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diff --git a/41192-0.txt b/41192-0.txt new file mode 100644 index 0000000..ca8014d --- /dev/null +++ b/41192-0.txt @@ -0,0 +1,10396 @@ +*** START OF THE PROJECT GUTENBERG EBOOK 41192 *** + +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 + + BAILLIÈRE, 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°-75° C., the gelatins (if swollen) +dissolve readily, whilst the keratins only dissolve at temperatures +above 100° 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 lævorotatory. They are coagulated by +heat and also by mineral acids, alcohol, and by many poisons. The +temperature of coagulation (usually about 72° 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° 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° 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 lævorotatory, +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 larvæ, 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 versâ_. 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 anærobes, but some may be anærobic by adaptation. It +is probable that ærobic 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. Ærobic +fermentation commences with the slaughter of the animal, and the +anærobic 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 anærobic 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°-90° 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° 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 +versâ_. 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 ægilops_). 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 _cæsalpina_. +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° C. (266° F.). The series may contain 5, 7, 9 or +even 12 vats, and the liquor obtained has a strength of 3° to 4-1/2° +Beaumé (22° to 33° 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° 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° 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° Beaumé, 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° 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° Bé +(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° F., and +afterwards at 140° F. with a higher vacuum. The liquor is concentrated +from 5° to 25° Beaumé (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°-50° Bkr. (6 - 7-1/2 per cent. tan), and after +settling is concentrated at 40°-50° 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° 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 dégras. 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° to 212° 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°-115° 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° 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° F. After softening in water +at 120° 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°-40° Bkr.) in about a +week, and are rocked in the first liquors. They next enter the handlers +(40°-55°) 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°-65°) 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° for +one week; second 75° for two weeks; third 80° for three weeks; and +fourth 90° 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°, is made from the best leach liquor, 65°, and extract +(chestnut with some oakwood or mimosa bark). After use it becomes the +second layer, 75°. The third layer, 80°, 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°. 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°-40°) by +diluting with 40° leach liquor and adding quebracho extract. The old +handler liquor is run to the suspenders (40°-20°), and finally used for +colouring off the offal in drum or paddle 18°. The suspender handlers +(65°-55°) 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° \ | + | | V | + | V 3rd layer, 80° | + | 2nd layer, 75° | | + | \ V | + | \ 1st layer, 70° | + \ \ / | + \ \ / | + \ V V | + \ (mixture)[-->bellies] | + \ / V + V V Suspender handlers-->(65°-55°)(shoulders) + Handlers, 55°-40° + | + 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°-25°) in about a week, rocking +in the early liquors, and then into large rounds of handlers (30°-45°) +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° + / | + / 3rd layer, 80° + / | + / 2nd layer, 75° + / | + / 1st layer, 70°-->[offal layers] + V + Handlers (45°-30°) + | + V + Suspenders (25°-10°) + + +=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°-40°) as usual one week, and then +pass into suspender-handlers (40°-60°) for two weeks, and thence to the +layers. In the first two of these (65° and 70°) they are actually in +suspension, a week in each liquor. They are then dusted down for ten to +eleven days, first in 85° and then in a 95° liquor, and finally for a +month in a liquor of 110°. 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°-40°) as usual one week, and then +through a round of suspender-handlers (40°-55°) 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° for 2 weeks; second, 110° for three weeks; and +finally, 120° 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°-40°) together in 4-5 +days, and go through a handler round (40°-55°) for 3 weeks, including +one duster. The bellies are removed after 2 weeks, and given three +layers (60°, 70°, 80°) of a week each. They receive, therefore, nearly 6 +weeks in all. The shoulders also have three layers (60°, 65° and 80°) +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°-60°) (80°-60°) | + \ / | + V V | + Offal handlers (55°-40°) | + | V + V Offal drums + Offal suspenders (40°-18°) (18°) + \ / + \ / + \ / + \ / + \ / + \ / + 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° 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°-40°)--very sour and mellow liquors--4 weeks in handlers (40°-55°), +4 weeks in dusters (60°), 4 weeks in round made from hemlock extract +(60°), and 20 weeks in six layers (60°-90°) 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° even where valonia and +gambier were also used, and rather more than 30° 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°-40°) for 2 weeks, +drummed for 12 hours in an 80° extract liquor, and finally in a neat +extract 200° 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°-105° 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°-30°) in 2-1/2 +weeks, and then pass to the handlers (30°-50°), in which they remain a +month; they are then put into suspension again and pass through the +suspender handlers (40°-55°), 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°, one week; second layer 60°, two +weeks; third layer 65°, four weeks; and fourth layer 75°, 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°-50°) 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°-67° and 67°-80°), +which takes a month, and thence to the layers. Three layers are given +(85°, 90° and 100°), 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° 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°-24°) of twelve pits arranged on the press system, +running two fresh liquors a week, and to give them two layers (24° and +28°) 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° 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°-30°), which takes up to three weeks, and +then in to handlers (30°-40°) 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°. Four layers +are given (50°, 55°, 60° and 65°), 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°-30°) in two +weeks, handlers (30°-45°) 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° 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°-55° and 55°-75°). 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°, in each of which they +remain one week. The tannage is completed by three layers: first, 85° +for one week; second, 90° for one week; third, 95° 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° Bkr., in the other +they were all 0° 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°), and passed to the first handlers (3°-30°) for three weeks. After +working through this set they pass through the second handlers +(20°-30°), 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°) for another week. + +_Type 2_.--In this process the goods are paddled, and then enter a large +handler round (8°-30°), 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°). 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°, and are then handled well for a +week in the first handlers (5°-20°). The goods then go through the +second handlers (20°-45°) in six weeks, and heavy goods may then receive +an extra floater (45°) 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°-18°) two weeks; second handlers (40°-45°) 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° 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°-40°), 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°-50°), 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° for one week; second +55° for two weeks; and third 60° 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°-85° +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°-20°) 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°-40°) for four weeks, and the heaviest goods given two +layers (40°) 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°-35°) three or four +days; second handlers (35°-60°) two weeks; forward handlers (60°-80°) +1-1/2 weeks; and floaters (80°-90°) 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°-20°) 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° to 50°. 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° 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°) to +which a little gambier has been added. The butts then go through the +first handlers (5°-15°), 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° 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°-25° 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 +(ærobic and anærobic) 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 "glacé 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° 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° to +95° 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°-20°) 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 versâ_, +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 _Otariidæ_ 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 _Phocidæ_ 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°-88° 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° 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°-70°) 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° 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°-15° and 15°-20°), 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°-24°). 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°-80° 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°-24° Beaumé). 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° 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°-11°) 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°-20°), 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° F. for about 3 hours in a paddle, +and scudded. They are drenched at a low temperature (68°-70° 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° 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° 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° F. They may then be +finished for basils, moroccos or roller leather as described above, but +are often finished as imitation glacé 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," "glacé 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° 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° 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° 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°-200° 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 "glacé 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 - 1/3×a) × 0.01 gram CrO{3} + and [(a×0.0033) - [(b - 1/3×a) × 0.01]] × 1.47 grams K{2}Cr{2}O{7} + + II. 10 c.c. of an acid bath contain (a×0.0033) grams CrO{3} and + [(b - 2/3×a) × 0.00365] grams HCl + + III. 10 c.c. of a neutral bath (a×0.0033) grams \ + > CrO{3} + _or_ (b×0.005) 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 formulæ +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° to 130° 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° 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° 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° +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° 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° 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° 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. =Glacé 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 +=Glacé kid=, to the manufacture of which this section is chiefly +devoted. To be quite strict, glacé 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 glacé 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 glacé upper leather is made from sheepskins, and often sold +as glacé 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 glacé sheep is by no means so good a leather as +glacé 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 glacé 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 glacé 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 glacé 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°-90° F., and are carefully +delimed in the puer liquor. After puering it is common to give a low +temperature drench (60°-65° 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 glacé 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° 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° 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° 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 glacé=, 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 glacé sheep follows the same general lines as +glacé 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 glacé sheep much the same methods are used as in the case +of glacé 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° 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°-195° 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° 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° 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° 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° +F. and added to the flour and oil. The tawing paste should be used at +about 105° 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° 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° 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° 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° 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° 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° 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° 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°-140° 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° to 120° 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° +to 118° 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°-210° 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° 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° 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° 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° to +37° Bkr. may be used, but drum tannages may be given using liquors of +14°-29° 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 glacé +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. "Suède" and "velvet calf" are also +tawed and retanned with gambier. + +Yet another case of combination tannage is that of sheepskins for glacé +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 glacé kid factories there is nothing yet +to correspond to the output of American glacé 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°-70° 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 versâ_. 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 (5×10^(-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°-30° C. and set at 18°-26° 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°, 15°, -21°, -30°, and -48° 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. Pöschl. + + "Grundzüge 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° to 7° Bé.). 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° 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° 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° 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° 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° 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. Pöschl. + + 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 (° F.) of cooling water; + T{o} = outlet temperature (° 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° to 80° F. If T{i} = 80° F. and Q/W = 70°, 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° 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° 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° 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° 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" × 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° C. (78° F.); Rideal considers 21° C. +(70° 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 "Lamellæ," +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° 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 larvæ of certain +bombycid moths. These larvæ 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 + + Glacé 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 + glacé 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 ægilops_, 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 THE PROJECT GUTENBERG EBOOK 41192 *** |