, and gives a blue-black with ferric salts. The tannin itself is -exceedingly similar to that of mimosa bark, but the material contains -about 2-1/2 per cent. of sugar, which makes it possible to employ oak -bark alone for making sole leather. It is noted for yielding a sound, -durable leather of good typical tan colour. Its tannin combines well -with hide and penetrates quickly. The fatal disadvantage of oak bark is -its weakness in tannin strength compared with other materials. This -results in heavy freight and heavy cost per unit tannin, bulky storage, -expensive handling in the factory, comparatively large bulk of spent -tan, after relatively greater trouble in extracting, and the -impossibility of making the strong liquors so necessary in these days to -produce good weight in a short time. No satisfactory extract has yet -been made from it. - -Pine bark, from _Pinas abies_, is one of the staple materials of the -Continent. It contains up to 14 per cent. of a catechol tan, and, unlike -most of this group, contains a high proportion of sugar and will give -good results alone. Hemlock bark has been the staple tanning material of -North America. It is obtained from the hemlock, or _Pinus canadensis_. -It contains up to 11 per cent. of tan and much phlobaphene, and yields a -characteristic red leather of good quality, but which rapidly darkens -with sunlight. It contains some sugar, but is usually employed in -conjunction with sulphuric acid or with sugary materials. Mallet bark -yields another catechol tan similar to that of mimosa, but somewhat less -astringent and more yellow in colour. Quebracho wood and mangrove bark -have been used, but are now made into extracts. - -=Leaching.=--Whatever class of leather is being made, and whatever blend -of tanning materials is being employed, the tannins must be efficiently -extracted by water in order to make the tanning liquors. This process is -called "leaching." The tanning materials, after being ground, crushed or -shredded, are placed in large pits arranged in "rounds," "sets," or -"batteries" of 6, 8 or 10 units, through which water is percolated -systematically, so as to secure a continuous extraction. Water itself is -added to only one of the pits of material. The liquor produced is passed -on to the next pit, and then to the next, and is continually gathering -strength. After passing thus through the series, the liquor becomes the -source of the strong extracted tan liquors which are used in the tannery -proper. With this system the stronger leach liquors are being acted upon -by fresh material, and the nearly "spent" material is being acted on by -the weakest liquors, and finally by water, thus ensuring a complete -extraction. In the press leach system, which is now practically -universal, the bottom of one pit communicates with the top of the next, -and the liquor presses round by gravity flow caused by a few inches -"fall." Liquor is thus constantly percolating downward through the -material in each pit. The "head leach" and "tail leach" are always -adjacent in a double row of pits, and when the material in the latter is -quite spent, it is "cast," and the pit is filled with fresh material. -The liquor is then pressed round into this pit by adding water to the -tail leach. Hot water is used to secure better diffusion. At least two -such sets of leaches ("taps" and "spenders") are necessary to spend the -material of the average tannery and to obtain liquors of the necessary -strength. - -=The Manufacture of Extracts.=--In addition to the use of the natural -tanning materials described above, modern leather manufacturers employ -also a variety of "tanning extracts," _i.e._ vegetable tanning materials -in which the tannin has been already extracted, and which are supplied -in form of a solid or concentrated liquid. Such extracts only need to be -dissolved in warm water in order to make a tan liquor, and the cost and -trouble of leaching is avoided. They are a great convenience as making -strong liquors of definite strength. Many vegetable tanning materials -are too weak in tan for the tanner to leach, and indeed to justify the -cost of importation have been made available by manufacturing an extract -at the source of the material. With such weak materials the extract -manufacturer has had to secure a much more complete extraction than in -ordinary leaching, and to concentrate his infusions by means of -steam-heated vacuum pans. With such experience he has naturally begun to -make extracts also from the stronger materials, such as myrabolans and -mimosa bark, and it is now possible to have a tannery without any -leaches at all. Tanners also have begun to realize the advantages not -only of more rapid and complete extraction, but also of doing the work -for themselves, and extract factories are beginning to appear as an -adjunct to the larger tanneries. The more complete extraction of tan -also involves a greater extraction of unwanted colouring matters, hence -decolorization is a feature of extract manufacture. - -=Chestnut Extract= is from the wood of the Spanish chestnut (_Castanea -vesca_), which contains 3-6 per cent. of a valuable pyrogallol tan very -similar to that of valonia. Its weight-giving and water-resisting powers -are as good as valonia, and its penetrating power is even better, so -that it forms an exceedingly suitable material for the modern short -tannage, and also for drum tannages. The extract is manufactured -extensively in France. The wood is stripped of bark and usually piled -for some months to dry and to allow the resins to become insoluble. Some -factories, however, use the green wood direct. There are two methods of -extraction, viz. in open vats and in closed vats under pressure. The two -methods yield extracts which differ in composition and properties. In -either case the vats have a capacity of up to 3,000 gallons, and hold up -to 6-1/2 tons of wood. They are arranged in series, as in leaching, and -the liquor passes in succession through all the vats over wood less and -less spent. The temperature is highest in the vat containing the fresh -water and nearly spent wood. In open vats of wood or copper the -temperature is near boiling-point, whilst in the closed autoclaves -(copper or bronze) the pressure reaches about two atmospheres and the -temperature about 130 deg. C. (266 deg. F.). The series may contain 5, 7, 9 or -even 12 vats, and the liquor obtained has a strength of 3 deg. to 4-1/2 deg. -Beaume (22 deg. to 33 deg. Bkr.). - -After extraction the liquor is allowed to stand, and much insoluble -matter settles out--resins, wood, fibre, etc. The clarified and settled -liquor is then passed through a cooler up to about 55 deg. C., and then run -into the decolorizing plant, a deep vat fitted with a copper steam coil -and mechanical stirrer attached to power. The best decolorizer is -bullock's blood, which is run into the vat and well mixed. The -temperature is next raised to about 70 deg. C., causing the blood albumin to -coagulate. It carries down with it a little tannin, but much colouring -matter. After standing a few hours the settled liquor is run off direct -to the evaporator. A multiple-effect evaporator is usually employed, and -the concentrated liquor, which has a strength of about 25 deg. Beaume, is -run into suitable oak casks. The extracts contain 27-32 per cent. -tannin. An extract made with open vats has about 7 per cent. soluble -non-tanning matters, whilst a "pressure extract" may contain up to 12 -per cent. of these "non-tans." Pressure extracts obtain also a better -yield of tannin, which more than compensates for the slightly lower -price. Open extraction yields, however, the purer product and an extract -with better penetrating powers, and is consequently the more suitable -for drum tannages. Chestnut extract is extensively used by the heavy -leather tanners. - -=Oakwood Extract= is manufactured from the wood of the common oak -(_Quercus robur_). The centre of the industry has been the oak forest of -Slavonia. The wood contains 2-4 per cent. of a tannin very similar to -that of chestnut wood, but somewhat more astringent. - -The manufacture is also similar to that of chestnut extract, but -decolorization is often omitted, and greater care has to be taken and in -other ways to keep the colour within limits. One of these is to strip -the wood more completely of bark. Another is to operate at as low a -temperature as possible, about 110 deg. C. The extraction is made in large -circular vats about 14 feet high and holding about two tons material. A -battery is composed of about eight vats or extractors. Open extraction -is used, and the liquor is passed forward after 2-3 hours' boiling, so -that the material is spent in about 24 hours. A liquor of about 5 deg. Be -(36 Bkr.) is obtained, and the strength of the material reduced from 4 -to 1/4 per cent. of tannin. Getting rid of insoluble matter is a -difficulty, and is attained by settling, by rapidly cooling, and then -passing through a filter press of wood. For evaporation a double-effect -vacuum pan is preferred, which operates first at about 113 deg. F., and -afterwards at 140 deg. F. with a higher vacuum. The liquor is concentrated -from 5 deg. to 25 deg. Beaume (s.g. 1.036 and 1.210 respectively). - -The extract has a much higher colour than chestnut, and is not used now -as much as some years ago. As the principal supply was German, it has -been unavailable. - -=Quebracho Extract= is made from the wood of the South American tree -_Loxopteryngium Lorenzii_, which contains about 20 per cent. of a -typical catechol tan. It is associated with a little catechin, much -phlobaphene, but practically no sugar. The tannin is very astringent, -penetrates quickly and gives a firm red leather which darkens on -exposure to light. It is not noted for weight-giving powers. The wood -itself, as chips or shavings, has been used in British tanneries, to a -limited extent, but the great bulk of the material is made into extract -chiefly in South America. The crude "extract," made by evaporating -aqueous infusions of the wood, is largely exported for refinement in -Europe. It is also refined on the spot to a large extent and converted -into solid extract containing 60 per cent. of tannin. - -The great difficulty with quebracho has been the disposal of the -phlobaphenes, and a great variety of quebracho extracts are now -available which deal with this problem in different ways. In some the -more soluble reds are simply left in the extract under the idea that -they are really tannins and may be of some use in some part of the -tanning process; in others they have been removed by settling and -filtration at appropriate temperatures and concentrations; in most, -however, they have been solubilized by treatment with alkalies, in the -presence of reducing agents, notably by heating with sodium bisulphite -in closed vats. The base combines with the phlobaphenes, which are made -completely soluble and available for tanning. Sulphurous acid is -evolved, and its reducing powers assist materially in retaining and -promoting a good colour in the product. Such "sulphited extracts" are -now extensively manufactured in this country from the imported "crude" -extract, and sold as liquid extracts containing 30, 35 or 40 per cent. -of tan according to the requirements of the buyer; "mixed extracts" -which are solubilized quebracho blended with about 15 per cent. of -myrabolans, are also used. - -By solubilizing quebracho with excess of bisulphite an extract is -obtained which possesses considerable bleaching powers, and such -extracts are also extensively manufactured for the "vatting" or -bleaching of heavy leather after tannage. The excess of sulphurous acid -not only bleaches the leather, but also swells it up and thus permits a -further absorption of strong tan liquor, which is conducive to good -weight. These bleaching extracts are usually of 36-38 per cent. strength -in tan. - -=Gambier= is an extract of the leaves and twigs of the eastern shrub -_Nauclea gambir_. It is a catechol tan of peculiarly mellow quality and -great practical value. It contains much catechin, but little -phlobaphene, and yields a beautifully soft leather, but without weight. -It is an exceedingly suitable material for the early stages of tanning, -and is much liked for tanning leathers that have to be curried, and is -widely used in the manufacture of upper leather. It is, however, an -exceedingly expensive tannin, and the extract is made in a very crude -way by Chinese and Malays without much supervision. Hence its strength -in tan and general quality is extremely variable. The plant is -cultivated for the purpose of extract manufacture, and prunings are -taken in the plant's third year. They are bruised and boiled with water -in the open. The infusion is strained, concentrated, and poured into -cooling vessels in which it sets to a paste. Two varieties of gambier -are well known, "cube gambier" and "block gambier." In the latter the -extract remains as a paste containing 25 to 40 per cent. of tannin. It -is sold in oblong blocks of 1 or 2 cwt., either wrapped in cocoanut -matting or in wooden boxes. Cube gambier is made by running the -concentrated syrup into trays 2 inches deep and drying in the sun. When -partly dry, it is cut up into 1-1/2-in. cubes and dried further on -cocoanut matting. The rough "cubes" as imported contain 40-50 per cent. -of tannin. - -=Myrabolans Extract= is now largely manufactured in this country. A -liquid extract of 25, 30 or even 35 per cent. strength is made for -home consumption, and a solid extract for export. The light colour, -high strength and easy extraction of the natural material have all -facilitated the task of the manufacturer. - -The material is extracted in open vats or stills of copper, which take -one ton or more of nuts. A battery of 4, 6 or 8 of such stills is -usually employed, and the temperature is kept well below boiling-point -except in the vats containing the nearly spent material. The liquors -move forward quickly, and the material is quite spent in 24 hours. The -material when cast contains less than 1/2 per cent. of tannin. The -liquor obtained is 40 deg.-50 deg. Bkr. (6 - 7-1/2 per cent. tan), and after -settling is concentrated at 40 deg.-50 deg. F. in a single effect vacuum pan, -which though more costly in steam is quicker than the multiple effects, -and gives the low temperature required. For solid extract the more -concentrated liquor is run direct into tarred bags, in which it soon -solidifies. - -=Hemlock Extract= is manufactured from the North American pines and -imported into this country to some extent. It gives a very red colour. - -=Mangrove Extract= is made from the bark of _Rhizophona Mangle_ and -other species of mangrove which grow freely in the tropical swamps of -West Africa, Borneo, etc. Much solid and liquid extract has been made -from this material, but is not very popular on account of its harsh -tannage and dark red colour. - -=Pine Bark Extract= (_Larch extract_) is made in Sweden from the Norway -spruce (_Pinus abies_). It is slightly sulphited and gives a good -colour. It is a liquid extract of about 30 per cent. strength, and is -sometimes used as a chestnut substitute. It should not be confused with -the so-called "spruce" or "pine wood" extract, which is a paper trade -bye-product and contains ligneous matters rather than tannin. - -=American Chestnut Extract=, made from the chestnut oak, is either a -liquid or a solid extract in powder form. It gives a wretched -brown-black colour, which is quite unsuited to the usual British needs. - - - - - THEORY OF VEGETABLE TANNAGE. - - -Vegetable tannage is a phenomenon of colloid chemistry. The old -arguments as to whether tanning was a chemical or a physical process -have been rendered obsolete by the advent of a new set of explanations, -which, though shedding light on many obscure points, have enormously -increased the complexity of the problem. In vegetable tannage an -emulsoid gel (pelt) is immersed in a complex emulsoid sol (tan liquor), -which immersion results, not in simple reaction or change, but in a -series of changes. - -One of these changes is _adsorption_. Pelt is a gel which possesses a -great development of surface. It not only exhibits like gelatine the -phenomenon of imbibition and dehydration to a very marked extent, but -also possesses a very fine fibrous structure due to its organic origin; -thus pelt possesses an enormous specific surface, further intensified by -the preparation processes previously discussed, which split up the hide -fibres into smaller bundles and into much finer constituent fibrils. -Tannins, on the other hand, are hydrophile colloids which in water form -emulsoid sols, and which may thus be expected to exhibit the phenomenon -of adsorption. A tan liquor usually contains several tannins in addition -to other closely similar substances, also in colloidal solution, and is -therefore a sol of considerable complexity. The immersion of pelt into a -tan liquor results in an adsorption, which consists essentially in an -inequality of concentration in the sol, the greater concentration being -at the interface. This inequality between the surface concentration and -the volume concentration of the sol, is due primarily to considerations -of surface tension and surface energy, and exists before the immersion -of the pelt. The surface layer having excess over the volume -concentration, any considerable extension of surface in a fixed volume -of sol must produce a very considerable decrease in the volume -concentration. This is what occurs when pelt is immersed in a tan -liquor, the immersion being the considerable extension of surface. It -should be especially remembered that the inequality of concentration is -in the sol, on the liquid side of the interface. In adsorption, the -substance adsorbed, _i.e._ the excess at the surface, is too frequently -regarded as bound to the solid immersed. This is because the excess is -in the layer which wets the solid and remains wetting it when the solid -is removed. Thus the immersion of pelt produces primarily only a change -in the distribution of the tannins in the liquor. It follows from this -that the adsorption is an equilibrium, and that if the sol be diluted, -the equilibrium will become the same as it would have been by immersing -the pelt directly into the dilute solution. Thus, if pelt be first -immersed in one tan liquor and then into a weaker one it will yield tan -to the latter solution. - -The chief object in heavy leather tanning is to obtain the maximum -possible adsorption in the minimum possible time, or in other words, to -obtain good weight quickly. The amount adsorbed is proportional to the -actual extension of surface, _i.e._ the adsorption is a function of the -specific surface of the adsorbent. Hence, to obtain good weight it is -necessary to develop in the pelt its maximum possible specific surface. -This is one of the objects of "plumping," which splits up the fibres. It -is attained also by the solution of interfibrillar substance in limes -and bates. - -The amount adsorbed is also a function of the volume concentration in -the sol after equilibrium is reached. Hence the better weights are -obtained with stronger liquors. - -The adsorption law is - - y/m = ac^(1/n) - -where y is weight adsorbed by the weight m of adsorbent, and c the -volume concentration after adsorption; a and n are numeral constants. -Hence weight is determined by the strength of the liquor which the goods -finally leave. The commencement of tannage is necessarily in weak -infusions, in order to secure the maximum diffusion into interior of the -fibres before they become heavily coated on the exterior. As the -equilibrium is being established in such liquors the volume -concentration diminishes, and thus makes it less likely that good weight -will be attained; hence it is necessary in practice to move the goods -constantly into fresh liquors of gradually increasing strength, and so -maintain the rate of adsorption and save time. A further consequence of -the adsorption isotherm is that as y varies as c^(1/n) and n is > 1, y -is increased appreciably only by a relatively large increase in c. -Hence, though stronger liquors give better weight, there is a limit -beyond which any further gain in weight is not justified by the enormous -increase in the concentration necessary to attain it. Such great -increase in c is impracticable not only on the ground of expense, but -also on account of the great viscosity of the sol. - -The amount of adsorption depends also upon the exact nature of the sol. -It has been previously pointed out that the tannins differ largely in -their penetrating and weight-giving powers. Some are readily adsorbable -and are deposited in great concentration at the surface of the fibre, -but for good weight it is necessary to use also the less adsorbable and -more diffusible tans, which penetrate the fibre itself. Hence it is -necessary for good weight to use a blend of materials, and so supply -many grades of liability to adsorption. It is particularly advantageous -to blend judiciously the two main types of material, the pyrogallol and -catechol tans. It is also necessary for good weight to present to the -pelt the more diffusible and less adsorbable tannins first, in order to -secure the maximum diffusion into the interior of the fibre before the -exterior of the fibre is heavily coated with the heavily adsorbable and -astringent tans. The least adsorbable materials are therefore used in -the early stages of tanning, and the most adsorbable materials at the -end of the tanning process. Thus gambier is added to the early liquors -(suspenders), solubilized quebracho to the later liquors (handlers), and -mimosa bark extract to the final liquors (layers). There is also another -excellent way of ensuring this progressive astringency of the liquors; -this consists in leaching the required blend of materials together (or -mixing them in the case of extracts) and presenting the mixed infusion -to the nearly tanned goods, which adsorb chiefly the more astringent -tannins. The liquor is then used for goods at a less advanced stage of -tanning, which again take the most adsorbable constituents. This is -repeated until the stage is reached when the fresh pelt is inserted into -the nearly exhausted liquor, which naturally contains only the least -adsorbable substances. This system is almost universal, and in practice -is known as "working the liquors down the yard." It has the additional -advantage of being a systematic method of economically exhausting -("spending") the tan liquors. When free acid is present in the tan -liquors, it tends to distend the fibres composing the pelt by a strong -and rapid adsorption. Thus distended or plumped the fibres present a -still greater surface for adsorptive operation, but the distension -naturally leaves less space between the fibres for the diffusion of the -sol. Hence acid or "sour" tan liquors give in the long run more weight, -but tan more slowly. Pelt tanned whilst thus plumped forms naturally a -thicker and less pliable leather. This occurs in tanning sole leather, -to a less extent with heavy dressing leather, and to a very small extent -in the case of softer dressing leathers. - -In addition to adsorption, there is another phenomenon of colloid -chemistry in operation, viz. the _mutual precipitation_ of the sols in -the liquid by the gels in the hide. In most sols the disperse phase is -electrically charged. The sol therefore possesses electric conductivity, -and migration occurs in the electric field to the cathode or anode -according to the nature of the charge. Oppositely charged sols -precipitate one another, the precipitate containing both colloids. The -maximum precipitation occurs when the + charge of one sol exactly equals -and neutralizes the - charge of the other. There is thus an electrical -equivalence; an amount of sol which is equivalent to a given amount of -the other. This is not a chemical equivalence, however, and the -precipitate is not a chemical compound in spite of its fairly constant -composition. The composition of the precipitate, indeed, is not quite -constant, for the optimum precipitation may not correspond exactly with -the electrical equivalence, being influenced by the number of particles -required, their size (dispersity), the rate of mixture, and the relative -concentrations of the sols. This mutual precipitation is exhibited by -emulsoids as well as suspensoids, but the charge (+ or -) on an emulsoid -is in many instances largely an accidental matter, being determined by -the medium in which it happens to be, its normal condition being -electrical neutrality. Gelatin and pelt are such emulsoids, and a -positively charged gelatin sol has been observed to precipitate a -negatively charged gelatin sol. It is thought, however, that gelatin is -primarily a positive sol. Pelt (whether delimed or not) is rapidly -acidified by the quickly penetrating and strongly adsorbed organic acids -of the old tan liquors and becomes positively charged before the tannins -are adsorbed. The positive charge increases with the acidity of the -liquor. Other emulsoids are not electrically neutral, but are -electrically charged and exhibit considerable conductivity. Into this -class fall the tannins, and in tanning it is thought that there is a -mutual precipitation of the negative tannin sol with the positive hide -gel, the precipitation of the negative sol being favoured by the acid -condition of the liquor. The effect of increasing acidity soon falls -off, however, as a saturation limit is soon reached. This mutual -precipitation of colloids in tanning is in reality but an extension of -the adsorption theory, which explains the predominant effect of H+, -and OH- on the electric charge by stating that these ions are more -readily adsorbed than other ions, and that as OH- is more readily -adsorbed than H+ most sols are negative to water. - -In addition to the adsorption phenomena described, there are in -vegetable tannage _secondary changes_ which are slow and "irreversible." -These changes are obscure and are difficult to investigate. Oxidation, -dehydration and polymerization have all been suggested, but there is -little direct evidence. Certain it is, however, that time renders the -tannage more permanent. It perhaps should be pointed out that in the -very strongest tan liquors the viscosity of the tannin sol is so great -that adhesion would be a better term than adsorption. There is no abrupt -division between the two phenomena. - -In the theory of vegetable tannage there is another factor the -importance of which has been strongly emphasized by the author, viz., -_lyotrope influence_. This has been most conveniently discussed in -connection with gelatin gels, but its effect on hide gels is analogous. -It has also an effect upon the diffusion and gelation of the tannin and -non-tannin sols. - -=Mechanical Operations.=--In the tanyard the liquors are almost -invariably divided up into sections, called "rounds" or "sets," in -which the mechanical operations are different in aim and method. In -the first pits entered by the goods there is rapid adsorption in spite -of the low concentration and small astringency, and the great aim is -to obtain evenness of action and a good level colour. It is also -necessary to maintain the rate of adsorption. All the aims are -attained by frequently moving the goods. Heavy leather is suspended -vertically in the pits of tan liquor and handled up and down as well -as forward from pit to pit. Such pits are termed "suspenders." In the -earliest suspenders it is indeed advantageous to have the goods in -constant motion. This is done by suspending on wooden frames which are -rocked gently by mechanical power; such pits are termed "rockers." For -dressing leather in which firmness and smooth grain are not so -essential, the goods may be paddled in the first liquors. This is -occasionally done with stronger liquors for the express purpose of -working up the "grain" pattern. The goods after passing through the -suspenders are usually passed to "handler" rounds, in which they are -moved less frequently. In these pits the goods are laid horizontally -one above the other. One advantage of handlers is that the goods -flatten thoroughly and straighten one another by their own weight; -another is that more goods can be placed in one pit than in -suspenders. They are not so convenient to work, however, as -suspenders, and the goods do not feed so rapidly. Hence the tendency -is now to tan more in suspension, and to economize labour by an -extension of the rockers. The handling of the goods is also saved by -pumping the liquors and by working rounds of suspenders or rockers -like the press leach system, with the difference that the stronger -liquor is pumped in to the head pit, and the liquor passes upwards -through the goods. - -Finally the goods are placed in "layers" or "layaways," in which they -remain undisturbed for a decidedly longer time. These pits contain the -strongest liquors of the yard, and their principal function is to -complete the tannage and give weight and firmness by the adsorption of -bloom, reds, etc., in the interior of the hide. The goods are placed in -horizontally, and are dusted in between with fresh tanning material -which maintains the local strength of the liquor and keeps the goods -somewhat apart. Drum tanning attains a more rapid penetration of the -pelt by giving constant motion in stronger infusions. It is of course -liable to result in an under-tannage of the interior of the fibre. After -the goods have been "struck through" in the ordinary way, however, -drumming in extract is increasingly used as a substitute for much labour -in handling, and also to save the time spent in the early layers. - - - REFERENCES. - - Procter, "Principles of Leather Manufacture," pp. 220-350. - - Bennett, "Manufacture of Leather," pp. 113-179. - - Bennett, "Celavima and Babla," L.T.R., 1914, 122. - - Dumesny and Noyer, "Manufacture of Tanning Extracts." - - - _Theory_:-- - - Meunier and Seyewetz, Collegium, 1908, 195. - - Stiasny, Collegium, 1908, 117-159, 289, 294, 337. - - Procter and Wilson, Collegium (London), 1917, 3. - - Wilson, Collegium (London), 1917, 97, 100, 105. - - Moeller, Collegium (London), 1917, 13, 38, 46, 103; and - _J.S.L.T.C._, 1917, 22, 56, 92. - - Bennett, _J.S.L.T.C._, 1917, 130-133, 169-182; 1918, 40; 1920, - 75-86; _S.L.R._, 1916, March. - - - - - SECTION IV.--FINISHING PROCESSES - - -After the tannage is complete, leather is hung up to dry. In the case of -heavy leather this drying must be very carefully carried out in order to -obtain a product of satisfactory appearance and saleable qualities. -Associated with the drying are many mechanical operations (scouring and -rolling) which assist very materially in imparting the desired -qualities. After tanning, however, the quality of the final product is -most strongly influenced by the amount of grease added in finishing. -Some grease is always used in finishing, partly because even sole -leather requires some measure of pliability and partly because a coating -of oil over the leather during drying prevents the loose tannin from -being drawn to the surface of the leather by capillarity, thereby -causing dark and uneven patches and a "cracky" grain. The added grease -is also a contribution to the "weight" of the finished article--a -primary consideration for heavy leather, which is usually sold by -weight. The finishing processes, indeed, tend to be dominated by this -consideration, and become a series of efforts to retain as much tannin -and add as much grease as are consistent with the requirements of the -class of leather being manufactured. Sole leather does not contain more -than about 2 per cent. grease, or its firmness is impaired. Belting -leather, in which considerable pliability is needed, may contain about 9 -per cent., whilst harness leather, which must be exceedingly tough and -durable, may contain up to 13 per cent. of fatty matters. Upper -leathers, which need to be soft and pliable as well as waterproof and -durable, are very heavily "stuffed" and often contain up to 30-40 per -cent. of grease. Sole leather is thus rather distinct from the rest, -which are called "curried," "stuffed," or "dressed" leathers. The actual -drying out before, after and between the various mechanical operations, -each have an appropriate degree of wetness. In this country the drying -is usually under the prevailing atmospheric conditions and is known as -"weather drying." The goods are suspended by hooks or strings or by -laying over poles in special sheds fitted with louvre boards by which -the rate of drying can be roughly controlled. Weather drying is cheap, -but exceedingly slow, and in unfavourable weather is very unreliable. -The goods, moreover, need constant attention to obtain an even result. -Steam pipes are usually laid along the shed floors, and are used in -winter and damp weather to accelerate the drying, and also in the final -shed stove to remove the last traces of moisture. Wet weather, however, -will not stand a high temperature, and steam drying is better avoided -when possible. Air-dried leather still contains about 14 per cent. of -moisture. Many systems of shed ventilation have been suggested to hasten -the drying and to secure a better control of the process. In one system -a screw fan is fitted at one end of a shed (without louvre boards) and -sucks air through the goods from an inlet at the other end. The air can -be heated by a steam coil near the inlet. In another system a -centrifugal fan blows air through an arrangement of pipes which -distributes it to the drying sheds, and discharges it close to the floor -by various branch pipes. The outlets are near the roof. A system of -dampers permits hot air, warm air and the used wet air to be blended in -the desired proportions. In America turret drying has been used. The -sheds are vertically above one another and have latticed floors. Heated -air is admitted at the bottom and rises through the goods up the -building just as in a chimney. For many of the finishing operations it -is important to obtain the leather in a uniformly half dry or "sammed" -condition. This may be done by careful drying, and wetting back the -parts that have become too dry with tepid water or weak sumac liquor, -and then leaving the goods "in pile" until of uniform humidity. It may -also be done by "wetting back" leather which has been completely dried -out. There are also "samming machines" which by means of rollers squeeze -out the excess liquor. Sole leather is dried out and finished -immediately after tanning, but dressing leather is often "rough dried" -out of tan liquors and wet back for finishing when required. Dressing -leather is often treated in different factories; tanners selling it as -rough leather and "curriers" finishing it. - -Scouring is one of the first operations in finishing leather. The grain -side is wet and worked with brushes and stones until the bloom and loose -tannin are removed. This process aims at producing a good even colour -and level surface, but is liable to cause a loss of weight. Dressing -leather is often scoured on both grain and flesh, and weak soap or borax -solutions are used to assist the process. In this operation hand labour -has been now quite superseded by machine work. A great variety of -machines have been devised. The mechanical working of leather takes -place in various parts of finishing. These operations, known as -"striking," "setting," "pinning," "jacking," may be carried out often by -the same machine as used for scouring, but with a change of tool. The -object of these operations is to get rid of wrinkles and creases, to -produce softness, pliability and area, and to remove superfluous -moisture, grease, dirt. The tools are of steel, brass, slate or -vulcanite. Scouring is often effected by putting the goods into rotating -drums together with extract and sumach. The bloom is removed by -friction, the colour is improved by the sumach, whilst the extract keeps -up the weight. - -In finishing sole leather firmness is enhanced by "rolling." A brass -roller passes to and fro over the goods with the exertion of -considerable pressure. The operation is carried out by machinery. - -Shaving is an important operation in the case of many dressing leathers. -Its object is to produce a uniform thickness of the leather and an even -surface on the flesh side. The sammed goods are laid over suitable beams -and shaved with special sharp knives which possess a turned edge. This -hand process, which demanded considerable skill, is fast becoming -extinct, and machine shaving is already almost universal on account of -its greater speed. The machines consist essentially of two rollers, one -of which is smooth, whilst the other is a spiral knife-blade cylinder -(cp. Section II.). The sammed goods are held in the hands and placed -over the smooth roller, which is raised to the cutting roller by a foot -treadle. A number of similar operations ("flatting," "whitening," -"buffing") are carried out by a suitable change of tool. In all these -operations good samming is important. - -Splitting is another important operation on tanned leather. In this -process the leather is cut parallel to its grain surface, thus yielding -two pieces with the same area as the original, the "grain" and the -"flesh split." It is essentially a machine operation, and is carried out -by presenting the carefully sammed leather to a sharp knife-edge, -towards which it must be constantly pressed. The "band knife" machine is -the most popular arrangement. The knife is an endless belt, which -continually revolves round two pulley wheels of equal size. In between -these the knife is horizontal, and is then used for splitting. The -sammed leather is pushed towards the blade by two feed rollers, and the -grain passes above the knife on to a small platform, whilst the flesh or -"split" passes below and falls to the ground. Emery grinders and thick -felt cleaners in the lower part of the machine keep the knife in good -condition. The adjustment of the machine is delicate and requires -considerable experience. With care splits may be obtained down to 1/16" -thick, and sometimes as many as 6 or 7 splits are obtained from one -hide. - -Oiling is still usually done by hand, and cod oil is still preferred for -many classes of goods. Of recent years there has been a great extension -of the use of sulphonated oils, which have the valuable property of -forming an emulsion with water or tan liquor. With these materials it is -easier to ensure the goods being completely covered with oil. The -penetration of the oil into the leather is also quicker and more -complete. These oils have often the disadvantage of leaving solid fats -on the exterior of the leather, which gives it an ugly smeared -appearance. - -Stuffing the dressing leathers is carried out in a variety of ways and -with a variety of materials. The old process of hand stuffing employs a -mixture of tallow and cod oil called "dubbin." This is made by melting -the ingredients together and allowing them to cool with constant -stirring to a nearly homogeneous salve. The dubbin is brushed thickly on -to the flesh side of the sammed leather, which is then hung up to dry. -As the moisture dries out the oils and soft fats penetrate the leather -and leave the more solid fats on the outside. The proportions of tallow -and oil are varied with the time of year and with the method of drying, -for if the dubbin be too soft it will run off the leather, and if too -hard will not penetrate it so well. - -Drum stuffing is a more modern development in which a higher temperature -is employed, about 140 deg. F. The drum is heated up by steam or by hot air, -and the sammed goods are then inserted and drummed for a few minutes -until they are warmed. The drum is fitted with a heated funnel -containing the melted grease, which is run in through the hollow axle. -After a half to three-quarters of an hour's drumming the grease is -completely absorbed by the leather. The drumming is continued for a -while until the goods have cooled. Whilst still warm they are "set out" -to remove creases and superfluous grease. Drum stuffing is not only -quicker than hand stuffing, but also makes it possible to use the hard -fats, and so make a leather which carries more grease without appearing -greasy. Thus in drum stuffing, paraffin wax and wool fat are used, and -their penetration assisted by small proportions of cod oil or degras. If -the leather be too wet the grease is not absorbed, whilst if it be drier -than usual the leather will take more grease, but the resulting colour -is not so good. There is also another method of stuffing which -originates from the Continent. It is known as "burning in" and involves -the use of still higher temperatures (195 deg. to 212 deg. F.). Wet leather -will, of course, not stand this temperature, so that it is first -necessary to make the leather absolutely dry. This is effected by drying -in stoves at temperatures up to 110 deg.-115 deg. F. There are two ways in which -the grease is applied. In one method the melted grease is poured by a -ladle on to the flesh side and brushed over until evenly distributed. A -second application of grease is made to the thicker parts. The hides are -then put into warm water (120 deg. F.) for about a quarter of an hour, and -then drummed for half an hour. In the other method the goods are -completely immersed in the melted fats for a few minutes in a -steam-jacketer tank at a temperature of 195 deg. F. After softening in water -at 120 deg. F. the goods are drummed. "Burning in" is used for the heavier -dressing leathers such as belting and harness. It does not give good -colour, but permits the employment of still more hard fats. - - - REFERENCES. - - Procter, "Principles of Leather Manufacture," pp. 223, 378. - - Bennett, "Manufacture of Leather," pp. 251-312. - - Bennett, "Principles of Leather Stuffing," _Leather Trades Review_, - 1911, 186. - - - - - SECTION V.--SOLE LEATHER - - -Leather for the soles of boots and shoes is a matter of essential -interest to all, and forms one of the best appreciated applications of -animal proteids to useful purposes. Methods for its manufacture are as -numerous as the factories producing it, hence all that can be done is to -describe broadly the general method which is typical of our time, to -classify the many varieties into types, and to indicate the recent -changes and present tendencies. - -Sole leather is mainly manufactured from butt pelt, and the great aim is -to produce a firm, thick, waterproof and smooth grained leather which -will bend without cracking. It must have a light tan colour to be -saleable, and contain as much weight as possible to be profitable. - -The modern mixed tannage of "sole butts" or "scoured bends" generally -utilizes ox-hides of the Scotch and English markets, though salted -Continentals and South Americans are also employed. After the usual -soaking a short and sharp liming is given. The special aim in liming -sole hides is to obtain the maximum plumping effect with the minimum -loss of hide substance. Both these achievements are necessary to obtain -good weight. The limes should be kept as clean as possible, which is -best obtained by putting clean hides into work. This reduces bacterial -activity and loss of hide substance. The "shortness" of the process is -attained by the use of sodium sulphide (from 2 to 16 ozs. per hide of -sulphide crystals), by which depilation may be accomplished easily in -about nine days. The amount of sulphide should be increased somewhat in -the short-hair season and in cold weather. Some factories take up to -about 12 days using less sulphide, whilst others will lime in about a -week by using the larger quantities. The amount of lime used varies -enormously, and is invariably in great excess of the actual -requirements. "Probably 2-3 per cent. on the green weight of the hides -is all that can be really utilized, the remainder being wasted."[5] This -amounts to about 2-1/2 lbs. lime per hide, but in practice it is more -frequent to find 7, 8, 9 or even 10 lbs. per hide being used. The excess -is innocuous, owing to the limited solubility of lime. Some excess is -desirable, to replace in the liquor the lime adsorbed by the goods in -plumping, to assist bacterial activity, and also because in -sharp lime liquors the undissolved portions do not remain so long in -suspension. The use of sulphide and other alkalies does not "make it -possible" to reduce the amount of lime used, it merely renders the -excess more superfluous. The use of sulphide not only shortens the -process, but also sharpens it, on account of the caustic soda produced -by hydrolysis. Usually for sole leather, however, it hardly sharpens it -sufficiently, and it is very common to add also caustic soda (or -carbonate of soda) to the limes. About 2 ozs. caustic soda (or its -equivalent in carbonate) is used per hide. The hides are limed generally -by the three-pit system, giving about three days in each pit. They -should be handled each day in the first pit (old lime) and once in the -other pits. - -[Footnote 5: Procter, "Principles of Leather Manufacture," p. 129.] - -Unhairing and fleshing by hand labour is still common, in order to avoid -great pressure on the plumped hide. Scudding should be very light, and -in some yards is entirely omitted. Only the lime on the surface of the -hide should be removed by deliming, and this immediately prior to the -insertion of the butts into the tan liquor. This is to ensure good -colour and yet keep the butts plump. Boric acid is the best for this -purpose, using 10-15 lbs. per 100 butts. The goods are inserted (and -preferably rocked) in a dilute solution for a few hours only. About the -same quantity of commercial lactic acid may be substituted for the -boracic. This deliming can also be accomplished by adding the acid to -the worst suspender in the tanyard. - -To obtain firmness and plumping it is necessary that the early liquors -in tanning should be more acid than for other leathers. With old methods -of tanning one could trust to the natural sourness of the liquors to -complete the deliming and replump the goods with acid. In such cases any -deliming was also unnecessary. In the modern yard, however, we get -"sweet" liquors coming down the yard, partly on account of the greater -proportion of extract used and partly because the liquors themselves are -not so old. Hence it is now practically always necessary to acidify -artificially the tan liquors. This may be done by adding a few gallons -of lactic, acetic, formic, or butyric acid to the handlers and -suspenders, especially in the winter and spring. It is now increasingly -common to place sole butts in a special acid bath after they have been -in tan liquor for about a week. This bath is often made from sulphuric -acid, and may be 1 or 2 or even 4 per cent. in strength. - -The actual tanning of sole butts lasts three to four months, and just -prior to the war the tannage consisted often of about one-third myrabs, -one-third valonia, and one-third extract. The myrabs and valonia were -leached together, and the extract added to the best leach to make layer -liquors of the required strength. Some mimosa bark was generally used -also, and now it is extensively employed to replace the valonia. The -most widely preferred extract is chestnut, but quebracho, myrabs extract -and mixtures have also a prominent place, and mimosa bark extract an -increasing importance. It is recognized that this tannage is if anything -too mellow, and that if only a smooth grain and plump butt can be -ensured in the first weeks of tanning, it is much better for sole -leather to employ the most astringent tans possible and the sharpest -liquors (_i.e._ liquors with a small relative proportion of soluble -non-tannin matters). Hence there is the tendency in sole-leather tanning -to employ fresh clear liquors for the butts and use up the more mellow -liquors on the "offal" (shoulders and bellies). - -Four types of sole butt tannage will now be described, all of which -illustrate the methods employed in a modern mixed tannage. - -=1.= The first type consists in a four-months tannage, in which the -liquors are worked down the yard. - -The butts pass first through the suspenders (20 deg.-40 deg. Bkr.) in about a -week, and are rocked in the first liquors. They next enter the handlers -(40 deg.-55 deg.) rounds of eight pits, six floaters and two dusters. Myrabs, or -a mixture with algarobilla is used as dusting material. The goods remain -in this set for two weeks, and should then be struck through. The -suspender handlers (55 deg.-65 deg.) are next entered, in which they remain up -to three weeks in suspension, being shifted forward on alternate days. -The goods now enter the layers, of which four are given: first 70 deg. for -one week; second 75 deg. for two weeks; third 80 deg. for three weeks; and -fourth 90 deg. for a month. The goods thus take sixteen weeks to tan, of -which ten weeks (62-1/2 per cent.) are in layers. - -The system of working the liquors is expensive, and is only possible if -the butt liquors can be spent out by the offal. The best or fourth -layer, 90 deg., is made from the best leach liquor, 65 deg., and extract -(chestnut with some oakwood or mimosa bark). After use it becomes the -second layer, 75 deg. The third layer, 80 deg., is also made from fresh leach -liquor and extract (chestnut with some myrabs or mixed extract). After -being used thus it is used for the first layer, 70 deg. The used first and -second layers are mixed together and used partly to form the belly -layers, and partly to make a sharp liquor for the handlers (55 deg.-40 deg.) by -diluting with 40 deg. leach liquor and adding quebracho extract. The old -handler liquor is run to the suspenders (40 deg.-20 deg.), and finally used for -colouring off the offal in drum or paddle 18 deg. The suspender handlers -(65 deg.-55 deg.) are made from fresh leach liquor and chestnut extract. They -are afterwards used to make shoulder layers. The course of the liquors -is shown in the diagram below. - -It will be seen that fresh leach liquor and fresh material are used to -each set except the suspenders, which must have some mellowness to -ensure plumping and smooth grain. Layer liquors are used twice only, and -then (when only five weeks old) pass to the handlers. These are further -sharpened by fresh leach liquor and fresh extract and dry materials. -The forward handlers are fresh liquors with fresh extract. This tannage -is fairly typical of high-class sole leather, in which the liquors are -worked down the yard, but worked towards the offal, which thus receives -liquors with relatively greater proportions of mellow tans and soluble -non-tans. - - - ----------Leach liquor------------------- - | / \ | - | V \ | - | 4th layer, 90 deg. \ | - | | V | - | V 3rd layer, 80 deg. | - | 2nd layer, 75 deg. | | - | \ V | - | \ 1st layer, 70 deg. | - \ \ / | - \ \ / | - \ V V | - \ (mixture)[-->bellies] | - \ / V - V V Suspender handlers-->(65 deg.-55 deg.)(shoulders) - Handlers, 55 deg.-40 deg. - | - V - Suspenders [-->offal] - - -=2.= The second type consists in a tannage of about four months, in -which the liquors are not worked down the butt yard. In this method also -there is an attempt to save much of the labour in handling, first by -shortening the time in the handlers by one week (as compared with the -above), and second by fusing the two progressive handler sets into two -sets of equal strength, through which the goods pass more slowly and -with less disturbance. - -The goods go through the suspenders (10 deg.-25 deg.) in about a week, rocking -in the early liquors, and then into large rounds of handlers (30 deg.-45 deg.) -for one month. The handlers consist of floaters and several dusters, in -which the butts are laid away with 1-3 cwt. myrabs. The goods next enter -the layers, of the same strength as in Type 1, and in which they remain -the same time. The total tannage is thus 15 weeks, of which 10 weeks -(nearly 67 per cent.) are in layers. - -The best or fourth layer is made up from leach liquor and extract, and -is then used successively as a third, second and first layer, and then -passes to the offal layers. The handler liquor is made entirely from -fresh leach liquor and quebracho extract, and is a sharp liquor of -greater strength than its Bkr. strength would indicate. The old handler -liquor is run to the butt suspenders. The course is represented thus:-- - - - Leach liquor - / | - / 4th layer, 90 deg. - / | - / 3rd layer, 80 deg. - / | - / 2nd layer, 75 deg. - / | - / 1st layer, 70 deg.-->[offal layers] - V - Handlers (45 deg.-30 deg.) - | - V - Suspenders (25 deg.-10 deg.) - - -=3.= The third type consists of a short three-month's tannage in which -the liquors are worked straight down the yard. To compensate for the -short time it is necessary to have stronger layer liquors in which the -goods spend a still greater proportion of their total time. The stronger -liquors involve a greater proportion of extract, particularly of -quebracho, which fact causes the whole of the liquors to be sharper than -their Bkr. strength indicates, and justifies them being worked straight -down the yard. - -The goods go through suspenders (20 deg.-40 deg.) as usual one week, and then -pass into suspender-handlers (40 deg.-60 deg.) for two weeks, and thence to the -layers. In the first two of these (65 deg. and 70 deg.) they are actually in -suspension, a week in each liquor. They are then dusted down for ten to -eleven days, first in 85 deg. and then in a 95 deg. liquor, and finally for a -month in a liquor of 110 deg. The total tannage is thus twelve weeks, of -which nine weeks (75 per cent.) are in layers. There is considerably -less handling than in Type 2, and it is more convenient, the goods being -in suspension. - -=4.= The fourth type is also a three-month's tannage. In this it is -attempted to obtain even greater weight with still less labour. The -layer liquors are kept much stronger by the more extensive use of -extract, and this makes it impracticable as well as too costly to run -these liquors down the yard. They are therefore repeatedly strengthened -with extract and used again. - -The goods go through suspenders (20 deg.-40 deg.) as usual one week, and then -through a round of suspender-handlers (40 deg.-55 deg.) consisting of fresh -sharp liquor from the leaches together with quebracho extract. They are -in this set two weeks, and then are laid away. They receive three -layers: first, 105 deg. for 2 weeks; second, 110 deg. for three weeks; and -finally, 120 deg. for a month. Of the twelve weeks, therefore, nine weeks -(75 per cent.) are spent in layers. In this method the goods are -immersed in 3 per cent. sulphuric acid after passing through the -suspenders. - -There is possible, of course, a tremendous number of variants of the -above types. The number of handler rounds is determined by the number of -butts being dealt with. With a large number it is more easily possible -to arrange for them to be in progressive strength as in Type 1. There -are also many systems of working the layers, of which the most notable -is to make the second or third layer from fresh leach liquor and -extract, and strengthen it with extract for the succeeding layers. It is -then used as a first layer and worked down the yard. - -The bellies and shoulders often go through separate sets of liquors, but -it is common to put them through suspenders, and even handlers together. -They receive, of course, a distinctly shorter tannage, and are often -drummed with extract before laying away or after the first layer. By way -of illustration, the course of the offal and their liquors may be given -in the case of Type 1. The shoulders and bellies are coloured off in a -paddle or drum with old butt suspender liquor, which is then quite -exhausted. They then pass through suspenders (18 deg.-40 deg.) together in 4-5 -days, and go through a handler round (40 deg.-55 deg.) for 3 weeks, including -one duster. The bellies are removed after 2 weeks, and given three -layers (60 deg., 70 deg., 80 deg.) of a week each. They receive, therefore, nearly 6 -weeks in all. The shoulders also have three layers (60 deg., 65 deg. and 80 deg.) -of 2, 3 and 4 weeks respectively. - -The course of the liquors is shown thus:-- - - - Butt layers Butt suspender Butt suspenders - (and + extract) Handlers (+ extract) | - | | | - V V | - Belly layers Shoulder layers | - (80 deg.-60 deg.) (80 deg.-60 deg.) | - \ / | - V V | - Offal handlers (55 deg.-40 deg.) | - | V - V Offal drums - Offal suspenders (40 deg.-18 deg.) (18 deg.) - \ / - \ / - \ / - \ / - \ / - \ / - V V - Drain - - -The tanned butts are piled for 2-3 days, sometimes rinsed to remove -dusting material, and then scoured either by machine or by drumming with -sumac and extract. This removes bloom, but causes some loss of weight. -"Vatting" or "bleaching" now follows, in which it is attempted not only -to bleach the colour of the leather, but also to impart as much weight -as possible. The vat liquor is made several degrees stronger than the -last layer by means of quebracho bleaching extract and good coloured -chestnut or myrabs extract. The liquor is kept warm by a steam coil, at -about 100 deg. F., but not much more without risk. The goods remain in the -bleach liquor 2-3 days and are then horsed or suspended to drain. Sumach -is sometimes used in the vats. A new vat liquor must be made up after -some weeks' use. The goods are sometimes rinsed in weak sumac liquor -before vatting to get good penetration, and sometimes after to ensure -good colour. - -The butts are next oiled and hung up in a dark shed and allowed to dry -slowly and evenly to an "india-rubbery" consistency and rather slimy -feel. They are then "struck out" by machine, wiped, re-oiled and again -hung up to dry, preferably with sulphonated oil. After a short drying -to a suitable and even condition they are "rolled on," and, possibly -after further drying, "rolled off" with greater pressure, and then dried -for a day or two with the help of a little steam. Finally they are -machine-brushed and sent to the warehouse, where they are weighed and -classified. - -The offal is often drum oiled. It needs more striking and is more -difficult to obtain in suitable condition for striking, rolling. It is -treated similarly to butts, but often also goes for dressing leather, -and may be split. It is of some interest to compare the above processes -with that once very popular manufacture of "bloomed butts" in the West -of England from South American salted hides. These receive a liming from -12-14 days, using 12-16 lbs. of lime per hide. They receive then a -tannage of about 9 months, comprising 3 weeks in suspenders -(20 deg.-40 deg.)--very sour and mellow liquors--4 weeks in handlers (40 deg.-55 deg.), -4 weeks in dusters (60 deg.), 4 weeks in round made from hemlock extract -(60 deg.), and 20 weeks in six layers (60 deg.-90 deg.) in which they were dusted -heavily with valonia. Oakwood extract was used for the layers, which -took 57 per cent. of the total time. The butts were scoured in a -much-dried condition, so that only the loose and surface bloom was -removed. No bleaching was given in the modern sense. - -In the old oak-bark tannage of sole leather up to 12 months were taken -for tanning, two-thirds to four-fifths of which time the goods were in -layers. The strongest liquor rarely exceeded 50 deg. even where valonia and -gambier were also used, and rather more than 30 deg. if not. - -It will be understood from the above that the tendency for many years -has been to shorten the time and the labour required for tanning. Drum -tanning is obviously the next stage in shortening the time. In one such -process the butts are put through suspenders (25 deg.-40 deg.) for 2 weeks, -drummed for 12 hours in an 80 deg. extract liquor, and finally in a neat -extract 200 deg. for 36 hours. Drum tanned sole leather, however, is not as -yet of good quality; the grain is not smooth, and the heavy weight -finish (striking and rolling) needed to counteract this tendency is -liable to cause poor "substance." The leather, too, readily wets and -goes out of shape. Possibly some drumming may be adopted to save time in -the early layers, but the most serious rival to the 3 months' tannage is -the waterproof chrome sole leather (Part III., Section V.). - - - REFERENCES. - - Parker, _J.S.C.I._, 1902, 839. - - Procter, "Principles of Leather Manufacture," p. 220. - - Bennett, "Manufacture of Leather," pp. 179, 259. - - Bennett, _J.S.C.I._, 1909, 1193. - - - - - SECTION VI.--BELTING LEATHER - - -The manufacture of belting leather is well illustrated by the tanning -and finishing of "strap butts." In general, the tannage presents many -points of great similarity with the tannage of sole leather; indeed, the -resemblance is so close that in some factories there is little -difference observed, and the currying and finishing operations are -relied on to produce the desired difference in final results. -Nevertheless, there is considerable difference in the type and ideal of -the two leathers, which may be expressed in trade parlance as a greater -"mellowness" for the belting leather, and in the best methods of -manufacture this fact is in evidence throughout the whole process of -manufacture. - -In liming, there need be little difference between sole and belting -hides, and a sharp treatment of 9-10 days, by the three-pit system, with -a day or two extra in the coldest weather, would meet ordinary needs. -For the conservation of hide substance and for the saving of time a -shorter liming is sometimes given, in which more sulphide is employed -than is usual for sole leather. Even the very short processes of liming, -1 to 3 days, which involve the use of strong solutions of sodium -sulphide, have been successfully employed for belting leather. The -tendency to harsh grain with such processes is not so serious a defect -with belting as with sole leather, and can be minimized by careful -deliming. American and Continental factories tend to favour the use of -those quick processes which employ warm water in addition to sulphide. -The hides after a short liming in sulphide limes are immersed in warm -water, which greatly accelerates both the chemical and bacterial -actions. For example, after about 3 days' liming, in which both old and -new limes are used as usual, the hides may be thrown into water from -100 deg.-105 deg. F., and will be ready for depilation in 7 or 8 hours. - -Even a stronger liming may be given, especially if the soaking is -unusually prolonged. Such processes undoubtedly save hide substance, and -the pelt is obtained more free from lime, but they have the disadvantage -that the natural grease of the hide is only imperfectly "killed" (_i.e._ -saponified or emulsified), and may interfere with the normal course of -the tannage. The plumping is also apt to be insufficient. On the other -hand, liming processes are also used in which a mellower liming or a -longer liming is preferred in order to produce the desired degree of -softness and pliability in the finished leather. Belting must not be too -soft, of course, and it will be clear that the required difference from -sole leather can be produced either in liming or tanning or partly in -both. These considerations also decide whether bating is to be omitted -or not. A hard astringent tannage in sour liquors after a sharp liming -might make bating essential, but in these days it is usual to avoid it -and produce the effect in other ways. A light bating of a few hours is -sometimes given, but it is more unusual to delime the grain thoroughly -with boric acid, using up to 20 lbs. per 100 butts. Crackiness is a -fatal defect in strap butts, so that a sound grain must always be -obtained. Generally speaking, therefore, strap butts receive more -washing in water, and rather more deliming than sole leather, even when -they are not bated. It is also usual to scud much more thoroughly, and -to round a larger proportion of butt, especially in length. - -The tannage is usually carried out with a blend which includes a much -greater proportion of the fruit tans, and correspondingly less of -extract. - -Distinctly more myrabs are used than in sole leather tannages, in the -dry material, and amongst the extracts chestnut is preferred to -quebracho, and myrabs to mimosa bark, though all these may be used in -some degree. In the past the most favoured extract has been undoubtedly -gambier, which gives a tannage which is easily curried and imparts the -required mellowness to the uncurried leather. The great expense of this -material, however, together with the advent of drum stuffing and shorter -tannages in stronger liquors, have tended to cause a considerable -reduction in the proportion used for strap butts, and to limit its -employment to the earlier stages of tanning. - -The same tendencies for reducing the time taken to tan, employing -stronger liquors, and securing economy of labour in handling, have been -evidenced in the tannage of strap butts as in sole butts. It is -nevertheless true that, broadly speaking, strap butts receive rather -more handling and rather weaker liquors than sole butts. A greater -amount of mechanical assistance is also employed with early stages -(paddling, drumming, rocking). This is less objectionable for curried -leather than for sole butts. The handling is more usually in suspension. -The liquors are usually worked straight down the yard as a greater -mellowness is needed in the early liquors than for sole butts. The offal -is given a separate tannage and often used for different purposes, -_e.g._ the shoulders for welting and the bellies for fancy goods. -Plumping with sulphuric acid is generally considered inadmissible for -strap butts. It has been shown that leather containing sulphuric acid -tends to perish after the lapse of a number of years. Sole leather will -be worn up before this effect is observed, but belting is an article -which is intended to last much longer, and the use of sulphuric acid is -consequently inadvisable. Plumping must be obtained, to a considerable -extent, but must be achieved by the organic acids (lactic, acetic, -formic and butyric acids). A few gallons of such acids are consequently -added to the handlers, especially in the winter and spring. Less may be -used in the autumn, when the layer liquors which fermented in the summer -months have worked down to the suspenders. A mixture of these acids is -usually better than any one alone, for they not only differ very -considerably in price, but also have different powers of neutralizing -lime and plumping the goods. Lactic acid (M.W. 90), Acetic acid (M.W. -60), and formic acid (M.W. 46) are each monobasic acids; consequently 3 -lbs. formic will neutralize as much lime as 4 lbs. acetic or 6 lbs. -lactic. Their plumping powers are somewhat influenced by the anion. In -determining what quantities to take, the commercial strength of the -acids must also be considered. Formic is often 80-90 per cent. pure, -acetic 60-80 per cent., and lactic 40-60, but may be as low as 25 per -cent. The blend must be adjusted accordingly. As strap butts do not need -the firmness of sole leather, less of these acids may be used than for -sole butts. - -The exact nature of the tannage and the strength of the liquors is -largely influenced by commercial considerations. If the manufacturer is -both tanner and currier, he need not go to such great expense in strong -liquors and in time in layers, for he can obtain some of this weight in -currying. If, however, the tanner sells the butts rough dried, he must -naturally aim at obtaining greater weight in tanning. - -The actual details of the tanning processes are as usual very varied, -but may be classified according to type, just as in the case of sole -butts. - -Illustrations will now be given. - -=Type 1=, which may be compared with Type 1 for sole butts, is a tannage -of about 5 months. The goods pass through suspenders (8 deg.-30 deg.) in 2-1/2 -weeks, and then pass to the handlers (30 deg.-50 deg.), in which they remain a -month; they are then put into suspension again and pass through the -suspender handlers (40 deg.-55 deg.), which takes 2-1/2 weeks. In this round -much gambier is added, and the goods are frequently handled. Four layers -are usually given, viz. first layer 55 deg., one week; second layer 60 deg., two -weeks; third layer 65 deg., four weeks; and fourth layer 75 deg., four weeks. -The tannage is thus 20 weeks, of which 11 weeks (55 per cent.) are in -layers. Extra layers may be given to heavier goods, using stronger -liquors made up with extract. All liquors work straight down the yard. - -The tannage consists of 35 per cent. myrabs, 35 per cent. valonia, 10 -per cent. Natal bark, and 20 per cent. extract, chiefly gambier, though -some chestnut and quebracho are used. - -=Type 2= represents the modern tendency to use stronger liquors and a -shorter time. The strap butts pass through the suspenders (22 deg.-50 deg.) in -1-1/2 weeks, during about a third of which time they are rocked. They -next pass through two sets of suspender-handlers (50 deg.-67 deg. and 67 deg.-80 deg.), -which takes a month, and thence to the layers. Three layers are given -(85 deg., 90 deg. and 100 deg.), in which the goods remain one, three and four weeks -respectively. The tannage is thus 13-1/2 weeks, of which 8 weeks (nearly -60 per cent.) are in layers. The liquors work down the yard. Longer time -may be given to heavier goods. The tannage consists of 40 per cent. -myrabs, 35 per cent. valonia or Natal bark, and 25 per cent. extract, -chiefly chestnut, though some gambier may be added to the suspenders. - -However tanned, strap butts are first dried out rough over poles. This -assists in making the tannage permanent, on account of secondary changes -discussed in Section III. They are next wet back for currying by -soaking in water or sumach liquor for a few hours and piling to become -soft and even. The first operation is "skiving," which is a light -shaving on the flesh side, carried out by a sharp slicker with a turned -edge. The butts are next scoured thoroughly by machine on both flesh and -grain, and sumached in a vat for several hours at 100 deg. F., after which -they are slicked out and hung up in a cool shed to samm for stuffing. -Hand stuffing is often still preferred, with tallow and cod oil. The -butts are next set out, and it is important that this should be -thoroughly done. Machines are now generally used, and the goods are -often reset after further drying. After drying out completely they are -given a light coating of tallow and laid away till wanted for cutting up -into straps, which is now done by machinery. - -A Continental method for making belting leather is to give 6 weeks in a -suspender set (70 deg.-24 deg.) of twelve pits arranged on the press system, -running two fresh liquors a week, and to give them two layers (24 deg. and -28 deg.) of 6 and 8 weeks. The material is chiefly pine bark, but some oak -bark, valonia, myrabs and quebracho are also used. The goods are stuffed -by "burning in," molten fat being poured on the flesh side. - - - REFERENCE. - - Bennett, "Manufacture of Leather," pp 194, 295. - - - - - SECTION VII.--HARNESS LEATHER - - -When discussing the question of oak bark (Section III.), reasons were -advanced for its decreased use and popularity. These were quickly -appreciated in the sole leather trade, but the obsolescence of oak bark -in the dressing-leather section was much more prolonged, partly because -there was less pressing need to obtain good weight in the actual -tanning, and partly because in some branches of dressing leather, such -as belting and harness, a leather was required of great durability and -toughness, for which qualities oak bark tannage had a deservedly high -reputation. Hence harness leather manufacture affords a good -illustration of the transition between the methods of the late -nineteenth and those of the twentieth century. With the use of oak bark -lingered the old methods of liming, bating and tanning in weak liquors -for a long time with plenty of gambier. Hence in this section it will be -necessary to observe a gradual transition of method, both in wet work -and tanning. It should be pointed out that this transition has not been -and is not going on in all factories at the same rate. Many factories -remain in which the old methods are still preferred at some stages of -the manufacture, and some remain in which many of the changes indicated -below have not taken place at all. The leather trade has always been -considered conservative in its methods, but it should be realized that -much of the prejudice in favour of old methods is due to the public, and -that after all tanners and curriers, like other business men, have to -suit their customers. The march of industry is not like a regiment in -line; it is rather more like nature, a survival of the most adaptable. - -Hides for harness leather are limed in various ways, of which the -following are types. - -1. A rather mellow liming of 10-15 days (longer than for sole leather), -in which nothing but lime is used, and a certain amount of old liquor -used in making up the new limes. The liming was carried out by the -one-pit system, but the goods and liquors were kept clean by a good -soaking process. Hence the loss of hide substances was not very great; -goods so treated were bated before tanning. - -2. A shorter liming than the above by the three-pit system. This saved -time (taking 9-10 days), saved hide substance, and ensured greater -regularity of treatment. The limes were about as mellow, but a little -sulphide (2-4 ozs. per hide) was used to assist the depilation, -especially during the short-hair season. These goods were also bated. - -3. A distinctly longer liming, 15-16 days, in mellower limes. This -differed from Type 1 also in the respect that greater regularity was -ensured by the three-pit system; a foot or two of old liquor was used in -making up the new lime. More hide substance was lost than in either of -the above processes, but this was deliberate, the object being to -dispense with bating, which is always light for harness hides. Thus a -longer and mellower but systematic liming was used as a substitute for -shorter liming and bating. No sulphide was used in this process. - -4. A short liming of 6-7 days, using up to 12 ozs. of sulphide per hide. -The object here is to save time and hide substance. The three-pit system -is preferred. Bating again becomes necessary, but the pigeon-dung bate -is replaced by artificial bates, less objectionable, quicker, and more -scientific in management. - -5. A still shorter process of about five days, using still more sulphide -(about 16-20 ozs. per hide), together with some calcium chloride to -reduce harshness. In such a method there is a tendency to revert to the -one-pit system, which involves rather less labour. The three-pit system -shows to a great advantage in the longer processes of liming when the -process is reduced to five days; there is little difference between the -two, for a one-pit system is a two-liquor method. Hence again an -artificial bate is used. - -The various methods of liming, together with analogous variations in -tannage, have resulted in great variety in bating. Sometimes up to three -days' bating has been given at 70 deg. F., but more often the goods are -merely immersed overnight, and then delimed with boric acid, but with -sulphide processes it is an advantage to use some of the commercial -bates of the ammonium chloride type, and finish off with boric acid. -Scudding is always more thorough than for sole or belting, the hides are -rounded into long butts which include most of the shoulder "harness -backs." The goods are sometimes bate shaved. - -A few tannages will now be outlined, in order of historic type. - -=Type 1= may be taken to represent the so-called "high-class" process in -which oak bark myrabs and valonia are the staple materials. A good deal -of gambier is also used, and a little myrabs and chestnut extract are -helpful in attaining the desired strength of liquor. The "backs" go -first through suspenders (8 deg.-30 deg.), which takes up to three weeks, and -then in to handlers (30 deg.-40 deg.) for four weeks, consisting of rounds of -clear liquor. They next go through a duster round, in which they are put -for a week with oak bark and myrabs into a liquor of 45 deg. Four layers -are given (50 deg., 55 deg., 60 deg. and 65 deg.), in which the goods remain for two, -three, four and five weeks respectively, oak bark being the chief -dusting material. The tannage is thus for twenty weeks. Light backs -receive less time in the layers (only 11 weeks). If the tanner is also -the currier, the fourth layers are omitted. He then saves five weeks and -gets the weight in the stuffing. - -=Type 2= is a tannage in which oak bark and valonia are replaced by -myrabs, mimosa bark and chestnut extract. It is therefore considerably -cheaper and probably no less durable. Expense is also curtailed in -handling. The harness backs go through suspenders (16 deg.-30 deg.) in two -weeks, handlers (30 deg.-45 deg.) in four weeks, and then receive four layers of -the same strength as in Type 1, but only one, two, three and four weeks -respectively. The last layer is omitted for light harness, and an extra -layer of 75 deg. is given if the tanner is not the currier also. Thus the -usual tannage is 16-20 weeks, of which 10-14 weeks (63-73 per cent.) are -in layers. - -=Type 3= is a tannage which may consist of myrabs (55 per cent.), -valonia or mimosa bark 25 per cent., and extract (26 per cent.). The -extract is chiefly quebracho, though some chestnut may be used. More -valonia and less myrabs may be used if desired (and when possible), and -myrabs extract will then replace quebracho and chestnut. The goods are -coloured off in drums or paddles, and then pass through two sets of -suspenders handlers (20 deg.-55 deg. and 55 deg.-75 deg.). They are handled up and down -very frequently in the first set and rapidly pass into stronger liquors. -The backs then receive three floaters at 80 deg., in each of which they -remain one week. The tannage is completed by three layers: first, 85 deg. -for one week; second, 90 deg. for one week; third, 95 deg. for two weeks. The -tannage is thus 11 weeks, of which 7 weeks involve little labour. If the -tanner is not the currier, still stronger liquors may be used. - -In all these tannages little or no acid is used for plumping, as the -natural acids of the liquors are sufficient to ensure what is necessary -in this direction for this class of leather. A little organic acid or -even boric acid may be used in the earliest liquors for deliming -purposes, when necessary. After tanning the goods are dried out and -sorted in the rough state. Harness is a somewhat broad term, and there -is scope for considerable variety in classification. The hides are -sometimes not rounded until after tanning. The finished article may be -any grade between heavy harness for artillery and leather for ordinary -bridles. - -In currying heavy black harness, the backs are soaked and sammed for -shaving. Lighter goods may be machine-shaved, but the heaviest are -shaved lightly by hand over the beam or merely "skived" with the shaving -slickers. The neck needs most attention, and it is often advisable to -stone by machine and split. The scouring should be thorough, on flesh -and grain. This is done by machine, and not only cleans the goods from -bloom, dirt and superfluous tan, but also assists in setting out. -Sumaching may be for several days, merely overnight or even only for a -few hours, being stoned after wetting back to temper. Hand-stuffed goods -get a coat of cod oil first, and during the drying are often well set -out. Drum-stuffed goods are well set out by machine, and after some -drying, stoned and reset by hand. It is now usual to buff the grain, -_i.e._ remove the coarser parts by light shaving. This prevents cracking -in the finished article. The goods are blacked with logwood, iron and -ammonia, thinly dubbined again, again well set out and tallowed. Setting -out, indeed, may be done at any convenient opportunity. The superfluous -grease is removed by slicking, scraping, brushing with a stiff brush, -and finally with a soft brush. - -For brown harness the goods are more carefully selected, more thoroughly -scoured and sumached, and bleached frequently with oxalic acid. They are -hand stuffed, stained twice, and after the usual setting out, glassing -and brushing, are finally rubbed with flannel. - -For bridle leather the goods are carefully shaved but are not stuffed, -being merely oiled with cod oil on flesh and grain. They are dried out -before scouring, and then sized, set out, stained and resized. The goods -are heavily glassed during the finishing. - - - REFERENCE. - - Bennett, "Manufacture of Leather," pp. 195, 297. - - - - - Section VIII.--UPPER LEATHERS - - -The manufacture of leather for the uppers of boots and shoes embraces a -bewildering variety of goods, suitable for anything between a baby's -shoe and a man's shooting boot. Almost all degrees of lightness, -softness, and waterproofness are in demand. A great variety of finish is -also involved, determined by the ingenuity of the currier and the -ever-changing fancy of the public. Even greater is the variety of -methods by which all these results are obtained by methods which -superficially seem quite different; the desired qualities being imparted -in one case largely by the tannage and in another case almost entirely -by the currying. Under such circumstances the selection of types becomes -a problem. - -The variety, moreover, commences from the earliest stages, the selection -of the raw material. Upper leather may be made from light calfskins, -heavy calfskins, kips (home and foreign), light dressing hides and heavy -dressing hides, which last may replace any of the former after splitting -to the required substance. In this section it will be necessary to take -kips as typical of the rest, and to use it in a rather broad sense, -including heavy calf and light dressing hides. - -Speaking quite generally, kips for upper leather receive usually a long -and mellow liming, a thorough bating and a sweet and very mellow tannage -in weak liquors. In currying they are well scoured and set out, heavily -stuffed and stained black, being sometimes finished on the grain and -sometimes on the flesh. These outstanding features of upper-leather -methods will be further illustrated by a brief account of the tanning -of kips (light hide and heavy calf), and outlining the best known types -of finish for butt, shoulder and belly. - -The goods receive usually a long and mellow liming of 14-16 days, using -only lime as a rule. In some factories lime liquors are used repeatedly -for successive packs to an almost indefinite extent. Dissolved hide -substance, ammonia, mud and dust, and bacteria accumulate for months and -sometimes for years. It is obvious that in such liquors "putrefaction" -is a more correct term than "liming" for the depilation. Such methods -have been used even in recent years, but there has now been a tendency -for some time to make the liming more methodical. Such old limes make a -leather which is empty, loose, and dull grained, but the defects are -minimized by the system of stuffing heavily and finishing the flesh, and -hence the ancient lime remained with surprising tenacity. Even so late -as 1903 we find that Procter with characteristic caution could write, -"Probably no lime ought to be allowed to go for more than three months -at the outside limit without at least a partial change of liquor." It is -within the writer's experience to find an upper leather factory with -limes which had never been emptied for over three years. In other -factories, however, there has been a revulsion of feeling with regard to -such processes, and it has been found advantageous to adopt a more -scientific routine, in which the lime pits are cleaned out at regular -intervals. There is little doubt that a mellow liming is desirable, but -this can be secured by blending some old lime liquors with fresh lime -liquor in a systematic manner. Similar considerations apply to the -question of working the various packs through the limes. It is clear -that with a mellow liming a one-pit system is quite possibly -satisfactory, but the revulsion of feeling against a lack of method -produced a method of liming more elaborate than usual, and it is now not -uncommon to find kips limed in a "round" of 6-8 pits, the goods passing -through each pit. They remain in one pit about two days, and are shifted -forward. In the green or old limes the goods are handled up and down. -The old limes are, of course, mellower than the new and exert the -desired softening effect. The working is quite analogous to that of a -round of handlers. Unhairing is sometimes assisted by the use of arsenic -sulphide. E.I. kips need a thorough soaking before any liming; several -days are usually needed. The old methods involving putrid soaks and -stocks may be considered out of date, and it is usual to soften back in -caustic soda or sulphide soaks with some assistance by drumming. A -little sulphide is sometimes added to the older limes to continue the -treatment. - -The goods are next thoroughly bated and delimed. The hen or pigeon dung -bate is still usual, and probably gives the best results, though closer -approximations have been made of recent years on artificial lines. Some -bating with solution of hide substance seems necessary for these goods. -The lighter goods are often drenched also to complete the deliming, -using 6 per cent. bran on the weight of pelt. The heavier goods are more -often treated with boric acid after bating, which not only delimes -completely and gives a soft relaxed felt, but also acts as antiseptic -and stops the action of the bate, a matter of some importance (see -Section II.). Lactic acid may substitute boric, in which case about 2 -per cent. on the pelt weight of 50 per cent. acid may be required. It is -important to avoid a strong solution and local excess, hence lactic acid -must be added gradually so that the liquor is never stronger than 0.2 -per cent. Drumming and paddling is an advantage in deliming. - -The tannage is light in most cases, partly because some of the finished -goods are sold by area, but partly also because even if sold by weight, -the weight is obtained quicker and more easily by stuffing, which course -is also often preferable to obtain the desired mellow feel, -waterproofness and durability. Hence it is seldom that strong liquors -are employed. The tannage is also mellow, on account of the softness and -pliability required; no acids are consequently employed, and no material -which is liable to yield sour liquors. Gambier is easily the first -favourite amongst the tanning materials, whilst oak bark comes second. -It should be observed, however, that a hypothetical tannage of equal -weights of cube gambier and oak bark is in reality a tannage by -four-fifths gambier and one-fifth oak bark, on account of the relatively -greater strength of the former. This observation is so apposite with -respect to some tannages that it is nearly correct to say that the -tannage is gambier and the oak bark an excuse for having leaches through -which the gambier liquors may be run occasionally to clear and to -sharpen slightly. No serious theoretical objection to such a method is -possible if the liquors are weak and the system of working the liquors -is scientific and the process carefully regulated. Upper-leather -tannages, however, have scarcely merited scientific praise. It is often -a case, not of poor methods, but of no method at all. The same lack of -system, principle, and regularity observed with regard to the limeyard -has been equally obvious in the tanyard, when perhaps the need was even -greater. Even a mellow tannage has varying degrees of mellowness -possible to it; there still remains the question of the soluble -non-tans. However, method in the upper-leather tanyard has often been -conspicuously absent. There has been many a factory where any one tan -liquor was as good as any other in the yard. In the writer's experience -are two such cases: in one the liquors were all 25 deg. Bkr., in the other -they were all 0 deg. Bkr. In such cases, handling the goods from pit to pit -is somewhat futile, and handling forward from set to set still more so. -Hence it is possible to find dressing leather tanned by putting it -slowly through one round of handlers, adding a few buckets of gambier -where it apparently is necessary. It is, from one point of view, -surprising to see what serviceable and excellent-looking upper leather -can be manufactured by such happy-go-lucky processes. It is, however, -also possible to see how this may occur. Gambier is a stable tan, and no -souring and little decomposition take place in gambier liquors. It is -also extremely mild and non-astringent, and is always used in weak -liquors. The hides, moreover, are completely delimed, and there is -little danger of bad or uneven colour. Tanning under these conditions is -at its easiest; it is almost more difficult to spoil the goods than make -them right. Under such conditions tanning deteriorated rather than -improved in method. When neglecting it made little difference to the -finished leather, it was neglected. - -This state of affairs, however, was embarrassing whenever a tanner -wished to try any other tanning material. The expense of gambier and oak -bark made valonia and mimosa bark into obviously desirable alternatives -and substitutes. Methods which would tan with gambier, however, would -not work with Natal bark or valonia, and many a tanner has had to revise -his method of tanning from end to end. The use of myrabs also raised the -problem of souring, and it has become evident that "working the liquors -down the yard" is as desirable a method for dressing leather as after -all other tannages. It will be clear from the above that types of -upper-leather tannages are less typical than for other leathers, but -nevertheless the more progressive manufacturers have for some years now -been working on sounder lines, economically and scientifically. In such -cases it is now usual to pass the goods through at least two sets of -handlers, and through liquors of gradually increasing strength. -Occasionally dusters or layers are given, especially for the heavier -goods. The tannage is nearly always commenced now by paddling the goods -in the oldest liquor. This paddling may be anything from half an hour up -to twenty-four hours. It is sometimes desired to work up a "grain," and -the old liquor is then often sharpened by the addition of fresh gambier -or leach liquor. - -The same tendency to save labour in handling is to be observed in upper -leather tannages as in sole and other dressing leather factories. There -is also a tendency to obtain rather more weight in tanning by using -stronger liquors, and in the heavier goods to shorten somewhat the time -taken. The following methods may be taken to illustrate modern processes, -in order of evolution. They all last about seven weeks. - -_Type 1_.--In this process the kips are first paddled in an old liquor -(3 deg.), and passed to the first handlers (3 deg.-30 deg.) for three weeks. After -working through this set they pass through the second handlers -(20 deg.-30 deg.), in which they are not handled quite so frequently. They are -in this set also three weeks. Heavy goods may then receive a floater -(30 deg.) for another week. - -_Type 2_.--In this process the goods are paddled, and then enter a large -handler round (8 deg.-30 deg.), through which they pass in five weeks. The goods -are handled frequently in the early stages. The tannage is completed by -one layer of two weeks (30 deg.). The layer is made by the ancient method of -putting the goods and dust alternately into an empty pit, and then -filling up with liquor from the best leach. Oak bark, valonia and myrabs -are used as dust, though sumach and gambler have been used. - -_Type 3_.--In this process an attempt is made to save handling and -obtain more complete tannage. The goods are paddled for three to five -hours in a rather sharp liquor of 10 deg., and are then handled well for a -week in the first handlers (5 deg.-20 deg.). The goods then go through the -second handlers (20 deg.-45 deg.) in six weeks, and heavy goods may then receive -an extra floater (45 deg.) for one week. - -In type 1 the leaching material is two-thirds oak bark and one-third -valonia; in type 2 it is half oak bark and half mimosa bark; in type 3 -it is one-third oak bark, one-third valonia or Natal bark, and one-third -myrabolans. In all cases the strongest handler is obtained from the -leaches, and made up to the required with strong infusion of gambier. -When the liquor has passed through the forward handlers, it is returned -to the leaches to clear and sharpen, and then run to the green handlers. -After passing through this round it is returned to the paddle, from -which it passes to the drain. The rest of the paddle liquor may be from -the forward handlers. It is often customary to obtain the best liquor -from the second leach, and allow the best leach to stand for a few days. -This allows the bloom to deposit in the leaches. The system secures the -result desired, but the deposition of bloom involves a loss of tannin, -which waste makes the system expensive. - -Heavier dressing hides are tanned by methods similar to the above, but -with floaters, dusters and occasionally layers added after they have -passed through two sets of handlers. Thus they may have first handlers -(8 deg.-18 deg.) two weeks; second handlers (40 deg.-45 deg.) for six weeks, making -twelve weeks in all. Lighter goods may receive two rounds, being two -weeks in each. - -After tanning, the kips are rounded usually into butts, shoulders and -bellies, to which different finishes are given. The currying may be -illustrated by selecting types, but it must be borne in mind that there -is much elasticity in this matter. Thus kips may be made into waxed -butts, satin shoulders and lining bellies, but also may be cut down the -back in "sides," both of which are finished limings. - -Waxed kip butts are a type of many similar upper leathers (waxed shoe -butts, waxed calf, waxed splits, etc.). The finish is on the flesh side. -The kip butts are soaked carefully, and shaved by machine. They are then -drummed in sumach for an hour or two, slicked out and sammed for -stuffing. The sumaching is also the scouring unless the goods be too -heavily bloomed. The samming is often done by machine. Drum stuffing -follows, wool fat and stearin being staple greases, with varying amounts -of degras and cod oil, and of tallow and cod oil. A little paraffin wax -and resin are also used sometimes. The goods are well slicked out and -dried. They may be now dubbined and laid away to mellow for whitening, -which consists of a careful shaving of the flesh by a turned-edge -slicker or by machine. The grain is stoned, set out and "starched," and -the butts grained by boarding the flesh. In the waxing, one of two -courses may be adopted. The butts may be blacked with lampblack and oil, -"bottom sized" with glue, soap and logwood, and then "top sized" with -glue, dubbin, beeswax and turpentine; or they may be given a -"soap-blacking" of soap and logwood and lampblack, applied by machine, -and sized once only. - -Dressing hide butts may also be given a grain finish, such as the "memel -butts" for heavy uppers. The butts are soaked, shaved or split, sumached -in drum, and preferably thoroughly scoured on flesh and grain. They are -then sammed and heavily stuffed in the drum. The grain is buffed and -stained black with logwood, ammonia and iron solution (curriers' ink). -The butts are then dried, set out, thinly sized and slowly dried. - -When dry on the face they are printed or embossed by machine to give the -characteristic memel pattern and dried out completely. They are then -grained four ways. The grain is finished by a coating of linseed oil -containing resin, and the flesh is whitened, French chalked and glassed. - -Shoulders for "satin" receive a currying which strongly resembles the -"waxed" finishes, but the smooth finish is on the grain side. The grain -is buffed, and blacked, dubbined, set and reset, with intermediate -drying, and is sized and finished by compositions similar to those used -for waxed leathers. The flesh is whitened. Satin hide and satin calf are -dressed similarly. - -Shoulders may also be finished for "levant." After soaking, splitting, -and shaving to substance, they are drum-sumached, machine-sammed, and -oiled up to dry. They are stained with logwood on the grain, and at once -printed with the typical "levant grain," blacked and dried out. They are -then softened by machine, seasoned with logwood and albumen, glazed, -grained and oiled lightly with mineral oil. It will be observed that -stuffing is omitted. - -Bellies may be dressed for linings. After soaking and splitting to the -required substance, they are bleached in a weak and warm solution of -oxalic acid, and drum-sumached at 110 deg. F. After slicking well out they -are hand-stuffed on the grain with dubbin and water, or merely oiled, -and hung up to samm. They are then set-out flesh and grain. If the grain -be coarse, it is buffed and reset. After drying out the flesh is fluffed -and the grain dusted with French chalk. - -In this section may be conveniently discussed the manufacture of legging -leather. Whilst in many respects a typical dressing leather there are -some rather important differences from the average upper leather. -Broadly speaking, the differences are that legging leather needs a -smooth grain, greater firmness and more thorough tannage on account of -the absence of stuffing. - -The liming and bating are somewhat similar to dressing leather, though a -shorter liming with sulphides and a milder bating would be in order. The -tannage is mellow, but not so much as is usual for upper leather. Thus -gambier is used, but more valonia and myrabs are employed, and the -liquors may be strengthened with chestnut and quebracho extracts. The -hides are rounded before tanning into long butts or backs, and the -tannage is commenced in suspenders (18 deg.-40 deg.), which are kept acid by the -addition of lactic or acetic acid, in order to obtain the required -firmness; the goods are three weeks in these liquors. The backs next go -through rounds of dusters (40 deg.-50 deg.), in which they are put down with oak -bark and Natal bark. They are six weeks in this section, and then pass -to the layers. Three layers are given, first 50 deg. for one week; second -55 deg. for two weeks; and third 60 deg. for two weeks. The tannage thus takes -fourteen weeks. - -In finishing, the goods are soaked and split, and then scoured flesh and -grain. They are heavily sumached, slicked out thoroughly, oiled up with -linseed oil and dried out. They are then next damped back, stoned and -flatted. After further wetting and tempering they are dressed with Irish -moss and tallow on the flesh, and with gum tragacanth on the grain. -They are glassed whilst drying out, and then stained twice and glassed -again. They are again brushed, seasoned and glassed by machine. - - - REFERENCE. - - Bennett, "Manufacture of Leather," pp. 197-201 and 301-308. - - - - - SECTION IX.--BAG LEATHER - - -Hides to be tanned for bag leather receive a treatment which is little -different in fundamental principle from that of dressing hides for upper -leather, except that the tannage is usually shorter. Hides for bags and -portmanteaux represent a type of dressing leather in which the -outstanding features are that the goods are split but not rounded. The -splitting is done at all stages, according to the requirements of the -tanner. Some tanners split "green," _i.e._ split the pelt itself. The -advantage of this is that the fleshes may then be treated in quite a -different way, _e.g._ pickled or given a much cheaper tannage. Other -manufacturers split after tanning, the advantage being that there is -much less material to handle. The general opinion, however, favours a -middle course in which the hides are split after being in the tan -liquors for a short time. The advantage of this course is that the hides -are easiest to split under these conditions--a great -consideration--being coloured through with tan, just a little plumped, -but not hard. A smoother flesh is obtained together with more even -substance. Here again, however, are differences; some tanners prefer to -split after two days, others after two weeks in tan. Much depends upon -the nature of the tan and the strength of the liquors. - -For this class of work, flat, spready and evenly grown cowhides are -obviously the most suitable material, and are invariably used. It is -important, however, that the grain be good, and free from scratches and -similar defects. The tannage must be sweet and mellow, _i.e._ contain no -acid and little astringent tan. Hence myrabolans and gambier have always -been the favourite tanning materials. A soft and mellow tannage is the -more important, inasmuch as the leather is not heavily stuffed with -grease in finishing. These types of method for tanning split hides will -now be outlined, and the nature of the currying then indicated. - -_Type 1_.--In this a long mellow liming of 15-16 days is given, much -like that described for harness leather in Section III., Type 3. -Only lime is used, but the liquors are not allowed to get dirty. The -three-pit system is much the best. The hides are trimmed at the rounding -tables, and then bated in hen or pigeon dung for three days at 75 deg.-85 deg. -F. The deliming is commenced by washing in tepid water before bating, -and is completed by a bath of boric acid, using up to 30 lbs. acid per -100 hides as necessary. In this and other processes for split hides it -is essential to obtain all the lime out, but to do no plumping with -acid. Lactic acid may also be used, but it is not so convenient to hit -the neutral point with it. - -The tannage consists of oak bark and myrabs together with gambier. These -may be partly replaced by Natal bark, valonia, and quebracho -respectively. It is sometimes desired to have a smooth finish, but -sometimes to work up a "grain." In the latter case the hides are first -put through colouring pits containing fresh leach liquor. In these they -are constantly handled for a few hours. A little experience indicates -which leach liquor will serve the purpose. The hides then go through the -"green handlers" (8 deg.-20 deg.) in two weeks. The liquor is the old forward -handler liquor made up with gambier. The hides may be sammed and split -up at this stage, but the heavier goods may be tanned further. These -heavies and the grains of the split hides now go through the "forward -handlers" (20 deg.-40 deg.) for four weeks, and the heaviest goods given two -layers (40 deg.) of two weeks each, and making ten in all. - -_Type 2_.--In this a shorter liming of 8-9 days is given with the help -of sulphide. No dung bate is used, but the goods are washed with water -and bated with ammonium chloride and boric acid. The tannage is chiefly -of myrabs, but some valonia or Natal bark may be used together with -chestnut extract and some quebracho. Gambier is used in the early -liquors. The goods are coloured off in drum or paddle and tanned in -several sets of handlers, viz. green handlers (15 deg.-35 deg.) three or four -days; second handlers (35 deg.-60 deg.) two weeks; forward handlers (60 deg.-80 deg.) -1-1/2 weeks; and floaters (80 deg.-90 deg.) for three weeks. The tannage is thus -6-1/2 weeks in all. The arrangement of pits is a matter of local -convenience, and the number of sets of equal strength is determined by -the number of hides being tanned. The hides are split green or after -passing through the green set. After tanning they are oiled with cod oil -and dried out. - -_Type 3_ is illustrated by American methods. The goods are tacked on -laths or racks with copper nails in order to ensure smooth grain. They -are then suspended in tan liquors. The tannage is largely with gambier -and in weak liquors, which also help to give smooth grain. The tendency -is to employ handler rounds involving a rather large number of pits, and -to work these on the press system. Handling is also saved by plumping -the liquors instead of shifting the goods forward, and by rocking the -suspenders instead of handling up and down. The hides are split after -about a month, and the heavier grains laid away in hemlock liquors. - -_Type 4_.--This is a rapid process throughout. The hides are limed in -6-7 days with the help of sulphide, and "bated" by washing in warm water -and then in cold to which hydrochloric acid is gradually added, -finishing off again in tepid water. The hides are now coloured off in -paddles, put through a small handler round (11 deg.-20 deg.) for half a week, -and then split. The grains are drum tanned in a mixture of chestnut and -quebracho extract, over a period of about three days in which the liquor -is strengthened gradually from 30 deg. to 50 deg. The fleshes are drum tanned -with the old grain liquors after strengthening with quebracho. - -The split hide grains for bag work, after tanning, are drummed in -sumach, rinsed, drained, and oiled up to dry out, with some setting out. -After wetting back they are shaved if necessary, hand scoured, and -heavily sumached again to get a light even colour. The goods are slicked -out, oiled up to samm, reset and dried out. They are next stained, -sammed, printed by machine, dubbined or tallowed, "grained" (see Part -II., Section I.), brushed and rubbed with flannel. - - - REFERENCE. - - Bennett, "Manufacture of Leather," pp. 202, 308. - - - - - SECTION X.--PICKING BAND BUTTS - - -It is the paradox of vegetable tannage that the less the pelt is tanned -the stronger is the leather produced. The manufacture of butts for -picking bands affords a good illustration. What is required is a leather -of maximum toughness, pliability and durability. Any factor reducing the -tensile strength of the leather is fatal. Hence, compared with most -other tannages, picking band butts are under-tanned. To ensure the -desired softness and pliability, moreover, it is necessary to have a -mellow liming, rather heavy bating, and a soft mellow tannage in sweet -and weak liquors. The required durability and the necessity for weak -liquors both point to oak bark as the most suitable tanning material, -assisted by some gambier in the early stages. - -A good quality hide is chosen, and given a long and mellow liming of -about 15-16 days. The one-pit system may be used, and the hides are put -into an old lime for about five days with frequent handling and then -placed in a new lime which is made up in a pit containing about a foot -depth of the old liquor. After about twelve days another 1/3 cwt. of -lime may be added. - -After unhairing and fleshing the goods are bated in pigeon dung for four -days at a temperature of about 78 deg. F., handling twice on the first and -last days. The bating is stopped and the deliming completed by paddling -with boric acid (15 lbs. per 100 butts). - -The tannage is commenced by paddling in a spent handler liquor (4 deg.) to -which a little gambier has been added. The butts then go through the -first handlers (5 deg.-15 deg.), which are rounds of ten pits in which the goods -are handled every day in the first week, and alternate days in the -second week, and are shifted forward twice a week in the next pit. The -goods are therefore in this set for five weeks. Gambier is added to -these liquors as needed. The butts next pass to duster rounds of four -pits, in which they are dusted down in a liquor of 20 deg. for four weeks -with 1-2 cwt. of oak bark. The liquor is obtained from the leaches, and -afterwards run alternately to the leaches and to the first handlers. As -many as six layers are now given of 20 deg.-25 deg. strength, in which the butts -are dusted down with 2-3 cwt. oak bark for three weeks. The layer -liquors are received from and returned to the leaches, which are made -from the "fishings" from the layers. The tannage lasts, therefore, 27 -weeks, of which 18 weeks (two-thirds) are in layers. - -Shorter tannages are now often given, using stronger liquors, much as in -ordinary dressing leather. - -The tanned butts are rough dried, and then wet in for shaving. They are -thoroughly scoured, flesh and grain. They are next drummed for -three-quarters of an hour in sumach, struck out and hung up to samm. -Hand stuffing is best, to avoid any tendering owing to high temperature, -but drum stuffing is also used. After setting out and stoning on the -grain they are stuffed with warm cod oil and laid away in grease for -several weeks, re-oiling occasionally. They may be stained before -stuffing. - - - REFERENCE. - - Bennett, "Manufacture of Leather," pp. 203, 310. - - - - - PART II.--SKINS FOR LIGHT LEATHERS - - - SECTION I.--PRINCIPLES AND GENERAL METHODS OF LIGHT LEATHER MANUFACTURE - - -The term "skin," like the term "hide," in its widest sense applies to -the natural covering for the body of any animal, but is generally used -with a narrower meaning in which it applies only to the covering of the -smaller animals. Thus we speak of sheep skins, goat skins, seal skins, -pig skins, deer skins, and porpoise skins. It is in this sense that it -will be used in this volume. The treatment of such skins to fit them for -useful purposes comprises the light leather trade. Whilst this branch of -the leather industry is certainly utilitarian, the artistic element is a -great deal more prominent in it than in the heavy leather branch. Thus -the light leathers are often dyed and artistically finished, and their -final purposes (such as fancy goods, upholstery, bookbinding, slippers, -etc.) have rather more of the element of luxury than of essential -utility. The total weight and value of the skins prepared, and of the -materials used in their preparation, are naturally considerably smaller -than those of the heavy leather trade. In the latter, moreover, one has -to consider the purpose in view from the very commencement of -manufacture and vary the process accordingly, but in light leather -manufacture one aims rather, in the factory, at a type of leather such -as morocco leather, and only after manufacture is it fitted to such -purposes as may be particularly suited to the actual result. These -results depend very largely upon the "grain pattern" which is natural to -the skin of any one species of animals. Hence in Part II. of this volume -it has been found most convenient to deal with the different classes of -skins in different sections. Just as the hides of ox and heifer were -much the most numerous and important of hides, so also naturally are -sheepskins the most prominent section of the raw material of the light -leather trade. This is the more true because the skin is valued for its -wool as well as for its pelt; indeed, the wool is often considered of -primary importance, and receives first consideration in fellmongering. -Unfortunately for the light leather trade, sheepskins, though most -numerous, do not give the best class of light leather, the quality being -easily surpassed in strength, beauty and durability by the leather from -goat or seal skins. - -In the wet work for the preparation of skins for tannage much the same -general principles and methods are embodied as in the case of hides, but -with appropriate modifications. As soft leathers are chiefly wanted, a -mellow liming is quite the usual requirement for all skins. It is also -usual to have a long liming, for some skins (like those of sheep and -seal) have much natural fat which needs the saponifying influence of -lime and lipolytic action of the enzymes of the lime liquors; whilst -other skins (like those of goat and calf) are very close textured and -need the plumping action of the lime and a certain solution of -interfibrillar substance. In consequence of the long mellow liming, -sulphides are not usually necessary, and indeed sodium sulphide is not -usually desirable, on account of its tendency to make the grain harsh. -It is used, however, for unwoolling sheepskins, in such a manner that -the grain is not touched. Similarly caustic soda is seldom required, and -the yield of pelt by weight is usually a small consideration. Systems of -liming show some variety. The one-pit system is very common, and is less -objectionable for a long mellow liming, but rounds of several pits are -also used, and in some cases even more than one round. This is obviously -conducive to regularity of treatment, and as the work involved in -shifting the goods is much less laborious than in the case of heavy ox -hides, it would seem a preferable alternative. The depilation of -sheepskins involves very special methods of treatment (sweating and -painting) on account of the importance and value of the wool, the -quality and value of which would be impaired by putting the skins -through ordinary lime liquors. The pelts, however, are limed after -unwoolling. - -In deliming light leathers the process of puering is widely used. This -consists in immersing the skins after depilation in a warm fermenting -infusion of dog-dung. In principle this disgusting process presents a -close analogy with bating, and indeed the two terms are both used -somewhat loosely, but there are nevertheless several points in which the -two processes are radically different. The dog-dung puer is a process -carried out at a higher temperature than the fowl-dung bate; it is also -a much quicker process, and the infusion employed is generally more -concentrated. Whilst the fowl-dung bate is always slightly alkaline to -phenolphthalein the dog-dung puer is always acid to this indicator, and -the course of the puering may be conveniently followed by testing the -pelts with it. The mechanism of the two processes is also probably -somewhat different. The mechanism of the dog-dung puer has been largely -made clear by the researches of Wood and others, and been found due -partly to a deliming action by the amine salts of weak organic acids and -partly to the action of enzymes from a bacillus of the coli class, which -received the name of _B. erodiens_, and which effects a solvent action -on the interfibrillar substance. As we have noted (Part I., Section -II.), the fowl-dung bate involves two fermentations, in each of which -(aerobic and anaerobic) several species of bacteria are probably active. -Wood found the bacteria of the bate to be chiefly cocci, and ascribed -part of the difference in mechanism by the nature of the media, which in -the bate includes also the urinary products. In the dog-dung puer, also, -a lipolytic action is probably an essential part of the total effect. -The puer gives a much more complete deliming and a much softer and more -relaxed pelt than the bate, it is therefore particularly suited to the -needs of light leather manufacture. The puering action has been imitated -fairly successfully by artificial methods. "Erodin" (Wood, Popp and -Becker) involves the use of _B. erodiens_ and a suitable culture medium -including organic deliming salts: "Oropon," "Pancreol" and others -involve the use of ammonium chloride and trypsin, together with some -inert matter. - -Light-leather goods are usually drenched after puering. They are also -often split green after the wet work. Sheepskins thus yield "skivers" -(the grain split), whilst the flesh split is often given an oil tannage -(see Part IV., Section III.). The greasy nature of sheep and seal skins -necessitates the processes of "degreasing." In the case of sealskins -this is done largely before liming, but with sheepskins either after -being struck through with tan, or after tannage is complete. Sheepskins -are often preserved in the pelt by pickling with sulphuric acid and -salt, which process forms a temporary leather. The fibres of the pelt -are dried in a separate condition, but the adsorption is easily -reversible and the pelts may be "depickled" by weak alkalies and -afterwards given an ordinary vegetable tannage. - -In the vegetable tannage of skins for light leathers, the same -theoretical considerations have force as in the heavy-leather section, -but the former has its own rather special requirements and aims. -Generally speaking, a softer and more flexible leather is required, but -these qualities must not be imparted by stuffing with grease as in the -currying of dressing leather, because a bright and grease-free result is -usually required. Hence it is important that a sweet mellow tannage be -given. The durability of the leather is also a primary consideration for -goods intended for bookbinding, upholstery, etc., and the tannage must -be arranged to impart this quality and avoid anything tending to cause -the perishing of the fibre. Thus oak bark is a popular tanning material, -and sulphuric acid very definitely avoided. The tannage must be fast, -and take the dyestuffs well, and for the production of light shades of -colour in dyeing must be a light-coloured tannage. All these qualities -are imparted by sumach, which also fits in excellently with the other -general requirements, such as softness, brightness and durability. -Hence sumach is the principal light-leather tanning material, but the -tendency is to employ other materials--oak bark, myrabs, and chestnut -extract--to do much of the intermediate tanning, so that the expensive -and useful sumach may be used for setting the colour and grain at the -commencement, and for brightening, bleaching and mordanting the leather -at the end of the tanning process. Weight is generally no consideration, -but area is often a definite aim, partly because some goods are sold by -area and partly because the striking out, setting out and similar -operations improve the quality of the leather by giving evenness of -finish. Leather well struck out, moreover, is less liable to go out of -shape. As the grain pattern is so important in the finished leather, -appropriate care must be taken during tannage. If a smooth or a fine -grain finish is wanted, for example, the goods must not be allowed to -get wrinkled, creased, doubled or unduly bent to and fro during the -tanning. For such goods, suspension, careful handling and even the "bag -tannage" may be desirable, whilst for coarser and larger grains paddles -or drums may be more extensively used. - -Amongst the finishing processes dyeing holds an important position. The -nature of the process has many points of similarity with that of -tanning. The great specific surface of pelt is probably more enhanced -than otherwise during tannage, at any rate with light leathers, owing to -the isolation of fibres, and consequently leather is as liable as pelt -to exhibit adsorption. The dyestuffs, on the other hand, are substances -very easily adsorbed. Some (like eosin and methylene blue) are -crystalloids, some (like fuchsin and methyl violet) are semi-colloids, -whilst others (like Congo red and night blue) are undoubted colloids -forming sols (usually emulsoid) with water as dispersion medium. The -crystalloids and semi-colloids may also be obtained in colloidal -solution, sometimes being so changed on the mere addition of salts to -the solution. In addition, the pelt has been mordanted with tannin. If, -however, leather has been kept long in the rough-tanned or "crust" -state, this may not be so effective, owing probably to the secondary -changes in tanning (Part I., Section III.), but such leathers are -usually "retanned" or prepared for dyeing by sumaching (which process -also incidentally bleaches). The tannin mordant assists materially in -the fixation of the dyes. In the case of basic dyestuffs, lakes also are -formed, _i.e._ there is a mutual precipitation of oppositely charged -colloids (+dye, -tannin). The dyeing of leather is thus a case of -colloid reactions even more complicated than that of tanning. - -Another finishing operation typical of the light leathers is "graining" -or "boarding." In this the skins after dyeing and drying are worked by a -board which is covered by cork, rubber, perforated tin or other -material, and so grips or "bites" the leather. The object of "graining" -is to work up the grain pattern by pushing or pulling a fold on the skin -with the board. The nature of the grain varies with the thickness and -the hardness of the skin, with the amount of pressure applied, with the -nature of the board, with the direction of the boarding and with the -total number of directions boarded. There is thus infinite scope for -variety of finish, and hence arise bold grain, fine grain, hard grain, -straight grain, cross grain, long grain, etc. The operation requires -considerable skill and experience. In the case of skins with little -natural grain (such as sheepskin) embossing and printing machines -impress the desired pattern. - -In seasoning, a dressing is applied containing essentially albumins and -emulsified fats, _e.g._ egg albumin and milk. Colouring matters are also -often added to intensify or modify the shade. After seasoning the goods -are usually "glazed" by a machine which rubs the seasoned grain with -considerable pressure, by a glass or hardwood tool, and so produces a -high gloss, for which the seasoning is very largely a preparation. Light -leathers are very lightly oiled with linseed or mineral oil. - - - REFERENCES. - - Procter, "Principles of Leather Manufacture," pp. 220, 394. - - Bennett, "Manufacture of Leather," pp. 36-41, 55, 85-90, 92-112, - 312, 332. - - Wood, "Puering, Bating and Drenching of Skins." - - Lamb, "Leather Dyeing and Finishing." - - - - - SECTION II.--GOATSKINS - - -Goatskins are amongst the most valued raw material for the manufacture -of light leather. The leather obtained from them is of the very finest -quality in respect to durability and adaptability to the principal -purposes in view. The texture of the fibres in goatskin is exceedingly -compact and very strong, whilst the grain exhibits naturally a -characteristic pattern which renders it most suitable for a grained -finish. Hence for purposes like upholstery, bookbinding, slippers, it -forms almost an ideal material. The tanning and finishing of goatskins -into "morocco leather" may indeed be taken as a quite typical example of -light leather manufacture. - -The skins are obtained from all quarters of the globe where goats exist, -and the excellent quality of the leather produced has created a demand -which is greater than the supply. This is due not only to the demand for -morocco leather, but also to the popularity of the goatskin chrome upper -leathers such as "glace kid" (see Part III., Section IV.). The large -American trade in the latter has produced the saying that wherever there -is a goat there is an American waiting for it to die! The European -supply of skins is somewhat limited. They are obtained from the Balkans -and Bavaria, in which case they are small, fine-grained and plump skins. -The Swiss goatskins are larger, and have also a fine grain; they are -well grown and well flayed. Scandinavian skins have a poor reputation, -being very flat. The African supply is important; Abyssinian skins are -exceedingly compact and tough, and are very suitable for "bold grain" -finishes. The Cape skins are particularly large, strong and thick, but -their quality is often impaired by the cure, the skins being flint-dry, -and, like hides so cured, prone to unsoundness. Large quantities of -goatskins also come from the East. Many of these are imported in a -tanned state (E.I. Goat). These skins are tanned with turwar bark, which -contains a catechol tannin. They are also heavily oiled with sesame oil, -and need degreasing. The tannage is also stripped as far as practicable, -and the skins retanned with sumach before finishing. They make good -morocco leathers for many purposes, but the primary catechol tannage -renders them ineligible for finishing under the specifications of the -Committee of the Society of Arts. The skins have a Persian or Indian -origin. India also supplies a large number of raw dried goatskins which -are small and of variable quality. These, however, are more extensively -used for chrome uppers. - -Goatskins are imported in either a salted or a dried condition. The -great aim of soaking is to obtain the skins in a thoroughly soft -condition. Hence the soaking is prolonged, and some mechanical treatment -is desirable in addition to various steepings in water. To be certain of -softness it is desirable to avoid the use of alkalies in the soak -waters, for although they cause hydration of the fibres by imbibition, -they also have a plumping effect which is not wanted at this stage. -Salted goatskins are first immersed in water and left until the -following day. This dissolves the salt. They are then stretched and -given a fresh soak liquor of water only to soften further, clean, and -remove the rest of the salt. This second water lasts only a few hours, -and the goods are then drummed well in running water. This not only -cleans quickly, but has an excellent softening effect. They are again -returned to a soak liquor, then softened mechanically by working them -over a beam. This treatment must be repeated, drumming again if -necessary, until the skins are perfectly relaxed and thoroughly -softened. If the treatment be very prolonged it becomes advisable to use -antiseptics in the soak waters after the first drumming. Solubilized (or -emulsified) cresols of the "Jeyes fluid" type are the most suitable -antiseptics, but too much must not be used or the sterilization affects -the liming, in which bacterial action is needed. Flint-dry skins are -left longer in the first soak, which should be of water only. They are -then given a fresh soak liquor containing 0.2 per cent. of sodium -sulphide. Sometimes a 1.0 per cent. solution of borax is used instead; -it softens excellently, is antiseptic, and avoids the plumping effect, -but is rather expensive. The goods are next drummed well, and resoaked -and worked as for salted skins. In either case the soaking takes about a -week. - -The liming of goatskins presents some points of contrast with the -methods used for other skins. These differences are due to the -exceedingly tight and compact nature of the skin fibres. This -compactness of texture makes it quite necessary to dissolve the -interfibrillar substance to a greater extent than usual, and also to -plump the fibres and split them into the constituent fibrils. These -effects are essential to obtain a rapid and complete tannage and a soft -leather. Too much bacterial action should be avoided, however, or the -brightness and soundness of the grain may be impaired, which would be a -fatal defect in such a leather. Hence the liming is long rather than -mellow, and sharp limes rather similar to those required for sole -leather are often used. Another result of the tight texture of goatskin -is that depilation is not easily effected. This feature is rather -intensified by the deepness of the hair-root. Hence it is usual to -employ sulphides to assist the depilation. In one method two rounds of -five pits are used. The skins are given about two days in each pit, so -that the liming lasts approximately three weeks. In the first round, -which consists of rather mellow limes, arsenic sulphide is used to -assist depilation. Up to 6 per cent. on the weight of lime is added -during slaking. This is a comparatively large amount of arsenic -sulphide, and the depilation is considerably hastened; the skins indeed -are unhaired after passing through this round, _i.e._ after about 10 -days' liming. In the next round the object is plumping, and caustic soda -(or carbonate) is added to the lime liquors in quantities comparable to -those suggested for sole leather (Part I., Section V.). In -this round the goods stay also for about 10 days. An alternative to the -above process is to hasten the earlier part of the liming by employing -sodium sulphide instead of realgar. More sulphydrate may be obtained in -solution in this way, and the unhairing may be in about half the time. -The sulphide of soda also commences the plumping action which follows in -the next round, but this alternative has the disadvantage that the skins -are unhaired whilst the pelt is swollen with sulphide, which renders the -grain both harsh and tender and consequently more liable to damage by -the unhairer's knife. - -Deliming is by puering and drenching, and is often associated with a -further mechanical working of the goods. The skins are inserted into a -puer liquor at 85 deg. F. and thoroughly pulled down. The caustic alkalies -should be completely neutralized. A slight cut into a thick part at the -butt end should develop no pink colour with phenolphthalein. The skins -should be thoroughly relaxed, and the swelling so much eliminated that -they are quite soft, weak and "fallen." The resilience and elasticity of -the plumped skins should have quite disappeared, and the impressions of -hand or thumb should be readily retained by the pelt. The grain should -appear white and possess a soft and silky feel. In this condition they -are again worked over the beam to soften further if possible. They are -then rinsed and again worked over the beam. Drenching follows with 10 -per cent. of bran on the pelt weight, the operation commencing at 85 deg. to -95 deg. F., and lasting till next morning. The skins are next scudded -thoroughly to remove all dirt, but carefully so as not to damage the -grain. - -In tanning, sumach and oak bark are the staple materials. Sumach gives a -much lighter colour, and hence it is used alone for goods that are to be -dyed the lighter shades, but oak bark is a "faster" tannage and more -preferable for dyeing in those cases where blacks and very dark shades -are wanted. For ordinary purposes a blend is usually employed. A feature -of oak bark, also, is that it tends to make a firmer leather, so that -the proportion used must be adjusted with this fact in mind as well as -the question of colour. For firmer moroccos the skins may pass through a -handler round of oak-bark liquors (10 deg.-20 deg.) in which a certain amount of -sumach is added to the liquors. The sumach is leached and assists both -in tanning and bleaching as the liquor works through the round. The old -liquor is run to a paddle, and the tannage is commenced by paddling the -drenched skins in this liquor. It is advantageous both for the tannage -and for the efficient "spending" of the sumach if this liquor be -slightly warmed. In the early pit liquors the goods are very frequently -handled. There is, however, the usual tendency of the times to save -labour in this direction, and hence it is common to have several paddles -with liquors of gradually increasing strength, followed by a shorter -round of handlers in which the handling is more infrequent. Instead of -paddles latticed drums may be inserted into pits containing liquors. -These, however, are not quite so convenient. In some tanneries, -especially where sumach only is employed, the tannage is in paddles -throughout. A new liquor is made up with fresh sumach and is used -repeatedly until exhausted. A three-paddle system sometimes obtains, in -which case the operation closely resembles the three-pit system of -liming (Part I., Section II.), and the skins pass through an "old" -liquor, a "medium" liquor and a "fresh" liquor. The goods need not be -paddled the whole day through, and indeed in the later stages this is -undesirable. The packs remain several days in each liquor and take up to -14 days to tan. Two to three bags of sumach are needed for about 20 -dozen goatskins. This method of tanning is efficient and convenient for -bold-grain finishes, on account of the constant tumbling and bending of -the skins which tends to work up a grain. For very soft leathers and -fine-grain finishes, however, the "bag-tannage" or "bottle tannage" is -favoured. In this method the pelt is stitched up by machine to form a -bag, grain outwards, leaving a "neck" in the hind shank. The bag is -nearly filled with a fairly strong infusion of sumach, inflated with air -and tied up at the neck. The bags are then placed into a vat of warm -sumach liquor, in which they just float. The bags are pushed down and -the liquor stirred up, so that the goods are in constant motion. After a -few hours they are piled on a rack, and the tan liquor of the interior -is caused to diffuse through the skins by the pressure due to the weight -of the pile. The bags are refilled with fresh and stronger sumach liquor -and the process is repeated. The skins are thus lightly but effectively -tanned in about 24 hours, and the leather has very fine grain and soft -feel. However tanned the skins are dried out after tanning, and sorted -in the "crust" according to size and colour. The larger skins are -preferred for upholstery and the smaller for fancy goods and -bookbinding. - -To illustrate the course of finishing operations, the case of hard-grain -morocco for bookbinding may be given as typical. The goods are wet back -with warm water and drummed for 1-2 hours in warm sumac to prepare for -dyeing. They are then struck out by machine, sammed and shaved. Dyeing -follows, with acid colours, in a drum. The goods are run first in a -little water and the dyestuff added very gradually through a hollow -axle. The acid required (preferably formic) is added later to develop -the full shade. Warm solutions are used, and the dye bath is practically -exhausted. The goods are next placed in cold water to wash off -superfluous liquor and free the skins from acid. They are then horsed to -drain, struck out and hung up to samm. They are seasoned with milk and -water and piled to temper. They are "tooth rolled" in the glazing -machine two ways: right-hand shank to left fore shank and _vice versa_, -and piled again. After wetting back again they are "wet grained" by hand -with a cork board in four directions: belly to belly, shank to shank, -and across as before, and finally from neck to butt. They are -immediately hung up in a warm shed to dry, and to fix the grain. They -are then softened by "breaking down" with a rubber board, top seasoned, -piled to temper and dry, brushed lightly, piled again, brushed more -heavily, and dried out. They are finally softened by graining in three -directions: shank to shank and across, and neck to butt. They are then -brushed again. If these skins are wanted for upholstery they are shaved -after dyeing, and nailed on boards to samm. They are also dried out in a -cooler shed or "stove," to ensure softness. - - - REFERENCE. - - Bennett, "Manufacture of Leather," pp. 39, 55, 89, 111, 204, 344, - 396. - - - - - SECTION III.--SEALSKINS - - -A special class of morocco leather is manufactured from the skins of -seals. This should not be confused with the "sealskin" of popular -parlance, which is manufactured from the skin of a different animal. All -the fin-footed mammals (_Pinnipedia_), except the walrus, are termed -seals, but they are divided into two families. The _Otariidae_ are known -by their possession of small but distinct external ears: into this class -fall the fur-seals whose skin is dressed with the fur on, for women's -jackets, muffs and caps. The _Phocidae_ are that family without external -ears: the skins of many species (_Phoce Greenlandica_, _Phoco barbata_, -etc.) of this family are unhaired and given a vegetable tannage, thus -forming the raw material of sealskin morocco leather. It is with the -latter that this section will deal. - -As the seal is a marine animal and is partial to the colder seas, its -skin is very oily. The skins are imported in a salted condition from -both the Arctic and Antarctic regions. North Europe, North America and -Newfoundland supply many skins, and the southern material is supplied -chiefly through the Cape. Sealskin shares with goatskin the properties -of compact texture, strength of fibre, and great durability, all of -which fit it for the manufacture of moroccos for upholstery, -bookbinding, etc. It is, however, readily distinguishable from goatskin -by its characteristic grain pattern. - -In soaking sealskins the object is not only to soften thoroughly, but -also to effect the recovery of as much seal oil as possible before the -liming commences. This is desired because the oil is in itself a -valuable bye-product, and because its removal is essential to a -satisfactory liming and tannage. The removal of the oil is materially -assisted by raising its temperature, so that the soaking of sealskins -is often done with warm water (85 deg.-88 deg. F.), after which treatment they -are laid over the beam and scraped with a blunt knife on both flesh and -grain. The oil flows away into a special receptacle. This treatment is -repeated until the bulk of loose oil is removed. The process is known as -"blubbering" or "brushing over." After some soaking the skins are -drummed to ensure softness. The skins are then fleshed. More oil may be -obtained from the fleshings. - -By fleshing before liming a more regular action of the lime is obtained. -This is necessary to "kill" the grease still remaining in the skin. A -long and mellow liming is given for the same reason. Fully three weeks -are given, and old limes are much preferred, partly to obtain the -maximum lipolytic action and partly to avoid the intense ribbing of the -pelt which new limes so easily impart to the older animals. These ribs -are very difficult to eliminate in the subsequent work. Some factories -find it necessary to finish up in new limes, however, in order to plump -and split the compact fibre bundles into their component fibrils. The -plumped pelt is also easier to split green. No sulphides are usually -employed. Sweating (see Section IV.) is sometimes used for -depilation, and in this case the ribbing of the pelt does not take -place. - -The puering is unusually thorough with sealskins. This is to obtain the -maximum softness and take full advantage of the lipolytic action. The -puer liquor is fully 95 deg. F., and the skins are paddled for about three -hours, or until fully pulled down and completely delimed. Scudding -follows, now usually by machine. The skins are then well drenched. The -action is intensified by the use of peameal in addition to the bran. -About 10 per cent. of the mixture on the weight of pelt is used. It is -customary, however, to drench at a lower temperature (68 deg.-70 deg.) than in -the case of goatskins (Section II.), but the goods are left in -the drench overnight only, as is usual in drenching. It is quite -possible that drenches worked differently may have also a somewhat -different fermentation and be due to other organisms than the symbiotic -bacteria discovered by Wood. It is equally possible that the acids -produced are also different, in relative proportion, if not in nature, -and that consequently there is a real difference in the practical -effect. In the Author's opinion, the great probability is that in the -drench are several fermentations, and that if the action be reduced by -lowering the temperature, but intensified by adding peameal to the bran, -some of these fermentations are encouraged at the expense of others. - -The tannage of sealskins depends upon the size of the skins, the purpose -for which they are intended, and whether they have been split or not in -the limed state. The largest and coarsest skins intended for boot -uppers, and those which have been heavily scratched on the grain and are -only suitable for enamels, are given a tannage which may last about 5 -weeks. The liquors are made from oak bark and mimosa bark, and are made -up to 35 deg. with gambier and possibly myrabolans extract. For fancy work -also heavy skins are used, but a softer tannage is needed. If for blacks -the tannage is with gambier and chestnut extract. Two sets of handlers -are given (10 deg.-15 deg. and 15 deg.-20 deg.), using only gambier in the green sets. -They are well sumached after tanning to bleach and to mordant. If for -colours, only sumach and oak bark are employed. The skins are first -paddled for 3-4 days in sumach liquors, in which they are coloured -through. The liquors may be warmed; this quickens the tannage and also -leaches the sumach. The skins are then split, and the grains pass -through a handler set with liquors made from oak bark (8 deg.-24 deg.). The -skins are in this set for 3 weeks, in the first half of which they are -very frequently handled. They are finished off by paddling for 1 or 2 -days in a fresh liquor containing much sumach, which mordants the skins -and bleaches the bark tannage. The flesh splits are given a drum tannage -in chestnut and quebracho extracts. If small skins are being tanned for -bookbinding purposes, sumach only is employed, and usually the tannage -is entirely in paddles. - -In finishing many types of grain may be obtained, in blacks and in -colours. The finishing of "black levant" may, however, be selected as a -typical case. The skins are soaked back, tempered, and either split or -shaved, according to their substance and the size of grain wanted. The -thin skins of course give the fine grains. Mixed tannages need scouring -and possibly sumaching. The skins are then oiled up with linseed oil, -sammed, set out and blacked. In this last operation the grain is brushed -over with a solution of logwood and ammonia, and afterwards with the -iron mordant which often contains glue. They are next hung up for a -while and then "wet grained" in four directions--belly to belly, shank -to shank, across, and neck to butt. After hanging up in a hot stove to -set the grain, they are cooled, fluffed on the flesh, and seasoned on -the grain with a solution of milk and blood. A little black dyestuff may -be added to the season. The season is well brushed in, the skins dried -somewhat, and then glazed. They are then grained four ways again as -above, dried out in the stove, and lightly oiled with warm linseed oil -on the grain. - - - REFERENCE. - - Bennett, "Manufacture of Leather," 40, 56, 90, 112, 206, 251, 312, - 346, 383. - - - - - SECTION IV.--SHEEPSKINS - - -The most numerous class of skins for light leathers is from the common -sheep. These skins have particular value inasmuch as they include the -wool as well as the pelt. This wool, which is actually the most valuable -part of the sheep's skin, is the raw material of our woollen industries, -and is one of the most important of animal proteids. We have, therefore, -in this section to consider this dual value of sheepskins, the proteid -of the epidermis (wool), and the proteid of the dermis (pelt); one the -raw material of the woollen industry, the other the principal raw -material of the light leather trade. The first problem is to separate -the two proteids. With other skins and hides the ordinary liming -processes were sufficient and appropriate, but in the case of sheepskins -the method is unsuitable, because the exposure of the wool to the action -of caustic lime and possibly other alkalies would seriously impair its -quality and reduce its commercial value. Hence this separation of wool -from pelt is usually quite a separate business, viz. that of the -"fellmonger," whose occupation it is to collect the sheepskins from -butchers and farmers, to separate the two important constituent -proteids, and to hand the wool in one direction to the "wool stapler," -who sorts it according to quality, and to hand the pelt in another -direction to the light leather tanner, who tans and finishes the pelt to -fit it for light upper work, fancy goods, etc. - -In the first instance, therefore, we have to consider the work of the -fellmonger, the separation of wool and pelt. In this work the wool -receives first consideration, and the raw material of the fellmonger is -usually classified accordingly into "long wools," "short wools," and -"mountain breeds." The skins vary very largely in quality of wool and in -quality of pelt, being influenced very strongly by the conditions under -which the sheep lived, and by the precise breed of animal from which the -skin has been taken. As in the case of hides (Part I., Section I.), -animals exposed to extremes of weather develop the best pelts, whereas -those sheep which have been carefully bred and reared for the sake of -their wool yield a thin and poor class of pelt. In Britain, and more -especially in England, are reared the finest and most valuable sheep. -This is evident from the prices paid for them by foreigners and colonial -breeders when seeking new blood for their flocks and fresh stock for -their lands. As much as 1000 guineas have been paid by an Argentine firm -for a single Lincoln ram. - -Long wools are obtained from some of the best and most extensively bred -animals. The "Cotswolds" are the largest, and probably the original -breed of England are still found on the Cotswold Hills. They have long -wool, white fleeces, white faces, and white legs, and have no horns. The -wool is fine, but the pelts are particularly greasy, especially along -the back. A later breed originating in the Midlands was called the -"Leicester" long wool. This breed gives a great cut of wool and much -coarse mutton. It is very extensively distributed in the North of -England and has been much crossed, so that many sub-breeds are now well -known, _e.g._ the "Border Leicester"--the general utility sheep of -Scotland--and the "Yorkshire Leicester" or "Mashams," much bred in -Wensleydale. "Lincolns" are another long wool found only on the -Lincolnshire Wolds. They also have white faces and shanks and yield a -large pelt with fine grain. They give a big crop of wool. "Devons" are a -smaller breed common in Somerset, Devon and Cornwall. They yield a -fairly long wool of great strength, but not quite white. Romney Marsh -sheep ("Kents") are also long wools. They have white legs, white faces, -a tuft of wool on the head, and no horns. The pelt is large and good. -"Roscommons" are an Irish cross breed with much Leicester blood. They -yield a long wool and a spready pelt. - -Short wools are typified by the "Down" sheep. These sheep are -extensively bred on the chalk lands which comprise a very large -percentage of the southern counties of England. The "South Downs" are -the best and most important, the breed being the general utility sheep -of England. They are small but well-shaped animals with grey faces, no -horns and fine close wool. The pelt is only fair, but the mutton is -excellent and provides the meat sold in our best shops. This breed has -largely stocked New Zealand. The "South Down" is a somewhat delicate -animal, and has therefore been largely crossed with Cotswolds and other -breeds. Many well-known cross-breeds are found in the eastern and -southern counties. The "Suffolks," for example, are found in the eastern -counties. They have black heads, faces and legs. "Oxfords" and -"Hampshires" are similar, but larger. "Shropshires" are another hardy -cross-breed, which yield a heavier fleece. All the cross-breeds are -larger than the South Down and yield bigger pelts. - -Mountain breeds yield wool of varying quality but give the best pelts. -The "Cheviots"--much favoured by the Scotch farmers--have a wool of -medium length but with much hair in it. They have white faces and legs -and no horns, and yield excellent pelts. The "Black-faced Mountain -Sheep" have longer wool but coarse, and yield good pelts. They are kept -in the hilly parts of North England and in the Scottish Highlands. -"Lonks" yield a large and good pelt, but very coarse wool. The mutton is -good. They are a very large breed with much curved horns and black -faces. There are also some small breeds, "soft wools," "Shetlands," and -"Welsh Mountain Sheep." The wool of the last two is poor, but the Welsh -pelts are valued for their fine grain. There are large numbers of -sheepskins also imported, from South and Central America, and from -Australia, New Zealand and the Cape. The colonies, however, have often -done their own fellmongering, and we have imported pickled pelts. They -now tan the skins also, and many tanned sheepskins are now imported. -There are also many Indian skins imported after tannage with turwash -bark (cp. E.I. Goat, Section II.). - -The depilation is brought about by "sweating" (or "staling") and by -"painting." The immediate object of both these types of method is to -avoid using any thing which will affect the wool. The sweating process -is the most ancient method of unhairing and is used in America for hides -as well as sheepskins. It consists of a more or less regulated -putrefaction. The loosening of hair or wool has long been accepted as -evidence that putrefaction had commenced in a hide or skin, and it is -the aim of the sweating process to stop the action at that stage, before -any damage has been done to the pelt. This aim is achieved rather -imperfectly by suspending the goods in closed chambers and regulating -the temperature and humidity by means of steam and water. Such chambers -are known as "sweat pits" or "tainting stoves". In the case of -sheepskins the "warm-sweat" system is generally used, and the operation -is carried out at 75 deg.-80 deg. F. A satisfactory yield of wool is obtained in -good condition, but the pelt is very liable to suffer bacterial damage -and show "weak grain." The skins are first cleaned by a few "soaks" in -clean fresh water, with intermediate help from a "burring machine" which -presents a rapidly revolving set of spiral blades to the wool, and in -the presence of a good stream of water quickly removes all dirt from the -wool. The skins then enter the tainting stove, and the operation is -commenced by a slight injection of live steam. In summer, about a week -is sufficient to loosen the hair, but in winter up to two weeks may be -necessary. Little control of the process is possible, and all that can -be done is to watch the goods carefully near the end of the operation. -In one variety of this method of unwoolling the skins are painted on the -flesh side with a creamy mixture of lime and water and piled for a day -or two until the pelt is distinctly plumped. They are then washed with -fresh water to remove the excess of lime, drained, and then enter the -tainting stove. By this method the pelts are obtained in better -condition and are less liable to damage by local excess of putrefaction. -In unwoolling the skins are placed over a beam and the true wool is -pulled out by hand. The wool is graded as it is pulled and different -qualities kept separate: ewe wool, lamb wool, hog wool, etc. The hair is -next removed from face and shanks by means of a blunt "rubbing knife," -and the pelt then immersed in water. - -In the other method of depilation, by painting, advantage is taken of -the loose texture of the sheepskin fibre and of the fact that the wool -root is nearly halfway through the skin. The flesh side of the clean -skin is painted with a creamy mixture of lime in a strong solution of -sodium sulphide (14 deg.-24 deg. Beaume). Care is taken to keep the depilatant -off the wool. The skins are folded flesh to flesh and left for a few -hours or until next day before unwoolling, according to the strength of -the sulphide solution. The depilatory action is entirely chemical, being -due to the solvent action of the sulphide on the hair root. The lime is -sometimes omitted. After pulling, the skins are opened up and washed in -fresh water. - -The various classes of wool are sold to the wool-stapler and so to the -woollen industry. As this is a mechanical rather than chemical industry, -its discussion is beyond the scope of this volume. However unwoolled, -the pelt still needs further treatment by the fellmonger. It needs -liming and unhairing. This is done in the ordinary way in pits of milk -of lime, through which the goods pass from old to new limes in the -course of about a week. This plumps the fibres, separates the fibrils -and kills the grease. Paddles are used also to save handling. Shearlings -are sometimes limed 9-14 days and unwoolled without sweating or -painting. After liming the skins are unhaired and fleshed, and placed in -clean strong limes until sold to the tanner. - -Sheepskin pelts are sometimes preserved by pickling. This consists in -placing them first in a solution of sulphuric acid (about 3/4 per cent.) -together with some common salt. The pelts swell up and imbibe the acid -solution. They are then placed in saturated brine, which causes a very -complete repression of the swelling, the pelts being apparently -leathered. In this condition or partly dried out they may be kept for -years. The forces at work in this phenomenon are somewhat complex (see -Part V., Section I.). The skins may be depickled by paddling in a -10 per cent. salt solution to which weak alkalies such as borax, -whitening, carbonate and bicarbonate of soda, etc., have been added. - -The leather manufacturer classifies sheepskins according to the size of -the pelts. The large skins are tanned for light upper leathers and -similar work. These are called "basils." Many large skins are also split -green into "skivers" which after vegetable tannage are finished for -fancy goods, bookbinding, etc. The fleshes are often oil-tanned for -chamois leather (Part IV., Section III.). Medium-sized skins such as are -obtained from the Down sheep are tanned for "roans," and finished as a -kind of morocco leather. Small skins are mostly "tawed" (Part IV., -Section I.) for glove leathers, but some are made into roller leather by -vegetable tannage. - -Basils, which represent the heaviest sheepskin work, are tanned and -finished in the following manner. The limed pelts are first bated -lightly at about 80 deg. F. for two days, scudded and drenched. They are -sometimes puered, but more often merely delimed with organic acids. In -this last case they are first paddled in warm water to remove excess of -lime, and a mixture of organic acids is very slowly added at definite -intervals. When nearly free from caustic alkali the skins are removed -and drenched overnight. There are two types of tannage. The West of -England tannage is similar to those noted for sealskins when oak bark -and sumach are employed (Section III.). There is also the tendency to -paddle more and handle less, and to use the stronger tanning materials -such as myrabs, gambier and other extracts. After about 12 hours' -tannage in paddles they are coloured through, and are then degreased by -hydraulic pressure. The skins are piled in the press with layers of -sawdust or bran between them, and the pressure applied very slowly. Much -grease runs out, for the natural sheepskin contains up to 15 per cent. -of oil and fat. Degreasing may be postponed till tannage is complete, -and the grease can then be extracted by solvents (benzene, acetone, -etc.). Degreasing after part tannage is usually considered preferable, -and the skins may be tanned out in pit or paddle in about a week. The -Scotch tannage is with larch bark from _Pinus larix_, which contains up -to 13 per cent. of a rather mellow catechol tan. This material has also -some sugars and yields sour and plumping liquors. The basils are paddled -in weak liquors (8 deg.-11 deg.) for about 2 days, and when struck through are -degreased by hydraulic pressure. They are then soaked back and tanned -out in stronger liquors (11 deg.-20 deg.), which takes up to one week. They are -then dried out and sorted in the crust. The finishing depends of course -upon the purpose in view. If for linings they are soaked, shaved, -sumached, struck out well, nailed on boards and dried right out. They -are next stained with a solution of starch, milk and red dyestuff. After -drying they are glazed by machine and softened with a hand board. For -fancy slippers the crust skins are starched and stained directly, then -"staked" (see Part III., Section II.), fluffed, seasoned and glazed. If -intended for leggings and gaiters a flesh finish is given. The skins are -soaked, stretched, shaved and sumached. They are then rinsed, drained, -sammed and stained. A brown stain mixed with linseed jelly is usual. -This is spread evenly over the flesh and glassed in. The skins are dried -out, restained if necessary, and staked to raise a nap. Basils for -gaiters are dyed in paddle and fluffed over the emery wheel. - -Skivers are split in the limed state and sometimes immediately -degreased. They are next puered at 85 deg. F. for about 3 hours in a paddle, -and scudded. They are drenched at a low temperature (68 deg.-70 deg. F.), but -often 2 or 3 days. They are again scudded and then rinsed and sent to -tan. The skivers are tanned in a few days by sumach liquors working the -goods up from mellow to fresh as usual. The liquors are warmed. Care -must be taken that the goods do not tear. A great variety of finish is -possible, but the "paste grain skiver" for fancy goods and the plain -finish for hat leathers are sufficiently typical. For paste grains they -are soaked and "cleared" for dyeing by immersion in very weak sulphuric -acid, excess of which is carefully washed out with water. Paddle-dyeing -follows, and is preferred to drum dyeing as the skins are so liable to -tear. After being struck out they are "pasted," by spreading on to the -flesh a glue jelly, using first the hand, then a stiff brush and finally -a cloth. The goods are then dried out. They are then seasoned, partly -dried and printed cross-grain. They are next grained two ways lightly; -shank to shank, and across, lightly tooth-rolled and glazed. They are -regrained two ways as before, dried out, and finally softened with a -graining board. They are sometimes sized on the grain to fix the pattern -and give a gloss. For hat leathers the skins are first soaked, sumached -and struck out. If for white or cream finishes they are now -lead-bleached. This consists of pigment dyeing with lead sulphate. They -are immersed alternately in lead acetate and in sulphuric acid solutions -until precipitation is sufficient. They are then dyed to shade. If for -browns it is common to mordant with titanium and use basic dyestuffs, -paddling afterwards in sumach to fix the dye. After dyeing the goods are -struck out again, starched, and dried out on boards. They are again -starched and rolled to give the plain finish. - -Roans are not split. They are degreased, puered, scudded and drenched -overnight at 95 deg. F. They are tanned with sumach usually in pits, and -take rather longer than usual to tan. They are finished in much the same -style as goatskins for morocco leather, but as the sheepskin has little -natural grain it needs embossing or printing according to the type -required. If for "hard grains," the skins are soaked, sumached, -seasoned, dried, glazed and damped back for printing. This is done by -the "hard grain" roller, and the goods are dried out to fix the pattern. -They are damped back, sammed, and grained in four directions (cp. -Section II.), dried out and boarded to soften. If for straight grains -they are printed with a straight-grain roller, or grained neck to butt. -After tooth rolling they are boarded, dried and glazed. They are -softened down and "aired off" in a cool store. - -Roller leather is a special class of sheepskin leather which is used to -cover the rollers used in cotton spinning. The essential requirements -are that a smooth plain finish should be given, and the leather must not -stretch or be greasy. For this purpose small sheepskins with a fine -small grain are chosen, such as those obtained from the Welsh mountain -sheep. The pelts are machine fleshed, short haired and often puered, but -the deliming is also brought about by organic acids also. The pelts are -drenched in pits fitted with paddles, which are used to stir up the -infusion occasionally. A thorough scudding is given. For the -smooth-grain finish it is necessary to tan in weak liquors, and to give -plenty of time so as to ensure complete penetration. An oak-bark tannage -is preferred, but a little extract is usual to assist. The goods are -coloured through in paddle, like basils, and are then degreased by -hydraulic pressure. This should be as complete as possible, and a little -heat is used to assist the escape of grease. The pressed skins, -moreover, must be quite freed from creases, and this is attained first -by paddling in warm water to remove sawdust, and then by drumming in -fairly hot water, in which they are left overnight. The skins are tanned -out in suspenders, taking about 3 weeks. The crust skins need careful -sorting, and are soaked and hand shaved. They are sumached in drum, -rinsed, struck out, sammed and set. The striking and setting should be -thorough, in order to get rid of stretch. They are next "filled" by -coating with linseed jelly or similar material, and dried out on boards -in a thoroughly stretched condition. They are then trimmed, seasoned and -rolled with a steel roller. They are then staked or perched, fluffed, -reseasoned, dried and glazed. They are carefully short-haired, glazed -again and finally ironed. - -E.I. sheepskins are imported in a tanned condition. These are soaked -back and the turwar bark tannage "stripped " as far as possible by -drumming with soda for 20-30 minutes at 95 deg. F.; after washing they are -"soured" in weak (1/2 per cent.) sulphuric acid solution, and retanned -with sumach paste for an hour, drumming at 100 deg. F. They may then be -finished for basils, moroccos or roller leather as described above, but -are often finished as imitation glace kid. In this case they are drum -dyed, lightly fat-liquored (see Part III., Section IV.), struck out and -dried. They are staked by machine, fluffed, seasoned and glazed. They -may be re-staked and reglazed if desired. - - - REFERENCES. - - A. Seymour Jones, "The Sheep and its Skin." - - Bennett, "Manufacture of Leather," pp. 30, 85, 107, 208, 349-354, - 385. - - - - - SECTION V.--CALFSKINS - - -Calfskins are the raw material for many classes of leather. The term -itself is rather broad. A calfskin may be obtained from a very young -animal and weigh only a very few pounds, or it may be anything just -short of a kip. Goat, seal, and sheep skins are obtained from adult -animals, but calfskins from the young of a large animal. Thus there are -many grades of quality, according to age, and the material must be -chosen with regard to the purpose in view. Some of these purposes have -already been discussed. Heavy calf is treated much like kip as a curried -leather for upper work. Even lighter skins are given the "waxed calf" -and "satin calf" finishes, and make upper leather of excellent quality. -To produce such leathers the treatment is much the same as described in -Part I., Section VIII. Calfskins were also used for very light upper -work, in which they were not so heavily greased in finishing, but rather -dyed and finished as a light leather. In this direction, however, the -vegetable tannage has been almost completely superseded by the mineral -tannages, first by "calf kid," an alumed leather (Part IV., Section I.), -and afterwards by the now popular chrome tannage of "box calf," "willow -calf," "glace calf," "dull calf," etc. (Part III., Section III.). In -this section, therefore, we have only to consider calfskins as used to -make a vegetable-tanned light leather, such as may be employed in -bookbinding and in the manufacture of fancy goods. For these purposes -the skins receive a mellow liming of 2-1/2 - 3 weeks. No sulphide need -be employed, as the goods are soon fit to unhair. In such a mellow -liming it is important that the bacterial activity is not too prominent, -and hence it becomes advantageous to work the liming systematically in -the form of a round of pits. To avoid over-plumping in the newest limes -some old liquor is used in making up a new pit, and its bacterial -activity is reduced by adding it to the new caustic lime whilst slaking. -Thus for a pack of 200-250 skins, 14-16 stone of lime may be slaked with -about 30 gallons of old lime, and the pit filled up with water. If it be -necessary to shorten the process and to use sulphide, this should be -added only to the tail liquors of the round, and with it should be -added, if possible, some calcium chloride to reduce the harshness of the -soda. The skins should be puered thoroughly to obtain the necessary -softness, bate-shaved if desirable, and drenched with 8 per cent. of -bran overnight. - -In tanning for fancy work and for dark colours, the goods are coloured -off and evenly struck through with sumach liquors, and then tanned -further with liquors made from oak bark, myrabolans or chestnut extract. -The methods are very closely similar to those used for goatskins and -sealskins (Part II., Sections II. and III.), and need not be described -in further detail. The tannage is finished off in sumach. For -bookbinding work, however, a pure sumach tannage is given, using liquor -slightly warm (70 deg. F.). Paddle tannages are common, but for bookbinding -the bag or bottle tannage is often preferred. The skins are sewn -together in pairs, grain outwards, and nearly filled with warm sumach -infusion, just as described for goatskins. They are then handled in old -sumach liquors for about 3 days, and piled to drain and press. At this -stage the bag is cut open, the goods worked on the flesh, and the -tannage is completed with separated skins in newer sumach liquors, -handling at least once a day for 4-5 days, as necessary. - -In finishing there is the usual variety, but a plain ungrained finish is -most typical, as the smooth and fine grain of the young animal lends -itself to this type of finish better than the skins of goat and seal, -and gives a better quality leather than those from the sheep. The crust -skins are wet back with water at about 110 deg. F., and, if necessary, -sammed and shaved. Sumaching follows, the operation being carried out in -a drum for 1-2 hours. The skins are then well struck out. Striking and -setting should always be thorough for a plain finish, and this case -forms no exception. Dyeing follows next, the paddle being often -preferred to the drum, which is liable to work up a grain. The dyed -skins are placed in cold water for a while and again well struck out. -They are often nailed on boards to samm, and are then set out, lightly -oiled with linseed oil and dried out in a cool shed. Seasoning follows, -with milk and water only. The operation may be done with either brush or -sponge, after which the goods are piled grain to grain and flesh to -flesh to regulate. They may be next perched to soften and fluffed if -desired. After top seasoning with milk, water and albumin the skins are -hung up for a while, piled to regulate and brushed, first lightly and -then more vigorously. They may be then oiled very lightly and dried out -in a cool stove to ensure a soft leather. - - - REFERENCE. - - Bennett, "Manufacture of Leather," pp. 55, 84, 105, 201, 207, 303. - - - - - SECTION VI.--JAPANNED AND ENAMELLED LEATHERS - - -The leathers which receive a japanned or enamelled finish are usually -vegetable tannages, and so may be discussed at this stage. They are -popularly known as "patent" leather, but for no obvious reason. The -chief object is to obtain a leather with an exceedingly bright and -permanent gloss or polish, and this is attained by coating the leather -several times with suitable varnishes. The great difficulties are to -prevent the varnish cracking when the leather is bent or in use, and to -prevent it peeling off from the leather. Almost all classes of vegetable -tannage are japanned and enamelled. Hides are split and enamelled for -carriage, motor car and upholstery leathers, and enamelled calf, seal -and sheep skins are used for boot uppers, toe caps, dress shoes, -slippers, ladies' and children's belts, hat leathers, and so on. Broadly -speaking, a japanned leather is a smooth finish and is usually black, -whilst an enamelled leather is a grain finish with a grain pattern -worked up, and more often in colours. Hence japanned leathers are often -made from flesh splits or leathers with a damaged grain. It is in any -case advantageous to buff the grain lightly, for this permits the -varnishes to sink rather deeper and get a firmer grip, and avoids the -too sudden transition from phase to phase which is one cause of -stripping or peeling. Many flesh splits, however, are printed or -embossed to give an artificial grain and are then enamelled, which tends -to fix the embossed pattern. - -Almost any method of preparing dressing hides for upper or bag work will -yield a suitable leather for enamelling and japanning (see Part I., -Section VIII.; and Section IX.). If anything the liming should be -somewhat longer and mellower in order to eliminate grease, as the -natural grease of the hide causes the stripping of some varnishes. In -finishing it is important to obtain even substance, or the varnish is -liable to crack. Hides are soaked and sammed in, and often split. -Sometimes they are split twice, giving grain, middle and flesh, the two -former being enamelled and the last japanned. Other goods are shaved -very smooth. The goods should be next thoroughly scoured and stoned to -get as much "stretch" as possible removed. They are often sumached, -washed in warm water, slicked out again and sammed. They are then -lightly buffed on the grain, and after oiling lightly are thoroughly set -out and dried. Embossing or printing for enamels is done before the -goods are quite dry. Considerable difference of opinion obtains as to -the best oil to use in the above oiling. Linseed oil is widely preferred -as being most likely to agree with varnishes made from linseed oil. Some -manufacturers of japans do not dislike the use of mineral oil, but -strongly object to cod oil, tallow or other stuffing greases as tending -to cause the varnish to strip or peel. Other manufacturers, on the other -hand, will not have leather with mineral oil in it, and indicate that -nothing but cod oil should be used. In all probability these various -preferences are determined by the nature of the varnish, which differs -widely in various parts of the globe. - -In this country the varnishes are made largely from linseed oil by -boiling it with "driers." This oil contains much triglyceride of an -unsaturated relative of stearic acid. The double bonds are very -susceptible to oxidation with the production of resinous bodies of -unknown constitution. This phenomenon is known as "drying the oil," and -has been extensively used in the manufacture of linoleums. The driers -are either oxidizing agents or oxygen carriers, such as litharge, -Prussian blue, raw umber, manganese dioxide, manganese borate, and -"resinate." Prussian blue is most preferred for British japans, as it -always materially assists the attainment of the desired black colour. -The exact details of the boiling, and the manufacture of the varnishes -is still largely the trade secret of the master japanners, and differs -indeed for the various stages of japanning. The varnish for the earlier -coats is boiled longer, and the drying carried further, than in the case -of the later coats. This is partly to obtain a product of such stiffness -that it will not penetrate the leather. The driers and the pigments -should be finely powdered and thoroughly mixed in. The boiling takes -several days when at a low temperature, but if done in 24 hours the -temperature may be up to 570 deg. F. In the later coats driers are often not -used, and the product is often mixed with copal varnish, pyroxylin -varnish, etc., which greatly help in obtaining smoothness and gloss. -Turpentine, petroleum spirit and other solvents are also used to thin -the varnishes. Before boiling, the oil is often purified by a -preliminary heating with nitric acid, rose spirit and other oxidizing -agents, which precipitate impurities and thereby assist in obtaining a -bright gloss. - -Before the application of the varnishes, the leather is first dried -thoroughly in a stretched condition. This is accomplished by nailing -down on boards which fit like movable shelves into a "stove," a closed -chamber heated by steam pipes. The temperature of the stove varies -widely in different factories, from 140 deg.-200 deg. F., according to the -nature of the varnishes. The first coat of warm and rather stiff japan -is laid over the hot leather in a warm room, being spread over first by -hand, then by a serrated slicker, and then again smoothed by hand. The -goods are then put into the stove for several hours to dry. When dry the -surface is pumiced and brushed and a second coat applied in a similar -manner, but with increased care. This is repeated with finer japans -until the desired result is obtained. Brushes are used to apply the -later coats. Up to seven coats may be applied for the production of a -smooth japan--three coats of ground japan, two coats of thinner japan, -and two coats of finishing varnish. - -After the stoving is complete, the product is given a few days under -ordinary atmospheric conditions to permit the reabsorption of moisture -to the usual extent. Enamelled leathers are then grained to develop the -pattern. - - - REFERENCE. - - Bennett, "Manufacture of Leather," p. 380. - - - - - PART III.--CHROME LEATHER - - - SECTION I.--THE NATURE OF CHROME LEATHERS - - -In these days the manufacture of chrome leather has attained a position -hardly less in importance than that occupied by the ancient method of -tanning by means of the vegetable tanning materials, and large -quantities of hides and skins are now "chrome tanned" after preparatory -processes analogous to those described in connection with vegetable -tannages (Part II., Section II.; and Part II., Section I.). - -Chrome leathers are made by tanning pelts with the salts of chromium, -and are typical of what are known as "mineral tannages," in which -inorganic salts are the tanning agents. Tannage with alum and salt (see -Part IV., Section I.) is one of the earliest mineral tannages, but is -now of relatively minor importance. Chrome tanning was first -investigated by Knapp (1858), who experimented with chromic chloride -made "basic" by adding alkali, but his conclusions were unfavourable to -the process. A patent was taken out later by Cavallin in which skins -were to be tanned by treating with potassium dichromate and then with -ferrous sulphate which reduced the former to chromic salts, being itself -converted into ferric salt. The product, which was a combination of -iron-chrome tannage, did not yield a satisfactory commercial leather. -Another patent, taken out in 1879 by Heinzerling, specified the use of -potassium dichromate and alum. This in effect was a combination -chrome-alumina tannage. The alum had its own tanning action and the -dichromate was reduced to chromic salts by the organic matter of the -skin itself and by the greases employed in dressing. The process, -however, was not a commercial success. In 1881 patents were obtained by -Eitner, an Austrian, whose process was a combination chrome and fat -tannage. The chrome was employed as "basic chromium sulphate" made by -adding common soda to a solution of chrome alum until a salt -corresponding to the formula Cr(OH)SO{4} was obtained. Such a solution -is now known to be perfectly satisfactory, but at first it proved -difficult to devise satisfactory finishing processes, and to supplement -the chrome tannage with the fat tannage. - -The first undoubted commercial success in chrome tanning was obtained by -the process of Augustas Schultz, whose patent was the now widely known -"two-bath process," in which the skins are treated successively with a -chromic acid solution and with an acidified solution of "hypo" (sodium -thiosulphate). The first bath was made up commercially of potassium -dichromate and hydrochloric acid, so that, strictly speaking, it -contained potassium chloride also. The second bath contained, in effect, -sulphurous acid, which reduced the chromic acid in the skin fibres to -the tanning chrome salts. Free sulphur is also formed in this bath and -in the skin, and contributes to the characteristic product obtained by -this process of tanning. Many minor deviations from the original process -of Schultz have been introduced, but the main features have been -unchanged, and this method of tanning is widely employed at the present -time for both light and heavy chrome leather. In 1893 tanning by basic -chromic salts was revived and the use of the basic chloride was patented -by Martin Dennis, who offered such a tanning solution for sale. The -validity of the patent has always been doubtful on account of the -previous work of Knapp and others, but the process itself was -commercially satisfactory, and the many variants of this and of the -basic sulphate tannages are now generally known as the "one-bath -process" in contradistinction to the variants of the Schultz process, -and are widely used for all classes of chrome leather. A one-bath -process which deserves special mention was published in 1897 by Prof. H. -R. Procter. In this the tanning liquor was made by reducing potassium -dichromate in the presence of a limited amount of hydrochloric or -sulphuric acid by adding glucose. Although a basic chrome salt is the -chief tanning agent thus produced, there is little doubt that the -organic oxidation products play an essential part in producing the -fullness and mellowness of the leather thus tanned, but their nature and -mode of action has not yet been fully made clear though lyotrope -influence is probable. - -More recently Balderston has suggested the suitability of sulphurous -acid as reducing agent. A stream of sulphur dioxide gas is passed -through a solution of sodium dichromate until reduction is complete. The -resulting chrome liquor has been favourably reported upon by some chrome -tanners. Bisulphite of soda has also often been used as the reducing -agent. Other organic substances are also often used, instead of glucose, -to reduce the dichromate. - -=Theory of Chrome Tannage.=--As to the theory of chrome tanning there is -still considerable difference of opinion and much room for experiment. -Some leather chemists regard the tannage as differing essentially from -the vegetable tannages. Mr. J. A. Wilson has even suggested that the -proteid molecule is in time partly hydrolyzed with the formation of a -chromic salt with the acid groups. The author, however, strongly favours -the view that in chrome tanning changes take place which are closely -analogous to those which occur in vegetable tannage, the differences -being mainly of degree. Thus the hide gel is immersed into a lyophile -sol--the chrome liquor--and there follows lyotrope influence, -adsorption, gelation of the tanning sol, as well as diffusion into the -gel, and finally also, probably, precipitation of the tanning sol at -this interface. - -In chrome tannage the lyotrope influence is much more prominent than in -vegetable tannage, but the effect is in the same sense, viz., to reduce -the imbibition of the hide gel. Thus the potassium sulphate in a chrome -alum liquor has its own specific action of this kind and contributes to -the leather formation. Unhydrolyzed chromium sulphate and the sodium -sulphate formed in "making basic" act also in the same sense. - -The tanning sol is probably chromium hydrate, formed by the hydrolysis -of chromium sulphate: it is a lyophile or emulsoid sol and is in -consequence very strongly adsorbed by the hide gel. This adsorption, -involving a concentration of lyophile sol, is the first stage in -gelation, which occupies a relatively more prominent place in chrome -than in vegetable tannage. Some diffusion into the gel also occurs, and -both the gelation and diffusion of the sol are affected by lyotrope -influence, but to a greater extent than in the vegetable tannage. Thus -far the analogy is almost complete. - -There remains the question of the precipitation of the tanning colloid -at the interface. This is a point which has not yet been thoroughly -investigated, and which offers considerable difficulty to a clear -understanding, but the matter may be probably summarized thus: the -adsorbed chromium hydrate is precipitated at the interface of gel and -sol to some extent, chiefly through the neutralization of its charge by -the oppositely charged ions of the electrolytes present, but possibly -also--in the last stages of manufacture by the mutual precipitation of -oppositely charged gel and sol. - -To illustrate the matter, the case of a basic chrome alum liquor will be -considered. The chromium hydrate sol is primarily a positive sol, just -like ferric and aluminium hydrate sols: _i.e._ in water they are -somewhat exceptional in that they adsorb H+ rather than OH-. To cause -precipitation therefore it is necessary to make the sol less positive -and more negative. The positive charge of the sol, however, is greater -than in water, because of the free acid formed in the hydrolysis, which -results in the adsorption of more hydrions by the sol. Hence to ensure -precipitation steps must be taken to reduce the adsorption of hydrions -by the chromium hydrate sol. In practice such steps are taken, and to -such an extent that there can be little doubt that the chrome sol is not -far from its isoelectric point. Amongst these "steps" are (1) making the -liquor "basic," _i.e._ adding alkali to neutralize much of the free -acid, which involves a considerable reduction in the stabilizing effect -of the hydrions; (2) the adsorption of hydrions by the hide gel when -first immersed in approximately neutral condition; (3) the operation of -the "valency rule" that the predominant ionic effect in discharging is -due to the multivalent anions. In this case the divalent SO{4}-- ions -assist materially in discharging the positive charge on the chrome sol; -(4) the final process of neutralization in which still more alkali is -added. The operation of the valency rule is the most complex of these -factors, for there is also to be considered the stabilizing effect of -the kations, especially of the trivalent kation Cr+++ from the -unhydrolyzed chromium sulphate. It is quite possible also that in the -last stages of chrome tanning there are "zones of non-precipitation" due -to the total effect of multivalent ions, and it is quite conceivable -that the chrome sol may change its sign, _i.e._ become a negative sol -and thus give also a mutual precipitation with the hide-gel. This is -particularly probable where a local excess of alkali occurs in -neutralization. However that may be, it is probable that most of the -tannage is accomplished by chromium hydrate in acid solution, and it is -therefore legitimate to conclude that adsorption and gelation have a -relatively greater part in chrome tannage. The operation of the valency -rule makes it easy to understand why basic chlorides do not tan so well -as sulphates; the precipitating anion is only monovalent (Cl-) and -chromic chloride contains no substance analogous to the potassium -sulphate of chrome alum and hence contains a less concentration of the -precipitating anion. Hence also the stabilizing influence of common salt -added to a basic alum liquor, the effect being to replace partially the -divalent SO{4}-- by the monovalent Cl-. Lyotrope influence, however, -may be here at work. - -It is possible to make out a rather weak case that the tanning sol is -not chromium hydrate at all, but a basic salt of chrome also in -colloidal solution, and to contend that this salt, like most substances, -forms a negative sol, but in practice not negative enough, hence the -desirability of alkali, divalent anions, etc. From this point of view -the analogy with vegetable tannage becomes more complete and the -stabilizing effect of the soda salts of organic acids becomes easy to -understand. - -It is highly probable that the electrical properties of the chrome sol -need closer investigation on account of the complexity due to the -prominent effect of multivalent ions. It is desirable to bear in mind -the remarkable phenomenon observed by Burton (_Phil. Mag._, 1905, vi, -=12=, 472), who added various concentrations of aluminium sulphate to a -silver sol (negative). He observed (1) a zone of non-precipitation due -to protection; (2) a zone of precipitation due to the trivalent kation; -(3) a second zone of non-precipitation due to protection after the sol -has passed through the isoelectric point and become a positive sol; (4) -a second zone of precipitation due to the precipitating effect of the -anion on the now positive sol. It seems to the writer that similar -phenomena may possibly occur in chrome tanning, for whatever the sol -actually is, it is not far from the isoelectric point. - -A few observations on the vegetable-chrome combination tannages will not -be out of place at this stage. Wilson refers to the well-known practical -fact that chrome leather can take up about as much vegetable tan as if -it were unchromed pelt, and considers this evidence that the two -tannages are of fundamentally different nature. "In mineral-tanned -leathers the metal is combined with carboxyl groups, while in -vegetable-tanned leather the tannin is combined with the amino groups. -This strongly suggests the possibility that the two methods of tanning -are to some extent independent of one another, and that a piece of -leather tanned by one method may remain as capable of being tanned by -the other method as though it were still raw pelt" (_Collegium_ -(London), 1917, 110-111). To the writer, however, it seems that the -facts are evidence for the contrary proposition, that the tannages are -fundamentally of the same nature. On the adsorption theory, one would -expect chrome leather to adsorb as much tan as pelt; the readily -adsorbable tan is the same, and the chrome leather is an adsorbent of -very much the same order of specific surface as pelt. The adsorption -theory would find it difficult to account for chrome leather not -adsorbing as much tan as pelt. It is quite conceivable that a chrome -leather could adsorb more tan than pelt, owing to the more complete -isolation of the fibrils by the chrome tannage and to their being coated -over by a more adsorbent gel. Adsorption is often deliberately increased -by a preparatory adsorption. Thus sumach-tanned goatskins are wet back -from the crust and "retanned" in sumach before dyeing, to coat the -fibres with a fresh and more adsorbent gel and so ensure the even and -thorough adsorption of the dyestuff. Mordanting fabrics has a similar -object,--the adsorption of colloidogenic substances which give rise to -an adsorbent gel on the fibre. Unless vegetable-tanned leather is so -much loaded with tan that its specific surface is effectively reduced, -one would similarly expect that vegetable-tanned leather would adsorb -the chrome sol. This, of course, is exactly the case of semi-chrome -leather. If, on the "chemical combination" theory, the vegetable tan -combines with the amino groups and the chrome with carboxyl groups, it -is natural to inquire which groups the dyestuffs combine with. As either -tannage does not interfere with the adsorption of dye, are we to -conclude similarly that tanning and dyeing are fundamentally different -processes? - -Those who favour this chemical combination theory, and who offer -equations for the formation of vegetable and of chrome leather, should -likewise suggest an equation for the formation of leather from pelt by -the action of dyestuffs--a practical though hardly an economic process. - -The remarks made earlier in this volume (Part I., Section III.) as to -the occurrence of what have been called "irreversible changes" -subsequent to the mutual precipitation of oppositely charged gel and -sol, are equally applicable to the chrome tannages. Chrome tannage was -once thought to embrace such irreversible changes, but the process can -now be "reversed" with ease. The reversibility of the chrome tannage is -an easier proposition than that of vegetable tannage, partly because the -leather is comparatively much less tanned, and partly because the -acidity or alkalinity of the stripping agent may be adjusted, as -desired, without the oxidation trouble. In approaching this question -from the theoretical side one must consider mainly whether to solate the -tanning agent to a positive or to a negative sol. Our imperfect -knowledge of the electrical forces in operation in the chrome tannage is -thus a serious drawback, but the evidence on the whole points to the -precipitation being effected by a negative sol near its isoelectric -point but in faintly acid solution. Hence, we should theoretically -expect that reversion should take place into a negative sol in nearly -neutral or even faintly alkaline solution. Thus, suitable stripping -agents for chrome leather would be the alkali salts of organic acids -(especially if multivalent). Now, Procter and Wilson have recently -accomplished this stripping of chrome leather by the use of such salts. -They approached the question from an empirical and practical point of -view and found that Rochelle salt, sodium citrate, and sodium lactate -would strip the chrome tannage with ease. This important and very -creditable achievement will have great practical and commercial -importance. Procter and Wilson have deliberately and carefully refrained -from offering an exact explanation of this reversible action, but point -out that all their stripping agents are salts of _hydroxy-acids_, and -strongly insist that these form soluble complexes with the chrome. -Whilst not denying this in the least, the present author would point out -that according to the views advanced in this book, the salts of organic -acids which do _not_ contain hydroxyl groups should, when combined with -a monacid base, also strip the chrome tannage. This he has found to be -the case. Thus the chrome tannage is reversible in solutions of ammonium -or potassium oxalate and of ammonium acetate. With these salts the full -effect of multivalent anions is not attained, so that somewhat strong -solutions are necessary. A 10 per cent. solution of ammonium acetate -shows some stripping effect after a few days, but a 40 per cent. -solution after a few hours. Saturated ammonium oxalate is only a 4.2 per -cent. solution, but shows a stripping effect in 2-3 days. Potassium -oxalate (33 per cent.) shows distinct stripping in 24 hours. Potassium -acetate and sodium acetate show only slight action, because the solution -is too alkaline, but strip if acetic acid be added until litmus is just -reddened. It is noteworthy from a theoretical point of view that a 40 -per cent. solution of ammonium acetate is distinctly acid, and indeed -smells of acetic acid. There can be little doubt that such stripping -actions are also connected with the solubility of the stripping agent in -the gel, for the liquid must pass through the walls of the gel to dilute -the liquid in the interior. This view fits in with the facts that -hydroxy acids and ammonium salts are particularly efficient, for the -tendency of chrome to form ammonia-complexes as well as hydroxy -complexes is well known. From this point of view we should not expect a -stripping action from a salt such as disodium phosphate, which would -form an insoluble substance. Actually sodium phosphate does not strip, -and indeed reduces the stripping power of ammonium acetate. Similarly, -we might expect some stripping action by ammonia and ammonium chloride, -with the formation of chrome ammonia complexes. This actually occurs, a -pink solution being obtained. Sodium sulphite does not strip, possibly -partly on account of its too great alkalinity, but is interesting -theoretically to observe that sodium sulphite as well as Rochelle salt -will strip salt stains (see Yocum's patent, _Collegium_ (London), 1917, -6; also Procter and Wilson, _loc. cit._). This points to the formation -of a negative sol, and suggests many other substances for removing salt -stains. - -=Special Qualities of Chrome Leather.=--A few words on the special -peculiarities of the leather formed by chroming will not be out of place -at this stage. One of the greatest disadvantages of the chrome tannage -has been the absence of what is known as the "crust" or "rough leather" -stage. In chrome tanning, the finishing operations have had to follow on -immediately after the tannage. Chrome leather, after tanning, may be -dried out like other leathers, but if thoroughly dried, or if kept in a -dried condition for any time, it will not "wet back" again with water. -Various suggestions have been made to overcome this difficulty but none -yet have found much favour in practice. The discovery of the -reversibility of the tannage, however, ought to solve this difficulty, -and the author would suggest that any of the substances used for -"dechroming" might also be suitable for "wetting in" chrome leather -which has been well dried out. A piece of chrome leather, dried out well -after neutralizing, and kept in a warm place for four years, wetted back -easily in ammonium acetate, in the author's laboratory. - -Another peculiarity of the chrome tannage is that any defects in the raw -material always seem more obvious in chrome than in vegetable leather. -This often necessitates the use of a better quality hide or skin. Weak -grain or loose grain becomes very obvious. The presence of short hair -which both unhairing and scudding have failed to remove also is usually -more evident. - -A more serious disadvantage of chrome leather is its tendency to -stretch. In the case of belting leather this feature is an obvious -nuisance, and has inevitably led manufacturers to use powerful -stretching machines upon the goods before they are marketed. In chrome -sole leather also there is a tendency to spread and throw the boot out -of shape. - -Further disadvantages arise from the fact that the chrome tannage is an -"empty" tannage. Compared with the vegetable tannage, very little of the -tanning agent is adsorbed. Hence there is little matter of any kind -between the hide fibres isolated during tannage. The inevitable effect -of this is that the leather has not the same solidity and firmness, and -needs filling out with other materials. A commercial consequence is also -that it is impossible to obtain the same yield of leather from any given -quantity of raw material. In trade parlance chrome tannage does not give -good "weight." Another consequence is that (even when well filled with -greases in finishing) chrome leather tends to be "woolly" on the flesh -side or at cut edges. - -On the other hand, chrome tanning has very many advantages over the -older process. The most obvious of these is the great saving in time. -Many chrome tannages involve only a day or two, and none more than a -week or two. A chrome leather factory therefore needs less capital on -account of the quicker turnover. If, moreover, the market be -unfavourable, a chrome tanner can stop or reduce his output in a very -short time, whereas a vegetable tanner is committed to many weeks' -supply of the goods he is manufacturing. Another notable advantage of -chrome leather is its durability. In the finishing processes more grease -is usually employed than in vegetable tannage, and this has a -preservative effect upon leathers which often get wet. Chrome sole -leather and hydraulic leathers are cases in point. Chrome leather will -also stand changes of temperature and friction much better than -vegetable tannages. The light chrome tannage results, further, in -yielding a leather which has great tensile strength, and it is not -surprising to find that chrome harness and chrome picking bands are -highly thought of. The empty nature of the tannage necessitates the use -of stuffing greases, but such large proportions of these may be used -that chrome tannage becomes obviously suitable if one wishes to produce -a waterproof leather. Hence the popularity of chrome tannage for -waterproof soling and hydraulic leathers. - -The advantages of the chrome process are very real, and very obviously -such as will appeal to manufacturers. Chrome leathers have now been for -some time in the forefront as far as boot-uppers are concerned, -especially for the best quality goods, in which the popular "box-calf" -and "glace kid" are so largely employed. There seems little doubt that -this will continue to be the case. It is an unfortunate fact that in -this important branch of tanning, British manufacturers have not quite -risen to the occasion. Their products have in the past been faced with -very serious competition from Continental and American manufacturers of -chrome uppers, and there can be no doubt that these competitors produced -a better article, and produced it more economically. The disorganization -of the Continental factories owing to the war should give British -manufacturers a valuable opportunity of putting such businesses on a -better basis. For sole leather also the chrome tannage makes constant -headway, and the relative proportion of it becomes gradually greater. A -great impetus to chrome sole leather has been given by the war -conditions of Britain. Owing to the submarine campaigns of Germany the -tonnage question became all-important, and the bulky imports of -vegetable tanning materials became a serious item. British tanners were -therefore encouraged to make more chrome sole and less vegetable sole. -The urgent need of leather for our armies also assisted in the same -sense. The production of chrome sole progressed therefore enormously -during 1917 and 1918, and although some reaction will doubtless occur, -there seems little doubt that chrome sole leather has taken a definite -and permanent leap forward. Once the general public fully appreciate its -qualities of waterproofness and durability its future will be assured. - -On the whole the position and prospects of chrome tanning are good. The -chrome tannages are making headway in all directions, and undoubtedly -threaten the existence of many of the older processes of vegetable -tanning. - - - REFERENCES. - - Procter, "Principles of Leather Manufacture," pp. 198-220. - - Bennett, "Manufacture of Leather," pp. 210, 355. - - Bennett, _J.S.L.T.C._, 1917, 176. - - Stiasny, _Collegium_, 1908, 117. - - - - - SECTION II.--GENERAL METHODS OF CHROME LEATHER MANUFACTURE - - -It has been previously pointed out that the chrome tannage is an "empty" -one; the primary principle in the wet work of goods for chrome leather -is to avoid anything which will make this feature more obvious. In the -vegetable tannages relatively larger amounts of the tanning agents are -used, and these fill the interfibrillar spaces; indeed, as we have seen -(Part I., Sections III., V. and VI.), effort is made to increase these -spaces and to fill them to their maximum capacity, thus yielding a -leather of which 50 per cent. is the tanning agent. In chrome tanning, -however, the tanning agent may only be approximately 5 per cent. of the -finished leather, so that any trouble taken to split the hide fibres or -to dissolve hide substance is usually not only superfluous, but also -calculated to enhance the "emptiness." The governing principle of all -the preparatory processes for chrome tannage is therefore the -conservation of hide substance, and this principle determines the -modifications of the processes of soaking, liming, and deliming, which -are in vogue. Now, in most of these processes there is usually some loss -of hide substance, and it is the particular problem of chrome tanning to -reduce this loss to a minimum in each stage. Whether the loss of hide -substance be due to alkaline or fermentive hydrolysis, or to solation of -the hide gel, the effect is increased by swelling, and in the wet-work -for chrome, therefore, any variations in the degree of swelling are -objectionable. The preparatory processes should be carried out with as -little change as possible in the volume and elasticity of the pelt. -Whether also the loss of hide be due to hydrolysis or solation, it is -increased by time, hence short processes are (other things being equal) -much to be preferred. Fermentive hydrolysis is minimized by cleanliness, -alkaline hydrolysis by avoiding strongly alkaline liquors, and solation -of collagen is reduced by both, and also by avoiding, as far as -possible, the presence of calcium and ammonium salts. - -Soaking should be quick and clean. The use of the paddle or drum gives -the greatest efficiency and also assists in procuring the softness so -essential to the bulk of chrome leathers. - -Liming chrome leather satisfactorily is almost an impossible ideal. -Every conceivable arrangement has some objection to it. The time of the -process may be shortened either by the use of sulphide or by the use of -mellow or old limes. To shorten time by the use of sodium sulphide -unfortunately involves the employment of more alkali than is desirable, -with a consequent plumping effect and tendency to harshness. If -sufficient sulphide be used to make the liming very short, then the -grease is not "killed" (saponified or emulsified). If the harshness and -alkalinity be removed by using also an excess of calcium chloride, then -the lyotrope influence of this substance enhances the solation of the -hide gel. On the other hand the use of old lime liquors avoids the -plumping effect, but increases considerably the bacterial activity, and -the bacterial enzymes produce both hydrolysis and solation of the pelt. -In practice what is generally done is to shorten time by both methods -and so to admit both disadvantages to a limited extent. This is -theoretically more sound than would appear, for in mellow limes sulphide -has less plumping power but is just as strong a depilatant; whilst, on -the other hand, a mellow liming shortened by sulphide is less -objectionable as there is some evidence that bacterial activity is -relatively less in the first few days. Hence a mellow sulphide liming of -7-10 days is very common in practice, but sometimes a 3-4 days' process -with more sulphide is also found satisfactory. - -It would seem probable that the real solution of the problem would be -found by a different process altogether. In this connection it is -interesting to note that a Continental proposal to unhair by enzyme -action only has been found most practicable with goods for chrome, and, -in the author's opinion, some development on these lines, in which a -lipolytic enzyme is used in addition to a proteolytic, might solve the -difficulty, and give a rapid depilation which dispenses with liming, -plumping and deliming with the consequent loss of valuable hide -substance. - -In the usual short, mellow, sulphide liming it is clear that there is -not much advantage in a "round" or "set" of pits. Hence the one-pit -system is popular on account of the less labour involved. - -The above remarks are less applicable in the case of chrome sole -leather. In this case weight is a great consideration and plumping is -necessary. In such leather the chrome tannage is supplemented by the use -of waxes, which fill up the spaces between the fibres and give solidity -and waterproofness to the finished article. With this leather an -ordinary sole leather liming in sharp liquors is not unsuitable, -handling the goods from "mellow to fresh," but there is, on the whole, a -tendency to shorten the process to about a week by using more sulphide. - -Processes for deliming pelt for chrome leather should also be chosen by -our guiding principle of hide substance conservation. Here again short -processes involving little change in swelling should be preferred. Now, -the ordinary bating and puering processes give (1) neutralization of -lime by organic acids combined with weak bases; (2) the solation of some -hide substance; and (3) a "pulling down" effect on the swollen pelt. -Now, neutralization is quite superfluous, as the acids of the chrome -liquor (one-bath or two-bath) can quite well accomplish this; the -solvent effect is undesirable altogether; and the "pulling down" effect -is also unnecessary if the goods are not plumped up. With any method of -liming, however, some plumping is obtained, and this creates a problem -of practical importance. In the huge quantities of pelt which go for -chrome upper leathers, a primary consideration is the soft, "kind," or -mellow feel of the grain in the finished leather. This is obtained only -by tanning the pelt when the grain at least is in a thoroughly deplumped -and inelastic condition. It is essential to delime not only so that the -alkaline plumping effect is completely removed, but also so that no acid -plumping effect succeeds it. The practical problem is to decide whether, -in any particular instance, dung puers and bates are necessary to obtain -this result. Bating is clearly not very desirable, on account of the -length of the process, during which hide substance would be lost -unnecessarily, and also because there will usually be a slight alkaline -swelling. Puering with dog-dung infusions is preferable; it is not such -a long process, the liquor is just acid to phenolphthalein, and the -action is more intense, and by puering for a short time only the loss of -hide may be confined to the grain and flesh only, whilst the desired -inelasticity of grain-pelt is soon obtained. Many large firms have -admittedly found themselves unable to dispense with puering, but others -have succeeded in substituting for it the use of non-swelling deliming -and lyotrope agents like ammonium chloride and boric acid. In all cases -it is futile to delime or puer the grain and then allow the goods to -stand until the centre lime has diffused outwards. The goods must pass -into the chrome liquors when in the correct condition. For heavy chrome -leather a surface deliming with boric acid is all that is necessary. -Even that is superfluous when the goods are to be pickled before -tanning. - -=Types of Two-bath Chrome Tannage.=--Although the original process of the -Schultz patent is quite a practicable one, many modifications have been -introduced. These modifications have been made to suit the class of -goods under treatment, to suit the particular mode of application which -is available or suitable, and to effect economies of chrome and other -material, and of time, and also to combine with the tannage a pickling -effect or a partial alum tannage. Other modifications arise from the -precise acid, neutral, or alkaline condition of the pelt, being for -example obviously necessary when pickled stock replace neutral pelts. -The many two-bath processes which have been found useful have been -classified previously by the author[6] into three types: (1) The -"Schultz type," in which such quantities of dichromate and acid are used -that there is no excess of free acid (other than chromic), but an excess -of unaltered dichromate; (2) the "Acid type," in which the chromic acid -is completely free and the liquor contains also some excess of mineral -acid also; and (3) the "Neutral type," in which neither of these main -constituents is in excess, just sufficient mineral acid having been used -to liberate all the chromic acid from the dichromate. - -[Footnote 6: "Types of Two-bath Chrome Tannage," _Leather_, 1909, -227-259.] - -Now:-- - - K{2}Cr{2}O{7} + 2HCl = 2KCl + 2CrO{3} + H{2}O - 204 73 - -Taking the commercial hydrochloric acid as a 30 per cent. solution, 73 -parts will be obtained in about 250 parts of commercial acid. Hence 294 -parts dichromate need 250 parts commercial hydrochloric acid for the -above reaction;[7] in other words, 5 per cent. dichromate needs 4-1/4 -per cent. commercial acid. Similarly 6 per cent. and 4 per cent. of -dichromate need 5.1 per cent. and 3.4 per cent. respectively of -commercial acid. If therefore such quantities be used we have the -so-called "Neutral type" of chroming bath. If less quantities of acid be -used we have the "Schultz type," and if greater quantities of acid be -used we have the "Acid type." The original Schultz patent used 5 per -cent. dichromate and 2-1/2 per cent. hydrochloric acid, and well -exemplifies its type, for there is much undecomposed dichromate. The -composition of some chroming baths in common use on a practical scale -are given below under the heading of their type:-- - - - ----------+--------------+--------------+-------+------------ - Type. | Dichromate. | Hydrochloric | Salt. | Aluminium - | | Acid. | | Sulphate. - ----------+--------------+--------------+-------+------------ - | 5 | 2-1/2 | -- | -- - | 5 | 2-1/2 | -- | 3 - Schultz | 5 | 2-1/2 | 5 | -- - | 5 | 2-1/2 | 10 | -- - | 6 | 3 | -- | -- - ----------+--------------+--------------+-------+------------ - | 4 | 4 | -- | -- - | 4 | 4 | 5 | -- - | 5 | 5 | 5 | 3 - Acid | 5 | 5 | 10 | -- - | 6 | 6 | 15 | -- - | 3 | 3 | 15 | 4 - | 2 | 4 | 10 | -- - | 4 | 15 | 24 | -- - ----------+--------------+--------------+-------+------------ - | 5 | 4-1/4 | 5 | -- - | 5 | 4 | -- | 2-1/2 - Neutral | Chromic acid | | | - | 5 | -- | 5 | -- - | 6 | -- | 8 | -- - | 4 | -- | 10 | -- - ----------+--------------+--------------+-------+------------ - -[Footnote 7: Commercial acids of course vary in strength, and the amount -needed varies accordingly.] - -All the figures are percentages of the weight of pelt. As K{2}Cr{2}O7 -has a molecular weight of 294, and Na{2}Cr{2}O{7}.2H{2}O a molecular -weight of 298, in practice they may be considered as interchangeable, -weight for weight. The sodium salt is cheaper and more often used. The -corresponding amount of chromic acid, 2CrO{3}, has an equivalent weight -of 200, hence any weight of dichromate may in practice be substituted by -two-thirds the weight of commercial chromic acid. Equivalent weights of -commercial sulphuric acid are sometimes used in place of hydrochloric. -The quantity depends upon the strength of the sulphuric acid used. -Aluminium sulphate, Al{2}(SO{4}){3}.18H{2}O (mol. wt. 666), may be -replaced by ordinary potash alum, K{2}SO{4}.Al{2}(SO{4}){3}.24H{2}O -(mol. wt. 948). In practice 7 parts of the former and 10 parts of the -latter may be considered equivalent. It should be remembered that both -these salts are hydrolyzed in solution, and therefore increase slightly -the amount of free acid present. Their presence decreases the amount of -chrome taken up, and as little or no alumina is found in the leather, -there is usually small advantage in their employment. The use of salt is -common but often unnecessary. It is considered desirable in baths of the -acid type to prevent swelling by the excess of acid, and in baths made -up from commercial chromic acid to replace correspondingly that normally -formed from the reaction of dichromate and acid. It is used also in all -baths which are intended to treat pickled goods. Like all electrolytes -its presence decreases the adsorption of chromic acid. - -All these conceivable modifications will make good leather, and the -choice of a process often depends largely upon market prices. On the -whole the tendency is to prefer the neutral or acid type, on account of -the greater ease and completeness with which the bath may be exhausted. -Pickled stock may be depickled before tanning, by a bath of salt, mixed -with borax, whitening, or basic alum solutions. It may also be placed -direct in the chroming bath, but the amount of acid thus added with the -goods must be determined and allowed for when making up the bath. No -allowance is usually necessary, however, if the "pickle" consist only of -alum and salt. - -The chroming operation is carried out usually in drums or paddles. Drums -are preferable because more concentrated baths may be used; these -solutions penetrate quicker and are easier to exhaust economically. They -are also preferable for hides and heavy skins. Paddles are preferable -where grain is important, and for light skins in which little time is -needed. Small variations in the ratio of chrome to pelt, or in -concentration of liquor, have little influence upon the resulting -leather. - -The analytical investigation and control of chroming baths is usually -simple. A suitable volume of liquor is titrated with N/10 thiosulphate -after acidifying with hydrochloric acid and adding potassium iodide. The -operation should be conducted in a stoppered bottle, and the liquor -allowed to stand for 10-15 minutes after adding the iodide and before -titrating. A little fresh starch infusion should be added towards the -end of the reaction. Each c.c. N/10 thiosulphate corresponds to 0.0033 -gram CrO{3} or 0.0049 gram K{2}Cr{2}O{7}. The same volume of liquor -should also be titrated with N/10 caustic soda and phenolphthalein. -Potassium chromate is neutral to this indicator, _i.e._ chromic acid -acts as a dibasic acid. Any excess of hydrochloric acid is also -titrated. More indicator should be added towards the end of the -titration, as it is often oxidized. Each c.c. N/10 soda corresponds to -0.005 gram CrO{3}, 0.01 gram "half-bound" CrO{3} (_i.e._ present as -dichromate), 0.0147 gram K{2}Cr{2}O{7}, or 0.00365 gram HCl. If _a_ -c.c. N/10 thiosulphate and _b_ c.c. N/10 soda be needed the type of -chroming bath may be seen at a glance-- - - - ---------------------------+-------------+----------------------- - If | The type is | The bath contains - ---------------------------+-------------+----------------------- - _b_ is greater than 1/3_a_ | Schultz | potassium dichromate - but is less than 2/3_a_ | | and chromic acid - _b_ is greater than 2/3_a_ | Acid | chromic acid and free - | | hydrochloric acid - _b_ equals 1/3_a_ | Neutral | chromic acid only - ---------------------------+-------------+----------------------- - - -If 10 c.c. chrome liquor require _a_ and _b_ c.c. of thiosulphate and -soda respectively-- - - I. 10 c.c. of a Schultz bath contain (b - 1/3xa) x 0.01 gram CrO{3} - and [(ax0.0033) - [(b - 1/3xa) x 0.01]] x 1.47 grams K{2}Cr{2}O{7} - - II. 10 c.c. of an acid bath contain (ax0.0033) grams CrO{3} and - [(b - 2/3xa) x 0.00365] grams HCl - - III. 10 c.c. of a neutral bath (ax0.0033) grams \ - > CrO{3} - _or_ (bx0.005) grams / - - -The second bath of the two-bath chrome tannage consists of a solution of -sodium thiosulphate acidified with hydrochloric acid. The reactions in -this bath are somewhat complicated, several occurring simultaneously. -Broadly speaking, the final result is due to (1) the reduction of the -chromic acid to a chromic salt by the sulphurous acid; (2) the -formation of a basic chromic salt owing to the excess of thiosulphate; -(3) the reaction of the added acid and thiosulphate to give free -sulphur, which is deposited in and on the leather. The relative -intensity of these effects is variable, according to the conditions of -operation, _e.g._ the amounts of chemicals used, their concentration, -the nature and condition of the goods, the time of application, the -manner of application, etc. In practice the most favourable conditions -are usually discovered empirically, but, broadly speaking, the goods are -usually added soon after the thiosulphate and acid are well mixed. There -is some evidence that the reduction is in steps, intermediate products -such as sodium tetrathionate and chromium dioxide are known to be -formed. The goods change from yellow to dark brown, then to green, and -finally to the familiar blue. The sulphur makes the final colour a -lighter blue than in the case of a one-bath tannage, hence the two-bath -process is often preferred for "colours." - -On account of the empirical character of this "hypo bath," it is -impossible to fix any exact relation between the quantities of material -used in the chroming bath, and the quantities of "hypo" and acid used in -the reducing bath. The following rules, therefore, must be understood as -rough approximations for practical use, and though they have been -empirically discovered their theoretical significance is often fairly -obvious. - - 1. The amount of hypo necessary is almost directly proportional to the - amount of dichromate used. In chroming with baths of the acid or - neutral type, the percentage of hypo should be about three times the - percentage of dichromate used. Thus 4 per cent. dichromate needs 12 - per cent. hypo; and 6 per cent. dichromate needs 18 per cent. hypo on - the pelt weight. In baths of the Schultz type a less proportion of - hypo may suffice, but the 10 per cent. hypo for 5 per cent. - dichromate, recommended by the Schultz patent, is generally considered - rather insufficient. - - 2. The proportion of hypo is increased somewhat for the heavier - classes of goods, and may even reach 20 per cent. of the pelt weight. - - 3. An increase in the proportion of hypo is usual with an increase in - the amount of free acid in an acid chroming bath. - - 4. The percentage of hydrochloric acid in the reducing bath is roughly - half that of the hypo, but is the most variable factor. The quantity - varies with the rate and mode of addition, the class of goods under - treatment, and the composition of the chroming bath. - - 5. In baths of the Schultz and neutral type it is better to add some - acid to the hypo bath before adding the goods, but this is less - essential for goods from an acid chroming bath. - - 6. In the case of goods from acid chroming baths, the amount of acid - used in the reducing bath is an inverse function of the excess of acid - in the first bath, _e.g._ take the following two processes:-- - - - ------------------------------------+------------------------------- - Chroming bath. | Hypo bath. - ---------------+--------------------+----------+-------------------- - Dichromate. | Hydrochloric acid. | Hypo. | Hydrochloric acid. - ---------------+--------------------+----------+-------------------- - 4 | 4 | 12 | 5 - 4 | 15 | 15 | 1 - ---------------+--------------------+----------+-------------------- - - - 7. There should be some excess of hypo at the end of the process. This - acts as a feeble alkali, and commences the neutralization. - -The process can be carried out in paddles or in drums as preferred, for -reasons similar to those applicable in the case of the first bath. On -the whole, however, drums are less popular for the second bath, for the -dilute solutions of the paddle effect some economy of sulphurous acid, -which is apt to escape into the air. A preliminary "hypo dip" is -sometimes used to prevent the "bleeding" of the chromic acid. The use of -many other reducing agents has been suggested as substitutes for hypo. -Sulphides, sulphuretted hydrogen, polysulphides, sulphites, bisulphites, -hydrogen peroxide, nitrous acid, lactic acid, etc., have been used, but -none are so easy to manipulate as thiosulphate. - -=Types of One-bath Chrome Tannage.=--The one-bath process is simpler -than the two-bath process inasmuch as only one kind of liquor is -involved, viz. one in which the chromium is in the chromic state. Hence -the variants of the one-bath process consist mainly of variations in the -composition of this liquor. The chief point of variation is in the -readiness with which chromium hydrate is adsorbed. This is determined by -the extent to which the chromic salt is hydrolyzed to form the tanning -sol and free acid, and by the concentration and nature of this free acid -as well as of other substances. It is difficult unfortunately to express -these factors in terms which are comparable under general conditions. -Chromic salts are usually hydrolyzed to some extent, but this extent is -very different even in water, according to the nature of the acid -radical. The degree of hydrolysis is also largely affected by the extent -to which the solution has been "made basic" by the addition of alkalies. -By the neutralization of the free acid in this way there is further -hydrolysis, the extent of which is again influenced by the nature of the -acid radical involved and other dissolved substances, especially of -organic matters. Again, the hydrolysis is largely affected by the -concentration of the solution even when the proportions of the -ingredients are constant, and this is practically important on account -of the necessity for exhausting the chrome liquors economically. Nor is -the matter entirely one of degree of hydrolysis, for (as we have noted -in the preceding section) the electrical condition of the chroming sol -is of great importance owing to the operation of the valency rule and -the possibility of zones of non-precipitation. The alkaline, neutral or -acid condition of the goods when first introduced has also its influence -on all these points. - -It will be readily understood, therefore, that there is some difficulty -in expressing the tanning power of a chrome liquor. As near as can be -yet said this is determined by (1) the concentration of the actual -tanning sol, and (2) its nearness to the isoelectric point. Now, these -points are not readily determined by analytical methods, and the best -that can yet be done is to determine the conditions which have large -influence upon these points. Thus the degree to which the liquor is -"made basic" by adding alkali is known, and can be expressed in formulae -by assuming that the acid neutralized by this alkali is replaced in the -chrome salt by hydroxy groups. Chromic chloride, Cr{2}Cl{6}, with the -addition of soda to correspond to half the acid formed upon complete -hydrolysis, would be considered then to be a solution of the salt, -Cr{2}(OH){3}Cl{3}. This has given rise to the conception of the -"basicity" of a chrome liquor, which may be expressed in many ways, the -most common of which in practice is the number of grams SO{4} still -combined with 52 grams Cr. Thus the salt corresponding to the -composition Cr(OH)SO{4} is said to have a basicity of 96. The practical -importance of such determinations of basicity has been much exaggerated, -for they are but a rough guide to the degree of hydrolysis of the chrome -and to the extent to which the sol is positive. Thus if the chrome salt -be actually a sulphate, a liquor of basicity 96 has about the same -_practical_ value as a chloride liquor of basicity 72, and in each case -the figures are of little significance if many organic substances be -present. If, however, as is usual in practice, there be approximately -the same acid radicals throughout the tannage and about the same -relative proportion of organic matters or of inorganic salts, then these -determinations have some practical value for comparative purposes. The -determination is itself simple: a portion of liquor is titrated direct -with caustic soda. The titration is at boiling-point, and is continued -until a permanent pink is obtained with phenolphthalein. The amount of -SO{4} corresponding to the soda required is then relative to the amount -of Cr in the same volume of liquor. A chromium estimation is therefore -also necessary and is most readily done by evaporating a portion of -liquor to dryness, igniting the residue and oxidizing the chrome to -chromate by heating in a muffle furnace with magnesia and sodium -carbonate in equal parts, or fusing in a blowpipe with sodium and -potassium carbonates in equal parts. The oxidized residue is dissolved -in hydrochloric acid and titrated with thiosulphate as described for the -two-bath process. - -Another attempt to determine the practical value of a chrome liquor is -the empirical test suggested by McCandlish, in which 10 c.c. of the -liquor is titrated with standard alkali until the precipitation point is -reached and a turbidity appears. The figure thus indicates approximately -the degree of nearness to the precipitation point and the amount of free -acid in the liquor. The author has found this a useful test taken in -conjunction with the basicity determination. It is best expressed in the -same units, _e.g._ grams SO{4} per 52 grams Cr. - -Another method is the determination of the hydrion concentration of the -liquor. This has useful possibilities for research work, but is usually -too laborious for rapid commercial control. The results, moreover, are -not less empirical, for the hydrion concentration of the liquor -indicates but imperfectly the electrical condition of the particles of -the tanning sol. - -In classifying one-bath liquors into types, it is best to take together -those in which the usual "basicity" and "acidity" determinations have at -any rate approximate comparative value, and this is determined in the -main by the method by which the liquor is manufactured. Broadly -speaking, there are three types of chrome liquor: (1) those made from -chromic salts by adding suitable amounts of alkali; (2) those made from -sodium dichromate by reduction with organic matter; and (3) those made -from sodium dichromate by reduction with sulphurous acid or its salts. - -Of the first type the most common is that in which chrome alum (a -bye-product of the dyeing industry) is the starting-point. To a solution -of this a solution of washing soda is gradually added, with constant -stirring, until the salt corresponding with the formula Cr(OH)SO{4} is -obtained. - -Now:-- - - K{2}SO{4}Cr{2}(SO{4}){3}.24H{2}O + Na{2}SO{4} - \___________________________________________/ - 998 - + Na{2}CO{3}.10H{2}O - \________________/ - 286 - = 2Cr(OH)SO{4} + K{2}SO{4} + CO{2} + 33H{2}O - -Hence, in practice, for every ten parts of chrome alum 2.86 parts of -soda crystals (or 1.06 parts anhydrous soda) are used. A convenient -"stock solution" is of 10 per cent. strength. Thus 10 lbs. of chrome -alum is dissolved, made basic, and made up to 10 gallons. To dissolve -the alum a mechanical stirrer is necessary, for the water must not be -more than warm. The disadvantage of this liquor is the limited -solubility of chrome alum and the need for its solution in the cold. -Liquors may be also made by dissolving chromium hydrate in hydrochloric -acid, and making basic to correspond to the formula -Cr{2}(OH){3}Cl{3}. Many preparations are on the market containing -both chlorides and sulphates with appropriate basicity. Chrome alum -liquors have been less often used in Britain of recent years owing to -the high price of chrome alum, caused in part by the presence in the -salt of potassium, all the salts of which have been scarce and dear -under war conditions. - -Of the second type Procter's "glucose liquor" is a good example. Use 5 -lbs. sulphuric acid, 6 lbs. sodium dichromate, and 7 lbs. of glucose, or -quantities in similar proportion. The dichromate is first dissolved, and -the acid added gradually. The glucose is then added cautiously on -account of the brisk effervescence of carbon dioxide. A glucose of good -quality is necessary, and the proportion to be used is not quite -definite, for sufficient only is needed to effect the reduction, and -this amount is influenced by the rate of addition and temperature of the -mixture. The reduction should be careful and regular, or the oxidation -products will be irregular and have a varying effect upon the tanning. -Molasses can be substituted for glucose, in amounts varying with its -strength. - -Of the third type the most common is that in which the dichromate is -reduced by sulphuric acid and sodium bisulphite. Solid bisulphite may be -used, but it is usually dear, and solutions are more commonly employed. -Into this type fall also the liquors formed by passing sulphur dioxide -gas into dichromate solution. Stock liquors of this type have the -advantage that strong solutions may be made (up to 18 per cent. -Cr{2}O{3}); they have the disadvantage that they are liable to contain -excess of free sulphurous acid. - -The method of application of chrome liquors is usually by paddling or -drumming the goods in solutions of appropriate strength--broadly -speaking, paddles used for lighter goods and plain finishes, and slowly -revolving drums for heavier hides and grained finishes. Heavy chrome -leather is often tanned in pits by suspension just as in vegetable -tanning. In such instances rockers may be usefully employed. - -In any case, the goods are successively brought into contact with -liquors of increasing strength, as in vegetable tannage, and the liquors -are thus most conveniently exhausted economically. The green goods thus -receive first nearly spent liquor and finish out of fresh strong liquor. -The goods may be, of course, handled from drum to drum, or from pit to -pit, but the modern tendency is to save labour by moving the liquors -instead. Thus in drum tanning the liquor is run out and pumped into the -next drum. In pits air ejectors have proved suitable, not only as lift -pumps, but also as agitators of the liquor in which goods are suspended. -The press system is also used. - -=Finishing Operations.=--In nearly all cases the chrome leather has -to be "neutralized" after tanning. This consists in removing the acid -"reversibly adsorbed". This removal is necessary to the finishing -processes, as well as to bring the tanning sol into condition for more -permanent tannage. Neutralization gets rid of soluble chrome salts as -well as free mineral acid, and is the final stage in rendering the -tanning sol less positive, and perhaps even negative. It is brought -about by the use of weak alkalies, of which borax is the easiest and -safest, but not the cheapest. Sodium silicate, phosphate, carbonate, and -bicarbonate have been also used, and a mixture of soda and an ammonium -salt has been suggested by Stiasny. Whitening has also been tried, but -is very slow-acting. Considerable economy in alkali may be effected by a -thorough washing of the leather before using the alkali. If the water be -hard, so much the better, and if warm water be available the process is -hastened. For most leathers it is necessary to remove excess of alkali -just as much as excess of acid, so that a thorough washing in water -generally follows the treatment with alkali. Anything from 1/4 to 3 per -cent. borax (or its equivalent) on the pelt weight may be used, and, -generally speaking, it is better to use solutions as dilute as -practicable in order to avoid local over-neutralization and tender -leather. - -Fat liquoring is a process which is very largely typical of chrome -leather manufacture; it consists in drumming the goods with an oil -emulsion, the grease of which is entirely taken up by the leather. It -thus strongly resembles drum stuffing (Part I., Section IV.) in method, -but the "fat liquor" is such that it mixes easily with water, and -usually contains soap in order to assist in this sense, and may -sometimes indeed consist of soap only. Mineral oil is also used -frequently in fat liquors. The object of fat liquoring is to give -softness, pliability, or waterproofness, and to feed the "empty" chrome -tannage. It is also used as a preparation for more complete impregnation -of grease, _e.g._ as in "stuffing" chrome harness, and in "dipping" -chrome sole leather. Fat liquors are usually made by dissolving the soap -in boiling water and gradually adding the oil with constant agitation. -Perfect emulsification is essential, and this is assisted by the use of -casein, albumen, gelatine, starch, egg yolk in addition to soap and -oil. Soda and borax also assist, and degras and sod oil are also useful -and are admissible where the leather is to receive a dull finish. The -operation of fat liquoring is greatly assisted by heat, and temperatures -of about 110 deg. to 130 deg. F. are usual. Chrome leather may be dyed before or -after fat liquoring: if before, the fat liquor sometimes tends to alter -the shade; if after, the dyeing tends to be uneven. Logwood extract and -iron salts are largely used for blacks. It is common to mordant chrome -leather with vegetable tanning before dyeing. Sumach and gambier are -often used for this purpose, and the usual "fixing agents" (tartar -emetic, titanium salts, etc.) may also be used. - -Of the mechanical finishing operations staking is the most -characteristic. It is now done entirely by machines, and the primary -purpose is to soften the leather, which otherwise dries out in a -non-pliant and stiff condition. In the staking machine, the "blade" is -fixed between two rollers, which are however on the other side of the -leather. The leather is held by the operator, and the machine "head" -pulls a fold of the leather over the blade. Seasoning and glazing are -also common for many chrome leathers. - - - REFERENCES. - - Procter, "Principles of Leather Manufacture," pp. 198-220. - - Bennett, "Manufacture of Leather," pp. 210, 312, 355, 375. - - Bennett, "Types of Two-bath Chrome Tannage," _Leather_, 1909, Aug. - and Sept. - - - - - SECTION III.--CHROME CALF - - -The tannage of calfskins by the chrome processes for the manufacture of -upper leathers is one of the most extensive branches of leather -manufacture. The deservedly popular =box calf= is typical of these -leathers, and the observations of this section are primarily applicable -to it. A chrome-tanned calf skin, fat liquored and blacked, provides as -suitable an upper leather as could be desired for ordinary boots. It is -at once supple and durable. It is also sufficiently waterproof, but can -be given a bright glazed finish. - -In regard to the wet work for chrome calf, the general principles and -methods discussed in the previous section are much to the point. It is -essential to avoid undue plumping and the loss of hide substance. The -skins should be washed clean as soon as possible. Three fresh waters are -desirable, the goods remaining only a short time in each. Salted skins -need more time, but the liquors must be kept sweet. Drumming the skins -in running water is very suitable for the first and last stages of -soaking. - -The liming should be short but not "sharp," _i.e._ mellow sulphide limes -are suitable, depilation being carried out after about 7 days. The -one-pit system is usual, but two liquors may be given, the green goods -being first inserted into a used liquor, and after handling reinserted -into the same pit with a new lime liquor made up with lime, sulphide and -a proportion of the old liquor. Scudding should be carefully done, as -hair on the finished leather is very objectionable. - -In deliming it is essential to have the grain of the skins thoroughly -relaxed and pulled down. The finished box calf should have a -characteristic soft and silky feel, and this is only attained by -procuring the inelastic pelt. It is not surprising that a light puering -is a popular method for attaining this, but there is also a tendency to -use artificial bates such as are made from ammonium chloride and -pancreatin, together with organic acids, or non-swelling acids like -boric acid. Drenching is also common after a preliminary deliming with -acid. The skins may be half or two-thirds delimed with lactic acid, -rinsed and drenched over night at 85 deg. F. with 6 per cent. bran on the -pelt weight. Less acid may be also used, in tepid water, and the drench -made up with 10 per cent. bran and a little pea meal. It is very common -to pickle the skins in 5 per cent. alum and 5 to 10 per cent. salt -before tanning. This is often of doubtful advantage, but sometimes -prevents drawn grain when the goods are moved rapidly into strong chrome -liquors. This pickling is said to give fullness to the leather. - -The tannage of box calf is usually by the one-bath process, though the -two-bath process gives quite as good a result and is sometimes used. -Again, drum tannages are the most popular on account of their speed and -the economy of chrome. The practical problem is to use up all the -chrome, and to tan quickly without "drawing" the goods. It is, in any -case, usual to commence the tannage in a used and nearly spent liquor -and finish in a fresh liquor. The most appropriate way depends largely -upon local convenience, the number of drums available, supply of labour, -etc. In a one-drum system the goods may be started in an old liquor, -which is run off when exhausted by the green goods. Fresh stock solution -is then added at intervals of an hour or two and the drumming continued -till tannage is complete, which is usually in less than 24 hours. The -remaining liquor is used to commence the tannage of the next pack. - -In another system the operation is similar except that the liquors are -weaker, and the goods are then removed and finished in another drum. A -three-liquor system, however, is often combined with a one-drum method; -the goods are thus not handled. The liquors are run off and pumped to -other drums, the once-used liquor to a drum containing goods already -treated with a twice-used liquor; the twice-used liquor to a drum -containing green goods, and the thrice-used liquor pumped to the drain. - -In any of these methods the chrome alum liquor is suitable, using 10 per -cent. alum and 3 per cent. soda on the pelt weight. The glucose liquor -has also proved very suitable for chrome calf, and the liquors made with -sulphurous acid or its salts have increasing popularity on account of -lower costs. Many tanners use bought liquors--"chrome extracts" which -are supposed to be specially devised to suit the tannage of chrome calf. -When thoroughly tanned through, as can be readily judged from a -sectional cut of the leather, and also by the strength of the liquor -remaining, the goods are horsed in pelt overnight, and are then ready -for finishing. - -In finishing box calf the neutralization should be thorough, or the acid -may cause trouble in dyeing and fat liquoring. Imperfect removal of -excess chrome salts may cause the formation of "chrome soaps" which are -very difficult to remove; the goods should therefore be well washed. -There are two general types of treatment before blacking. In one, the -skins are first well washed with water at 110 deg. F., neutralized with -about 3 per cent. borax, and well washed again. Striking follows and is -usually very thorough, partly because it assists in producing evenly the -characteristic box grain, and partly because the finished leather is -sold by the square foot. Machine striking is now almost universal, and -may be done several times at different stages in the drying. When half -dry ("sammed") the skins are shaved by machine and, at this stage -usually, weighed. Dyeing and fat liquoring then follows. In the other -type, the goods are merely washed, and then struck out, sammed, shaved -and weighed. The skins are then neutralized, washed and immediately dyed -and fat liquored. The advantages of this latter course are that the -goods remain in the drum for the last four processes, which is -economical of labour, and also that by neutralizing immediately before -dyeing and fat liquoring there is less danger of a further diffusion of -acid. - -In dyeing logwood extract is largely used, occasionally a little fustic -is used also, and by using a "striker" of iron and copper sulphates a -good black is obtained. Logwood is often used also in conjunction with -coal-tar dyestuffs. The goods are first warmed in the drum up to 140 deg. -F., and the dyestuff solution gradually run into the drum whilst it is -revolving. Up to 3/4 hour may be necessary to exhaust the bath, the -goods being constantly drummed. The fat liquor is then run in similarly, -and the drumming continued until the grease is all absorbed by the -leather, which may take another hour. The skins are horsed till next -day, during which time the grease penetrates more completely. - -The skins are now dried out, sometimes by suspending from the hind -shanks and sometimes by nailing on boards or wooden frames. They are -damped back for staking by leaving for 1-1/2 to 2 days in moist sawdust. -After staking they are dried strained in a "stove" at about 105 deg. F. - -In finishing off, the grain is "cleared" by sponging with 10 per cent. -lactic acid, and seasoned with a mixture of milk, blood and black -dyestuff. When dry on the surface the skins are glazed by machine, and -grained two ways--neck to butt and belly to belly. They are usually -reseasoned, dried out, reglazed, regrained, lightly oiled with mineral -oil, and finally trimmed. These various operations are fairly typical, -but there is obviously ample scope for divergence. Thus one may fat -liquor before dyeing, and the skins may be staked before drying out, and -may be re-staked after glazing. - -Much so-called "box calf" is not made from calf skins. A very close -approximation, however, is obtained from rather older animals, and -"box-kip" is largely manufactured by similar methods. Light hides are -also widely used, being similarly treated except that they are split -and also cut into two along the spine. The finished article is sold as -"box-sides." To yield the characteristic grain pattern, the goods are -frequently printed and embossed. Even the flesh splits are sometimes -made into box calf imitations, some filling material being used and an -artificial grain pattern embossed. - -=Willow calf= typifies the chrome calf which is finished in colours. The -soaking, liming and deliming processes are the same as for box calf. The -tannage, however, is generally by the two-bath process on account of the -lighter colour thereby obtained. This colour is largely due to the -deposition of sulphur in and on the leather in the second bath. - -In one tanning process the skins are first pickled in 2 per cent. -hydrochloric acid and 10 per cent. salt. They are then drummed in -solution containing 2 per cent. dichromate (strength 1 in 60) for about -half an hour. A solution containing 4 per cent. dichromate, 3-1/2 per -cent. hydrochloric acid, and 5 per cent. salt is gradually added, and -the skins drummed until well struck through. They are then horsed -overnight and struck out and passed through a "hypo dip,"--a 2 per cent. -solution of thiosulphate,--and then into the reducing bath, which -contains 10 per cent. of thiosulphate, to which 5 per cent. hydrochloric -acid is added. - -Another process employs paddles instead of drums. The chroming liquor is -made up with 4-1/2 per cent. chromic acid and 10 per cent. salt. The -bath is exhausted by commencing the tannage of a succeeding pack. The -skins are reduced as in the last process. - -In another process the "acid" type of chroming bath is used. The skins -are paddled with a solution containing 5 per cent. dichromate, 5 per -cent. hydrochloric acid, 2 per cent. aluminium sulphate, and 10 per -cent. salt. In the reducing bath 14 per cent. hypo and 4 per cent. -hydrochloric acid are used. - -In yet another process the skins are pickled first in 5 per cent. -aluminium sulphate, 7-1/2 per cent. salt, and 3 per cent. sulphuric -acid, and are then dried out and sorted. The tannage proper is in the -drum, using 6 per cent. dichromate, 5 per cent. hydrochloric acid, and 5 -per cent. salt. In the reducing drum 15 per cent. hypo is used and 4-1/2 -per cent. hydrochloric acid. - -Whichever process of tanning has been used, the skins are neutralized -and washed thoroughly, as for box calf, sammed and shaved. In dyeing, -the skins are first mordanted with a filtered infusion of leaf sumach, -used at 110 deg. F. for half an hour. As fixing agent, 4 oz. tartar emetic -per dozen skins is then added and the drumming continued for half an -hour. The goods are washed, struck out and drum dyed at 140 deg. F. with -basic colours, and immediately fat liquored. In the fat liquors olive -oil and castor oil, with the corresponding soaps, have been popular, but -substitutes are now used on economical grounds. The skins are next -horsed a while, well struck out again and dried strained. They are now -finished off as for box calf, except that it is usual to grain only one -way--neck to butt--and the season should consist of milk, water and -albumin only, though sometimes other mucilagenous matters are added. As -with box calf, the finishing may be varied in many ways. The skins may -be dyed with acid colours after fat liquoring. For pale shades direct -dyes are used without a mordant. For darker shades of brown and red, the -dyewoods are used both as mordants and ground colours, and titanium -salts are useful as fixing agents. - -Both the "box" and "willow" finish are largely a matter of public taste, -and the fashion varies from time to time on such points as to whether -the grain should be one way or two ways, and whether it should be faint -or bold. There are also other common finishes besides the typical box -grain. =Glace calf= is made much in the same way as box calf, but there -is no graining at all. The goods are usually seasoned and glazed three -times. Small skins are preferred for this finish. =Dull calf= is also a -plain finish. The leather contains more grease, and the fat liquor is -made up with greater proportions of degras. The goods are not seasoned -or glazed, but ironed, "sized" with gum, oil, soap and logwood, and -after brushing are dried and rolled. In both these plain finishes a -one-bath paddle or pit tannage is common, in order to ensure the smooth -finish. - - - REFERENCES. - - Procter, "Principles of Leather Manufacture," p. 198. - - Bennett, "Manufacture of Leather," pp. 55, 84, 105, 227, 360-363, - 375. - - Bennett, "Theory and Practice in Wetwork of Chrome Calf," _Shoe and - Leather Reporter_, Sept., 1909. - - - - - SECTION IV.--CHROME GOAT AND SHEEP - - -Immense quantities of goat and sheep skins are chrome tanned for upper -leathers. Most of them are manufactured into the well-known and popular -=Glace kid=, to the manufacture of which this section is chiefly -devoted. To be quite strict, glace kid should be made from kid skins, -but actually comparatively few of such skins are used, they being -reserved rather for glove leathers. The popular upper leather is made -from goatskins. - -Chrome goat is deservedly popular; it is an ideal upper leather for -shoes and light boots. As compared with chrome calf (thickness and other -factors being equal), it is not only softer and more pliant, but also -more durable. It is usually, however, not quite so thick, and perhaps -therefore not quite so warm and waterproof. The popularity of glace is -probably enhanced by the brighter and more glassy finish than is usual -with box. - -As the supply of goatskins is unfortunately too limited, an even more -widely used glace upper leather is made from sheepskins, and often sold -as glace kid. From what has been previously said as to the quality of -goat and sheepskin leathers (Part II., Sections II. and IV.), it will be -readily understood that glace sheep is by no means so good a leather as -glace goat. It is perhaps as soft, but is more spongy and loose -textured, and is neither so waterproof nor so durable as chrome goat. -The ubiquitous sheep, however, provides an immense supply of raw -material, and the resulting leather, which should strictly be regarded -as a glace kid imitation, finds a ready sale. When well finished it is -indeed a good imitation in respect of appearance, and this fact, -together with its comparatively low cost, causes it to meet an undoubted -public need. - -The production of glace goat will first be considered. The soaking -process is quite similar to that before described for the production of -goatskin moroccos (_q.v._) and need not be here repeated. The liming is -similar in many respects also, but from what was said in Section II. -about the undesirability of excessive plumping of pelt for chrome -leather, it will be clear that caustic soda should be omitted from the -limes. The liming should also be shorter for glace than for moroccos, -and this is attained both by using a greater proportion of sulphide and -by using mellower lime liquors, preferably the latter, as soft pelts are -better ensured. Calcium chloride has sometimes been added to the limes: -this reacts with the soda from the sulphide, yielding salt and probably -precipitating lime, and has its own lyotrope influence, thus reducing -the plumping effect possibly in two ways. To obtain either effect it is -necessary to use considerable amounts of calcium chloride. As goatskins -are so tight fibred, a longer liming and a greater loss of collagen is -permissible than with most pelts for chrome. The deliming operations -should be exceedingly thorough in order to obtain the desired softness -and the smooth grain. Puering is largely used to the full extent, _i.e._ -the goods are thoroughly pulled down at 85 deg.-90 deg. F., and are carefully -delimed in the puer liquor. After puering it is common to give a low -temperature drench (60 deg.-65 deg. F.), which of course acts slowly over a day -or two. The skins must be well scudded after puering or after drenching; -sometimes after both. The drenching is often substituted for purely -deliming processes, of which may be mentioned the use of boric acid and -also the use of warm solutions of the commercial organic acids (lactic, -formic, acetic, butyric, etc.), together with calcium chloride. In place -of the chloride, a salt of the acid may be employed, and the deliming -bath may be regenerated by oxalic acid and used repeatedly. Sometimes -puering is omitted and the desired result obtained by washing in warm -water, nearly deliming with warm solutions of organic acid, washing -again and drenching. Skins are also washed often after drenching. - -In tanning chrome goat for glace the two-bath process is mostly -preferred. This is partly because the sulphur deposited in the reducing -bath assists materially in producing the mellowness and fullness which -are so essential, and partly because a large proportion of skins are -finished in colours. The two-bath process also lends itself to a paddle -tannage, which is necessary for the smooth grain finish. One or two -illustrative processes may be given. - -One process presents many points of resemblance to the first process -suggested for willow calf in Section III. (_q.v._). The skins are first -pickled in a paddle with 2 per cent. hydrochloric acid and 10 per cent. -salt, and then pass into the chroming paddle, which contains at first -only 2 per cent. dichromate. Subsequently 4 per cent. dichromate, 3-1/2 -per cent. hydrochloric acid, and 5 per cent. salt are added to the -paddle liquor, and the skins paddled until well struck through. After -being horsed overnight the skins are struck out by machine, passed -through a hypo dip if desired, and reduced with 12 per cent. of -thiosulphate and about 5 per cent. of acid. The skins may be left -overnight in the hypo paddle, and the excess of thiosulphate, which is a -feeble alkali, commences the neutralization. - -In another process the chroming bath is made up of 5-1/2 per cent. -chromic acid and 6-1/2 per cent. of salt, and to this paddle liquor 2 or -3 per cent. of aluminium sulphate may be added if desired. The reduction -is with 14 per cent. hypo and 7 per cent. hydrochloric acid. A little of -the acid is added to the reducing bath; when the liquor turns milky, the -skins are rapidly inserted, and the rest of the acid gradually added. - -In the finishing processes the mechanical operation of "striking" is -very prominent, on account of the necessity of obtaining area and smooth -grain. The skin of goats has rather a tendency to bold grain, and this -enhances the need of striking. Most manufacturers lay great stress upon -thorough neutralization and washing. An important point also is that the -staking should be carried out at the proper condition of dryness. If -either too damp or too dry, the requisite mellow feel is not obtained. -There is, of course, ample scope for variation and ingenuity, and the -following processes for blacks and colours must be taken as broadly -typical. - -The skins from the reducing bath are first machine-struck, and then -immediately neutralized with one per cent. borax until this is -thoroughly used up, and the skins are then paddled for many hours in -running water. They are again struck out and lightly shaved, possibly -after a little drying. There is a tendency to save time by using a -stronger borax solution, and by using warm or tepid water, and some -factories save borax by washing well first in warm water. If for blacks -a common plan is to dye grain and flesh a violet-blue and then black the -grain only with logwood and iron. The skins are drum dyed blue with a -coal-tar dyestuff, drumming half an hour in the solution at 110 deg. F., and -again struck out. They are then paired or pleated, and rapidly passed -successively through three vats containing respectively cold weak -ammonia, a logwood and fustic infusion at 120 deg. F., and a solution of -ferrous sulphate containing a little copper sulphate. The skins must be -immediately washed well to remove excess of iron. Instead of this -process the skins may be passed through vats containing coal-tar blacks. -Instead of blue backing the skins may be drum-dyed black on flesh and -grain with either coal-tar blacks or with logwood and iron. In the -latter case the skins must be drummed in water for an hour to remove -excess of iron. However dyed, the skins are often struck out again after -dyeing, and sammed slightly for fat liquoring. Neatsfoot oil is a -popular ingredient of the fat liquor. The skins are drummed dry for a -few minutes in a hot drum, and the fat liquor added at 130 deg. F., and the -drumming continued after the grease has been taken up in order that it -may be thoroughly distributed. The skins are struck out again, rapidly -dried out, and wet back for staking in damp sawdust. The staking should -be thorough, and, if necessary, repeated when the goods are rather -drier. - -In finishing off the skins may be fluffed if desired, and are then -"cleared" by sponging with 10 per cent. lactic or acetic acid. They are -then seasoned and glazed after some drying. This is repeated until the -required gloss has been obtained. They are finally oiled lightly with a -mixture of linseed and mineral oils. On finishing =dull kid= a heavier -fat liquor is given, in which degras is used, and the skins are not -seasoned and glazed, but are ironed and oiled. In finishing for -=coloured glace=, the skins are mordanted before dyeing by the use of -dyewood extracts, antimony and titanium salts being used as fixing -agents. The fat liquor should contain less soap and more egg yolk, and -for fancy shades even egg yolk only is sometimes used. - -The production of chrome glace sheep follows the same general lines as -glace goat. There is less difficulty in obtaining smooth grain, so that -"striking" is perhaps less prominent, and drum tannages are preferred, -whether one bath or two bath. The skins are received after -fellmongering (see Part II., Section IV.) and need thorough puering to -remove scud, and may be then rinsed through boric acid. Pickling is very -common with these goods. In the pickled state they are often sorted out -before tanning. The pickling is usually a one-bath process in which -vitriol and salt or else alum and salt are used, but sometimes all three -substances. The skins may indeed be received in a pickled state. They -may be depickled by paddling with salt and borax, bicarbonate, or basic -alum solution. They may also be tanned without depickling if the -composition of the pickle be allowed for in the first chroming liquor. A -commonly used pickle consists of 3 per cent. aluminium sulphate and 9 -per cent. salt. If these goods are to be dried out, flour also may be -used with the pickle, which thus becomes practically a light preliminary -alum tannage (see Part IV., Section I.). A commonly used acid pickle is -of 5 per cent. commercial sulphuric acid and 25 per cent. salt. - -The delimed or depickled stock may be tanned as now described. The -two-bath process may be used with drums. The chroming bath contains 5 -per cent. dichromate, 5 per cent. hydrochloric acid, and 10 per cent. -salt. After the skins are thoroughly penetrated they are horsed -overnight and reduced with 20 per cent. thiosulphate, up to 7 per cent. -of hydrochloric acid being added after half an hour in thiosulphate -only. - -Alum pickled or tawed skins are wet back by drumming for about an hour -in water, and are then tanned by the one-bath process in drums. Only a -few hours are needed. Towards the end of the operation about 1/2 per -cent. of bicarbonate of soda may be added to the chrome liquor. Acid -pickled skins may be wet back with 10 per cent. salt, and depickled by -adding a basic alum solution and the chrome tannage superimposed after -about half an hour without handling the goods. The basic chrome alum -liquor is suitable for this purpose. - -In finishing glace sheep much the same methods are used as in the case -of glace goat. Sheepskins are perhaps more lightly fat liquored, being -naturally soft and porous. Degreasing is often necessary to obtain an -even finish. As sheep gives an empty pelt and chrome an empty tannage, a -slight retannage is often given in gambier, especially for blacks, in -which case the skins are well mordanted. This retannage makes the -leather less stretchy. Logwood and iron blacks are usual. For colours, -fustic or sumach are the usual mordants, with tartar emetic to fix. If -for glove leathers, skins pickled in alum and salt or tawed should be -preferred, and flour may be used in the fat liquor. - -Sheepskin splits are sometimes given a chrome tannage and finished as -=chrome chamois=. This leather may be used for linings, but not for -polishing silver on account of the sulphur originating from the -reduction bath. The splits are puered heavily, and pickled in 6 per -cent. vitriol and 24 per cent. salt. They are paddled in this pickle -liquor, and 4 per cent. dichromate added in successive portions. The -fleshes are horsed overnight and reduced in 15 per cent. thiosulphate, -to which a little hydrochloric acid is added if needed. - -In finishing the splits are washed in warm water, neutralized in weak -soda, and washed again. They are sammed by machine striking, and fat -liquored, using much soap. They are then horsed, struck and dried out. -They are staked several times after damping back, drying out again -between stakings. They are finally fluffed. - - - REFERENCES. - - Procter, "Principles of Leather Manufacture," p. 198. - - Bennett, "Manufacture of Leather," pp. 55, 84, 105, 230, 364. - - Bennett, "Theory and Practice in Wetwork of Chrome Goat," _Shoe and - Leather Reporter_, Sept., 1910 - - - - - SECTION V.--HEAVY CHROME LEATHERS - - -The term "heavy chrome leather" is taken to include chrome sole leather, -chrome strap and harness butts, waterproof chrome upper leathers, motor -butts and picking band butts. These will be discussed in turn. - -=Chrome sole leather=, as stated in Section I., has made headway in -Britain during the European War, the Army authorities having recognized -its great advantages in durability and waterproofness. At the time of -writing, however, its manufacture has received a set back, and many -factories are reducing their output. The primary cause of this is that -the Army purchases have largely ceased, whilst the general public have -not yet been educated to its value. Men who take chrome uppers for -granted talk of chrome sole as a "leather substitute" with an -implication that it is of inferior value. It must be recognized, too, -that there is some interested opposition to its development. Cobblers -and bootmakers complain that it ruins their tools, being so hard to cut. -Now, it is manifestly impossible for it to be soft to cut and hard to -wear out; the complaint is therefore an excellent testimonial. There is -also a stupid fear that an article which lasts twice as long will reduce -repairs and retail sales by 50 per cent. Even the manufacturer has -sometimes a suspicion that a demand reduced in proportion to durability -will not be balanced by an extended export trade. These points of view -will become minor considerations when the public realize its relative -economy, and when the community as a whole grasp that a durable article -is a natural asset. Meanwhile credit is due to those firms who persevere -in their pioneering work of educating the public. - -The manufacture of chrome sole leather presents many analogies with the -vegetable tannages. The soaking and liming should be about identical, -but the hides for chrome are generally given more sulphide and the -depilation is reduced to about a week. The methods used for deliming -differ widely in different factories. Some delime completely with -mineral acids, some even pickle in acid and salt, whilst others merely -delime the grain with boric acid. The last is really quite sufficient. -Again, in tanning one finds similar divergences of method. Drum tanning -is practised, but tannage in pits by suspension is more usual, though, -as this last involves more dilute liquors, it involves also greater time -to tan. In drum tannages a few days only are sufficient. In pit tanning -at least a week is given, but sometimes up to a month, according to the -strength of the final liquor and the rate of progress of the goods into -stronger liquors. Liquors containing over 1 per cent. of chromium may -easily be spent out so as to contain only 0.01 per cent. Labour and time -are saved in pit tanning by the use of rockers. The press system of -avoiding handling, however, so complicates the analytical control that -its advantage is doubtful, a better way being to shift the liquors by an -air ejector, which may also be used as an agitator of the liquor and -thus abolish the need for rockers ("Forsare" patent). Chrome butts are -tanned out in suspension. No floats or layers are used. The -neutralization need not be so thorough as for light chrome uppers, as -dyeing is not practised and trouble does not arise with emulsions made -from sulphonated oils. Thorough washing is advisable, and the butts are -usually then cut into bends and may be oiled before drying if desired. -The bends are dried strained, to obtain flatness and smooth grain, for -no machines, such as strikers and rollers, are usually employed. It is -necessary to dry very thoroughly, for the bends are waterproofed by -dipping the dry leather into molten waxes. The most commonly used wax -and the cheapest is paraffin wax with a m.p. of about 127 deg. F. It is -rather a brittle wax, however, and as the finished leather consists of -up to one-third of the wax, it is better to use at least some proportion -of hard fat, Japan wax or ceresin wax, to obtain a stuffing material -with less crystalline texture. The use of 10-30 per cent. rosin in the -stuffing grease is also usual. This prevents the leather from being so -slippery when in wear. The stuffing should take place at temperatures -from 150 deg.-195 deg. F., according to the melting-point of the grease -employed. The bends are taken out and laid in pile to cool and set in a -flat condition, and are then finished. - -The chrome tannage of butts for strapping and harness backs, and for -motor butts and picking bands may be similar to that for chrome sole, -but drum tannages are more common and the two-bath process is often -used. In the latter case the acid chroming bath is preferred, using 6 -per cent. of dichromate and of acid, with up to 15 per cent. of salt, -and reducing with 15 per cent. thiosulphate and acid as needed. This -process assists in the production of the light colour which is preferred -in the case of some of these leathers. - -Strap butts after tanning are very thoroughly washed with cold water in -pits, and repeatedly struck out by machine between the washings. They -are then oiled with heavy mineral oil, and stretched by powerful -machines. They are dried and curried during the stretching. Degras, wool -fat and vaseline are greases used, and the drying and stretching -finished off at 120 deg. F. They are then fluffed on the flesh, -French-chalked and heavily rolled. - -Harness backs are neutralized, machine sammed, and lightly fat liquored -with 4-1/2 per cent. soap. They are then struck and oiled with heavy -mineral oil and dried for stuffing. Hand stuffing, drum stuffing, and -"burning in" are all used (see Part I., Section IV.). Stearin, paraffin -wax, ceresin wax, wool fat, sod oil and mineral oil are the greases -employed. The butts are blacked after stuffing with lamp black and oil, -glassed well and buck-tallowed on the grain. - -Motor butts are fat liquored lightly, using soap only. They have to be -softened, therefore, during the drying by being mechanically worked. A -boarding machine is repeatedly used during the drying. They are -finished off with French chalk on flesh and grain. - -Picking band butts are neutralized by using warm water and then borax -solution, and are then sammed by machine and very heavily fat liquored -with cod oil and tallow and hard soap, to which degras may also be -added. Up to 20 per cent. of greases (on the pelt weight) may be used. -They are well drummed in this, struck out, French chalked, and dried -out. They are softened finally by machine. - -Waterproof chrome upper leathers are manufactured usually from hides -tanned by the two-bath process, which is said to give a mellower -leather. The neutral type of chroming bath is common. The butts are -neutralized, machine sammed and struck, and then fat liquored with 2 per -cent. each of neatsfoot oil and soft soap. They are then sammed, shaved -and blacked on the grain with logwood and iron, and dried further. They -are stuffed then by brushing with an abundant amount of concentrated fat -liquor. This gives the waterproofness. They are staked after drying -further, and often grained three ways. A further waterproof finish is -given consisting of a fat liquor containing beeswax. They are finally -brushed and re-oiled with linseed oil, to which some mineral oil may be -added. This leather is much the most durable type for a shooting boot, -or where waterproof uppers are desirable. - - - REFERENCES. - - Procter, "Principles of Leather Manufacture," p. 198. - - Bennett, "Manufacture of Leather," pp. 234, 368. - - - - - PART IV.--MISCELLANEOUS TANNAGES - - - SECTION I.--ALUM TANNAGES - - -The use of alum for making pelt into leather is several centuries old. -It was the first case of what are called "mineral tannages." The tannage -is closely analogous in theory to the chrome tannages discussed in Part -III., and if soda be added to ordinary potash alum in solution, a basic -alum liquor is obtained which is quite capable of yielding a -satisfactory leather, and which is thus a strict analogy of the basic -chrome alum liquor described in Part III., Section II. The range of -basicity which is practicable is very limited, however, and it is much -more usual to use common salt with the alum instead of soda. The alum -is, of course, hydrolyzed and free sulphuric acid is quickly adsorbed, -whilst the colloidal solution of alumina is adsorbed also but more -slowly. The adsorbed acid tends to swell the pelt and to cause it to -take up the alumina less readily. The function of the salt is to repress -the swelling by a pickling action. The actual result is thus partly due -to the alum tannage and partly due to the temporary tannage given by the -pickle. Hence such tannages are not firmly "fixed," nor is the result -water-resisting, for much of the tanning material may be washed out. If, -however, such leathers be stored for a time in a dry condition, the -alumina becomes much more firmly fixed, owing probably to a further -dehydration of the alumina gel deposited upon the fibres. The tannage is -thus relatively more "irreversible," and such storage is practised in -commerce for this purpose, being known as the "ageing" of the leather. -It will be understood that it is possible to use too great a proportion -of salt, the hygroscopic nature of which would keep the leather moist -and thus interfere with a glossy finish. About one-third the weight of -the alum used is usually sufficient. - -All that has been said in Part III. as to the empty nature of the chrome -tannage is equally applicable to the alum tannages. It is as necessary -therefore to employ filling agents. A fat liquor is quite satisfactory -for many purposes, but is too dark coloured and greasy for glove -leather. Egg yolk is the favourite emulsion in these cases. It contains -about 30 per cent. of an oil very similar to olein and in very perfect -emulsion. Olive oil is also largely olein and is also used, being -emulsified by the egg yolk and effectively reducing the proportion -required of this expensive material. Flour is also used as a filling -agent. It acts also as a whitening agent and as an emulsifier. Its use -enables the tanner to obtain the required fullness without so much -greasiness. Thus softness and fullness may be obtained, and yet a glossy -finish be possible. It will be clear that the more flour is used, the -more oil may also be used. - -The materials mentioned, viz. alum, salt, flour, egg yolk and olive oil, -are all mixed together into a paste with some amount of water. The goods -are drummed in this paste and then dried out. This operation is known as -"tawing." The goods are then "aged" for several weeks and finished as -required. - -The manufacture of "glove kid" from lambskins and kid skins is the most -typical example of alum tannage. Lambskins are unwoolled very usually by -painting the flesh with a mixture of lime and sodium sulphide. There -must not be too much of the latter on account of its tendency to give -harshness, a fatal defect in glove kid. The addition of calcium chloride -is desirable, and the skins, which should be pulled as soon as possible, -should be quickly placed in soft water or weak lime. For kidskins a set -of lime liquors may be used, and in preference to sodium sulphide red -arsenic is employed. About one per cent. realgar on the weight of the -lime is used, but more often larger quantities are preferred, even up to -6 per cent. The liming is thus shortened to 4 or 5 days. Fresh lime -liquors are sometimes used without any sulphides. Another method is to -place the skins in a paste of lime to which realgar has been added in -slaking. In any method it is necessary to saponify or emulsify the -grease on the grain, or difficulties occur in dyeing and finishing. - -Skins which are to be tawed for glove kid are both puered and drenched. -They are heavily puered at 70 deg. F. for 3 hours, or even longer for the -heavier skins. After scudding they are drenched with 10 per cent. bran -and some pea meal at 95 deg. F. for a few hours only. - -In preparing the tawing paste, the flour should be mixed with tepid -water; the egg yolk should also be diluted with tepid water slightly, -and strained if necessary, and then added to the flour. The oil is then -carefully mixed in. The alum and salt are dissolved separately at 110 deg. -F. and added to the flour and oil. The tawing paste should be used at -about 105 deg. F. For every hundred medium-sized lambskins there will be -required: 10 lbs. flour in 2-1/2 gallons water, 1 quart preserved egg -yolk, 3-3/4 lbs. alum and 1-1/4 lbs. salt. The skins are drummed in this -for an hour or so and dried out on poles rapidly, but not with great -heat. This is essential to get "stretch." They are next wet back, -staked, dried and staked again. They are then "aged." - -To wet back for dyeing and finishing the skins are drawn through warm -water and then drummed in water at 95 deg. F. for 15 minutes to wet evenly -and thoroughly. This liquor, which contains much of the tawing material, -is run off and replaced by the dye solution, _e.g._ fustic or turmeric, -with which the goods are drummed for half an hour. Iron, chrome or -copper salts may be used for saddening. After this "bottom" colour is -obtained, a coal tar colour is added for "topping" and the drumming -continued until the required shade is obtained. The excess liquor is now -run off, and the materials lost in soaking are replaced by drumming -further with egg yolk and salt for 15 minutes. This is known as -"re-egging." Blacks are obtained with logwood and iron. After -re-egging, the skins are dried out and staked. They are "seasoned" with -a weak emulsion of soap and oil, dried, oiled lightly with linseed oil, -ironed, re-oiled and finally brushed. Whites are undyed, and 10 lbs. -French chalk per 100 skins is used in re-egging. - -"Calf kid" is a once popular but now obsolete upper leather made by -tawing calfskins. The skins were well plumped in limes, delimed by -washing and drenching, tawed much as for glove kid, split, dried out -rapidly, staked and aged. They were finished dull and black with soap -and wax. - -The various white leathers used for belts, laces, whip lashes, aprons, -covers for stoppered bottles, etc., are very usually made with an alum -tannage. Alum, salt and flour only are used. Whitening is also mixed in -and acts as neutralizing agent as well as pigment dye. - -Wool rugs are manufactured from suitable sheepskins by an alum tannage. -They are first well cleaned, using soap on wool and flesh. They are next -degreased by painting with fuller's earth paste and drying. They are -tawed by painting the flesh with a strong solution of alum and salt, or -even by rubbing on the solid salts. They are dried out, aged and sorted -for suitable colours. The dyeing is rather difficult, as many artificial -dyestuffs are of no use. It is usual to bleach the skins first in a weak -solution of bleaching powder, and afterwards to dye with infusions of -the dyewoods, _e.g._ logwood, fustic, sandalwood, terra japonica, -quercitron bark, turmeric, indigo, etc. Vat dyeing is usual. After -dyeing, retanning with alum and salt is necessary, on account of the -loss of these in bleaching and dyeing. Rugs are usually finished black, -white, grey, brown, walnut, crimson, blue or green. - - - REFERENCES. - - Procter, "Principles of Leather Manufacture," pp. 184, 236. - - Bennett, "Manufacture of Leather," pp. 239, 371. - - - - - SECTION II.--FAT TANNAGES - - -For the manufacture of a permanent leather the essential requirements -are that the fibres of the hide or skins gel should be dried in a -separate condition, and that they should be coated by some waterproof or -insoluble material. Many substances fulfil the first but not the second -of these conditions. For example, the dehydration only may be -accomplished more or less by salt (as in curing hides), still better by -salt if a little mineral acid be used (as in pickling), and by other -salts such as potassium carbonate and ammonium sulphate, and dehydrating -agents such as alcohol. Such "temporary leathers," however, are not -water-resisting, as the second requirement has not been fulfilled, viz. -the coating of the fibres with some more or less waterproof material. -Thus if pelts dehydrated with alcohol be treated with an alcoholic -solution of stearic acid, the second condition is fulfilled and a -permanent leather is obtained. - -Now, many tanning agents accomplish these two requirements only -imperfectly. As we have noted in the preceding section, the alum-tanned -leathers are not very water resisting, and much of the tannage will wash -out. Leathers made by the vegetable tannages usually contain some excess -of vegetable tanning matters which are soluble in and removed by water, -though much tannin can no longer be thus removed, owing to the mutual -precipitation of the oppositely charged tannin sol and hide gel. The -necessity for fulfilling the second requirement mentioned is one reason -for the practice of following these tannages by applications of oil, fat -or of both. In this way the isolated fibres are not only dried -separately, but are coated with a typical water-resisting material. - -In the fat tannages an attempt is made to fulfil this second requirement -without the use of any specific "tanning agent" for producing the first -requirements; _i.e._ an attempt is made to dry the fibres separately in -an "untanned" condition, and to coat them simultaneously with fat so -that a permanent leather is obtained. It is only possible to do this, if -the pelt is constantly during drying subjected to mechanical working, -_e.g._ by twisting, folding, bending, drumming, staking, etc. The -resulting leather is often called "rawhide leather," and presents a real -advantage over other leathers in its great tensile strength. Where -toughness is an essential quality, there is much to be said for the fat -tannages. It is also possible, of course, to effect compromises between -ordinary tannages and the straight fat tannages; thus picking band -butts, which must be tough, are often very lightly tanned with oak bark -or chrome, and then given what is practically a heavy fat tannage. In -the most typical of fat tannages, moreover, it is often common to -"colour" the goods by a brief immersion in a weak vegetable tan liquor. -Further, the employment of fats in the currying of dressing leather is -in effect a fat tannage superimposed upon the vegetable tannage. (See -Combination Tannages, Section VI.) - -The fat tannage is undoubtedly one of the earliest methods for making -leather. Prehistoric man discovered that the skins of animals killed in -hunting could, by alternately rubbing with fats and then drying -slightly, be eventually converted into a useful leather, whereas without -the fat it was stiff and horny. Even yet similar methods are in use, -thongs of raw hide being continually twisted during drying, with -intermittent application of fats. - -In the modern fat tannages drums are used to give the necessary -mechanical working to the goods. The raw hide leather produced in the -U.S.A. is made by drumming the nearly delimed goods with tallow and -neatsfoot oil. In this country the fat tannages have been typified by -the "Crown" and "Helvetia" leathers. The hides are thoroughly limed in -mellow limes, and after the beam work are delimed by drenching, scudded, -and sometimes fleshed again, and then coloured off in tan liquor. After -partial drying, they are drummed warm for some hours to ensure isolation -of the fibres. After further drying they are coated with the tanning -paste, which consists essentially of soft fats and flour to produce -partial emulsification. Equal parts of soft fats and of flour may be -used, to which may be added smaller proportions of degras, cod oil, -mutton tallow, salt, together with about 25 per cent. water. The goods -are coated with this mixture, drummed, and dried further, and this -routine repeated as often as necessary to fill the interstices -thoroughly with fat. The temperature in the drum may reach 95 deg. F. In -finishing an attempt is made to stuff further with grease. The goods are -thoroughly set out, dried a little, and coated again, flesh and grain, -with a mixture of tallow, cod oil, glycerine and degras, and dried -further. The excess grease is slicked off and the goods again set out -and grained. They are then dried out. - - - REFERENCES. - - Bennett, "Manufacture of Leather," pp. 245, 246 and 376. - - Procter, "Principles of Leather Manufacture," p. 378. - - - - - SECTION III.--OIL TANNAGES - - -There are very obvious analogies between the fat tannages discussed in -Section II. and the oil tannages now to be dealt with, but there is -nevertheless a distinct departure in principle involved. In the oil -tannages the mechanical treatment is generally more vigorous, and the -"drying" process is conducted at a much higher temperature, with the -result that there is a vigorous oxidation of the oil. This results in -the formation of insoluble oxidation products which coat the fibre and -play an essential part in the production of a permanent leather. Pungent -vapours are evolved in the drying operations, amongst which is acrolein -and probably also other aldehydes, and it is thought by Procter that -these aldehydes also are essential tanning agents and typical of the -process (cf. Section IV.). Fahrion considers that the tanning action is -due solely to unsaturated fatty acids with more than one double linkage. -Garelli and Apostolo, however, believe that the tannage is due to a -coating of fatty acid whether saturated or not. These observers made -leather with stearic and palmatic acids in colloidal aqueous solution. - -The manufacture of chamois leather from the flesh splits of sheepskins -comprises the largest and most typical branch of the oil tannages. The -sheep pelts are split in the limed state, and the fleshes are given -another sharp liming which may last up to a fortnight. They are next -"frized," _i.e._ scraped over the beam with a sharp two-handled knife, -to remove roughness and loose fat. The goods are next thoroughly washed -in running water and drenched. A paddle drench is often preferred, and -if not used the handling should be frequent. Paddling drenching reduces -the time required from about 16 hours to about 6 hours. An hour or more -in a hydraulic press removes superfluous liquor and some more grease. -The fleshes are separated, cooled and then stocked for 30 minutes to -equalize the moisture in them. After removing from the stocks they are -sprinkled on both sides with cod oil and thrown back into the stocks for -a few hours. They are then dried cold for a day or two. The stocks used -are similar to those once popular for softening dried hides during -soaking, and consist of two heavy hammers which fall alternately upon -the goods which are contained in a curved box below. The result is a -mechanical kneading action. The fleshes are again sprinkled with cod -oil, restocked for a few hours and dried again, this time at 100 deg. F. -They are then repeatedly sprinkled, stocked and dried, the last -operation being conducted always at an increasing temperature until -finally the final "heater" is even up to 160 deg. F. As the operation -proceeds it is advantageous to hang the splits also nearer one another, -and in the final "heater" they are quite close. The next stage is to -pack the goods quickly into suitable boxes and allow them to "heat," -_i.e._ to oxidize further. This is a rather critical stage in the -process, and to prevent overheating ("burns") it is often necessary to -open out and repack into another box, with possibly some little -intermediate cooling. They are turned over thus repeatedly until the -oxidation is complete, and then spread out to cool. - -The fleshes are now a dark brown colour, and are next treated to remove -excess of oxidized oil products. The goods are dipped through water at -110 deg. F. and then subjected to hydraulic pressure. The grease and water -which exude are allowed to separate by settling, and the thick yellow -oil so obtained, known as "degras," forms a valuable material for -leather dressing, as it more readily emulsifies with water than many -oils, and impart this quality to other greases mixed with it. A further -quantity of a similar oil is obtained by paddling the goods with a weak -soda solution. The liquor obtained is treated with sulphuric acid to -neutralize the alkali, and the grease recovered is known as "sod oil." -The fleshes are now well washed with hot water (140 deg. F.), fat liquored -with cod oil and soft soap, machine sammed, either by a wringer or a -centrifuge, and then dried out. - -Much chamois leather is also made in France by closely similar methods. -The skins are usually oiled on tables and folded up before stocking. -Other marine oils (seal, whale, etc.) replace cod oil. Generally -speaking the oxidation is more moderate, and the grease from the -hydraulic press (_moellon_) is mixed with other fish oils to form -commercial degras. An inferior quality of degras is obtained by -subsequent treatment with soda. - -The crust chamois obtained as above has only to be thoroughly staked to -soften, "grounded" and "fluffed" to raise the nap, and then trimmed, and -the ordinary wash-leather is obtained. - -If intended for glove leathers superior skins are selected. These are -fluffed carefully upon emery wheels, using first a coarse surface and -eventually a fine surface so that a fine velvet effect is attained. The -skins are next bleached. - -In the "sun bleach" or "grass bleach" the goods are soaked in a 1-1/2 -per cent. soft soap solution and exposed to sunlight after being wrung. -They are bleached in about 3 days in summer, but nearly a fortnight may -be necessary in winter. - -In the permanganate bleach, which is less tedious, the skins are first -degreased by soaking in a warm 3/4 per cent. solution of soda crystals -and then drumming for 30 minutes in water at 95 deg. F. They are then -paddled in a 1/8 per cent. solution of commercial permanganate for an -hour at the same temperature, rinsed through water, and the brown -manganese dioxide is then removed by paddling or drumming the goods in a -3 per cent. solution of sodium bisulphite to which hydrochloric acid is -added as required. The goods are well washed in warm water, and are then -"tucked," _i.e._ placed in a vat of boiling water containing a little -soft soap, just for a few seconds. The goods shrink and curl up, and -they are then dried out at 120 deg.-140 deg. F. to fix the tuck. They are then -staked, fluffed, and dyed. - -In dyeing with coal tar colours the alizarin colours may be used after -mordanting with chrome alum. Direct dyes, natural dyestuffs and pigment -dyes are also used. The goods are struck out after dyeing, lightly fat -liquored with commercial egg yolk, dried out at 110 deg. to 120 deg. F., staked -and fluffed on the face side. - -Buff leather is a similar leather made from hides. They are limed mellow -for a fortnight, unhaired, fleshed, and then limed again for another -week in sharp limes. The grain is then split off, and the goods rinsed -and scudded, slightly delimed and hung up to dry. They are then treated -in much the same way as fleshes for chamois, but lime is often added to -the cod oil used in stocking. - -Buck leather is a similar product obtained from deerskins, but much mock -buck is made from cheaper raw material. - - - REFERENCES. - - Bennett, "Manufacture of Leather," pp. 247-250 and 376-379. - - Procter, "Principles of Leather Manufacture," p. 378. - - - - - SECTION IV.--FORMALDEHYDE TANNAGE - - -The use of formalin for hardening gelatin has long been known, but it -was left for Payne and Pullman to devise a commercial process for -tanning pelt into leather by means of formaldehyde (H.CHO) solutions. -Their process, which was patented, specified the use of alkalies in -conjunction with formaldehyde or other aldehydes. The function of the -alkalies is not very obvious, for it has been shown that formaldehyde -will tan also in neutral and in acid solution. The precise action of the -aldehydes is also as yet somewhat obscure, but it is noteworthy that -very small proportions of formalin will give a complete tannage. It is -probable that the action of formaldehyde is not perfectly analogous with -that of its homologues, for it is a most reactive substance, and will -certainly with proteids undergo reactions which are not analogous to -those with other aldehydes. The leather obtained by tanning with -formalin is quite white and resembles buff leather, but has advantages -over the latter in that no bleaching is necessary. - -According to the patent specifications the pelt should be drummed in -water and the tanning liquor--a solution of formalin and sodium -carbonate--added gradually at 15-minute intervals. Up to 6 hours for -light skins, and up to 48 hours for heavy hides, are required for -complete tannage. The temperature is raised during the process from 100 deg. -to 118 deg. F. The tanning liquor may be made from 16 lbs. of commercial -formalin (36 per cent. formaldehyde) and 32 lbs. soda (80 per cent. -Na{2}CO{3}) in 10-15 gallons of water. This should be added, one -gallon at a time, to 4 cwt. pelt in 100-120 gallons of water. After -tannage is complete the goods should be paddled with a 1-1/2 per cent. -solution of ammonium sulphate to remove the soda, and "nourished" in a -solution of soft soap and salt, about 2-1/4 per cent. of each on the -weight of pelt. The goods are then dried out, and may be finished like -chamois, buff, and buck leathers (Section III.). - - - REFERENCES. - - Payne and Pullman, English Patent 1898, 2872. - - Bennett, "Manufacture of Leather," pp. 250 and 379. - - - - - SECTION V.--SYNTHETIC TANNING MATERIALS - - -In spite of much valuable work on the constitution of the vegetable -tannins and the compounds usually associated with them, such as that of -E. Fischer, K. Freudenberg and their collaborators on gallo-tannic acid, -and that of A. G. Perkin on ellagic acid and catechin, we are still in -the dark with respect to the constitution of the tannins which are of -commercial importance, and any synthetic production of these materials -is thus out of the question as yet. Attempts, however, have been made to -produce artificially substances which possess similar properties to the -tannins and which may be used for converting pelt into leather. Into -this category fall some of the earlier attempts to synthesize -gallo-tannic acid by heating gallic acid with condensing reagents. - -The first commercial success in this direction was attained by Stiasny, -who produced condensation products of the phenolsulphonic acids, to -which products he gave the general name of "syntans" (synthetic -tannins). The Badische Co. placed one of these products on the market as -"Neradol D," and later took out subsidiary patents for the manufacture -of similar products by slightly differing methods of productions. Since -the outbreak of the European War such patent rights have been suspended, -and several British firms have been manufacturing synthetic tanning -materials by similar methods, but doubtless with developments and -improvements of their own discovery. These products (_e.g._ Cresyntan, -Maxyntan, Paradol, Syntan, etc.) are now in use in many factories, and -assist rather than substitute the vegetable tannins in producing leather -of the desired colour and quality. - -These synthetic tanning materials resemble the vegetable tannins in the -following respects. They are organic acids containing phenolic groups. -They are semi-colloidal, passing slowly through semipermeable membranes. -They precipitate gelatin, basic dyestuffs and lead acetate, give a -violet-blue colour with ferric salts, and convert hide into an undoubted -leather. They differ from the vegetable tannins in that they contain -sulphur and sulphonic acid groups, but they agree in that both are -aromatic derivatives. In each case the tanning effect is diminished by -alkalies, but the synthetic materials are the more sensitive. - -=Methods of Manufacture.=--There are, broadly speaking, three types of -method by which these condensation products are produced, viz., -condensation by formaldehyde, condensation by phosphorus trichloride or -similar reagents, and condensation by heat alone. Illustrative methods -will now be given. - -Condensation by formaldehyde was the first method used. The procedure is -given by the Austrian patent 58,405. A phenol, _e.g._ crude cresylic -acid, is heated with the equivalent amount of sulphuric acid for a few -hours to 100 deg.-210 deg. C., cooled, and formaldehyde added slowly whilst -cooling and stirring, in the proportion of one molecule of formaldehyde -to 2 molecules of phenol. The free mineral acid is neutralized, and the -resulting product is the syntan "Neradol." By this procedure only -water-soluble products are obtained, but an alternative process is to -heat the phenols in slightly acid solution, and then to render soluble -the resinous products obtained by treating with sulphuric acid. The -proportion of formaldehyde to phenol used led Steasny to conclude that -the resulting products were diphenyl-methane derivatives which -polymerize to form molecules of considerable size. The formaldehyde -supplies the "carbon bridge." This view was criticized by A. G. Green as -too simple, and he suggested the alternative theory that polymerization -does not take place at all, but that more advanced or higher -condensation products are formed; he thought that o-hydroxy-benzyl -alcohols were first produced, that these condensed with another -molecule, and afterwards the process was repeated. The result was a -"colourless dyestuff." This view receives some support from the other -types of method of manufacture. - -With the use of other condensing reagents the procedure may be as in the -process of the B.A.S.F. (Fr. pat. 451,875-6), thus: 225 parts of -o-cresol-sulphonic acid are heated to 60 deg. C. for 4 hours with 262.5 parts -of phosphorus oxychloride. The excess of oxychloride is removed by -distillation under reduced pressure and the residue washed with dilute -hydrochloric acid. - -Condensation by heat alone is illustrated by the method given in the -same patents, thus: phenol-p-sulphonic acid is heated to 130 deg. C. for 24 -hours under a pressure of 20 mm. or in a current of dry air at -atmospheric pressure. The product may be used direct or may be purified -by dissolving in water, neutralizing with caustic soda, filtering and -evaporating to dryness. A white powder is obtained which tans when its -solution is acidified. An alternative is to mix phenol with sulphuric -acid and heat the mixture to 140 deg. C. for 72 hours under 20 mm. pressure -and purify as before. - -=Methods of Use.=--The synthetic tanning materials may be put to many -uses. When well manufactured they make practically a white leather, and -this fact makes a valuable opening for their use in connection with -light leather tannages and the dressing of rugs. It is also claimed that -they improve the colour usually obtained in the ordinary vegetable -tannages. If used in the suspenders to the extent of 5-10 per cent. they -are said to brighten the colour throughout the tannage. If used in -bleaching and finishing they are said to lighten the colour of the -finished leather. About 5 per cent. on the weight of the goods may be -added to the bleach or vat liquors; they may be also mixed with sumac -during finishing, and in effect act as a sumac substitute; solutions are -also brushed over the grain before oiling, with a view to obtaining good -colour. It is also claimed that their use prevents vegetable-tanned -leather from becoming red under the action of sunlight. The syntans are -also used to lighten the colour of chrome leather, even of chrome sole -leather after it has been dipped. - -It is claimed also that syntans produce a tough leather, and if used for -heavy leather in the early stages they give a tough grain and assist in -avoiding a cracky grain. On this account they are also recommended for -retanning E.I. tanned kips. When used in heavy leather suspenders they -are said to get rid of lime blast (CaCO{3}) and to quicken the tannage, -_i.e._ to enable the same weight to be obtained in less time. Procter -suggests that a tannage of commercial value might be obtained by -blending them with wood pulp extract. - -If used alone for tanning a series of pits containing liquors of 4 deg. to -37 deg. Bkr. may be used, but drum tannages may be given using liquors of -14 deg.-29 deg. Bkr., the goods being tanned in 6-8 hours. About 30 per cent. of -syntans are said to be necessary for complete tannage. - - - REFERENCES. - - E. Stiasny, "A New Synthetic Tannin," _Collegium_, 1913, 142-145. - (See also _J.S.C.I._, Abs. 1913, 500.) - - E. Stiasny, "Syntans--New Artificial Tanning Materials," - _J.S.C.I._, 1913, 775. - - - Patents:--Austrian 58,405. - German 262,558, Sept. 12, 1911. - French 451,875, Dec. 13, 1912; 451,876, Dec. 13, 1912; - 451,877, Dec. 13, 1912. - - - - - SECTION VI.--COMBINATION TANNAGES - - -The formation of leather being due to the adsorption of colloidogenic -substances at the interface of the tanning liquor and the hide gel, -there is the obvious possibility that several such substances may be -used simultaneously, and that the resulting leather may be due to the -combined effect of these substances. Indeed, the average vegetable -tannage consists of such a combination tannage, each tanning material -contributing its own individual tannin and characteristic astringent -non-tannins. There is evidently also the possibility that the different -_types_ of tannage discussed above might be used either simultaneously -or successively, and that a leather might be obtained which combines to -some extent the qualities of each of the types in combination. It is -such a case that is generally called a "combination tannage." There are -many conceivable combinations, and in this section will be chiefly -discussed a few which have demonstrated some commercial possibilities. -Some of these have already received notice in the preceding sections. -The manufacture of curried dressing leathers is a combination of -vegetable and fat tannages. The manufacture of waterproof chrome uppers -illustrates a combination of chrome and fat tannages. The use of -"syntans" in conjunction with vegetable tanning materials is also a -combination tannage. The case of chamois leather is possibly a -combination of aldehyde tannage with fatty acid tannage. Two-bath chrome -leather is a combination of chrome, sulphur and fat tannage. -Formaldehyde and vegetable tannage is also a known possibility. It is -clear that there are possibilities of endless complexity, and that what -normally may appear as a simple tannage is in reality a very complex -combination tannage. From this standpoint one might instructively -consider the successive adsorptions involved in a goatskin tanned first -with syntans, then with oak bark, "retanned" in sumac, mordanted with -chrome, dyed with coal-tar dyestuffs and finally oiled with linseed oil. -It will be easily seen that in a very strict sense nearly all tannages -are combinations. - -Usually, however, the term "combination tannage" is confined to those -cases where the main tanning agents not only differ in type, but where -none are in predominant quantity. A typical case is that of "semichrome -leather," in which a vegetable tannage is succeeded by a chrome tannage. -E.I. tanned sheep and goat skins are rather heavily "stripped" of their -vegetable tannage and heavy oiling, by drumming with warm soda -solutions, and after washing with water are chromed with the one-bath -process; they are neutralized, dyed, fat liquored and finished for glace -upper leather. - -In a precisely similar way kips and split hides which have received -vegetable tannage are stripped and retanned in chrome and finished as -for box calf, of which they are a good imitation. Such vegetable-chrome -combination tannages possess many of the properties of chrome leather. - -To chrome the pelt first and afterwards to subject it to vegetable -tannage is also an obvious possibility, but has not yet been made a -commercial success in this country, but has been increasingly used in -the U.S.A. during the War. - -Another typical case of combination tannage is the dongola leather -produced by the use of gambier and of alum and salt. This is a -vegetable-alum combination, and yields a good quality leather for light -uppers, gloves, etc. Goatskins for "glazed dongola" are paddled tanned -in gambier liquors, and alum and salt are subsequently added. They are -tanned in 24 hours, well washed, and are fat liquored without ageing. -The E.I. tanned skins may also be stripped with soda, and retanned in -alum and salt, using flour also if desired. Dull dongola are first tawed -and then retanned in gambier liquor. "Suede" and "velvet calf" are also -tawed and retanned with gambier. - -Yet another case of combination tannage is that of sheepskins for glace -uppers, which are first tawed thoroughly with alum, salt and flour and -dried out for sorting, and are then retanned in chrome by the one-bath -process, and finished as usual. Closely related to this is the method of -"pickling" in alum and salt and then chrome tanning. - -Another case is the combined one-bath, two-bath method of chrome -tanning. The goods are chromed by a one-bath liquor containing -dichromate (say 2 per cent.), and then pass into a reducing bath. There -is not much advantage in such procedure, however. - -From a strictly commercial point of view the "dongola" and "semichrome" -leathers have proved the most successful combination tannages, but there -seem to be possibilities in combinations of the vegetable tannins with -synthetic tanning materials. - -Many other substances are known to tan, _e.g._ iron salts, cerium salts, -sulphur, quinones, fatty acids, the halogens, etc., etc.; hence there is -always the possibility that new useful combination tannages may be -discovered. - - - REFERENCES. - - Bennett, "Manufacture of Leather," pp. 243, 374-5. - - Procter, "Principles of Leather Manufacture," p. 236. - - - - - SECTION VII.--THE EVOLUTION OF THE LEATHER INDUSTRY - - -The leather trades are amongst the oldest of all industries, but their -evolution has been much more rapid during the last two or three decades -than at any other period of their history. The European War, moreover, -has caused the commencement of another period of rapid development, and -it is the aim of this section to point out some of the principal lines -of change which have already become apparent. - -Many of these lines of evolution in the methods of manufacture have been -previously discussed in their appropriate sections. They may all be -summarized as attempts at more economical production. Prominent amongst -them is the persistent effort to attain quicker processes. During the -last twenty-five years the time necessary to produce the heavy leathers -has been reduced from 12 months to as many weeks. The tendency is to -reduce the time further still, but this is of course increasingly -difficult to accomplish. On the other hand, it is more urgent to strive -in this direction than ever, because a needless week involves more -capital lying idle than ever before. Moreover, as most leather factories -are now large works, a saving even of 24 hours has become a serious item -in economic production. Hence in liming, bating, tanning, drying and in -warehousing there are increased efforts to make a quicker turnover. - -A good illustration of this "speeding up" in modern tanneries is the -adoption by all large factories of much more rapid methods of extracting -tannin. On the old press-leach system liquors may be percolating through -the material for possibly a fortnight. The extract manufacturer reduces -this operation to about two days. Steam generated from the spent bark is -used to heat the extracting vats, and to work a vacuum pan or evaporator -whereby more water can be used and a more complete as well as a more -rapid extraction obtained. The evaporator also makes easy the -preparation of the strong liquors used in modern tanning. - -Hand-in-hand with quicker production and manipulation are the attempts -to obtain a larger turnover. It is realized that the big business -attains cheap production. Even before the war the smaller factories were -disappearing. A small tannery must now either extend or close down. This -has been better realized in the heavy than in the light leather trades. -In the sole leather tanneries very often many thousand hides per week -are put into work, but in the glace kid factories there is nothing yet -to correspond to the output of American glace factories, which sometimes -reaches three or four thousand dozen a day. - -Another very prominent feature of factory evolution is the increased use -of labour-saving machinery. This practice has been in operation for a -considerable time, but with marked acceleration during the last few -years owing to the labour shortage occasioned by military service. This -development of machine work has largely dispensed with that labour which -involved any skill or training. The journeyman currier is now -practically extinct. In the beam house, too, fleshing, unhairing and -scudding are rapidly becoming machine instead of hand operations. Many -devices are now being adopted also which reduce the quantity of -unskilled labour needed. Instead of "handling" the goods from pit to -pit, modern tanneries aim at moving the liquors. Thus in the "Forsare" -and "Tilston" systems of liming, hides are placed in a pit and lie -undisturbed until ready for depilation, the soak liquors and lime -liquors being supplied and run off just as required, whilst these -liquors are agitated as often as desired by means of a current of -compressed air. This agitation replaces the "handling" up and down once -practised. In the tanyard proper the same tendency is at work, "rockers" -are increasingly preferred to "handlers," and an inversion of the press -leach system permits the exhaustion of tan liquors by a gravity flow, -and so avoids the handling forward from pit to pit. There is also a -tendency to install lifts, overhead runways, trucks on lines, motor -lorries, etc., to replace carrying, barrowing, carting, etc., and so to -arrange the tannery that the minimum transport is needed. - -All these lines of evolution involve more intensive production, and -necessitate much more careful supervision. It is not surprising, -therefore, that the industry now feels that scientific oversight and -administration are essential. A dozen years ago the trade chemists were -largely unqualified men, whose work lay solely in the laboratory, and -consisted mainly in the analysis of materials bought. To-day all large -tanneries have qualified chemists, and it is realized that they are the -practical tanners. Their function is so to control the manufacturing -processes that all waste is avoided, and so to correlate and co-ordinate -the manufacturing results with the analytical and experimental records -of the laboratory, that constant improvements are made in the methods of -production. The extended use of machinery, and the necessity for economy -in coal and power, give the engineer also very large scope for useful -work. Modern business conditions, moreover, have made necessary more -skilful clerical work and accountancy in the large offices of a modern -tannery. - -In the creation of cordial relationships between capital and labour in -the leather trades, there has been unfortunately little progress. The -leather trade is not a sweated industry. Its workers have always enjoyed -reasonable hours of work. In most factories an approximate 48-hour -working week (involving no night work) has long been in operation. The -industry, however, is not one in which high wages obtain. The average -tannery worker receives a wage which is never much above the level of -subsistence. This is mostly due to the fact that he is usually a quite -unskilled labourer, and is therefore on the bottom rung of the labour -ladder. In addition to this the work itself is often distressingly -monotonous, and makes little demand upon the intelligence of the worker. -The trade consequently offers little attraction to the intelligent -labourer. The old system of apprenticeship is now quite obsolete, partly -owing to the rapidity of the changes in the methods of manufacture, -partly to the specialization of labour which results from the -development of large factories, and partly also, because to understand -modern tanning involves a better general education than most workmen -receive. It is indeed frequently difficult to find competent -under-foremen for the different departments of the modern leather -factory. Until recently leather workers have been either unorganized or -badly organized, and their views and complaints have been confused and -sporadic, but during the war period there has been a very rapid -extension of trade union movements, and consequently a more articulate -expression of the demands for "democratization" as well as "a greater -share in the fruits" of the industry. In the leather trades, however, -the gulf between the unskilled labourers and the wealthy employers is -perhaps unusually wide, and there is little disposition on the part of -capital to recognize the equity of either of the above demands of -labour. Generally speaking, the leather trade firms are not public but -private companies. There is absolutely no trace of "co-partnership" or -"profit-sharing" schemes, or of co-operative production. There is little -recognition that the trades' prosperity should be shared in any way by -the workpeople, and still less recognition of any right to a voice in -industrial conditions. This condition of affairs has an ominous reaction -upon the attitude of labour, which believes that it is producing great -wealth but not obtaining much more than subsistence. It is not the -function of this volume to pronounce a verdict upon the wages question -or upon the democratization of the leather trades, but one may be -permitted earnestly to hope that if such be the future lines of -development, there will be also, as an absolutely essential part of any -such schemes, a much higher standard of education amongst the workers, -for this is the only satisfactory guarantee that the voice of labour in -council will have any practical value, or that higher wages will be at -all wisely used by the recipients. - -In his instructive and valuable volume on "The Evolution of Industry," -Prof. MacGregor points out that modern industry has evolved three -outstanding types, viz. the Co-operative Movement, the Trusts, and the -methods of Public Trading. He also suggests that these types tend to -blend. In the leather industry co-operative and municipal production -are unheard of, but the industry has certainly developed along the lines -of the large trusts. Large businesses have replaced small, and later -still have formed local federations, which in turn have combined to form -the "United Tanners' Federation." War conditions have certainly -stimulated evolution towards the trust type. The United Tanners' -Federation has become possessed of powers which were not originally -contemplated, such as the purchase and distribution to its members of -hides, bark, extract, sulphide and other materials. How far some of -these arrangements will be permanent is problematical, but one -beneficial result is that the allied trades have certainly realized more -thoroughly their unity of interests. This is shown by the much freer -collaboration of the tanners, and by the encouragement now given to -similar collaboration between their chemists. More evidence is found in -the proposals for combined research. - -There is also considerable reason to believe that there is some movement -in the direction of partial State control. There is little doubt that -evolution along trust lines will make this less difficult and possibly -more desirable. The country cannot afford the spectacle of a Leather -Trust permanently at war with a Labourers' Union. The public has -realized that the well-being of the leather industry is vital to the -national safety. It has realized that the leather trades are great -producers of national wealth, and that increased production with the -development of the export trade will materially assist to restore the -country's financial position. It has realized also its own right to -protection from bad leather and from exorbitant prices. On all these -grounds it is probable, though there may be some reaction from the -present position, that the State, which has already got its fingers in -the pie, will refuse to draw them out altogether. The Imperial aspect of -the question affords some further justification for this attitude. The -leather trades operate very largely upon imported material, and it is -clearly desirable that there should be close co-operation between the -home industry and the colonial supplies of material. Here too the war -has also given a great stimulus in this direction. Indian myrabolans has -long been a staple tanning material. South African wattle bark has -during the last few years replaced almost completely, and probably to a -large extent permanently, Turkish valonia. There has also been great -increase in the imports of Indian kips and of South African hides, and -it is not at all an impossible proposition to maintain a self-contained -Imperial Leather Trade, should this be necessary. French chestnut -extract, and quebracho extract, however, are much too valuable tanning -materials to exclude for merely sentimental reasons. These instances -indicate possible advantages in Imperial co-operation, but also show the -need for caution in the elaboration of such schemes. - -Although a partial, and indeed increasing, measure of State Control is -probable, there has been as yet no serious proposal to nationalize the -leather industry. Such a proposition, indeed, is hardly ripe even for -discussion. Until the nationalization of transport and of mines is a -proved success, and until the merely distributive undertakings of the -municipalities (_e.g._ of coal and of milk and other foods) are past the -experimental stage, any proposition to nationalize the leather trades -seems premature. It is noteworthy, however, that in Queensland, -Australia, the Government have the right to commence and to administer -State Tanneries. - -Any progress in the direction either of democratization or of -nationalization, has been certainly postponed by the sudden and -unprecedented trade slump which commenced in the earlier part of 1920. -This depression, in spite of heavy falls in the prices of raw materials, -has made economic production a much more difficult problem. It has -undoubtedly given a further stimulus to evolution towards the trust -type, and created a further tendency towards the closing of the smaller -factories, and the employment of labour-saving devices. When the general -fall in prices has made an appreciable fall in the cost of living, some -reduction in the leather workers' wages, together with more efficient -work, will also contribute to the solution of the difficulty. It is -chiefly to be desired, however, that the export trade should be -restored. The realization of this hope depends largely upon the -establishment of peace and prosperity abroad, and the consequent -stabilization of the various foreign exchanges. - - - - - PART V.--GELATINE AND GLUE. - - - SECTION I.--PROPERTIES OF GELATINE AND GLUE - - -Many of the chemical properties of gelatine, especially those which -distinguish it from other proteins, have been described in the -Introduction to this volume, and need no further comment. In this -section its colloid nature and behaviour will chiefly be considered, for -these points have greatest importance from the standpoint of industrial -chemistry. - -It is hoped, moreover, that this section will be of interest not only to -the chemist concerned in the manufacture of gelatine and glue, but that -it will be of value also to those concerned in leather manufacture. The -difference between the "collagen" which composes the hide fibre and the -high-grade gelatines is so small that for many practical purposes it may -be considered negligible. Thus the description of the behaviour of a -gelatine gel is very largely applicable to a hide gel also. - -Gelatine has been crystallized by von Weimarn by evaporating a dilute -solution in aqueous alcohol whilst in a desiccator containing potassium -carbonate, the temperature being maintained at 60 deg.-70 deg. C. The carbonate -takes up water only, and the concentration of the alcohol therefore -slowly increases until the gelatine is no longer soluble. Gelatine is -usually found and known in the colloid state, however, and its behaviour -in this state only is of practical importance. - -The fundamental idea of modern colloid chemistry is that colloids are -heterogeneous systems, usually two-phased, in which one phase is liquid -and the other phase either liquid or solid. The latter phase, which is -divided into small separate volumes, is known as the "disperse phase," -whilst the other is the "continuous phase" or "dispersion medium." The -"dispersity" is the degree to which the reduction of the dimensions of -the disperse phase has been carried, and is best expressed numerically -in terms of "specific surface," _i.e._ surface area divided by volume, -but it is also often expressed as the thickness or diameter of a film or -particle. When the dispersity is not high, we have ordinary -"suspensions" and "emulsions," which with increasing dispersity merge -into the typical colloids. By analogy, colloids have been divided into -"suspensoids" and "emulsoids," when the disperse phase is solid and -liquid respectively. The classification, however, has not been found -satisfactory, for some systems in which the disperse phase is -undoubtedly liquid, exhibit characteristic properties of suspensoids, -and _vice versa_. A more satisfactory division, therefore, is found in -the presence or absence of affinity between the two phases, the systems -being termed "lyophile" and "lyophobe" respectively. If water be the -continuous phase the terms "hydrophile" and "hydrophobe" are often used. -Broadly speaking, the lyophile colloids correspond to the emulsoids, and -the lyophobe colloids to the suspensoids. Gelatine is a typical -hydrophile colloid. - -Another fundamental idea of colloid chemistry is that the great -extension of surface involved in a high dispersity causes the surface -energy to be no longer a negligible fraction of the total energy of the -system, and that the recent advances in knowledge respecting surface -phenomena may be called in to assist in the explanation of the special -properties of the colloid state. Particles which exhibit the Brownian -movement, about 10^(-5) cm. diameter, down to the limit of microscopic -visibility (10^(-3) cm.) are termed _microns_. Particles less than this, -but just visible in the ultra-microscope (5x10^(-7) cm.) are termed -_submicrons_. Particles still less, approximately 10^(-7) cm., have been -shown to exist, and are termed _amicrons_. The dimensions of molecules -such as may exist in true solutions are of the order of 10^(-8) cm. A -colloid sol may contain particles of various sizes. Thus a gelatine sol -(like other lyophile systems) contains chiefly amicrons, but submicrons -are also observable. - - - - - 1. THE CONTINUOUS PHASE - - -Owing to the contractile force of surface tension, it is concluded that -the surface layer of a liquid is under very great pressure, much greater -than the bulk of the liquid. Any extension of the surface of the liquid -naturally causes a corresponding extension of the proportion of liquid -which is thus compressed. If in a beaker of water there be placed a -porous substance, such as animal charcoal, there is a great extension of -the surface of the water, and a corresponding increase in the amount of -compressed water. If instead there be substituted a large number of very -small particles of a substance, a still further increase in the amount -of compressed water is involved. As the specific surface of the -substance inserted is increased, and its amount, the proportion of -compressed and denser water increases also, until it is a practically -appreciable percentage of the total volume. It is clear also that the -extent of the zone of compression will be determined also by the nature -of the substance with which the water is in contact at its surface, -_i.e._ by the extent to which it is hydrophile, and this indeed may be -the more important factor. - -Now in a gelatine sol we have the necessary conditions for a system in -which the compressed water bears an unusually large ratio to the total, -owing to the enormous surface developed by the minute particles of the -disperse phase (amicrons) and to the unusually wide zone of compression -surrounding each particle caused by the strongly hydrophile nature of -gelatine. It should be pointed out that these zones of compression do -not involve any abrupt transition from the zone of non-compression, the -layer nearest the particle is under the greatest pressure, and the -concentric layers under less and less pressures, the actual compression -being thus an inverse function of the distance from the particle. Now if -there be a gradual increase in the concentration of the sol, the time -will come when these zones of compression begin to come in contact, and -the system will then show a considerably increased viscosity. With -further increase in concentration the zones of compression will overlap -throughout the system, and when the layers under considerable pressure -are thus continuous, the whole system will acquire a rigidity much -greater than water and approaching that of a solid body. This is a -gelatine gel, or "jelly." With increasing concentration the jelly -becomes increasingly rigid, and if it be eventually dried out under -suitable conditions it forms what is practically solid -body--gelatine--which, however, still contains from 12 to 18 per cent. -of water. - -It will be clear that, in the case of gelatine jellies (_e.g._ of 3-10 -per cent. strength), an increase in temperature will cause an increase -in the kinetic energy of the particles and effectively reduce the zones -of compression. Indeed, they may be reduced to such an extent that they -are no longer in contact, and the rigidity due to the continuous contact -of the layers of great compression will then disappear; as we say -usually, the jelly melts. On cooling, the decreased kinetic energy of -the water molecules results in the return of the state of compression, -with rapidly increasing viscosity and eventual gelation; as we say -usually, the jelly sets. Neither of these changes takes place at a -definite temperature (like a melting-point), and in "melting" (solation) -or in "setting" (gelation) the temperature-viscosity curve is quite -continuous. By various arbitrary devices, however, approximate melting -and setting points may approximately be determined. The results also -vary somewhat with the concentration of the gel or sol. Gels between 5 -and 15 per cent. strong melt about 26 deg.-30 deg. C. and set at 18 deg.-26 deg. C. - -On this view, we must regard a gelatine gel as a continuous network of -water under great compression, and in this network are zones of still -greater compression, which surround the particles of the disperse -phase--the gelatine itself, and zones of less compression which in a -weak gel, at any rate, have a compression equal to or much the same as -the normal state of compression in water. - -One consequence of this system is, that when a piece of gelatine -swells, there is a considerable enlargement in the zones of compression; -in other words, some, at least, of the imbibed water is compressed. Now -the compression of water means that work is done, and when gelatine -swells, therefore, we expect--and actually find--that heat is liberated -(5.7 cal per g. gel). Hence also by the Le Chatelier theorem, we -expect--and find--that gelatine swells best in _cold_ water. Further, -the compression of water involves a decrease in volume, and we therefore -expect--and actually find--that the volume of the swollen jelly is -appreciably less than the volume of gelatine plus the volume of water -imbibed. - -Another consequence of such a compressed system is that a gelatine -jelly, even in water, will have a surface tension towards water just as -the water itself has such a tension to the water vapour above the -liquid. This interfacial tension of the jelly will of course have a -contractile effect, and will tend to resist swelling and to limit it as -far as it possibly can. This force, tending to contract the jelly and -resist imbibition is therefore one of the main influences at work in the -swelling of gelatine, and is one of the two principal factors which -determine the extent of the maximum swelling when equilibrium is -established. The force tending to resist swelling is, in the ultimate, -just surface tension. Its actual magnitude depends, of course, mainly -upon the extent of compression in the dispersion medium of the gel, and -will be a resultant which is a function of this compression. The -magnitude will thus vary with the average compression in the continuous -network of compressed water. It will be obvious that as the jelly swells -the power of resisting the swelling will decrease, and the interfacial -tension with the external water will tend to disappear. If the force -tending to swell were great enough the swelling would continue until the -zones of compression were no longer in contact and the gel would become -sol. - -As suggested above, it is probable that the extent of the zones of -compression is determined by another factor in addition to the great -development of surface. That factor is connected if not identical with -that power which makes the system lyophile, and is evidently connected -also with the solubility of the disperse phase, and may indeed be -electrochemical forces tending to form a series of hydrates, or at least -to cause an orientation or definite arrangements of the water molecules -in the zone of compression. This idea receives some support from the -hydrate theory of solution, and the zones of compression and orientation -are the colloid analogue of the hydrates supposed to exist in solutions -of electrolytes. The extension of such zones on cooling are then -analogous with the series of hydrates formed, for instance, by manganese -chloride with 2, 4, 6, 11, or 12 molecules of water when crystallized at -temperatures of 20 deg., 15 deg., -21 deg., -30 deg., and -48 deg. C. respectively, the idea -being that the salts most hydrated in solution crystallize with most -water. - -As the compression is the result of two factors, one of which depends -upon the nature of the disperse phase, we expect--and find--in other -lyophile systems a considerable variation in their power of gelation. -Some indeed, though very viscous, _e.g._ egg albumin, never quite set -like gelatine, and others (_e.g._ agar-agar) set to a stiff gel from a -much weaker sol than gelatine. When the zones of compression are large, -as in gelatine, the magnitude of the compressing force on the outermost -part of the zone is relatively small, and it is not surprising that time -is necessary for the victory of this force over the kinetic energy of -the water molecules. Hence we find a 5 per cent. jelly sets readily on -cooling, but its elasticity increases steadily for many hours after it -has set. This phenomenon, known as hysteresis, we should expect--and -find--to be much more marked in a case where the zone of compression is -unusually large (_e.g._ an agar gel). We should also expect--and -find--that hysteresis is more marked in a high-grade gelatine than in a -low-grade gelatine where both eventually form gels of equal elasticity. -We should expect too--and we find--that hysteresis is more prominent in -weak gels than in strong. These points are of obvious importance in -testing gelatine by its elasticity, _e.g._ the well-known "finger test." - -There are also other facts and considerations which have an important -bearing upon the point under discussion. It is necessary ultimately to -regard true solutions of electrolytes and other bodies as heterogeneous, -though perhaps of a rather different order. From this point of view -molecules and ions existing in an aqueous solution will present a -surface and have associated zones of compression analogous with those -suggested for the minute particles of gelatine. - -Now recent investigations have shown that the essential physical -properties of water are affected by dissolved substances in a definite -manner and to a fixed extent, and that these substances exhibit a -sequence in order of their effect. This sequence is also exhibited in -the essential properties of water as solvent and as dispersion medium -for colloid sols. The sequence is known as the "lyotrope series." Thus -the numerical value of the compressibility of aqueous solutions is -reduced below that of water by salts which, with the same kation, -exhibit an effect in the following order:-- - - CO{3} > SO{4} > Cl > Br > NO{3} > I - -This same order is observed, in the effect on the increased values for -the surface tension, density and viscosity of these solutions. On the -other hand, the kations have a similar sequence of effects, - - Mg < NH{4} < Li < K < Na < Rb < Cs - -which appears when salts of the same anion are chosen. It is not -surprising to find that this lyotrope series exhibit an analogous -influence on the chemical reactions of water, _e.g._ the hydrolysis of -esters. In the hydrolysis by acids SO{4} retards the action, the other -anions and the kations accelerate it, in the lyotrope order. In the -hydrolysis by bases the series is reversed. Similarly the lyotrope -series exert the same order of effect upon the inversion of cane sugar -and other reactions. - -This lyotrope influence has also been shown to exert considerable effect -in the behaviour of lyophile sols. With the lyophobe sols the addition of -foreign substances apparently affects the disperse phase only, but with -the lyophile sols the effect on the continuous phase is also important, -and may overshadow the other. Now, in gelatine and in hide gels and -tanning sols we are dealing with lyophile systems, and there are many -points of behaviour in which lyotrope influences become prominent. -Similar effects are observed upon other lyophile sols (_e.g._ albumin, -agar-agar, etc.) which differ widely in chemical nature. Thus the -salting out of albumin (reversible precipitation) is influenced by -sodium salts in lyotropic sequence as follows. The anions hinder -precipitation; in order of precipitating power they are: - - citrate > tartrate > SO{4} > acetate > Cl > NO{3} > ClO{3} > I > CNS - -The sulphates illustrate the kation effect, which is independent and -which favours precipitation: - - Li > K > Na > NH{4} > Mg - -If the experiments be carried out in faintly acid solution this order of -effect is exactly reversed, iodide and thiocyanate having the greatest -effect and citrates the least. The coagulation temperature of albumin -and the coagulation by other organic substances are similarly influenced -by the lyotrope series. - -Lyotrope influence also exerts a powerful effect on the behaviour of -gelatine sols and gels. The gelation temperature is influenced thus:-- - - raised by SO{4} > citrate > tartrate > acetate - - lowered by Cl < ClO{3} < NO{3} < Br < I - - The kation effect (small) is Na > K > NH{4} > Mg - -Other lyotrope substances raise or lower the temperature thus:-- - - glucose > glycerol--(H{2}O)--alcohol < urea - -The effect on gelation is also illustrated by the change of viscosity of -the sol with time. The same lyotrope order is found. - -In the salting out or precipitating of gelatine with salts, the order of -anions is lyotrope: - - SO{4} > citrate > tartrate > acetate > Cl - -Also the osmotic pressure of gelatine sols is markedly lowered by -neutral electrolytes in lyotrope sequence: - - Cl > SO{4} > NO{3} > Br > I > CNS - -Similarly lyotrope influences are shown in the modulus of elasticity: -substances which favour gelation increase elasticity, whilst substances -which favour solation decrease elasticity. The order is again lyotrope. - -The permeability of the gel is affected by lyotrope influences; alcohol -and glycerol reduce diffusion through gelatine (or agar); and urea, -chloride and iodide increase it. (Similarly the diffusion of sols -through "semipermeable" membranes is affected by lyotrope influence.) -The lyotrope series also influence the optical activity of gelatine sols -and the double refraction of strained gels. - -The swelling of gelatine (and other gels) is very strongly influenced by -the lyotrope substances and merits more attention than it has received. -Hence this lyotrope influence exerts a profound effect in the -manufacture of gelatin, and perhaps even greater in the manufacture of -leather. This is only to be expected. If a gel comprise a continuous -network of compressed water, as suggested above, the presence of other -substances in the gel which cause increases or decreases in the -compression must modify accordingly the properties which depend upon -this state of compression, such as the viscosity of the melted gel, the -rate of gelation, the elasticity of the gel, and the rate and extent of -its imbibition. This indeed we find to be the case. Now the substances -which affect the compressibility, surface tension, etc., of water -_least_, _i.e._ the substances producing little or no compression of -water, are just those which reduce the compression of water in a -gelatine jelly, and cause a decreased viscosity, elasticity, surface -tension, etc., and which therefore naturally allow the gel to swell more -than in pure water. Conversely, the substances which cause the greatest -compression of water, the greatest increase in its surface tension and -viscosity, are also the substances which increase the compression, -viscosity, elasticity, and surface tension of gels, and which therefore -hinder imbibition. The effect on swelling is as follows:-- - -Sodium sulphate > tartrate > citrate > acetate; > alcohol > glucose > -cane sugar; (water) chlorides-potassium < sodium < ammonium; -< sodium chlorate < nitrate < bromide < iodide < thiocyanate < urea. - -As the amount of compression will depend upon the amount of substance, -we expect--and find--that the effect is usually additive, and that -suitable mixtures of substances having an effect in the opposite sense -will produce no change. - -The interpretation of lyotrope influence is of course somewhat -speculative, but considered as a surface phenomenon, the surface -specific of the molecules and ions of the lyotrope substance must be one -of the factors involved. One naturally also connects the effect with -solubility and the tendency to form hydrates in solution, the zones of -compression being zones of orientation and of electrochemical -attraction. The hydrate theory of solution again affords an instructive -commentary. The fact that, broadly speaking, the polyvalent anions and -the monovalent anions also group themselves together, suggests that -electrical forces are at work, and the order of effect of monovalent -anions almost suggests that what are called "residual valencies" are in -operation. It is difficult to resist the conclusion that in the lyotrope -influence, in the crystallizing of salts, and in the formation of a gel, -we have zones of compression and orientation which are manifestations of -the same forces--surface and electrical; the chief differences in the -case of gelatine being that the zones are larger and that the electrical -effect is perhaps of less definite magnitude. - -However these things may be, the fact of water compression determines -the rigidity of the gel, and the changes in this compression of the -continuous phase determine the surface tension resultant which hinders -swelling, and which is one of the two main factors fixing both the rate -at which gelatine swells in water, and the final volume attained by the -gel. - -Before leaving this point, it is desirable to note the effect on the -swelling of gelatine of the extremes of this lyotrope influence. -Substances like iodides, thiocyanates and urea prevent a gelatine sol -from setting to a gel at all, and a piece of gelatine in such solutions -swells rapidly until it solates. On the other hand, sulphates, -tartrates, etc., make a stiffer gel on account of the enhanced -compression. Gelatine in such solutions may swell, but at a much slower -rate than in water and with a decreased maximum extent. A gelatine gel -may in such solutions not only fail to swell at all, but actually -contract and in some cases, indeed, be practically dehydrated. If a gel -be in a very concentrated solution of such a substance, it may be that -the lyotrope compression in the external solution is greater than the -compression in the dispersion medium of the gel; in which case the -surface tension effect is reversed, and the external solution tends to -increase in volume and the gel to contract. Hence we find that the -saturated solutions of such substances as ammonium sulphate and -potassium carbonate will dehydrate a gel almost completely, and will -also, by a similar action on pelt, make a kind of white leather. It is -important to remember this contractile effect of strong solutions of -salts, because it is very easy to confuse this effect with a similar -result produced in another manner, viz., by a reduction of the force -tending to swell. - - - - - 2. THE DISPERSE PHASE - - -A very important feature of the colloid state is that the particles of -the disperse phase appear to possess an electric charge, and if this -charge be removed a colloid sol no longer remains such, but -precipitates, flocculates, coagulates, etc. As to the origin of this -charge several theories have been advanced, but the most generally -accepted is that it is a result of the adsorption of electrically -charged ions by the particles of the disperse phase. The enormous -specific surface possessed by this phase renders it particularly liable -to such adsorption. This view harmonizes well also with the general -behaviour, of colloid sols and gels, in endosmosis, kataphoresis, -precipitation, etc. According to this point of view the particles of the -disperse phase are surrounded by a surface layer in which these ions are -in much greater concentration than in the volume concentration of the -dispersion medium. The hydrion and hydroxyl ion are particularly liable -to such adsorption. In the case of a lyophile colloid, like gelatine, -the charge may be either positive or negative, according to the nature -of the predominant ions in the dispersion medium, and the amount of -adsorption is determined by the concentration of these ions in -accordance with the adsorption law. - -In effect, therefore, the particles of the disperse phase each carry an -electric charge of the same nature, and as similarly charged bodies -repel one another, the particles of the disperse phase will tend to -separate and to occupy a bigger volume. It is the author's opinion that -this repulsion of similarly charged particles is the cause of the -swelling of gelatine. The amount of charge and force--tending to -swell--is due possibly to several ionic adsorptions, which may be -considered to operate independently, and the power of repulsion is -determined by the nett charge, which in the case of a "positive colloid" -is positive, and in the case of a "negative colloid" is negative. As -ions possess different electric charges, the charge on the disperse -phase is subject to the valency rule. - -Now the repulsive force between two similar and similarly charged bodies -is proportional to the amount of charge and is inversely proportional to -the square of the distance between them. The amount of charge on a -colloid particle will be determined by the dispersity--best signified by -the specific surface (s)--and by the operation of the adsorption law - - y = mac^(1/n) - -The distance between the particles varies with the degree -of swelling, and is determined by the cube root of the volume of the gel -(_v_). Hence if F be the force tending to make the gelatine swell, we -may write - - F = Q/(d^2) = (sy)/v^(2/3) - -Now with all electrolytes, even with water, we have both positively and -negatively charged ions, and y is consequently determined by the -difference in the amounts adsorbed. Hence in the case of an electrolyte -with an equal number of oppositely charged ions -y = ma{1}c^(1/n{1}) - ma{2}c^(1/n{2}), where a{1}, a{2}, and -n{1}, n{2}, are the appropriate constants for the particular ions -concerned. Hence at constant temperature, pressure, etc., we may write - - F = [ sm( a{1}c^(1/n{1}) - a{2}c^(1/n{2}) ) ] / v^(2/3) - -The force tending to make a piece of gelatine swell is proportional to -its mass, which is perhaps fairly obvious. The swelling force is also an -inverse function of the volume of the gel, and as swelling proceeds -therefore the force tending to swell further decreases. The force -tending to swell is proportional to the specific surface of the disperse -phase, other factors being constant. To illustrate this one has only to -imagine that one particle of the disperse phase be split into two -particles each carrying half the original charge. It is clear that a new -repulsive force becomes operative, which did not before influence the -swelling, and that the distance between the particles is halved. In the -swelling of gelatine, however, we may consider the dispersity constant -for constant temperature, and if we consider unit mass we see that the -force causing swelling depends upon the operation of the adsorption law -and upon the degree to which the gel is already swollen. - -In the swelling of (say) one gram of gelatine to its maximum, both the -contractile force of surface tension and the expanding force of -electrical repulsion are in operation. At the commencement the latter is -much the greater force--hence the rapid imbibition. Both these forces -decrease in magnitude as the swelling proceeds, but the force tending to -swell decreases at a more rapid rate, and the time comes when it has -decreased to the precise value of the force tending to resist swelling. -At this point equilibrium is established and the maximum swelling -attained. Obviously this maximum will in many cases be determined -largely by the value of a{1}c^(1/n{1}) - a{2}c^(1/n{2}). -This factor, therefore, demands particular consideration. - -Now, unfortunately, the adsorption law constants for the different ions -have not yet been numerically determined, so that we are still somewhat -in the dark as to the operation of ionic adsorptions. It is -possible, however, to form conclusions of a qualitative or relative -order, and these are such as to throw much light upon the question at -issue. In the first place, we know that in general the various ions are -not usually very widely different in the extent to which they are liable -to be adsorbed. If this were otherwise, the valency rule would hardly -operate so well in endosmosis, kataphoresis, and precipitation. In -consequence we must expect the differences between the ions to appear in -small rather than in large concentrations, the amounts adsorbed being -under those conditions more affected by changes in the volume -concentration. At the larger concentrations, therefore, the value of -a{1}c^(1/n{1}) - a{2}c^(1/n{2}) is small, and the force causing -swelling often tends to zero. - -There are, however, noticeable differences at lower concentrations. Thus -we know that if a substance be primarily a positive colloid, it will -absorb kations more readily than anions. As gelatine falls into this -class, we may therefore conclude that usually a{1} > a{2}. Further, -it often happens that very adsorbable substances are less affected by -concentration changes, and in the case under consideration, therefore, -we should expect that n{1} > n{2}. Moreover, we know that the hydrion -and hydroxyl ion are much more readily adsorbed than other ions, _i.e._ -have a large value for _a_. Hence in the case of gelatine we expect that -a{1}c^(1/n{1}) - a{2}c^(1/n{2}) will have a comparatively large -value when one of the ions is H+ or OH-. Also we know that organic -anions are usually much more strongly adsorbed than inorganic anions, -and hence that in such cases a{1} is more nearly approached by the -value of a{2}. It should be emphasized perhaps, at this point, that -these various considerations are not based upon any facts relating to -the phenomena of imbibition in gels, or in gelatine in particular, but -are based upon the behaviour of colloids in endosmosis, kataphoresis, -electrolytic precipitation, adsorption, etc. - -[Illustration: FIG. 1.] - -Now if we select a few simple figures which are in accord with the above -considerations, we can examine the value of the factor -a{1}c^(1/n{1}) - a{2}c^(1/n{2}) in a purely illustrative and -typical way, and at any rate form some idea as to the manner in which it -is likely to vary. The figures might be:-- - - - Ion. | _n_. | _a_. - -------------------------+---------+--------- - Hydrion _or_ hydroxylion | 20 | 10 - Kation of a metal | 15 | 7 - Organic anion | 10 | 8 - Inorganic anion | 6 | 6 - - -For the sake of simplicity we can assume that these ions are all -monovalent. The ions adsorbed by unit mass will then be 10c^(1/20), -etc. If these hypothetical adsorption isotherms be plotted as usual we -get the fairly typical curves shown in Fig. 1. - -Now in practice there are always two of these ions, each giving its own -specific effect in opposite senses, and the difference -( a{1}c^(1/n{1}) - a{2}c^(1/n{2}) ) represents the nett charge -adsorbed. Hence we have the following combinations:-- - - - Inorganic acid 10c^(1/20) - 6c^(1/6) - - Organic acid 10c^(1/20) - 8c^(1/10) - - Alkali 10c^(1/20) - 7c^(1/15) - - Inorganic salt 7c^(1/15) - 6c^(1/6) - - -If we plot these values of nett adsorption against the concentration we -obtain the curves shown in Fig. 2. - -[Illustration: FIG. 2.] - -On the assumption that the nett charge adsorbed is the dominant factor -in determining the maximum swelling at equilibrium, one must therefore -regard the curves of Fig. 2 as representing the changes in volume of the -swollen gel as the concentration is increased. Now in _type_ these -curves correspond to those obtained by experiment from hydrochloric -acid, acetic acid, caustic soda, and common salt. The maximum swelling -with hydrochloric acid increases rapidly with the concentration at first -and then rapidly decreases, though not at such a great rate. The -swelling with acetic acid increases less rapidly and to a less maximum, -but decreases more slowly. With common salt there is a slight swelling -followed by contraction. Caustic soda gives a rapid increase in volume -at first, afterwards much less so, and finally yields an exceedingly -slow decrease. The correspondence of these facts with the type-curves -inevitably suggests that the phenomenon of swelling might be accounted -for, in part at least, along these lines. - -Of course it is not likely that the simple figures selected for the -illustration of the argument are either relatively or absolutely -correct. Thus we know that the adsorption curve for hydrions and -hydroxylions are not likely to be quite identical, as assumed above. As -gelatin is primarily slightly positive, it is probable that the values -of _a_ and of _n_ for hydrion adsorption will be relatively slightly -greater. The relative values supposed, however, are near enough to -illustrate the contention that the type of the maximum volume curve can -be explained on this assumption of different adsorption isotherms for -each of the ions. - -If the remarks on the compression of the continuous phase be recalled, -it will be obvious that in the present paragraphs we have been giving -the question of equilibrium-volume a rather one-sided consideration. The -volume of the gel when equilibrium is established may be determined in -type by the nett charge adsorbed by the disperse phase, but it will be -modified also by the lyotrope influence of the particular substance on -the continuous phase. When gelatine swells in solutions the influences -on both phases are always in operation, and either upon occasion may -become predominant. In the case of neutral organic substances, such as -cane-sugar, the lyotrope influence is the determining factor. In the -case of neutral salts the predominant influence is decided by the place -occupied by the salts in the lyotrope series. If at either end of the -series the lyotrope influence is uppermost and the effect of ionic -adsorptions is practically swamped. Thus sodium sulphate and sodium -iodide hinder and promote imbibition respectively as could be expected -from their strong lyotrope power. On the other hand, in the case of -sodium chloride, which has comparatively feeble lyotrope influence, the -relatively different adsorptions of its ions comes to the fore. With -acids and alkalies the relatively large adsorption of the hydrion and -hydroxylion causes this to be the predominant influence, but we must -concede the possibility that purely lyotrope influences may be at work -in some cases, and especially at the greater concentrations. Indeed, it -is sometimes a difficult problem to decide whether an increase or -decrease in swelling is due to lyotrope or adsorptive influence, but, -broadly speaking, we can expect strong lyotrope effects at either end of -the series and also at large concentrations, and we can expect strong -adsorptive effects in dilute solutions, in the middle of the lyotrope -series and in the case of alkalies and acids. - -For much of the above explanation of the nature and behaviour of -gelatine, the author must himself take responsibility, and in this -section he has freely quoted from his own papers upon the subject (see -References). He claims that his view of a gelatine gel as involving a -network of compressed water, liable to modification by lyotrope -influence upon the continuous phase and by ionic adsorptions of the -disperse phase, is most in harmony with the recent advances in our -knowledge of colloids; that much of the theory is a necessary corollary -of those discoveries; and also that he has found this view to be a sound -guide in practice, both in tanning and in gelatine manufacture. - -Many other theories have been advanced, but most are generalizations -over too limited a field, and from experiments with only a few -substances, and show little or no correlation with the wider facts of -colloid behaviour. That of Procter, for example, discards altogether the -idea of a two-phased structure of the gel as an "unproved and rather -gratuitous assumption," dismisses surface tension considerations as -"more complicated and less verified," and adsorption as "wholly -empirical," whilst it ignores lyotrope influence and the analogy with -agar gels completely. Procter's theory applies mainly to the swelling of -gelatine by acids, which swelling he considers to be due to the osmotic -pressure of the anion of a highly ionizable salt formed by the chemical -combination of the acid with gelatine. On this assumption, mathematical -considerations show that the electric charge on the gelatine is given by -the expression z = sqrt(4ex + e^2), where z = the amount of ion taken -up, x the concentration of the surrounding solution, and e the excess -concentration of diffusible ions in the jelly. - -The property of gelatine and glue which is chiefly used in classifying -them into grades of different commercial value, is the strength of the -jelly obtained as compared with any arbitrary standard gelatine. An -enormous number of other physical tests have been devised, but none are -nearly so simple or so reliable. Gelatine is unfortunately very liable -to hydrolysis even by water, and long before any amido-acids, etc., have -appeared there is a change to a not greatly hydrolyzed product -(sometimes called [beta] gelatine) which has lost the power of setting -to an elastic gel. It is thus the lyophile nature which has been -altered, and the fall in elasticity corresponds to the fall in power of -compressing water, which is proportional to the concentration of [alpha] -gelatine. Now the elasticity of a gelatine gel varies as the square of -the concentration. Hence if one so arranges the concentrations of -standard and unknown samples that gels of equal elasticity are obtained, -the concentration of [alpha] gelatine is the same in both gels, and the -_relative_ amounts of [alpha] gelatine in the original samples are -inversely proportional to the weights used to give gels of equal -elasticity. The "strength" of a gelatine or glue is therefore usually -stated as the number of grams of a standard gelatine which will yield a -gel with elasticity equal to that from 100 grams of the gelatine or glue -being tested. Elasticity is matched by lightly pressing with the -finger-tips. - -It is also possible to grade samples of gelatine and glue by the -estimation of "peptones," whose amount indicates the degree of -hydrolysis. Nitrogen is estimated by Kjeldahl's method in the sample and -in the precipitate obtained by saturating a solution with zinc sulphate. -The difference is calculated as peptones by multiplying by 5.33. Trotman -and Hackford say that the results are in the same sequence as those of -the finger test. The method, however, is much more laborious than the -"finger test." - -Gelatine is also graded according to the results of bleaching and -clarifying, but with quite arbitrary standards, largely determined by -the fancy of the customer. - -Chemical analyses, involving estimations of ash, lime, fat, acid, water, -insoluble matter, and poisonous metals, _e.g._ arsenic, copper, zinc and -lead, are of value for special cases according to the destiny of the -goods. Special physical tests, such as "breaking strain" and "foam -test," are also of some little value in special cases. - - - REFERENCES. - - "The Chemistry of Colloids," W. W. Taylor. 1915. - - "Handbook of Colloid Chemistry," W. Ostwald. 1919. - - "Chemistry of Colloids," Zsigmondy and Spear. 1918. - - "Introduction to the Chemistry and Physics of Colloids," E. - Hatschek. - - "Surface Tension and Surface Energy," Willows and Hatschek. - - "Chemistry of Colloids," V. Poeschl. - - "Grundzuege d. Dispersoid Chemie," von Weimarn. - - "The Lyotrope Series and the Theory of Tanning," Bennett, - J.S.L.T.C., 1917, p. 130. - - "The Swelling of Gelatine," Bennett, J.S.L.T.C, 1918, p. 40. - - "The Swelling of Gelatine," Procter, _J.C.S. Trans._, 1914, =105=, - 313; and _Koll. Chem. Beihefts_, 1911, =2=, 234. - - "The Swelling of Gelatinous Tissues," Procter, J.S.C.I., April 16, - 1916. - - "Summary of Procter's Views, and Bibliography," Collegium (London), - p. 3, 1917. - - "Lyotrope Influence and Adsorption in the theory of wet work," - Bennett, J.S.T.C., 1920, p. 75. - - For the "finger test," see-- - - "Glue and Glue Testing," Rideal, 2nd ed., p. 158. - - "Leather Trades' Chemistry," Trotman, p. 241. - - - - - SECTION II.--RAW MATERIALS AND PRELIMINARY TREATMENT - - -The raw materials for the manufacture of gelatine and glue may be -classified according to their origin. The preliminary treatment, which -comprises chiefly purifying and cleansing operations, is varied -according to type of manufacturing process for which it is a -preparation. - -In the case of hide or =skin gelatine=, the raw material is a -bye-product of the leather industry. After the hides or skins have -passed through the preparatory processes which convert them into "pelt" -(see Part I., Section II.), they are so trimmed that all that is left -will make a useful leather. These "trimmings" or "roundings" include -ears and noses, the udders of cows and heifers, and also include parts -from the butt, belly and shanks which are collectively termed "pieces." -The operation of fleshing (Part I., Section II.), in which fat and flesh -are cut from that side of the hides and skins which was next the flesh, -also involves cutting into the collagen to some extent, and these -"fleshings" comprise another very large class of raw material. The -fleshings obtained by hand labour contain distinctly more hide -substances than those obtained by machine work, and their commercial -value to the gelatine manufacturer is of course proportionate to the -collagen content. Some hides and skins are split in the pelt (Part I., -Section IX.; Part II., Sections II., III. and IV.), and the "flesh -split," though sometimes made into leather, is also used in making -gelatine, a high quality being obtained from such material. Minor -sources of material are tendons and cartilages, and also hides and skins -which have been too much damaged by partial putrefaction or by accidents -to make sound leather. Of course the material from the hides for heavy -leathers form the greater bulk of raw material for skin gelatine which -is thus derived principally from ox hides but sheep and goat skin pieces -have also an important place. The skins of other animals, such as dogs, -cats, hares and rabbits not usually made into leather can also be -depilated and used for making skin gelatine and glue. Horse hide -fleshings and pieces are sometimes used, but are notorious for the poor -quality of their product. They seem to contain less [alpha] gelatin. All -these materials are of course readily putrescible and must be put "into -work" without much loss of time. When it is impossible to convey them -from the tannery to the gelatine factory quickly enough, _e.g._ foreign -material, the "glue stock" is dried out completely and sold in that -condition. In the manufacture of pickers from limed pelt there is some -superfluous material, and this is cut into shavings and dried. This -"picker waste" also forms a useful source of raw material. Skin gelatine -material is not very strong in gelatine-substance. The fleshings, -pieces, etc., contain much water, even up to 80 per cent. This, however, -is very variable, and only a practical test or a hide substance -determination can indicate the commercial value of any particular -material. This value, moreover, is determined not only by the yield and -quality of the gelatine which can be obtained, but also by the yield of -grease, the valuable bye-product. - -The preliminary treatment of material for skin gelatine consists -essentially of liming and of washing. The object of each process is to -purify. Liming has much the same action on hide pieces, etc., as on -hides, and indeed the liming treatment is somewhat superfluous on -cuttings from well-limed hides. The material is plumped up and the -partially hydrolyzed products are taken into solution. Lime also acts as -mild antiseptic, stops any putrefaction and liberates ammonia formed by -fermentation in transit to the factory. When plumping is particularly -wanted (as in wetting in dry stock) caustic soda is sometimes used as an -assistant (_cf._ dried hides). Sodium sulphide has also been used -for this purpose. The liming is in brick pits, an excess of undissolved -lime being always used. It is advantageous frequently to disturb or -agitate the goods in the lime pits. Up to ten weeks liming has sometimes -been given, but about three weeks is now generally considered -sufficient, and the tendency is to shorten the time. The lime and soda -have also a detergent action on soiled stock, and they probably assist -in hydrolyzing the pigments of the hair roots and sheaths. They also -saponify and emulsify the grease, and it is obvious, therefore, that -liming can be carried too far. Slaked lime, of course, must always be -used. - -After liming the soaked, softened and plumped stock is washed as -thoroughly as possible. To do this it is necessary to supply repeated -batches of clean cold water. Some manufacturers, however, use the warm -water from the evaporators. Wooden vats or brick pits with arrangements -for agitation, for draining off and for inspection, are used for this -purpose. The agitation may be carried out by means of revolving shafts -or drums with projecting curved spokes or vanes. An American patent -(Hoeveler's glue stock washer) involves the use of a paddle wheel. It is -combined with a settling tank to gather particles of stock. In the -washing the chalk, excess lime, dirt, etc., are quickly removed and a -slow deliming process is commenced. The sediment from the washers and -wash waters has some value in making fertilizers. Deliming cannot be -carried on further than certain limits by water alone. Hence acid is -often added to finish off the process. Hydrochloric acid has the -advantage of forming soluble salts, but if they are not removed -completely their lyotrope influence is to weaken the gelatine. Sulphuric -and sulphurous acids are even cheaper, and the lyotrope influence of -their salts is in the opposite sense. The latter also has the advantage -of destroying sulphides, an important advantage for food gelatines. -Whatever acid is used, however, it is evident that an abundance of pure -cold water is the fundamental requirement of a pure product. It is a -sound maxim in gelatine manufacture to avoid, if at all possible, the -addition of any soluble substance, for it is always present in a more -concentrated state in the finished article. Thus if its solubility be -even moderate, one is likely to attain supersaturation in the "cake" -and consequently a dull product. Further, lyotrope influences can never -strengthen a gel very much, but may and often do weaken it very -considerably. Hence the aim of most manufacturers in the preliminary -treatment is so to delime that a nearly neutral and salt-free product is -obtained. An exception is the case of skin gelatine in which excess of -sulphurous acid is used. This process has for its object not only -deliming and purifying, but also a bleaching action. - -In the case of =bone gelatine=, the raw material is such that there are -much longer and more elaborate preparatory processes. This arises from -the fact that about half the bones of animals consists of mineral -matter, chiefly calcium phosphate. Bones, of course, vary in composition -to some extent, and those from younger animals contain distinctly less -of the mineral constituents. Approximately speaking, bones have the -following average composition:-- - - - Gelatinous matter 21-1/2 per cent. - Fat 12-1/2 " " - Calcium phosphate 48 " " - Calcium carbonate 3 " " - Alkali salts, silica, etc. 2-1/2 " " - Water 12-1/2 " " - -------- - 100 " " - - -It will be seen, therefore, that the manufacture of bone gelatine and of -a comparatively large proportion of phosphate involves the recovery and -purification of much fatty matter. The manufacturing processes are -naturally subject to considerable variation. One respect in which they -differ is the stage in which grease is removed. Sometimes this is simply -done as the need and occasion arise, and it is skimmed out in the acid -or water extractions, but it is now more usual to have a special -"degreasing" process. There are, moreover, two quite distinct types of -manufacture. In one of these (the boiling process) the routine bears -some resemblance to that for skin gelatine. In this process the bones -are washed and cleansed and then immediately subjected to extraction -with water. This removes the gelatinous matter and leaves the phosphate -and earthy matters behind. Grease may be removed before the water -extraction, but is also sometimes removed by skimming off during the -extraction, as is usual in the case of skin gelatine. This procedure is -now not much favoured unless only a low-grade glue is required. In the -other type of manufacture (the acid process) the material is first -degreased, and then the mineral matter is extracted or dissolved by -acids, leaving the gelatinous matter behind for subsequent refinement -and solution. The acid process has long been preferred for high-class -bone gelatine, and hence needs further discussion. - -The degreasing operation was once brought about by steaming only, but is -now accomplished with the assistance of fat solvents. - -The object of cleansing is not only to remove dirt, but also fleshy -matter which often adheres to the bones. This may contain a little -gelatine, but consists mainly of other proteins and insoluble fibre, -neither of which are wanted in the water extraction. The mill consists -of a large cylinder of stout wire gauze. This revolves round the axis of -the cylinder, and the bones are fed in at one end by a hopper and are -discharged at the other. The revolution of the mill causes the friction -which polishes off the fleshy matter. The dirt and flesh fall through -the gauze and are sent to the fertilizer factory. The polishings are -sometimes further separated by a similar machine. Raw bones may thus -yield nearly 60 per cent. of degreased bones, and about 56 per cent. -cleansed bones ready for extraction, and 3 or 4 per cent. "bone meal." - -The next stage is the extraction of the mineral matters by acid, for -which purpose hydrochloric acid has proved very suitable, as both -phosphate and carbonate of lime are dissolved by it. The usual -counter-current system of extraction is used [_cp._ Leaching and extract -manufacture, Part I., Section III., p. 35]. The process is methodical -and regular, the acid liquor passing successively through a battery -of six vats in such a manner that the liquor richest in lime salts comes -into contact with the bones most recently charged; the fresh acid thus -acts upon the nearly extracted bones. The hydrochloric acid used is of 8 -to 10 per cent. strength (5 deg. to 7 deg. Be.). Stronger acid is apt to -hydrolyze ("rot") the gelatine, whilst weaker acid takes longer time. -The process takes 8 to 10 days, though up to 14 days is sometimes given, -and, on the other hand, the process has been occasionally reduced to 4 -days. The gelatinous matter undissolved has the shape of the original -bone, but is much swollen. When the acid liquor is saturated with lime -salt, the liquor is drawn off from below the vats and sent to the -phosphate precipitation tanks. The phosphate is usually precipitated by -adding just sufficient milk of lime to neutralize the hydrochloric acid. -The precipitated phosphate is then well washed by decantation to remove -calcium chloride. It is then drained, and dried at a low temperature. As -a large bulk of phosphate is obtained it is often filter-pressed and -dried quickly in long revolving chambers through which a current of air -is passed. The phosphate is sometimes also precipitated by ammonia. It -is then more easily washed and dried, and the ammonium chloride is -recovered and may be used to regenerate ammonia, or be sold as a -valuable bye-product. Sometimes the acid liquor is not used for making -precipitated phosphate, but is evaporated with animal charcoal and -silica and then distilled to make phosphorus. - -The next stage is the purification by washing of the gelatinous matter -which remains. The vat is filled up with pure cold water and the -material allowed to steep for six or seven hours. The acid and salts -remaining diffuse outwards into the water. This is drained off and -replaced by fresh water, and the procedure repeated half a dozen times -or as often as necessary. The end is said to be determined by the -absence of a precipitate on adding silver nitrate to the wash water, or -by the absence of any action on blue litmus paper. It will be seen, -however, that there are two actions involved, one being the removal of -calcium chloride and the other the removal of excess acid. The former -is the easier, and is almost necessarily brought about by the latter. -Hence in some factories the neutralization is brought about, therefore, -by the addition of a certain quantity of soda, or more usually by lime, -and the material is sometimes submitted to a veritable liming by which -it remains in milk of lime for about three weeks, the lime liquor being -renewed several times. The product is finally washed again to remove -excess lime. This is carried out in a rotating vessel through which -passes a continuous stream of water. If a slightly acid gelatine is -required, however, the lime and liming are both superfluous, and the -procedure is simply to wash as thoroughly as possible and then to -immerse the material in a 1 per cent. sulphurous acid solution for 3 -hours to bleach, and then to proceed with the water extraction or -solution of the gelatine. The hydrochloric acid used for these processes -should be as pure as possible, and the degreasing as thorough as -possible, for, if not, a gelatine with a bad odour is liable to be -obtained. - -Instead of using hydrochloric acid for the solution of mineral matter, -sulphurous acid is sometimes employed, and has the advantages that its -bleaching effect is thereby obtained throughout the process, and that it -is recoverable for subsequent use. The Bergmann process, most generally -favoured, is described very concisely by Rideal thus: "A sulphurous acid -solution is made to circulate over the bones in a series of closed -tanks, the solution being continually enriched with sulphurous acid from -a cylinder of the liquefied gas. The resulting liquor, containing an -acid calcium phosphate and calcium bisulphite, is heated by steam in a -leaden digestor, when the excess of sulphurous acid is liberated and -passes back to the tanks, while neutral calcium phosphate and sulphite -are precipitated. The latter is decomposed by an equivalent of -hydrochloric acid, setting free the remaining sulphurous acid, which is -returned to the tanks, leaving calcium chloride in solution, and neutral -calcium phosphate in suspension." Not more than 5 per cent. of -sulphurous acid is said to be lost in this process, and the gelatine is -more thoroughly bleached. It is subsequently well washed before -extraction. - -=Recovery and Purification of Grease.=--The degreasing operation, which -is applied usually to bones and to skin glue scutch, was once brought -about by steaming only, but is now accomplished with the assistance of -fat solvents, though in the latter case steaming together with -mechanical centrifugal force has proved sufficiently successful. On the -Continent carbon disulphide was once largely used as solvent, and in -this country benzene has been employed, but their low volatility and -high inflammability, as well as their expense, make both these -substances somewhat unsuitable, and it is now usual to make use of -petroleum oils, whether Scotch, American or Russian. A fraction which -boils about the same temperature as water is usually employed, and all -of it must be volatile under 280 deg. F. Before the actual grease extraction -the bones should be sorted over and unsuitable substances (horns, -gravel, iron, etc.) removed. They are also usually put through a mill -and roughly crushed or broken. The actual grease extraction plant -consists of large copper vessels which will each take 5 tons of bones. -These extractors are arranged in sets so that the degreasing is -proceeding in some whilst the others are being emptied and recharged. -The doors for charging and emptying must be securely fastened. When the -extractor is charged the solvent is run in and heated by a steam coil -which eventually causes it to distil. After some hours the remainder, -which has dissolved much grease, is run off, and a fresh lot of solvent -is added and heated up. After four such extractions only about 1/4 per -cent. of grease remains in the bones. To remove the remainder of the -solvent high-pressure steam (80 lbs.) is blown through the bones. The -extractor is then opened and the degreased and somewhat dried bones are -mechanically conveyed to the cleansing mill. The grease solutions -obtained are subjected again to steam with a view to removing the -solvent and obtaining it for repeated use in this sense. The efficient -distillation and recovery of the solvent is indeed an essential element -in the success of the process. - -The greases obtained, whether by the use of fat solvents or by skimming -off during extraction, or in any other way, are mixed together as is -appropriate to their origin and purity, and subjected to further -purification, the object of which is to remove gelatinous and albuminous -matters, and to decompose lime or soda soaps. The precise methods of -purification are, of course, dependent mainly upon the impurities known -to be present, but the readiest method is to give the grease further -steaming or boiling with water, and so effect by washing and by solvent -action the elimination of non-fatty matters. In many cases it is found -advantageous to employ mineral acids or oxidizing agents to assist the -process. The process may be repeated as often as is desired. - -The recovered and purified greases are often of a high standard of -purity, and the best are quite fit for edible purposes. The large -extension of the margarine industry in this country has indeed caused a -larger proportion than ever of this bye-product to be so used. In some -cases it is found commercially advantageous to submit the grease to -action of the filter press, and so to separate it into solid and liquid -portions, the former containing a much larger proportion of stearin, and -the latter of olein. Much of the grease from the gelatine trade is also -found suitable for soap manufacture, and is therefore a valuable source -of glycerine. - -=Other Raw Materials.=--Whilst hide pieces and fleshings, and animal -bones, comprise the principal raw material for the manufacture of -gelatine and glue, there are also minor sources of raw material which, -though often not suitable for gelatine manufacture, will yield a -satisfactory glue. Thus the skins, bladders and bones of fish form the -source of "fish glue." Sole skins, indeed, when deodorized by chlorine -and decolorized by animal charcoal, are made into gelatine. The bladders -of some fish (_e.g._ the sturgeon) are washed, purified and dried with -rolling to make "isinglass," a form of natural gelatine in which the -original fibrous structure is retained. There is a limited demand for -this material for clarifying purposes by brewers, wine merchants and -cooks. - -Leather waste may sometimes be used to make a low-grade glue. -Vegetable-tanned leather offers much difficulty unless very lightly and -recently tanned. The tannage must be stripped by drumming with weak -alkalies, _e.g._ borax, sodium sulphite, or weak soda. Chrome leather -may be stripped easily and completely by Rochelle salt and other salts -of hydroxy acids (Procter and Wilson), and also by ammonia acetate, -oxalate and similar salts (Bennett), also by certain organic acids -(Lamb). Processes are patented by which chrome leather is digested with -lime to make glue, the chromium hydrate being insolubilized. Viscous and -tenacious substances are also obtained from some vegetable matters and -are called "glue." - - - REFERENCES. - - "Glue and Glue Testing," S. Rideal, D.Sc., 2nd ed.; Skin Gelatine - and Glue, pp. 25-48; Bone Gelatine and Glue, pp. 59-66. - - "Gelatine, Glue and their Allied Products," T. Lambert, pp. 11-52. - - "Encyclopedie chimique," Fremy, tome x. - - - - - SECTION III.--EXTRACTION - - -The term "extraction" is applied to that essential process by which the -gelatinous matter from whatever raw material is used, is actually -dissolved in water and removed from the rest of the material. Extraction -is often termed "boiling" or "cooking." Whether one is treating hide -fleshings and pieces or whether one is dealing with raw or acidulated -bones, the general principles of extraction are much the same, and most -of this section is equally applicable to any class of material. - -The chief principle of extraction is so to arrange the process that both -the material and the extracted liquor are maintained at high -temperatures for the shortest possible time. As we have observed, -gelatine is readily hydrolyzed by hot water, and as hot water is needed -for its extraction or solution, care must be taken to remove the -solution as soon as possible from the source of heat. In practice this -can only be done somewhat imperfectly, as it is necessary to obtain a -gelatine sol of several per cent. strength before removing it from the -extraction vessel. The stronger this sol is made before removal, the -less the time, trouble and expense is incurred in evaporation -subsequently, but the more is the exposure to heat with consequent -weakening of the gelatine. Hence in practice it is necessary to -compromise. The matter is complicated further by the necessity of -obtaining a clear sol, for which it is desirable that the sol obtained -in extraction should not be too concentrated, as impurities settle and -filter much more readily from weaker and less viscous sols. - -It will be understood, therefore, that whatever material is being -extracted, the most favoured procedure is to extract in fractions. The -first fraction, which is least exposed to hydrolytic decomposition, -produces the highest quality products, and the subsequent fractions -(nearly always two more, and sometimes several) yield products which -gradually become of inferior quality owing to the number of times the -raw material has been re-heated. - -Within limits, the precise temperature of extraction does not have the -importance one would expect. Lambert suggests the temperature of 185 deg. F. -as suitable for both skin and bone gelatine, and most manufacturers -would, on the whole, endorse this. If, however, a higher temperature be -preferred, the hydrolytic action is increased in intensity but decreased -in its time of operation, whilst if a lower temperature be adopted the -decomposition is retarded in speed, but is increased in totality because -of the longer time needed to obtain a suitable strength of liquor. Thus, -with care, much the same result is obtained by extraction at near -boiling-point for a short time as by extraction at 160 deg. F. for a long -time. The higher temperatures have the definite advantage of speed, -whilst the lower temperatures have the advantage that one may choose to -be satisfied with a weaker extract, and so gain a little in the strength -of the gel, by throwing more work on the evaporator. One other point -should, however, be borne in mind in this connection, viz. that a -gelatine sol kept at temperatures above 185 deg. F. begins to deteriorate in -colour. Whilst, therefore, much depends upon the precise class of -material, it is broadly true to say that the higher temperatures are -advantageous for glue, whilst the lower temperatures are preferable for -the highest quality gelatine. - -Extraction in open vats is used both for skin and bone gelatine. It is -usually preferred when it is intended to extract at the lower -temperatures, and it is usually adopted also when the material is such -that the extraction is comparatively rapid, as for example in the case -of skin gelatine and bones by the acid process. The vats themselves are -often constructed of wood, in which case they are heated by a copper (or -brass) steam coil. They may be constructed also of iron, cast or -wrought, the former being cheaper, less liable to corrosion, but more -liable to fracture. In the case of iron vessels the heating may also be -done by a steam coil beneath a false bottom, but it is sometimes -arranged that iron vats are heated by a steam jacket, and even by a -hot-water jacket. Heating in either wood or iron vessels has been -brought about by direct application of raw steam, but the results are -both uncertain and unsatisfactory owing to local overheating. Whatever -appliances are used agitation of the material or liquor is advantageous. - -Extraction in closed vats is also used. This is generally associated -with extraction at higher temperatures, and more often also with the -manufacture of glue than of gelatine. It has been used on the Continent -for skin glue, and in this country for bone gelatine and glue by the -"boiling" process. In this system of working the vessels are usually -made of 3/8-inch steel plates, and will take a charge of 3 to 5 tons of -material. It is claimed for the system that there is a lessened steam -consumption as well as lesser manipulation, that strong liquors are more -easily and quickly obtained, and that the material may be more -thoroughly exhausted. Extraction is sometimes made by steam and water -playing alternately on the material, but many manufacturers prefer the -use of direct steam, keeping the pressure at 15 lbs. for about 2 hours. -The pressure is then reduced considerably and the process finished off -by spraying the material with water. From such a procedure a 20 per -cent. glue sol may be obtained. It is common to work such extractors in -couples or in batteries of four to six. It will be readily understood -that the process is suitable for making bone glue when the phosphate has -not been dissolved. The high temperature is in this case almost -necessary to ensure thorough extraction. It will be equally clear that -the process is not so suitable in the manufacture of a strong gel. - -As alternatives to the systems of fractional extraction, several -processes have been devised in which the extraction is continuous. - -Amongst these is the tower system, in which the material is placed upon -a series of perforated shelves arranged inside a steam-tight cylinder or -tower. Water is admitted from the top and trickles down over the -material whilst steam is admitted from the bottom. Superheated steam is -sometimes used. The material may thus be digested with a minimum amount -of water, and the sol passes out of the apparatus and from the action of -heat soon after it is formed. From bones the sol obtained is of good -colour, but is somewhat dull. Several variants of this process have been -patented. - -Another continuous system of extraction is that involving the use of the -Archimedean screw. The material is fed into one end of a cylinder -carried along and discharged at the other end by the screw. The cylinder -is of metal gauze and is steam jacketed. (Lehmann's patent, 1912.) - -Continuous systems, involving a battery of digestors connected by pipes, -have also been devised. Arrangements are made of course for admitting -water and steam as required. - - - REFERENCES. - - "Glue and Glue Testing," by S. Rideal, D.Sc., 2nd ed., pp. 47-56 - and 61. - - "Gelatine, Glue and their Allied Products," by T. Lambert, pp. - 21-24, 40, 42-44, 49 and 51. - - "Encyclopedie chimique," Fremy, tome x., p. 83. - - - PATENTS. - - Edison: U.S.A. patent, 1902, 703204. - - Bertram: English patent, 1892, 951. - - Dorenburg: German patent, 1911, 239676. - - Lehmann: French patent, 1912, 441548. - - - - - SECTION IV.--CLARIFICATION AND DECOLORIZATION - - -After the raw material has been appropriately prepared and an aqueous -extract or gelatine sol obtained therefrom, there are certain -refinements necessary before the weak sol is evaporated. These purifying -processes include (1) clarification, (2) decolorization, and (3) -bleaching. Whilst most manufacturers have more or less successfully -solved the problems involved in these processes, the practical methods -that are in common use have been evolved and elaborated in a purely -empirical way, and the underlying principles have been very imperfectly -recognized, and indeed often confused and misunderstood. Hence it is -even yet not uncommon to find these terms rather loosely used, and it is -one aim of this section to define and distinguish these various -operations in principle as well as in practice. - -Clarification consists essentially in the removal of suspended matters, -with the consequent production of a sol or gel which is bright, clear, -and apparently homogeneous. Bleaching consists essentially in destroying -the colouring matters of the sol by chemical action, such as oxidation -or reduction. Decolorization involves the removal rather than the -destruction of colouring matters, and does not therefore imply a -chemical action in the ordinary sense. - -Clarification may be now considered more particularly. It is necessary -in this connection to consider what is meant by "suspended matter." The -modern view is that the difference between a true solution and a muddy -liquor or an emulsion is one chiefly of degree. If the particles of -matter in suspension or emulsion (the disperse phase) be reduced in -size they eventually merge into colloidal sols which are sometimes -analogously named "suspensoids" and "emulsoids," if further reduced in -size into "suspensides" and "emulsides," and with further reduction into -true solutions. On this view not only suspensions and emulsions, but -also sols, solutides and solutions are all heterogeneous. Now in -practice the clarifying of a gelatine sol involves only the removal of -the particles which are evident to sight. What is needed is that the -product should make a sol or gel which to the naked eye appears to be -optically clear both to reflected and to transmitted light. If desired, -the limit could be expressed in terms of dispersity or specific surface. -Now it is a comparatively easy matter to remove the coarser substances -which often pass into the sol, _e.g._ undissolved portions of raw -material or the insoluble portions, such as the hair, the grain (hyaline -layer), and the elastic fibres of skin gelatine material, and the fibres -which even remain in extracting acidulated bones. A more difficult -proposition is the removal of still finer particles which may be almost -said to be in colloidal solution, but which at any rate are so large -that they cause a visible opalescence or even a turbidity of the -gelatine sol. A more difficult task also is the removal of minute -particles of grease, which are an exceedingly common cause of turbidity -and which are often very effectively emulsified in the sol. - -Now at this stage it is necessary to point out that besides the -difference in the size of the particles of the disperse phase, there is -another important difference involved, viz. that the particles of a -colloid sol carry an electric charge owing to the adsorption of -electrically charged ions of the electrolytes (salts, acids or alkalies) -present. If this charge be removed the colloid is precipitated -(coagulated, flocculated) and is then filtered off with comparative -ease. This precipitation can be brought about by a reduction or -elimination of the potential difference between the disperse phase and -the continuous phase. The electric charge given by the adsorbed ions may -be reduced by dilution, for dilution causes a lessened adsorption of the -charging ions. Hence the well-known practical fact that it is more -satisfactory to filter a dilute gelatine sol. Further, the electric -charge may be reduced also by causing the adsorption of an ion of -opposite charge. This is the principle underlying the precipitation (of -any colloid) by adding electrolytes. It is essential here to consider -which ions are most likely to be adsorbed, and also to bear in mind what -charge they carry. Now the hydrion (H+) of acids and the hydroxyl ion -(OH-) of alkalies are most strongly adsorbed, so that to precipitate a -negative sol, acid is very effective, whilst with a positive sol an -alkali is an appropriate precipitant. Further, it is known that organic -ions are usually more strongly adsorbed, hence when precipitating from -an alkaline sol (negative sol), one should preferably select an -inorganic or mineral acid rather than an organic acid. Thus in -clarifying an alkaline gelatine sol, hydrochloric or sulphuric acid is -to be preferred to acetic or lactic acid. Again, it is necessary to -remember that a divalent ion carries twice the charge of a univalent -ion, hence the precipitating power of an electrolyte depends upon the -valency of the ion whose electric charge is opposite to that on the sol -(Hardy's valency rule). Thus a negative sol is most easily precipitated -by a monobasic acid. Thus hydrochloric acid is better than sulphuric, on -account of the stabilizing effect of the divalent SO{4}-- ion on a -negative sol. In such a sol, also, the valency rule indicates that the -multivalent kations, _e.g._ iron, Fe+++; chromium, Cr+++; and aluminium, -Al+++, should have great precipitating and clarifying effect. This of -course is known to be the case, aluminium salts having long been used. -The rule indicates, also, that aluminium chloride would be better than -the sulphate or than potash alum. Another feature of precipitation -worthy of mention is the phenomenon of "acclimatization." This describes -the fact that when the precipitating reagent is added very slowly, or a -little at a time, a larger amount must be used, and the slower the -addition the greater the excess required. Hence in precipitating matters -from an alkaline gelatine sol the acid, if practicable, should be added -all at once. In any case it is clear that one should aim at filtering a -gelatine sol when it is near the iso-electric point, which is stable -enough for gelatine itself, but a point of instability for many -undesired impurities. Yet another phenomenon of colloid chemistry is -concerned, viz. "protection." The particles it is desired to precipitate -not only adsorb ions of electrolytes, but also the gelatine sol itself, -and the particles, thus covered by a layer of a stable emulsoid sol, -attain much of the stability of this gelatine sol. Unfortunately for -gelatine manufacturers, gelatine possesses very great powers as -"protective colloid," and this no doubt greatly enhances the practical -difficulty of obtaining a clear and bright sol or gel. Here again -dilution of the sol reduces the adsorption and correspondingly reduces, -to some extent, the difficulty. - -With regard to the turbidity or opalescence in a gelatine sol due to -minute globules of grease, the case presents some analogy to the coarser -colloid solutions, but the analogy has its limits, for an emulsion of -grease is not an emulsoid sol. Doubtless the grease globules exhibit -adsorptive phenomena, in which case the valency rule comes into force; -the gelatine, also, by lowering interfacial tension, assists in -protecting the emulsion; but grease emulsions are certainly stabilized -in alkaline media (hence the detergent effect of soap, soda, borax, -etc.), and it is undoubtedly easier to separate the emulsion by making -the medium acid. Hence the practical fact that an acid sol is more -easily clarified from grease than an alkaline or even than a neutral -one. - -The next stage in clarification is the separation of precipitated -matters and of the coalesced particles of grease. This may be attained -by the two processes usual in such a problem of chemical engineering, -viz. sedimentation and filtration. After precipitation, therefore, the -sol should be allowed to stand for some hours, during which time the -precipitate not only flocculates but also settles to the bottom, and the -globules of grease coalesce further and rise to the top, from which they -may be skimmed off. Sedimentation alone is both too slow and too -incomplete to be sufficient for proper clarification, and in these days -it is always supplemented by the use of the filter-press. This -well-known appliance can easily be adapted to the local requirements of -the manufacturer. As speed of working is an essential requirement it is -necessary to have a large filtering surface, and this may be done either -by increasing the number of plates in the press or by increasing the -area of the plates used. The large plates, however, are often cumbrous -and inconvenient, and if of metal are very heavy. The plates may be -constructed of well-seasoned wood, or in the case of alkaline gelatine -and glues, even of iron. The framework is in any case usually iron. Acid -gelatines and glues may have wooden plates, but "acid-proof" alloys are -sometimes used to make them. Where it is essential to filter quickly two -presses may be arranged _in parallel_, thus doubling the active -filtering surface. When it is essential to obtain the highest possible -clarity, two presses may be worked _in series_, which, in effect, means -that the sol is filtered twice. In using the filter press for gelatine -and glue it is most necessary to observe the most scrupulous -cleanliness, and the plates must be frequently washed and sterilized. -Rideal recommends weak chlorine water or bleaching powder solution for -this purpose. - -The process of _decolorization_, by which colouring matters are removed -without being chemically altered or destroyed, usually precedes or takes -place concurrently with the filtration. The underlying principle of this -operation is adsorption. The colouring matters are usually in colloidal -solution and most frequently are emulsoids, hence they are substances -which are known to be exceedingly susceptible to positive adsorption. It -is probable, also, that in a gelatine sol are particles which cause -turbidity, though not coloured, and which are capable of being adsorbed. -Hence the adsorption of colouring matters not only makes the sol more -colourless, but in all probability makes it brighter and clearer. -Further, decolorization by adsorption probably also involves the removal -of the last traces of emulsified grease. It will be clear, therefore, -that in the improvement in brightness and colour of a gelatine sol, -adsorption fulfils a triple usefulness. The ordinary processes of -dyeing fabrics or leather are adsorption processes, and the -decolorization of gelatine sols consists essentially of the same -process, except that the concentration of the dyestuff is much less, and -the liquor remaining, instead of the adsorbent, is the primary -consideration. - -Decolorization of gelatine sols may be effected by any substance with a -large specific surface. Indeed, a great variety of adsorbents are -actually used in practice, and each factory has its favourite material -or mixture, and its favourite mode, place, and time of application, -determined partly by the nature of the adsorbent and partly by the -precise form of apparatus used. Amongst the adsorbents which have -received special favour are sand, kieselguhr, asbestos, animal charcoal, -wood pulp fibre, albumin and alumina. Sand is very effective, but a -comparatively large weight is needed, and its cleansing for repeated use -is troublesome. On the other hand, it may be completely renovated by -ignition. Kieselguhr is a very powerful adsorbent, and only a little -will do much good; it is, however, hardly sufficient alone. Animal -charcoal has great specific surface, but its pores are very small for -viscous liquors, and its use is less suitable in the case of gelatine -than in the decolorization of liquors which may be boiled. Wood pulp -fibre is a very popular decolorizing material, not only in gelatine but -also in other trades. Its short, woolly fibres give a clarifying as well -as a decolorizing effect. It may thus act as a mechanical filter for -suspended matter and grease, as well as an adsorbent for colouring -matters present as sols. Its two functions, however, are often confused. -It may be regenerated for repeated use by careful washing, and special -pulp-washing machines are manufactured and sold for the purpose. -Detergents are usually employed in the wash waters. Asbestos is also a -good adsorbent, and its long fibres make it much less liable to -non-operating "channels" and "bursts." It also has the advantage that, -if desired, it may be regenerated by ignition. It forms a very useful -mixture with pulp fibre. - -All the above decolorizing materials are insoluble and hydrophobe, and -act in virtue of their finely divided conditions, which causes them to -have a large specific surface; but there is another type or branch of -substances, whose effect is due to surface action of rather a different -type. These are the hydrophile gels. In a gelatine sol the colloid -particles have largely adsorbed the colouring matters which it is -desired to remove. This adsorption, which is after all only an -equilibrium, is reduced by introducing another very strong adsorbent. -This latter, by adsorption from the continuous phase, reduces the -adsorption of colouring matters by the gelatine particles. In the case -under discussion another lyophile colloid is introduced, and after -bringing about such an action is removed by appropriate means. The use -of albumin has long been known for such a purpose, its special advantage -being that after its admixture and adsorptive action, it may easily be -removed by raising the temperature above 70 deg. C., when coagulation takes -place, and by subsequent mechanical filtration. The coagulated albumin -takes down the adsorbed colouring matters. Albumin has been used in this -way not only for gelatine and glue liquors, but also for tanning -extracts (Part I., Section III.) and other commercial preparations. Into -this class of decolorizing agents fall the insoluble inorganic gels -which have been advocated by W. Gordon Bennett, _e.g._ alumina cream. -Freshly precipitated alumina hydrate is a colloid gel with very -considerable adsorptive powers. It has also the advantage that it is -quite insoluble, easily removed in filtration, and has a powerful -adsorptive action upon other objectionable impurities, especially the -poisonous metals, arsenic, copper, zinc and lead. Its use is an -undoubted advantage when in addition to the other clarifying agents and -adsorbents. It is conceivable, in some cases, that when alum is employed -as clarifying agent in an alkaline gelatine liquor, some alumina may be -formed, and as such contribute to the total effect. - - - - - SECTION V.--BLEACHING - - -The adsorption law indicates that however much colouring matter is -removed from the volume concentration (continuous phase) there must -always be some left. After all that the decolorization processes can do, -there still remains much colour that can only be removed by a chemical -action of the ordinary sense. The amount of colouring matter of this -kind is not large, but it is a deep red-brown, and when the gelatine sol -has been evaporated and dried out the final product, if untreated, -possesses this typical colour, and is known as glue. If, before -gelation, a chemical bleaching action is applied to destroy this -pigment, the product may be then dried out in a nearly colourless -condition and is known as gelatine. Gelatine, therefore, is simply -bleached glue. Many other definitions have been given, and many -elaborate distinctions drawn, but the fact of bleaching is the essential -difference. In these days when gelatine is so valuable, the higher-grade -products are nearly always bleached, and the term "glue" is consequently -more often applied to a lower-grade product, and is sometimes used in a -sense implying this fact. - -If it be desired to manufacture gelatine, it is fairly obvious that the -task is lightened by observing the axiom that prevention is better than -cure. If steps are taken to prevent the presence or development of such -colouring matter, a great advantage is attained, for not only is the -problem of bleaching easier, but also quicker and less expensive in -chemicals. The nature of the colouring matters is but imperfectly -investigated, but in the case of skin gelatine the pigment of the hair -roots and epidermis is doubtless one factor. A long liming is said to -assist in its destruction, possibly because this completes the loosening -of epithelial structures and possibly because the alkali causes some -hydrolysis of the pigment. In both skin and bone gelatine sols, however, -there is a considerable tendency to develop the brown colouring matter -typical of glue. This tendency is enhanced by an increase in temperature -and also by the presence of acid or alkali. These facts seem to indicate -that its development is associated with a partial hydrolysis of the -gelatine in some direction. Rideal says this colouring matter is allied -to caramel. In harmony with this is the experience that its development -is greatest in products which have been "burnt," _i.e._ subjected to -unusually high temperature. The practical maxims which arise from these -considerations are fairly obvious and widely known, viz. to conduct the -extraction and evaporation at as low a temperature as possible and in as -neutral a condition as practicable. The temperature is particularly -important during evaporation (see Section VI.). - -Fortunately for manufacturers of gelatine, the colouring matter to be -attacked is very susceptible both to reduction and to oxidation, and -both types of bleach are widely used in practice. It is somewhat curious -that the same colouring matter should be destructible both by reduction -and by oxidation, but there is no doubt that each type gives a perfectly -satisfactory bleaching action and can result in a practically colourless -gelatine. On the other hand, the reduction is the more unstable -reaction, for the glue colour slowly develops again in the gelatine on -keeping it, even in a dried condition. Gelatine bleached by oxidation, -however, retains its colour quite well, and even tends to improve with -keeping. It is quite possible that quite different reactions are -involved in the two processes, but in the light of the above facts it is -somewhat surprising to observe Rideal's statement that reduction -followed by oxidation has been successful in practice. - -Although there is a wide choice of reducing and of oxidizing agents, -those which are suitable for application to gelatine cover a very -limited field. This limitation arises not so much from the -ineffectiveness of the bleach, as from the other effects of these -substances upon the purity of the product and upon the elasticity of the -gel which it can yield. Especially important is the lyotrope influence -of the bleaching agent. Many reactive substances are ruled out simply -because they either insolubilize the gelatine or weaken the gel it -makes. Others are inadmissible on account of their poisonous nature. It -must never be forgotten that whatever is used in bleaching is, like the -gelatine itself, much concentrated during evaporation and drying. Its -possible percentage in the finished product should be considered, and -also the possibility that in these finishing operations what is present -may not remain in solution, owing to supersaturation. - -=Bleaching by Reduction.=--Of all the reducing agents suggested, -sulphurous acid has proved to be much the most suitable and successful. -It has been used with equal success both for bone and for skin gelatine, -but on the whole has proved more suitable for the former. - -Sulphurous acid can fulfil in this instance a double function, viz. that -of acid solvent for the bone phosphate, and that of bleaching agent -also. As it penetrates the bone material, dissolving the phosphate, it -also exercises its bleaching influence on the gelatinous part of the -material. Changes of liquor tend to complete both actions, so that a -counter-current system is found most convenient. The "acid process" for -the manufacture of bone gelatine has been previously described (Section -II.), and the use of sulphurous acid in this connection is typified in -the Bergmann process. In this process bleaching is in effect merely a -continued treatment. - -In the case of skin gelatine, also, sulphurous acid may fulfil a double -function, viz. that of deliming agent as well as of bleaching agent. In -such instance it is necessary to use excess of bleaching acid, some -acting as deliming material and the remainder as bleaching agent. As it -is desirable to get rid of the lime and soda salts, several changes of -liquor are given to the goods, possibly with intermediate washing. Here -again approximation to a counter-current system is of advantage, as the -employment of used bleach liquors for deliming purposes effects -considerable economy of sulphurous acid. Indeed, there need be no waste -acid at all. - -Whether the material be for bone or skin gelatine, however, it will be -seen that the extraction is conducted in an acid condition and the -resulting sol is also acid. Most usually the decolorization and -filtration processes are also conducted with such an acid sol. From what -has been said (Section IV.) of the value of dibasic inorganic -acids as clarifying agents, it will be understood that the presence of -sulphurous acid at this stage is of great advantage in the production of -a clear and bright gelatine. Indeed, it is well known in trade circles -that sulphurous acid gelatines are usually of exceptional clarity and -brightness. - -The disadvantage of sulphurous acid processes is also found in the same -fact that both sol, gel and cake are in an acid condition. To complete -the bleach it is sometimes necessary to add sulphurous acid to the sol -after extraction, or even after evaporation, but this is to be avoided -if possible. Usually the ideal attempted is that the bleaching action -should be as much as possible before extraction; the excess of -sulphurous acid is then washed off just before the extraction, as far as -practicable, and the rest is boiled off during extraction. The ideal is -practically never attained, for the acid is strongly adsorbed, and the -result is that the finished article is always an acid gelatine, and -sometimes indeed very decidedly such. The acid condition is -objectionable in the case of some forms of filter press on account of -the solvent action on the metals, and is objectionable in evaporation -for similar reasons. Acid gelatines are also objectionable for many -purposes for which gelatine is usually sold, and this limits the -commercial possibilities of the product thus obtained. - -Sulphurous acid is itself, of course, a gas, and whilst the gas itself -has been used for treating the material (_e.g._ bones), it has been -found not only more convenient but also more effective to use an aqueous -solution. This is mainly because it is possible to attain a greater -adsorption in a liquor. Unfortunately, however, sulphurous acid is not a -very soluble gas, and although 8-10 per cent. solutions may be, with -great care, obtained, they are really supersaturated and readily yield -the gas, even with slight mechanical agitation. Solutions even of 2 to 3 -per cent. strength are also liable to this, and the general experience -is that 1 to 2 per cent. solutions are most economical and convenient -for practical purposes. As the freight on weak solutions is prohibitive, -the manufacturer using sulphurous acid is faced with the necessity -either of purchasing cylinders of sulphur dioxide liquefied by pressure -or making the gas and solution himself. The former is the most -convenient course when only small amounts are required, but the latter -preferable for a gelatine factory of any size. Sulphurous acid is easily -manufactured by burning sulphur and leading the fumes by induced draught -up a scrubber down which water slowly trickles. Forced draught may also -be used, as in the Sachsenburg plant. - -Of the other reducing agents which have been used, sodium hydrosulphite -(Na{2}SO{2}) deserves mention. It is a very powerful reducing agent, -and has been found most useful when employed as an assistant to -sulphurous acid. This reagent is usually added to the sol, after -evaporation and before gelation. It is sold as a white powder, usually -under trade names. Sometimes a mixture of bisulphite and powdered zinc -replaces it, but this is objectionable for pure food gelatines. Its use -also involves an impurity in the finished article, and a greater amount -of "inorganic ash." - -=Bleaching by Oxidation.=--Many oxidizing agents have been suggested for -bleaching gelatine, but most of them have some practical disadvantage. -Most of them contradict the maxim (previously noted) that it is -desirable to avoid adding any soluble substance, as this involves a -permanent impurity, possibly concentrated to supersaturation in the -finishing processes, and possibly involving a disadvantageous lyotrope -influence. There is another objection to oxidizing agents also; whilst -their bleaching action on the pigments is undoubted, some of them have -also a special action upon the gelatine itself which is in reality akin -to tanning, and may indeed involve an insolubilization of the gelatine. -Thus, chlorine gas (which Meunier patented for tanning) has been used -for bleaching gelatine, but the conditions of success have not yet been -thoroughly elucidated, and it is problematical indeed whether the -process is consistent with best results. Hypochlorites and bleaching -powder have also a similar action, which has been utilized with some -success in practice. Rideal suggests that a suitable concentration for -these reagents is 1:2000, and emphasizes the care necessary. An -advantage of all these chlorinations is the formation of the strongly -antiseptic chloramines, which preserve the gelatine from putrefaction. -Ozone has also been tried as an oxidation bleach for gelatine, but not -successfully, partly on account of difficulties in controlling the -quantity used. Peroxide of soda has also been used, but it is not only -alkaline, but liable to contain sodium hydrate and carbonate as -impurities, and this involves neutralization either before use or in the -gelatine sol, and the consequent presence of sodium salts in the -finished article. Peroxide of calcium is open to the same objections, -except that calcium is more easily removed from the sol than sodium. -Rideal's suggestion for removing this lime, viz. precipitation by a -current of carbonic acid, merits attention in this and in other -directions also. Rideal also states that in the case of an acid bone -gelatine, a good peroxide of lime is almost an ideal reagent for -bleaching, inasmuch as "the lime carries down phosphate, several -impurities and colouring matters." It thus acts as bleach, as -neutralizing agent, and as precipitant, and the precipitate itself is a -strong adsorbent. On account of its freedom from bases, and because its -residue is simply water, peroxide of hydrogen has been found of great -service in practice, and in most factories it has shown itself superior -not only to the other peroxides, but also to all other oxidizing -agents. Its application is simple, a concentrated solution being added -to the gelatine sol before or after evaporation. It is the most -"fool-proof" of all the oxidizing agents used in bleaching, and it -yields the purest product. Its bleaching action is perfectly -satisfactory, but only in a non-acid sol. Hydrogen peroxide is -moderately stable in acid solution, and its bleaching action is best in -slightly alkaline solution. An acid sol bleaches too slowly, or not at -all; an alkaline sol induces evolution of oxygen and consequent waste. -The great disadvantage of peroxide of hydrogen is its great expense, -which is enhanced by an increasing demand for it in other industries. A -minor disadvantage is its instability, which leads to loss in transit -and storage. It is sold usually in strengths indicated by the volume of -oxygen obtained from unit volume of the solution, when treated with -permanganate in a nitrometer (_e.g._ "15 vols. peroxide"). - -It is a fortunate feature of both the oxidizing and reducing agents -usually employed in bleaching, that they have considerable antiseptic -power. This assists materially in preserving the gelatine from -putrefaction during the critical period between extraction and -concentration. - - - REFERENCES. - - "Glue and Glue Testing," S. Rideal, D.Sc., 2nd ed., pp. 61-66, - 78-82. - - "Gelatine, Glue, and Allied Products," T. Lambert, pp. 29, 30, 49, - 51. - - "Chemical Engineering," _J.R. San. Inst._, No. 2, 1910. S. Rideal. - - On adsorption phenomena: - - 1. "Chemistry of Colloids," Dr. W. W. Taylor. - - 2. "Chemistry of Colloids," V. Poeschl. - - 3. "Chemistry of Colloids," Zsigmondy and Spear. - - 4. "Chemistry and Physics of Colloids," E. Halschek. - - 5. "Surface Tension and Surface Energy," Willows and Hatschek. - - - - - SECTION VI.--EVAPORATION - - -The evaporation of the weak gelatine sols (3-9 per cent.) obtained by -the processes described in previous sections into sols of such -concentration (20-55 per cent.) that they readily set to a stiff gel on -cooling, is now an essential feature of gelatine manufacture, and is one -of the most important processes. - -In the early days of this industry, manufacturers aimed at obtaining a -concentrated sol, as this saved time in drying, and so reduced the -possibilities of putrefaction. The advent of evaporation has reduced -these possibilities to a minimum, and has also enormously reduced the -space required and the capital outlay needed in the drying sheds. It -has, in addition, given the practical advantages involved in dealing up -to the last minute with a much less viscous liquor. As the liquors -extracted are weaker, the extraction is more complete and the -decolorization more easily effected. - -The earliest attempts at evaporation were not very successful, partly on -account of the prolonged "stewing" which ruined the setting power, and -partly because of the poor economy of heat. Thus in the open evaporators -the sol was maintained at a high temperature for a long period, and this -process only proved suitable for low-grade products. - -A great stride forward was made by Howard's invention of the Vacuum Pan. -This made it possible to undertake concentration at much lower -temperatures, a most important improvement in the case of gelatine and -other organic matters easily damaged by heat. The process, however, was -still slow, and the sol exposed to heat for a long time, as must be the -case when evaporation takes place in bulk. These disadvantages were -still fatal to the production of the highest-grade gelatine. There were -also the practical difficulties of entrainment ("blowing over"), in -which parts of the sol were carried away by the escaping vapour, and -also of "incrustation" which so rapidly reduces the heating efficiency -and evaporative capacity of the machine. The vacuum pan, however, -presented two decided advantages--evaporation at a low temperature, and, -as a corollary, the possibility of utilizing exhaust steam to attain -this temperature. - -Whilst the vacuum pan was a satisfactory machine for many branches of -chemical engineering, the problem of evaporation was still unsolved for -gelatine liquor because of the "stewing" involved, until the advent of -the "film evaporator," which dealt with the liquor not in bulk, but in a -continuous stream. In this way the product was only exposed to heat for -a comparatively short time. Many evaporators of this type came into -being, and rapid improvement was made in the constructional details. The -film evaporators retained usually the advantage of evaporation _in -vacuo_, so that it was now possible to evaporate gelatine sols by -exposure for a short time to a comparatively low temperature. Of this -type of evaporator, the Lillie, Yaryan, Schwager, Claassen, Greiner, -Blair Campbell, and the Kestner machines are well-known examples. - -A further advance in solving this problem was the application of the -principle of multiple-effect evaporation. The vapour driven off during -evaporation possesses of course many heat units, and is of very -considerable volume. In multiple-effect evaporators this vapour is used -to work a similar evaporator, and the evaporated liquor passes -immediately into what is practically a second machine, and is further -evaporated by the heat from the vapour just driven from it. Such an -arrangement would be termed a double-effect evaporator. The vapour from -the second effect may of course be similarly used to operate a third -effect, and the vapour from this to work a fourth effect, and so on. -Thus, we may have triple effect, quadruple effect, etc., even up to -octuple effect. The great advantage of multiple-effect evaporation is in -the saving of costly steam. Reavell gives the following figures to -illustrate the economy thus obtained:-- - - - WATER EVAPORATED PER 100 UNITS STEAM. - - -----------+-----------+-----------+-------------- - Single. | Double. | Triple. | Quadruple. - -----------+-----------+-----------+-------------- - 95 | 150 | 220 | 300 - -----------+-----------+-----------+-------------- - - -There is naturally a limit beyond which the capital cost of the machine -neutralizes the advantage of steam economy, and it is seldom that -octuple effects are used. There are probably more triple effects in use -than any other machine. - -An essential and important part of the modern evaporator is the -"condenser," in which the vapour from the last effect is conducted into -water (jet condensers) or over cooled surfaces (surface condensers), -with a view to producing and maintaining the vacuum. - -A lasting vacuum cannot be maintained without an air-pump, as air is -often introduced (1) with the steam, having entered the boiler dissolved -in the feed water; (2) by leakage from the atmosphere into the condenser -and the connected vacuous spaces; and (3) in jet condensers, in solution -with the circulating condenser water. That from the first two sources -may be reduced, but the third is beyond control: hence if high vacua are -necessary, surface condensers are to be preferred. Dissolved air is -usually 5-20 per cent. of the water volume, and is least for sea-water. -It should be noted that water leaving a surface condenser is in a very -air-free state, and therefore particularly suitable for boiler supply. -Apart from the capital cost of a condenser the chief cost of maintaining -a vacuum is in pumping the circulating water, of which up to 70 lbs. is -usual per lb. of steam condensed. - - If W = weight of steam condensed (lbs. per hour); - Q = weight of cooling water circulated (lbs. per hour) - T{i} = inlet temperature ( deg. F.) of cooling water; - T{o} = outlet temperature ( deg. F.) of cooling water; - then - T{o} = T{i} + 1050(W/Q) - -It will be understood that for high vacua, low temperature of cooling -water (T{i}) is more important than copious supply (Q/W). It is -advantageous, however, to choose a site yielding plenty of cold water, -such as a river or canal side. Otherwise it is often necessary to use -cooling towers or spray nozzles. The cooling is by evaporation (= 60 to -80 per cent. of W), cold water replacing that evaporated, and yielding -water 75 deg. to 80 deg. F. If T{i} = 80 deg. F. and Q/W = 70 deg., a vacuum of -28.34" is possible, but the 0.34" should be allowed for the partial -pressure of the air, determined exactly by the air entering and by the -displacement of the air-pump. - -Another feature of the modern evaporator is the "heater" or -"calorifier," by which the liquor to be evaporated is led in a -continuous rapid stream through heated tubes immediately prior to its -entry into the first effect. It is the aim of the heater to raise the -temperature of the liquor to the temperature of evaporation, and so to -avoid this being necessary in the first effect. The heater thus further -avoids stewing, ensures steady running, and effectively increases the -capacity of a machine. - -It is noteworthy that superheated steam is not desirable for working an -evaporator. The principle of evaporation by steam is not merely that the -temperature of the liquor is raised to boiling point; it is that in the -condensation of the heating steam its latent heat is yielded to the -liquor being evaporated. To evaporate quickly, therefore, the heating -steam must condense rapidly. Hence, as superheated steam has a rate of -condensation 20-30 times slower than saturated steam, the latter is much -to be preferred. A slight superheating, however, may be justifiable -where the steam has any distance to travel before use. It is the fact -that it is the latent heat of steam which is mainly utilized which gives -steam its great practical advantage over hot non-condensable gases. -Steam in condensing yields an enormously greater number of heat units -per lb. than hot waste gases. Steam has also the advantage of more -constant temperature. - -The capacity and efficiency of an evaporator depends upon a good many -factors, some of which are worthy of discussion at this point. - -The transference of heat and the amount of evaporation are directly -proportional to the mean temperature difference between the heating -steam and the liquor being evaporated. These temperatures, however, both -vary somewhat, the steam losing part of its pressure and temperature as -it passes along the heating surface; the liquid generally increases in -temperature. The mean difference in temperature, moreover, is not the -arithmetic mean between the smallest and largest temperature -differences, but is given by the following expressions, which yield -results not wide apart:-- - - If [theta]{a} = temperature difference at commencement; - [theta]{e} = " " " end; - and [theta]{m} = mean temperature difference; - - then - - [theta]{m} = ([theta]{a} - [theta]{e}) / - log([theta]{a} / [theta]{e}) - - or = ([theta]{a} - [theta]{e}) / - [ n(1 - [nth root of]([theta]{e} / [theta]{a})) ] - -This mean temperature difference is in practice usually spoken of as the -"temperature head" or "heat drop." It will be clear that this -temperature head is increased by using steam at higher pressure -(temperature), and by evaporating under reduced pressure. Since most -liquids have their boiling points reduced about 40 deg. C. by operating _in -vacuo_, the advantage of the vacuum is apparent. It should be remembered -that the temperature head has not the same value in any part of the -scale: it has more value higher up the scale, because the steam is -denser and more heat units come in contact with a given area in a given -time. It must also be remembered that whilst the pressure gauge is a -most useful indicator of steam temperature, it is not necessarily -accurate. The pressure in the hot space is the _sum_ of the pressures of -air and steam, and since the temperature (the important condition) of -the hot space depends upon the pressure of the _steam_, and not on the -sum of the pressures, the temperature in a steam space is always rather -lower than would be supposed from the pressure indicated by the gauge. - -The transference of heat is influenced by the velocity of both the -heating fluid and the fluid being heated over the heating surface. The -more rapidly each fluid moves, the more rapid is the transference of -heat, because a greater number of particles of both fluids are brought -to the heating surface in any given time. This is popularly known as the -effect of "circulation," and is illustrated by the advantage of stirring -a liquid being heated in bulk. In the film evaporators the circulation -is through tubes at high speed (up to 2 miles a minute), and the maximum -effect in this sense is thus obtained. The increase in heat transference -is not directly proportional to the increase in velocity, but in a lower -ratio, sometimes approximately the square root of the velocity. In such -a case, if either velocity be quadrupled, the heat transference is -doubled. Other advantages of high velocity are that the heating steam -more readily sweeps away condensed steam from the heating surface, and -the high-speed film similarly "scours" away "incrustations" on the -interior of the tubes. - -The transference of heat is also proportional to the conductivity of the -metal forming the heating surface. For gelatine liquors, copper tubes -are almost invariably employed, the advantage being great even when -price is taken into consideration. The following conductivity -coefficients illustrate this point (calories per hour through 1 sq. -metre of metal 1 metre thick, with a temperature difference of 1 deg. C.):-- - - - Copper...330 - - Iron.....56 - - Steel....22-40 - - Tin......54 - - Zinc.....105 - - Lead.....28 - - -The coefficient of heat transmission decreases the more with increasing -thickness of wall, the worse conductor is the metal. For copper tubes, -however, this decrease is usually unimportant. - -The transference of heat is also much influenced by the viscosity of -the liquor being evaporated; the greater the viscosity, the lower the -coefficient of heat transmission. Unfortunately for this process of -evaporation, gelatine sols are exceedingly viscous, and thus the -difficulty in obtaining a concentrated sol is thus greatly enhanced. - -The transference of heat is often greatly hindered by incrustations of -the tubes, which incrustations generally conduct heat very badly. Thus -the relative heat conductivities of copper and chalk are as 1000:5. - -The amount of heat transferred is of course determined also by the area -of the heating surface. The amount of evaporation needed thus determines -the number of tubes (of standard size) in the evaporator, and thus the -capacity of the machine. An evaporator should have its heating surface -area chosen with a view to the duty required of it. - -In practice the working of an evaporator is often not a very difficult -matter, and large numbers of machines are operated by unskilled labour. -Troubles generally arise from inconstant steam pressure, incrustation, -leakages of air, which reduce the vacuum, the temperature head, and -hinder heat transmission. For the evaporation of gelatine liquors the -Yaryan, the Kestner, and the Blair-Campbell film evaporators are the -most widely used. The velocity of the liquor through some of these -machines is so great that occasionally no vacuum is used. The -temperature obtained is high (200 deg. F.), but the time is very short, if -rapid cooling of the evaporated liquor is arranged. - - - REFERENCES. - - "Evaporating, Condensing and Cooling Apparatus," by E. Hausbrand. - Scott, Greenwood & Son (1916 Ed.). - - "Evaporation," by E. Kappeschaar. Norman Rodger (1914). - - "Evaporation in the Chemical Industry," by J.A. Reavell, - M.I.Mech.E., _J.S.C.I._, 1918, April 11th. - - "Glue and Glue Testing," S. Rideal, D.Sc., pp. 56-59. - - "Gelatine, Glue, and their Allied Products," T. Lambert, pp. 26-29. - - "Notes on Condensing Plant," J.M. Newton, B.Sc., _J. Junior Inst._ - Engineers, Aug., 1912. - - - - - SECTION VII.--COOLING AND DRYING - - -The conversion of a gelatine sol into cakes of gelatine has been much -simplified by the advent of the evaporator. Before this machine was used -much trouble was experienced with putrefaction, and in hot and thundery -weather, especially on the Continent, it was often necessary to suspend -operations. Evaporation has, however, materially contributed to the -possibility of rapid and satisfactory cooling and drying. - -From the time the weak sol is decolorized and bleached, the finishing -processes consist essentially in the removal of water. This is now -usually done partly by evaporation of the sol, and partly by the -desiccation of the gel. There is an obvious elasticity in method, and -factory practice does actually vary considerably in the relative -proportions of these two alternatives. Some factories evaporate to a 20 -per cent. sol, approximately, and rely upon drying sheds and lofts to -complete the desiccation: other factories evaporate up to a 55 per cent. -gelatine sol, and so can manage with less shed room. Something depends -upon local conditions, but the main issue is between the cost of steam -in evaporation and the cost of land and buildings required for sheds. On -the whole the modern tendency is to evaporate more, for this course has -the additional advantage of speed, involving both a quicker turnover and -less liability of putrefaction. Lower-grade products need relatively -greater evaporation to form a gel of equal rigidity. - -After evaporation and bleaching, the concentrated sol is first cooled -rapidly until it has set to a stiff gel, then cut up into cakes -according to the size required, these being dried out on network frames -arranged in tiers, through which a draught of air is usually forced or -induced. This general description is of course applicable to many -factories with innumerable variations in detail, most of which -variations originate in local convenience and are unessential parts of -the manufacture. - -An essential principle is that the cooling or gelation should be done -rapidly, not only to avoid putrefaction but also to avoid the action of -heat on the elasticity of the gel. A hot sol or gel is liable to -hydrolysis and loss of setting power, and should have its temperature -quickly reduced, but a warm sol or gel (say 100 deg. F.) is most liable to -putrefaction, so that the cooling should be continued quickly. On the -other hand, the gel should not be frozen. For cooling purposes a copious -supply of cold water is most usually employed, but some factories have -installed refrigerators. These plants operate by the rapid evaporation -of liquefied gases such as carbon dioxide, sulphur dioxide, or ammonia, -so arranged as to cool a solution of common salt, which forms the -circulating liquor and is returned after use to the refrigerator. Where -such plants are used, it is natural that their use should be extended to -the drying sheds to cool the air entering in the height of summer. In -some factories the cooling is attained neither by cold water nor cooled -brine, but merely by cold air. - -The kind of vessel in which gelation is induced varies widely in -different factories. For lower-grade products metal boxes are used, -heavily galvanized iron being the most common material. If the liquor be -muddy, deep boxes are preferred, but if clear, rapid cooling is best -attained by having them long and shallow, and so exposing a relatively -greater area to the cooling action. In either case the boxes may contain -up to 1/2 cwt. of jelly. Lambert mentions boxes 24" x 6", which are 5" -deep; Cavalier suggests rectangular moulds holding 30 litres. In place -of galvanized sheet iron, boxes of sheet zinc or of wood lined with zinc -are sometimes used. In any case the most scrupulous cleanliness should -be observed in all cooling-house work, and in some factories the most -elaborate precautions are taken for cleansing vessels, tools, floors, -etc., and even for their disinfection and sterilization. Iron, tinned -iron, and copper cooling vessels are ruled out on account of their -tendency to rust and tarnish, and the last is unjustifiably expensive. -Many of these vessels are unsuitable for pure food gelatines in which -traces of copper, zinc and arsenic are held to be very objectionable. -For the best gelatines, therefore, a very shallow vessel (1/4" to 1/2" -deep) with a sheet glass bottom is preferred, and the concentrated sol -is run on to this for gelation. - -Glue (or gelatine) which has set in this way is sometimes called "cast -glue." That which sets in metal boxes in blocks is termed "cut glue," -because the blocks of jelly need subsequently to be cut into slabs of -the desired size and shape. Jelly blocks may be cut by hand with the -"wire knife" which yields a characteristic wavy appearance to the -finished product. This may also be done by machinery, the block of gel -being placed on a series of correctly spaced wires and forced through -the network by hydraulic pressure. A cutting machine (Schneible) has -also been used to cut up blocks of jelly into slices of the required -thickness, but these machines have not made great headway in this -country. It will be clear that cast glue is cooled more rapidly than -glue in blocks; it is therefore not surprising to note Lambert's -statement that the former comprises the larger proportion on the market. - -The cut or cast cakes are next placed upon network frames, and a series -of such frames are placed on a bogey. The bogey is run along tram lines -into the drying tunnel, through which air is forced or induced by a fan. -Many such bogeys are, of course, passed into each tunnel, and as many -tunnels as required may be constructed. Care is necessary to expose the -cakes evenly to the action of the air. It is mostly necessary to warm -the air at the inlet by means of steam pipes and so increase its drying -power. This is especially necessary in winter or wet weather. In summer, -however, it is often arranged that the air is cooled before entering -the sheds. This is accomplished by passing the air through pipes from a -refrigerator. When heated air is used, it is stated by Lambert that the -maximum temperature should be 25.5 deg. C. (78 deg. F.); Rideal considers 21 deg. C. -(70 deg. F.) should be the maximum. In all cases the drying power of the air -is easily ascertained from a wet-and-dry bulb thermometer, and the -amount of air passing along the shed from a wind gauge. Lambert states -that drying normally occupies four to five days. The final product is -still a gel, of course, and contains from 10 to 18 per cent. of water. -It appears, however, very hard and solid. The dried cakes are removed -from the frames and transferred to the warehouse, where they are sorted -according to quality and packed in bags or tin-lined boxes. Some -material is ground to powder. - -The network of the drying frames has been made from many materials. -Cotton or string netting is very common, but is liable to sag and to get -dirty. It also has a short life. Ordinary galvanized iron soon loses its -galvanizing cover, and the iron then is liable to rust. Attempts have -been made to use sheet zinc and other alloys, which are cut or punched -into nets with square or diamond-shaped holes. These were found to warp -and break. Rideal's conclusion, which is confirmed by the general -experience, is that the best material is a heavily galvanized iron wire -netting. He suggests that it should have 15 to 25 per cent. of its -weight of zinc, and that it should be strengthened by stiffer ribs -arranged both longitudinally and transversely. - -Many attempts have been made, and many patents taken out, with the -object of making the cooling, cutting, and drying processes as -continuous and as quick as possible, and with a view to saving labour, -which is rather costly at this stage. These attempts, however, have only -met with indifferent success. A common idea is that a continuous supply -should fall upon a revolving appliance, and be instantly congealed in a -thin state, which last lends itself to more rapid desiccation. Vacuum -drying has also been attempted. - - - REFERENCES. - - "Glue and Glue Testing," S. Rideal, D.Sc., pp. 68-74. - - "Glue, Gelatine, and Allied Products," T. Lambert, pp. 30-35. - - _Chem. Zeit._, 1911, 85, 17 (Cavalier). - - - PATENTS. - - Eng. Patent (1894) 11,426 (Hewitt). - - Eng. Patent (1898) 2,400 (Brauer). - - Fr. Patent (1909) 398,598 (Lehmann), _J.S.C.I._, 1909, 897. - - U.S. Patent (1912) 1,047,165 (American Glue Co.). - - - - - SECTION VIII.--USES OF GELATINE AND GLUE - - -Gelatine and glue have both been put to an immense variety of uses, and -the list is constantly extending. Indeed, no one who considers the -following account of their applications can doubt that gelatine and glue -have become a necessary part of our civilization. - -Gelatine for edible purposes certainly forms a very considerable part of -the total used, and great pains are now taken to obtain a pure product. -Thus, a gelatine with more than 1.4 parts per million of arsenic, or -more than 30 parts per million of copper, is not considered good enough -for "pure food." The food value of gelatine, compared with other -proteids, is exceedingly low; its use in this connection has no -connection with the "calories" of heat energy it will yield. It is used -almost entirely because of its property of forming a gel. Table jellies -form, of course, one popular use of gelatin, but the manufacture of -sweets makes also a great demand upon the gelatine trade. Culinary -operations often require a little gelatine, especially is it used in -pies and soups. An extension of the same idea is found in its employment -for many manufactured foods, _e.g._ tinned meats, meat extracts, and the -concentrated foods. The use of gelatine in connection with the first of -these received a big impetus during the war period. In gelatine for any -of these purposes, the presence of excess of sulphurous acid is -objectionable, as its taste is easily noticed. - -Gelatine for medicinal purposes finds an ever-growing number of -applications. Gelatine capsules for holding greasy liquids and solutions -of nauseous drugs are increasingly popular, for the dose may be -swallowed without unpleasantness. In making these capsules some sugar -is also used, and the finished article is often protected from -atmospheric moisture by treatment with a weak solution of alum. In a -similar way pills are often coated with a 33 per cent. gelatine sol. -Such pills are not only pleasanter to swallow, but are less liable, -after being dried, to stick together in the box. Alcohol solutions of -drugs (or essences, perfumes, etc.) may be suitably stored in gelatine -instead of metal tubes. Medicated wines are detannated by gelatine -before the addition of drugs which would have been precipitated by the -tannin. The British Pharmacopoeia specifies four kinds of "Lamellae," -which are small discs of gelatin and glycerin, each containing a minute -but definite dose of some powerful alkaloid. Glycerin jelly is a mixture -of gelatin glycerin with some water. It is used for chapped and rough -hands; the mixture is also used for glycerin suppositories, and for -mounting microscopic sections. The mixture also forms the basis of -gelato-glycerin, used in nasal bougies, and of glyco-gelatin for -medicated lozenges. Gelatine insolubilised by formalin (formo-gelatin) -has been used for making tabloids, wound dressings, and artificial silk. - -Gelatine is in constant demand for bacteriological work, for which -purpose a high-grade product is desired. Nutrient media for the culture -of bacteria are solidified by 10-15 per cent. of gelatin, and the growth -of colonies of bacteria often show typical formations. By inoculating -into a melted and sterile quantity and setting quickly in a flat dish -after mixing, the number of bacteria in the volume introduced can be -judged from the number of colonies which develop. Bacteria are also -distinguished often as "liquefying" or "non-liquefying" according to -their type of culture on nutrient gelatine media. Gelatine for such work -should be neutral and of high clarity. - -The gelatine required for photographic purposes is also a high-class -product. It should be neutral, colourless, and free from chlorides and -other mineral salts. Grease also is objectionable. Gelatine is used in -the numerous carbon processes, in which the principle is that gelatine -is made insoluble in water by the action of potassium dichromate under -the action of light. It is used also in Poiteoin process for copying -engineering drawings, which is based upon the power of a ferric salt to -render gelatine insoluble so long as it is not exposed to the actinic -rays. - -Gelatine is used in the manufacture of the "crystalline glass" used for -decorative purposes. Advantage is taken of the immense contractile force -it exerts on drying. When ground glass is coated with gelatine, and the -latter dried, it tears away the surface of the glass itself, and leaves -peculiar fern-like patterns. Inorganic salts dissolved in the sol -influence the nature of the pattern obtained. - -Gelatine is used also very largely in the textile trades, for finishing -coloured yarns and threads, for sizing woollen and worsted warps, and -for thickening the dyestuffs used in printing fabrics. It is also used -for finishing white straw hats; as a size in the manufacture of -high-class papers, and as a wax substitute for covering corks and bottle -necks. - -Glue is used instead of gelatine in all cases where colour is not a -matter of much moment. The fact that it has not been bleached makes no -difference to its suitability in such a case, and the cost is -substantially reduced. Thus, for dark-coloured straw hats, textiles, -sweets, papers, and in all suitable woolwork, glue is used in place of -the more expensive article. - -A very large quantity of glue is used in the manufacture of matches, -where it functions as the material binding the "head" to the stem. A -15-50 per cent. sol is used, containing nitrate or chlorate of potash as -oxidizing agent. The mixture is kept at 38 deg. C. and the phosphorus -cautiously added, and when this is emulsified, the friction ingredients -(sand, glass, etc.) are also added. The glue acts also in preventing -premature oxidation. Glue is also used in making the match-boxes, and -similarly in making sand, emery, and glass papers and cloths. - -There is a large consumption of glue by joiners, carpenters, -cabinet-makers, and all kinds of woodwork and fancy work. It is used in -the manufacture of furniture of all kinds, of pianos, organs, billiard -tables, panels, picture frames, and of toys and brushes. Mixed with -white lead, chalk, and sawdust, it forms a composition used for mirror -frames, rosettes, etc. Glue is used for veneering, for mosaics, plaques, -trays, fingerplates, leather wall coverings, and for staining floors. - -There is also a considerable sale for glue in bookbinding, for which a -sweet, light-coloured, and strong product is required. It has been found -particularly suitable for leather bindings where the grain has been -artificially printed or embossed, and in finishing and gilding. - -The compositions used for printing rollers all contain gelatine or glue -together with sugar or glycerin and possibly oil and soap. They are -often hardened with formalin. Similar mixtures are used for the beds of -hectographs. - -Glue (together with waste leather) is used in the manufacture of -imitation leather and leather substitutes. Cotton and wool fibres are -often incorporated, and sometimes textile fabrics. - -Much glue is converted into "size," which is a weak gel used as a -filling rather than as an adhesive agent. A low-grade glue is often -therefore preferred for such purposes, as having "body" rather than -"strength." Size is often sold in cake, but sometimes in the form of the -gel itself, in which case it may never have been evaporated. Indeed, -size is often overboiled glue, made by crude and out-of-date methods. It -is largely used in the paper trade, and for wallpapers, millboards, -papier-mache, paper and cardboard boxes, etc. Mixed with logwood and -iron, and possibly alum, it formed the "blue size" once largely used by -bootmakers as a foundation for blacking, and is similarly used in -currying. Size is also used in making oil paints and varnishes. -Distemper is a size with which is incorporated whiting or gypsum and -coloured pigments. In all applications of size, it is common to use -antiseptics. Salicylic acid has been widely used in this sense. -Low-grade glue is used for the manufacture of cheap brushes and for -fly-papers. - -Innumerable patents have been taken out and mixtures invented for the -production of plastic materials, which frequently involve gelatine or -glue. Thus, gelatine and glue are used in making plaster casts, and for -imitation ivory, wood, stone, and rubber. Many of these inventions have -been investigated by Rideal, who points out the features common to most -of them. Usually a viscous sol is thickened by the addition of inert -fibres and powders, and with the object of making the preparation more -waterproof it is customary to incorporate oils, fats, waxes, tars, and -resins before the gel is set. The surface is hardened by "tanning" with -formalin or tannin solution, finally painted or varnished. - -Equally innumerable are the inventions, recipes, and patents for making -glues that shall remain liquid. The convenience of this ideal is -obvious, but many of the suggestions are useless. It is quite easy to -incorporate into a gel substances which keep it liquid--any soluble -substances with a lyotrope influence of the iodide type will do -this--but these also prevent the glue setting when used. Even in small -quantity they will influence the tenacity of the joint. Other methods -depend upon a partial hydrolysis of the protein. Amongst the most -successful of these attempts are to dissolve 3 parts of glue either in -12-15 parts saccharate of lime, or in 9 parts of 33 per cent. acetic -acid. - -Many special glues and cements are made from commercial glue, according -to the purpose required. "Marine glue" contains no glue; it is made from -shellac and rubber mixed with benzene or naphtha. Its advantage is -waterproofness. - - - REFERENCES. - - "Glue and Glue Testing," S. Rideal, D.Sc., 2nd ed. - - "Uses of Glue," chap. iii. p. 83. - - "Uses of Gelatine," chap. iv. p. 100. - - "Special Glues," p. 108. - - "Liquid Glues," p. 119. - - "Gelatine, Glue, and their Allied Products," T. Lambert. - - "Uses of Glue and Gelatine," chap. ix. p. 80. - - "Liquid Glues and Cements," chap. viii. p. 69. - - - - - SECTION IX.--THE EVOLUTION OF THE GELATINE AND GLUE INDUSTRY - - -The manufacture of gelatine and allied products has received a great -stimulus in this country from the circumstances arising from the -European War. The large restriction of continental--especially French -and Belgian--supplies of gelatine, led to greater demands for the -British-made product, and resulted not merely in a period of greater -prosperity, but in a period in which much greater efforts were made to -supply a high-grade article in larger quantities. Most manufacturers -strove to make high-class gelatine rather than low-grade glue, great -extensions were made, and many new businesses were established. The -development of the leather trades, more particularly in respect of -greater production, caused a bigger supply of raw material for skin -gelatine, and the slaughter of home animals for food caused a more -plentiful supply of bones. At the same time it was realized that greater -production not only reduced working costs, but also that a bigger -turnover in any one factory involved a proportionately less capital -outlay. These facts tend to counterbalance the heavy freight on the raw -materials. Production is thus not only on a larger scale but more -intensive. - -One of the greatest difficulties of this industry is to produce a -regular or standard article, for the raw material is so exceedingly -variable in quality; that for skin gelatine tends also to become less -valuable. In such a case, as Rideal has truly remarked, to ensure that -supplies to customers shall be always "up to sample," which is often a -matter of contract--"exact and regular working, strict cleanliness, -observance of temperatures and other physical data, and scientific -supervision", are clearly necessary. "Rule of thumb" is never quite -certain to produce the same article twice. In past years British methods -of manufacture have been far too empirical. As in other industries, -"rule of thumb" must inevitably be replaced by scientific principle. The -advances in colloid chemistry of this last decade or so have, in the -author's opinion, supplied the clue to this line of development. In the -preceding pages emphasis has been laid upon the importance of the -adsorption law, the lyotrope series, and the valency rule. The -manufacturer or supervisor who understands and can apply these -generalizations will find his task vastly easier and his factory more -efficient. Much remains to be learnt, however, and the industry would -certainly benefit by research work, for which there is a fertile field. - -There is also considerable room for improvement in the methods of -chemical engineering usually employed. Whilst the heat engineers have -certainly done much to solve the question of evaporation and drying, -there is still great scope in the more economical application of heat in -extraction, and the last word can hardly have been said on the problem -of clarification and decolorization. There is indeed almost as much -scope for research by the chemical engineer as by the colloid chemist. - -The industry also exhibits, in common with the leather and many other -trades, the same tendency to save labour, both by careful arrangement of -the factory and by the installing of mechanical labour-saving devices. -Thus, lifts, runways, hoists, and trucks are increasingly used to move the -solids, and pipes and pumps to move the liquors. As ever, there is scope -for the mechanical engineer. - -If some of these problems are vigorously tackled during the present -reconstruction period, there is little doubt that the gelatine and glue -industry will be in a much better position to cope with all possible -competition in the future. - -From what has been said in Section VIII. as to the wide uses of -gelatine and glue, it will be seen that general prosperity in trade is -conducive to better trade conditions in the gelatine and glue industry. -It is similarly true that a general trade slump affects the glue trade -adversely. The severe trade depression which commenced in 1920 has had -this effect, and has made economic production much more difficult as -well as more essential. As often is the case, the larger factories and -firms can better face the difficulties, and there can be little doubt -that if the depression be long continued there will be a tendency for -the smaller factories to be closed down and for the larger firms to -unite. As in the leather trade, both the War boom and the Peace slump -have caused the gelatine and glue trade to develop along the lines of -the great trusts. It may be reasonably expected, moreover, that these -will be intimately connected with the leather trusts. This fact, -together with the heavy freight charges on the raw material, tends also -to make the skin glue factories gravitate towards the leather centres. - - - - - PART VI.--MISCELLANEOUS PROTEINS AND BYE-PRODUCTS - - - SECTION I.--BYE-PRODUCTS OF THE LEATHER TRADES - - -In the leather trades by far the most important and valuable -bye-products are obtained from the hides and skins themselves, and all -these are obtained before the tannage proper is commenced. The leather -trades use only the dermis (corium) or true skin for the manufacture of -leather, and as we have noted (Part I., Section II.) this prepared and -purified dermis is called "pelt." The cuttings and trimmings from the -pelt form the most valuable bye-product of the leather trades, and are -the raw material of the gelatine and glue industries (Part V., Section -II.). Many portions of the pelt, indeed, such as ears, noses, and cows' -udders, are quite useless for any other purposes. Other portions, such -as cheeks, faces, and even bellies, may be made either into glue or -leather according to the state of trade. Hardly less important to the -same industry are the cuttings of adipose tissue removed in "fleshing" -the hides and skins. These, though yielding less protein, yield also, -however, the valuable animal greases (Part V., Section II.). To obtain -both these products in a purer condition the removal of "flesh" after -"soaking," but before "liming" (Part I., Section II.), has been favoured -by some, especially in America. - -Amongst the epithelial structures of the hides and skins, we have -several protein bye-products which have some commercial value. The -horns of cattle are now almost invariably removed before reaching the -leather manufacturer, but have some little value. This part of the -epidermis is not solid keratin. A "pith" is easily removed after boiling -in water. The outer parts, too, are often coarse and somewhat damaged, -but if removed by scraping reveal often a rather beautiful structure of -varying colour. There is some opening for this product in the -manufacture of small articles of horn, but much of it, together with -hoofs, is roasted and crushed for making fertilizers. The hair of -cattle, goat, etc., has also a commercial value. This is removed after -liming, and needs subsequent purification (Part I., Section II.). The -hair is well washed with water, using either repeated changes or a -continuous supply, the operation being carried out in paddles or similar -machines which stir up the hair in the water. When clean, the hair is -transferred to a centrifugal machine or "spinner," in which much -adhering water is removed. This is a great assistance in drying out, -which is the next and final operation. In drying, the hair is laid upon -steam-heated boxes or pipes, and a current of warmed air passed over or -through it by means of a fan. It is better to have the hair "turned" -occasionally. This ensures quicker as well as more even drying. The -product is made up into large bales and sold for the manufacture of -felts, mattresses, etc. White hair is usually kept separate and commands -a larger price. The power consumed in driving the washing machinery, the -centrifuges and the drying fan, together with the fuel required for the -drying steam, and the labour involved throughout, make it doubtful -whether this bye-product is worth either the capital outlay or the -working costs necessitated. Many manufacturers avoid this treatment -altogether, therefore, and the wet limed hair is sold direct to the -fertilizer factory. A less price is obtained, but much expense is saved. -Especially when the animals have only their short summer coats, this -course is preferred. - -In the case of the wool from sheepskins the product is much more -valuable. The wool, indeed, is often the primary consideration. -Unfortunately this sometimes results in the neglect of the pelt. The -removal of wool from sheepskins forms a special industry known as -"fellmongering," which has been previously described (Part II., Section -IV.). Pains are taken to clean the wool even before removal from the -pelt, by the liberal use of water and the "burring machine." There is -much variation in quality, and care is taken to keep the various grades -separate, even during the "pulling" operation. From the fellmonger the -wool passes to the "wool stapler," and forms the basis of one of our -most important mechanical industries, the manufacture of woollen cloths. -Wool is also removed from sheep by the periodic shearing, and in this -case does not reach the fellmonger at all. - -Apart from the raw material itself, there are few bye-products of the -leather trades which are of commercial importance. The sludge from the -pits of the limeyard contains, in addition to much lime and chalk, a -certain proportion of protein matter. This is derived partly from the -blood and dung associated with the hide, partly from the solution of the -corium hide substance, partly from the solution of the softer keratins, -and partly also undissolved and loose hair. This bye-product is rather -difficult to deal with, as it will not easily dry. It is indeed -sometimes a problem to dispose of it, except in rural districts, where -the farmers appreciate its manurial value and will usually cart it away -for a nominal fee. Where possible, it is better to let it drain and -settle on land, and pile it up in heaps to dry further. Soak-pit sludge -has a distinctly greater value as manure, on account of the greater -proportion of dung proteins. As some lime is often used in these pits, -the product is a really useful fertilizer. - -The only other bye-product of the leather trades is waste leather -itself. For small pieces of leather there is always some little opening -in producing small articles, such as washers for taps, etc., and there -is also the possibility of shredding or pulping and making an artificial -leather. The best leather substitutes, indeed, are made from waste -leather. Nevertheless, there is always a certain amount of waste leather -which only finds an outlet in the fertilizer factory. Such material is -usually steamed or roasted to make it brittle, and then crushed in a -disintegrator. It is then mixed in with other materials, but is -sometimes solubilized by the action of sulphuric acid. Leather seldom -contains less than 30 per cent. protein. - - - REFERENCES. - - "Chemical Fertilizers and Parasiticides," S.H. Collins, M.S., - F.I.C. (Companion volume in this series on Industrial Chemistry.) - - "Wool Wastes," Part II., Section V., p. 75. - - "Hoofs, Horns, Leather," Part III., Section II., p. 115. - - "Gelatine, Glue, and Allied Products," T. Lambert. - - - - - SECTION II.--BYE-PRODUCTS OF THE GELATINE AND GLUE TRADES - - -From the skin gelatine and glue trades the most valuable bye-product is -the grease, which is obtained from the "fleshings" of the adipose -tissue. These fleshings are themselves a bye-product of the leather -trades. The recovery and purification of this grease has been dealt with -previously (Part V., Section II.). In the skin glue trade the only other -bye-product is the residue from the extraction process (Part V., Section -III.). This residue is known usually as glue "scutch," and is composed -of the proteins of the skin which are insoluble in hot water. These -insoluble portions are obtained from all layers of the skin. There is -much hair often in scutch, the hyaline or glassy layer (grain), and the -elastic fibres of the corium are also insoluble, and a proportion is -derived from the fibres of the adipose tissue on the flesh side. All -these portions are fairly rich in nitrogen, and the scutch has, -therefore, considerable value to makers of fertilizers. It is liable to -contain also a percentage of grease, which is usually removed by -steaming under hydraulic pressure. This process recovers a valuable -bye-product and increases the manurial value of the scutch. There is -always left in scutch some of the gelatinous skin substance which, -strictly speaking, should have been removed during extraction. There is, -however, a practical limit beyond which it does not pay to do this. When -this limit is reached the cost of steam in extracting, and also in -evaporating and drying, together with the loss of time and labour -involved by occupation of the plant, is greater than the value of the -possible product. - -From the bone-glue industry, the grease is similarly a valuable -bye-product, but there is also another of equal importance, viz. the -phosphate of lime, which comprises about half the raw material. As -previously described in Part IV., Section II., this is usually -extracted after the grease, by solution in weak hydrochloric acid. The -solution is neutralized in lead-lined vats with milk of lime, a -precipitate of di- and tri-calcium phosphates being obtained. Calcium -chloride is left in solution, and the precipitate should be, therefore, -well washed if it be desired to have dry phosphate. The bone-glue -industry is, generally speaking, much more intimately connected with the -fertilizer trades than the skin-glue trades, indeed the extraction of -the bones for glue is not always advisable, in which case the protein -matter as well as the phosphatic matter of the bones are employed for -making "bone manures." For details of this industry the reader is -referred to a companion volume in this series on "Chemical Fertilizers." - - - REFERENCES. - - "Chemical Fertilizers and Parasiticides," S.H. Collins, M.Sc. - - "Bones," Part II., Section V., p. 72. - - "Precipitated Bone Phosphate," Part III., Section III., p. 157. - - "Bone Manures," Part III., Section V., p. 173. - - "Gelatine, Glue, and Allied Products," T. Lambert. - - - - - SECTION III.--FOOD PROTEINS - - -Although there are those who consider that animal proteins are both -undesirable and unnecessary as foods, it is nevertheless true that man -is almost universally a carnivorous animal. The animal world provides -mankind with one of its chief sources of food, and especially of protein -foods. Protein foods are unquestionably essential, and animal protein -foods differ chiefly from those of vegetable origin in the fact that -they contain generally much more protein. Of the proteins noted in our -Introduction, the keratins have no value as foods; the gelatins have -some value as culinary material, but little actual food value; whilst -the albumins comprise practically all the useful animal food proteins. -Whilst the actual flesh of animals is the principal source of food -proteins--both as to quantity and food value--other parts of animals, -_e.g._ kidneys, liver, blood, brains, tongue, are used and relished. The -most important sources of animal food proteins are from fish, fowl, -sheep, cattle, and pigs, the meat from these being roughly in the same -sequence as to digestibility. There are, however, many other animals of -which the flesh is quite edible, but most of the above are specially -farmed and propagated primarily for their food value. - -As the animal food proteins are exceedingly putrescible, they are -usually consumed within a short time of the animal being killed. It is -perhaps natural, therefore, that many efforts have been made to discover -means of preserving such foods. These efforts form the basis of some -important industries, and though they can hardly be included as chemical -industries, it will not be out of place in this volume to point out that -these efforts present analogies with, as well as differences from the -methods used for preserving hides and skins (Part I., Section I.). The -curing of hides and skins is a temporary preservation from putrefaction -until the opportunity is convenient for the permanent preservation -(_i.e._ tannage). The preservation of meats is analogous to curing -inasmuch as more drastic treatment might indeed make them -non-putrescible, but would also render them indigestible and unsuitable -for food. Thus drying, salting, drying and salting, pickling and -freezing, are just as suitable for preserving food proteins as for hide -and skin proteins. Hence we have dried meats, salt bacon, pickled beef, -frozen mutton, etc. To a limited extent smoking (fish, bacon, etc.) has -been employed as a cure. When it has been applied to skins it is usually -combined with a fat tannage. There is, however, one method of -preservation of proteins, inapplicable to skins, which has been -eminently successful and useful for food proteins, viz. sterilization by -boiling. The food has been placed in tins, hermetically sealed, and -thoroughly sterilized. Hence have appeared corned beef, tinned tongue, -sardines, etc., which merely illustrate the immense possibilities -involved. A noteworthy advantage of this method of preserving animal -food proteins, is that the food is already cooked and prepared for -immediate consumption. - -Another line of effort is the preparation of concentrated foods. Just as -animal foods are on the whole more concentrated in protein than -vegetable foods, so these prepared animal foods are more concentrated -than animal flesh, and generally also more soluble. Such preparations of -animal protein are obviously useful when there is difficulty in -swallowing and when journeys are necessary into regions of poor food -supply. It is a little doubtful, one must say, whether the concentration -is as great in some cases as is claimed. - -Yet another industry based upon the animal proteins is the manufacture -of meat-extracts. These are not merely concentrated extracts of animal -flesh, but contain especially the stimulative properties of animal food -proteins. There is now little doubt of the value of these preparations -as stimulants, and it is claimed for them that they not only have food -value, but also that they increase the food value of other foods used -with them. Together with these products may be classed all the -miscellaneous tonic foods, in which proteins are blended with -carbohydrates and often also with drugs. These aim at the cure of -specific disorders, such as nervous debility, sleeplessness, etc. Their -claims are often extravagant. Amongst all the multitude of prepared -foods, there deserve particular mention the partly predigested foods. In -cases where the digestive functions are weak or disordered these -products have been of real service. - -One of the most useful and valuable of animal food proteins is obtained -from hen eggs. The "white" of eggs is almost pure albumin, and there is -much protein in the yolk also. Eggs are now produced and imported by the -million, and form a most important item in the country's dietary, the -protein being in a very easily digestible form. - -It is also necessary to refer to the importance of cows' milk as a -source of animal food protein. The amount of protein in milk (4-5 per -cent.) is not large, but it is united with fats, carbohydrates, salts, -and vitamines in such proportions, that milk is about the only article -which may reasonably present a claim of being a complete food. Milk, -moreover, forms the staple diet of infants and young children, so that -its protein is certainly of great importance. As an infant food, cows' -milk is not altogether ideal. Even when the proportions of fat, -carbohydrate, and protein have been adjusted to resemble human milk, -there remains the difficulty that some of the proteins of milk -(especially the casein) are too indigestible for young infants. This -difficulty has been only partly surmounted by those industries engaged -in manufacturing infant foods. Some claim to remove the bulk of the -casein; others to have rendered it digestible by treatment with enzymes; -others, again, simply claim to supply concentrated cows' milk. Tinned -milk, generally concentrated to some extent, now forms a useful addition -to animal food products. The casein of milk also finds some outlet for -industrial purposes. When treated with formaldehyde it yields an -artificial horn much used for the preparation of imitation -tortoiseshell. Skim milk is treated with caustic soda or carbonate of -soda, the casein precipitated by acid, pressed, impregnated with -formaldehyde, and dried. The product is termed "galalith." It can be -distinguished from real tortoiseshell by the action of fuming nitric -acid (see _J.C.S.I._, 1909, 101). - -The utilization of the blood of animals, which is very rich in protein, -as a foodstuff has long been known, but has met with a good deal of -prejudice in this country. This prejudice has arisen not merely from the -objection to blood as food, but also from the fact that such foods have -been particularly liable to putrefaction and hence to cause poisoning. -The shortage of all foodstuffs occasioned by the European War did much -to overcome this prejudice, and there were considerable developments in -the manufacture of black pudding and similar preparations of animal -blood. The same circumstances made it necessary to consider more -seriously the possibilities of other butchers' offal as human food, and -resulted in new preparations of tinned animal proteins being placed on -the food market. - -The author would like to record his opinion that by no means the last -word has been said on the question of drying as a method for preserving -animal food proteins. There is much to be said for this method on every -ground in theory, and it is evidently an increasing success in practice. -Dried milk has been followed by dried eggs, and in view of the success -of the method when applied to fruits and vegetables, there seems a -prospect of better success in respect of dried meats. After all, animal -food proteins are chiefly lyophile colloids, and though desiccation -presents some practical difficulties, the subsequent imbibition -(assisted perhaps by lyotrope influences) seems to be the ideal method -for restoring preserved protein to its original condition. - -In conclusion, it will be interesting to note in the subjoined table, -the relative importance of the different sources of supply of both -animal and vegetable food protein. The figures are taken from the report -of a Committee of the Royal Society. They show the average quantities of -food materials (imported and home produced) available for the United -Kingdom during the five years 1909-1913 inclusive, together with the -amounts of protein, fat, and carbohydrate present and the energy value. -This information formed the basis of the Committee's recommendations as -to economy of protein during the war shortage. These recommendations -included the more economical production of meat by slaughtering cattle -younger and the saving of 55,000 metric tons of protein annually by -adopting cheese-making as a general practice in place of butter-making. - - - | Metric tons. | Energy - +--------------------------------| value, - | Protein. | Fat. | Carbo- |millions of - | | | hydrate | calories. - ---------------------------+----------+---------+-----------+----------- - Cereals | 549,000 | 63,000 | 3,628,000 | 17,712,000 - Meat | 356,000 | 799,000 | -- | 8,890,000 - Poultry, eggs, game, | 42,000 | 31,000 | -- | 461,000 - and rabbits | | | | - Fish | 91,000 | 17,000 | -- | 531,000 - Dairy produce, including | 199,000 | 686,000 | 258,000 | 8,253,000 - lard and margarine | | | | - Fruit | 9,000 | 14,000 | 222,000 | 1,077,000 - Vegetables | 120,000 | 10,000 | 1,031,000 | 4,812,000 - Sugars (including cocoa, | 5,000 | 18,000 | 1,572,000 | 6,633,000 - etc.) | | | | - Other cottage and farm | | | | - produce | 67,000 | 13,000 | 551,000 | 2,655,000 - - - - - SECTION IV.--MISCELLANEOUS ANIMAL PROTEINS - - -The excreta of animals include animal proteins of great importance to -agriculture and horticulture, forming the staple supplies of manure. The -manure of animals should contain not only the solid waste material and -undigested food, but also the urine, which contains much nitrogen, and -hence makes considerable difference to the value of the product as a -fertilizer. If the animals are fed on rich foods, the manure obtained is -correspondingly richer, especially in its protein content. - -The value of dung manures depends not merely upon the protein content, -but also upon its content of phosphate and potash, as well as other -organic matter. The protein breaks down into simpler nitrogenous -compounds, and eventually, through ammonium carbonate, it becomes -nitrate. Nitrogenous manures darken leaves and increase growth -considerably. Dung manures are deficient in phosphates and potash and -are of value partly as nitrogenous manures producing growth, and partly -as dressings of organic matter for soil. From both points of view it is -desirable that the manure should be well decayed. Fresh dung manures are -both wasteful and injurious to soil, except perhaps to very stiff clays. -They are wasteful inasmuch as much ammonia escapes, and injurious -inasmuch as they cause the "denitrification" of the valuable nitrates -already in the soil. When possible dung manures should be kept under -cover. Free exposure to air and rain will sometimes reduce its value by -one half. It should be stored until "sweet," and until the straw has -rotted and become "short." This takes usually several months. A ton of -well-rotted farmyard manure contains very approximately 10-12 lbs. -nitrogen, about the same amount of potash, and about half that quantity -of phosphates. It is, however, very variable. Horse manure is rather -richer than cow manure, but more liable to loss on storage. Pig manure -is intermediate between them. Sheep manure is distinctly richer in -protein, and has therefore greater value as nitrogenous fertilizers. -Poultry droppings are richer still, perhaps partly because they include -the urinary products. When fresh they contain 18-25 lbs. nitrogen, 12-24 -lbs. phosphate, and 6-12 lbs. potash per ton. When dried they have about -double the value. Pigeon manure is even richer, and the pigeon loft -scrapings have a manurial value about double that of dry hen manure, and -eight times that of farmyard manure. Guano is much decayed droppings of -sea birds on the tropical coasts of Africa and America. The supplies are -now quite exhausted, and the market guanos are chiefly artificial -fertilizers. - - * * * * * - -There is one other animal protein which must be referred to before this -volume is concluded, viz. silk. This is obtained from the cocoon of the -"silkworm," which is the general name given to the larvae of certain -bombycid moths. These larvae feed on the leaves of the mulberry, and when -ready to pupate produce a considerable supply of a soft and delicate -thread which is wound round about the larva itself. This is the raw -silk, and it is unwound from the cocoon in a machine called the -"silk-reel," and may then be wound into a thread. Two or more threads -twisted together form "thrown-silk." Silk threads are also woven into -cloth of characteristic texture and appearance. This protein thus forms -the raw material of one of the most important textile industries. - - * * * * * - -From the fish trade there is much animal protein, which is useless for -food purposes and which, to avoid nuisance, it is necessary to convert -promptly in fertilizers. During the herring season there is the disposal -in this way of the heads, tails, and the guts. Many fish are -incidentally caught which, being valueless as food, are yet useful as -manure. After the extraction of oil from fish livers the residue is -suitable for a similar purpose. These residues are steamed, dried, and -ground up, forming fish manure, rich in nitrogen and often also in -phosphate. - - - REFERENCES. - - "Chemical Fertilizers and Parisiticides," S. H. Collins, M.Sc. - - "Organic Nitrogen Fertilizers," Part III., Section II., p. 105. - - "Fish Manure," p. 110. - - - - - INDEX - - - Acclimatization in colloid systems, 236 - - Acid, ellagic, 29 - gallic, 29 - sulphurous, 227, 243 - - Acid process for bone gelatine, 243 - - Acids, for deliming, 23 - for pickling, 114 - in sour liquors, 29, 44 - - Adsorption, law of, 43 - methods of clarification, 234 - nature of, 41 - of ions by gelatine, 211 - - African hides, 15 - - Albumins, 4, 240, 274, 277 - - Algarobilla, 32 - - Alum, 236, 240 - - American hides, 14 - - Animal excreta, 279 - - Arsenic sulphide, 20 - - Asiatic hides, 14 - - Astringency of liquors, 44 - - - Bacteria in soaks, 16 - limes, 20 - bates, 24 - tan liquors, 29 - - Bag leather, 86 - - Band-knife splitting, 52 - - Bark, hemlock, 34, 40 - mallet, 34 - mangrove, 35, 41 - mimosa, 33 - oak, 34 - pine, 34, 41 - willow, 32 - - Basic dyestuffs, 97 - - Basils, 115 - - Bating, 24, 94 - - Belting leather, 65 - - Blair-Campbell evaporator, 249 - - Bleaching leather, 62 - glue, 241 - - Block Gambier, 40 - - Bloom, 29 - - Boiling process for glue, 223 - - Bone gelatine, 223 - manure, 273 - meal, 224 - - Bones, 223 - - Bookbinding leather, 104, 106, 117, 120 - - Boric acid, 23 - - Bottle tannage, 103 - - Box calf, 156 - - Bridle leather, 71 - - British hides, 8 - - Brushing leather, 63 - - Buck leather, 181 - - Buff leather, 181 - - Buffing, 52 - - Burning in, 54 - - Butt, 22 - - Bye-products of the gelatine trade, 272 - of the leather trades, 268 - - - Calcium sulphydrate, 22 - - Calf skins, 76, 120, 156 - - Casein, 276 - - Cast glue, 257 - - Catechin, 32 - - Catechol tans, 32 - - Caustic soda, 18, 20 - - Centrifugal fan, 50 - - Chamois leather, 181 - - Cheeks, 268 - - Chemistry of colloids, 201 - - Chestnut extract, 36 - - Chlorine bleach for glue, 246 - - Chrome calf, 156 - goat, 163 - hide, 170 - sheep, 163 - - Chrome tannage, 127-174 - finishing operations, 153 - general methods, 139 - history of, 127 - one bath, 149 - special qualities of, 136 - theory of, 129 - two-bath process, 142 - - Clarification of gelatine, 234 - - Coefficient of conductivity, 253 - - Colloid chemistry, 201 - - Combination tannages, 191 - - Concentrated foods, 275 - - Condenser water, 251 - - Conductivity coefficient, 253 - - Continental hides, 14 - - Crown leather, 178 - - Cube gambier, 40 - - Curing hides, drying, 13 - dry-salting, 13 - freezing, 13 - salting, 12 - sterilizing, 14 - - Currying, 49 - - Cut glue, 257 - - - Decolorization of glue, 238 - - Deerskins, 92, 181 - - Degreasing bones, 224, 227 - leather, 115 - - Deliming, 23 - - Depilation, 19 - - Divi-divi, 32 - - Dongola leather, 191 - - Drenching, 25, 95 - - Dressing leather, 24, 65-92 - - Drum stuffing, 53 - tanning, 63 - - Drying gelatine and glue, 255 - hides, 13 - leather, 50 - - Dung bates, 24 - manures, 279 - puers, 94 - - Dyeing leather, 96 - - - Ears, 220, 268 - - Eggs, 276 - - Elastic fibres, 6, 272 - - Ellagic acid, 29 - - Enamelled leather, 123 - - Enzymes, 24, 25, 94, 95 - - Erodin, 94 - - Evaporation, 37, 248 - - Evaporators, Blair Campbell, 249 - Kestner, 249 - Yaryan, 249 - - Evolution of gelatine industry, 265 - of leather industry, 194 - - Extraction of gelatine and glue, 230-233 - of grease, 115, 224, 227 - of phosphate, 224 - of tannin, 35 - - Extracts of meat, 275 - of tanning material, 37-41 - - - Faces, 220, 268 - - Fan drying gelatine, 257 - leather, 50 - - Fat liquoring, 154 - tannages, 178 - - Federation of Tanners, 108 - - Fellmongering, 113 - - Fermentation in bates and puers, 24, 94 - in drenches, 25 - in limes, 20, 21 - - Fertilizers, 269, 279 - - Filter press, 238 - - Finger test, 218 - - Finishing chrome leather, 153 - heavy leather, 49 - light leather, 96 - - Fish glue, 228 - manure, 280 - - Fleshing, 22 - - Flocculation, 237 - - Food proteins, 274 - - Foods, concentrated, 275 - dried, 277 - - Formaldehyde tannage, 185 - - Fractional extraction of glue, 230 - - - Galalith, 277 - - Gallic acid, 29 - - Galls, 32 - - Gambier, 40 - - Gelatine, bleaching, 241 - clarification of, 234 - decolorization, 234 - drying of, 255 - evaporation of, 248 - extraction of, 230 - properties of, 200 - raw material for, 220 - uses of, 260 - - Glace calf, 156 - goat and sheep, 163 - - Glazing, 97, 155 - - Glove leather, 174 - - Glue (_see_ GELATINE) - difference from gelatine, 241 - - Goatskins, 99, 163 - - Graining, 97 - - Grease in bones and scutch, 224, 227 - in skins, 115 - - Guano, 278 - - - Hair, removal of, 22 - - Handlers, 47 - - Hard-grain morocco, 117 - - Harness leather, 71, 170 - - Heavy leather, 7-92 - chrome leather, 170 - - Helvetia leather, 179 - - Hemlock bark, 34 - - Hides, American, 14 - African, 15 - Asiatic, 14 - British, 8 - Continental, 14 - dried, 13 - dry-salted, 13 - fresh, 8 - frozen, 13 - salted, 12 - - Hoofs, 268 - - Horns, 268 - - Hyaline layer, 272 - - Hydrophile colloids, 240 - - Hydrophobe colloids, 240 - - Hydrosulphite of soda, 245 - - Hypo bath, 128, 147 - - - Imitation box calf, 159 - glace kid, 163 - - Imperial aspect of leather trade, 198 - - Increase in strength of tan liquor, 44 - - Incrustation, 254 - - Influence of Lyotrope series, 206-209 - - Intensive production, 194, 265 - - Interfibrillar substance, 24 - - Iron and logwood, 75, 83, 109 - - - Jacking leather, 51 - - Japanned leather, 123 - - Jelly, 203, 258 - - - Keratins, 4 - of epidermis, 272 - - Kestner evaporator, 249 - - Kid skins, 163, 174 - - Kips, 8, 76, 159 - - - Lactic acid, 23 - - Lambskins, 110, 163, 174 - - Larch bark extract, 41 - - Layaways, 47 - - Layer, hyaline or glassy, 272 - - Layers, 47 - - Leaching, 35 - - Leather, definition of, 27 - - Legging leather, 76 - - Levant grain, 109 - - Lime, function of, in depilation, 20 - - Liming for chrome leather, 127 - glue pieces, 220 - hides, 18 - leather, 83 - skins, 92 - - Liquor, chrome 127, 129, 143-153 - lime, 18-22 - tan, 35 - - Logwood, 75, 83, 109 - - Lyophile colloids, 201 - - Lyophobe colloids, 201 - - Lyotrope series colloids, 206-209 - - - Machine fleshing, 23 - scudding, 23 - shaving, 52, 82 - - Mallet bark, 34 - - Mangrove bark extract, 41 - - Mean temperature difference, 252 - - Meat extracts, 275 - - Mellow lime liquors, 18-21 - tan liquors, 44 - - Memel butts, 83 - - Milk, 276 - - Mimosa bark, 33 - - Miscellaneous proteins, 266, 279 - tannages, 174 - - Mixed tannage of sole leather, 55 - - Mordants, 96 - - Morocco leather, calf, 120 - goat, 99 - seal, 106 - sheep, 110 - - Motor butts, 170 - - Multiple-effect evaporation, 249 - - Myrabolans, 30 - - - Nature of chrome leather, 127 - of leather, 27 - - Nett adsorption, 215 - - Neutralization, 153 - - Nitrogen in proteins, 1 - value of manures, 280 - - Noses, 268 - - - Oak bark, 34 - - Oakwood extract, 37 - - Offal for sole leather, 63 - - Oil tannage, 181 - - One-bath chrome tannage, 149 - - One-pit system of liming, 19 - - Open-vat system of extraction, 231 - - Oxidation method of bleaching, 245 - - - Paddles for washing, puering, dyeing, and tanning, 17, 94, 96, 103 - - Parker, on valonia, 31 - - Pelt, preparation of, 16, 92, 139 - - Peroxides for bleaching, 246 - - Phlobaphenes, 33 - - Phosphate of lime, 223, 225, 273 - - Picking band butts, 90 - - Pickling foods, 275 - skins, 114 - - Pigskins, 92 - - Pine bark, 34 - - Plumping, 19, 44 - - Precipitation, 236 - - Predigested foods, 276 - - Preparation of pelt, 16, 92, 139 - - Press leach, 35 - - Principles of chrome tannage, 139 - clarification of gelatine, 234 - liming, 92 - vegetable tannage, 41 - - Procter, definition of leather, 27 - glucose chrome liquor, 152 - on gelatine swelling, 217 - on pickling, 115 - - Properties of chrome leather, 127 - gelatine and glue, 200 - - Protective colloid, 237 - - Proteins, classification, 3 - composition of, 1-3 - food, 274 - miscellaneous, 266, 279 - of dermis, 271 - of epidermis, 272 - - Puering, 94 - - Purification of grease, 227 - - Putrid soaks, 18 - - Pyrogallol tans, 28 - - - Qualities of chrome leather, 127 - gelatine and glue, 200 - - Quebracho extract, 38 - - _Quercus aegilops_, 30 - _robur_, 34 - - Quick processes of evaporating, 248 - of tanning, etc., 194 - - - Rabbit skins, 221 - - Raw material for gelatine, 220 - heavy leather, 7 - light leather, 92 - - Reds, 33 - - Reduction bleaching of glue, 243 - - Refrigerator, 258 - - Roans, 117 - - Rockers, 46 - - Roller leather, 118 - - Rolling leather, 51 - - Round of pits, 19, 46 - - Rounding pelt, 22 - - - Salted food proteins, 275 - hides, 12 - - Samming, 50 - - Satin leather, 76 - - Schultz chrome tannage, 128 - - Scouring, 51 - - Scudding, 22 - - Scutch, 271 - - Sealskins, 106 - - Seasoning, 97 - - Semi-chrome, 191 - - Sharp limes, 19 - - Shaving, 51 - - Shearlings, 114 - - Shedwork on gelatine, 257 - on leather, 50 - - Sheepskins, 110, 163, 174, 181 - - Short processes, 47, 194 - - Silk, 280 - - Skins, 92 - - Skivers, 116 - - Sludge from lime pits, 270 - - Smoked foods, 275 - - Soaking hides, 16 - - Soda, 18, 20 - - Sodium sulphide, 18, 20 - - Sole leather, 55 - - Sour tan liquors, 44 - - Split fleshes, 76, 181 - hides, 86 - - Splitting, 52 - - Staking, 155 - - Stocks, 18 - - Stoning, 51 - - Stove drying, 105, 109, 125 - - Strap butts, 65 - - Striking leather, 51 - - Stuffing leather, 49, 53 - - Substance, interfibrillar, 24 - - Sulphide of arsenic, 20 - soda, 18, 20 - - Sulphurous acid, 227, 243 - - Sumach, 31 - use in dyeing, 104, 117 - use in finishing, 62, 84 - use in tanning, 102 - - Suspenders, 46 - - Sweating, 113 - - Swelling of gelatine, 201-220 - of pelt, 19 - - Syntans, 188 - - Synthetic tanning materials, 187 - - - Tannage, alum, 174 - bag, 103 - bottle, 103 - chrome, 127, 139 - combination, 191 - drum, 47 - fat, 178 - formalin, 185 - oil, 181 - with synthetic materials, 187 - of bag leather, 86 - of bridle leather, 71 - of belting leather, 65 - of harness leather, 71 - of bookbinding leather, 99, 110, 120 - of morocco leather, 99, 106, 110, 120 - of picking band leather, 90 - of sole leather, 55 - of upper leather, 76 - of roller leather, 118 - - Tannage, chrome, of calf, 156 - of goat and sheep, 163 - of hides, 170 - - Tannage, vegetable, heavy hides, 55-90 - skins, 92-123 - - Tanning, theory of, 41 - chrome, theory of, 129 - - Tannins, catechol, 32 - classification of, 28 - properties of, 27 - pyrogallol, 28 - - Three-paddle system of tanning skins, 103 - - Three-pit system of liming, 19 - - Tissue, adipose, 271 - - Two-bath chrome tannage, 142 - - - Udders, 220 - - Unhairing, 22, 23 - - Upper leather, 76, 115, 120, 123 - - - Vacuum on condenser, 250 - pan, 248 - - Valency rule, 131, 236 - - Valonia, 30 - - Vatting sole leather, 62 - - Vegetable tannage, 41 - of hides, 55-90 - skins, 92-123 - tanning materials, 28 - - Velocity effect on heat transference, 253 - - - War, effect on methods, 194, 265 - on supplies, 11-13, 33, 277 - - Warble fly, 10 - - Waste leather, 270 - - Wattle or mimosa bark, 33 - - Waxed leathers, 76-86 - - Weather drying, 49 - - Willow bark, 32 - calf, 156 - - Wood, J. T., action of puer, 94 - - Wool, 110, 269 - - - Yaryan evaporator, 249 - - - Zones of compressed water, 202-205 - - - - - -End of the Project Gutenberg EBook of Animal Proteins, by Hugh Garner Bennett - -*** END OF THIS PROJECT GUTENBERG EBOOK ANIMAL PROTEINS *** - -***** This file should be named 41192.txt or 41192.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/4/1/1/9/41192/ - -Produced by Juliet Sutherland, Joanna Johnston and the -Online Distributed Proofreading Team at http://www.pgdp.net -(This file was produced from images generously made -available by The Internet Archive/Million Book Project) - - -Updated editions will replace the previous one--the old editions -will be renamed. - -Creating the works from public domain print editions means that no -one owns a United States copyright in these works, so the Foundation -(and you!) can copy and distribute it in the United States without -permission and without paying copyright royalties. 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