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diff --git a/old/63421-0.txt b/old/63421-0.txt deleted file mode 100644 index ef67448..0000000 --- a/old/63421-0.txt +++ /dev/null @@ -1,4762 +0,0 @@ -Project Gutenberg's Glass and Glass Manufacture, by Percival Marson - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: Glass and Glass Manufacture - -Author: Percival Marson - -Release Date: October 9, 2020 [EBook #63421] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK GLASS AND GLASS MANUFACTURE *** - - - - -Produced by deaurider, Barry Abrahamsen, and the Online -Distributed Proofreading Team at https://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - - - - - GLASS - - - - ------------------------------------------------------------------------- - - - - - _Reprinted June, 1918._ - _Reprinted June, 1919._ - - - - ------------------------------------------------------------------------- - - -[Illustration: - - AN OLD GLASS HOUSE, A.D. 1790 - _Frontispiece_ -] - - ------------------------------------------------------------------------- - - - _PITMAN’S COMMON COMMODITIES - AND INDUSTRIES_ - - - - - GLASS - AND GLASS MANUFACTURE - - - - - BY - PERCIVAL MARSON - - CONSULTANT UPON REFRACTORY MATERIALS, ETC., - HONOURS AND MEDALLIST IN GLASS MANUFACTURE. - - - - - LONDON - SIR ISAAC PITMAN & SONS, LTD., 1 AMEN CORNER, E.C.4 - BATH, MELBOURNE AND NEW YORK - - ------------------------------------------------------------------------- - - - - - PRINTED BY SIR ISAAC PITMAN - & SONS, LTD., LONDON, BATH, - NEW YORK AND MELBOURNE - - - - ------------------------------------------------------------------------- - - - - - PREFACE - - -Who is not acquainted with glassware in some form or other? From the -early days of the Ancient Egyptians the art of glassmaking was known, -and it is now one of our most important industries, supplying as it does -many articles for our common domestic use and convenience. Glass windows -have introduced comfort and convenience into every home; for by their -means light is admitted into our dwellings without the wind, rain and -cold, and we enjoy the blessings of the one without the inconveniences -of the others. The purposes for which glass can be used are manifold; -and in domestic articles it contributes largely to our cleanliness and -health. In the use of spectacles, table glass, mirrors, bottles, and -many other goods our dependence upon glass becomes very evident. The -degree of proficiency attained in the manufacture of glass is still more -remarkable when we consider the various kinds of glassware used in -physical, chemical, astronomic, medical, and other scientific -investigations. Many of the wonderful results of the present times would -not have been attained without the aid of glass in supplying the needs -of our scientific investigators. Before August, 1914, few people -realised the important part glass occupies in the production of war -munitions. The importance of optical glasses for telescopes, gun sights, -and microscopes is well known. Again, glass plays an essential part in -every ship, locomotive, motor-car, aeroplane, and coal mine, and if -defective glasses were supplied there would be a great loss in our -industrial efficiency. The manufacture of high explosives or special -steels could not be carried on without the supplies of laboratory -glassware to enable the chemist to carry out his delicate tests. - -Upon the outbreak of the present war our supplies of certain types of -glassware were not made in Great Britain, but imported from abroad, and -it was owing to the energy and enterprise of a Scottish glass -manufacturer, with some assistance from a well-known scientist, that a -start was made in making these much-needed goods, and what might have -been a serious crisis was averted. Professor Herbert Jackson and the -Institute of Chemistry placed at the disposal of glass manufacturers -numerous formulas for the special glasses that were urgently required, -and later on this work was recognised by the Government; and now the -investigations are being continued by a committee, with the assistance -of the Government, under the control of the Ministry of Munitions. This -committee is now rendering the greatest assistance to manufacturers in -the general development of the glass trade and the reclamation of the -ground lost in previous years. There is now every hope that Britain may -raise again to eminence and perfection this very important industry of -glassmaking. One of the chief objects of this volume is to supply within -a small practical treatise the general available information upon glass -manufacture, much of which, although familiar to many manufacturers or -those engaged in glass works, will be of great assistance to those who -are commencing a study of this very interesting and complex subject. - -Few people have any idea of the vast and enormous trade done on the -Continent in the manufacture of glassware for export to Great Britain -and British Possessions abroad, and on this account it is essential that -so important a subject as glass manufacture should form some part in the -technical education of our universities and trade schools, so that a -section of the rising generation may be taught to understand the -manufacture of such a necessary commercial product, and assist in -recapturing the trade from the Continental glass works in supplying our -needs. That some progress has been made along these lines is evident by -the establishment at Sheffield University of a school in Glass -Technology, and it is to be hoped that similar schools will be -established in other centres, staffed by capable instructors and -supported by the co-operation of the glass manufacturers. - -The author gives in an Appendix the literature accessible to those who -wish for further information upon the subject, and trusts that, in the -presentation of these notes, in response to the demand for such a book, -a useful purpose will have been served by introducing the first -principles of glass manufacture to those interested. - -It affords me great pleasure to acknowledge the valuable aid that has -been rendered me by Mr. S. N. Jenkinson, Professor Herbert Jackson, and -Mr. Frederick Carder, to whom I am much indebted. - -My thanks are also due to the following firms: Messrs. Melin & Co., -Crutched Friars; The Hermansen Engineering Co., Birmingham; The Glass -Engineering Co., Edinburgh; and Banks & Co., Edinburgh, who have kindly -supplied me with illustrations. - - PERCIVAL MARSON. - - CRAIGENTINNY, - - EDINBURGH. - - ------------------------------------------------------------------------- - - - - - CONTENTS - - - - ------- - - - CHAP. PAGE - - PREFACE V - - I. HISTORY 1 - - II. THE CHEMISTRY OF GLASS-MAKING AND THE 4 - MATERIALS USED - - III. THE CHEMICAL AND PHYSICAL PROPERTIES OF 15 - GLASS - - IV. THE COMPOSITION OF THE DIFFERENT KINDS 24 - OF GLASS - - V. COLOURED GLASS AND ARTIFICIAL GEMS 28 - - VI. DECOLORIZERS 32 - - VII. THE REFRACTORY MATERIALS USED 36 - - VIII. GLASS HOUSE FURNACES 43 - - IX. GLASS-MELTING POTS AND THEIR MANUFACTURE 59 - - X. LEHRS AND ANNEALING 71 - - XI. THE MANIPULATION OF GLASS—GLASSMAKERS’ 76 - TOOLS AND MACHINES - - XII. CROWN, SHEET, AND PLATE GLASS 89 - - XIII. TUBE, CANE, AND CHEMICAL GLASSWARE 96 - - XIV. OPTICAL GLASS 104 - - XV. DECORATIVE GLASSWARE 108 - - XVI. ENGLISH AND FOREIGN METHODS OF GLASS 118 - MANUFACTURE COMPARED - - APPENDIX 123 - - INDEX 125 - - ------------------------------------------------------------------------- - - - - - LIST OF ILLUSTRATIONS - - - ------- - - - PAGE - - AN OLD ENGLISH GLASS HOUSE, A.D. 1790 _Frontispiece_ - - HORIZONTAL CRACKING-OFF MACHINE 1 - - INTERIOR VIEW OF AN ENGLISH 44 - GLASS-MELTING FURNACE - - EXTERIOR VIEW OF AN ENGLISH 46 - GLASS-MELTING FURNACE - - - SIEMENS SIEGBERT REGENERATIVE - GLASS-MELTING FURNACE— - - FIG. A. CROSS SECTION 48 - - FIG. B. SECTIONAL PLAN 49 - - Fig. C. SECTIONAL ELEVATION 50 - - - A MODERN GLASS HOUSE. HERMANSEN’S 52 - CONTINUOUS RECUPERATIVE GLASS-MELTING - FURNACE, COVERED POT TYPE - - HERMANSEN’S CONTINUOUS RECUPERATIVE 53 - GLASS-MELTING FURNACE, 8-POT TYPE - - HERMANSEN FURNACE— - - FIG. A. SECTION THROUGH GAS PRODUCER 54 - - FIG. B. CROSS SECTION THROUGH GAS 55 - PRODUCER - - FIG. SECTIONAL PLAN 56 - - - “THE HARLINGTON” BOTTLE-MAKING MACHINE 79 - - GLASS WORKER’S CHAIR 81 - - GLASSWARE BLOWN IN MOULDS, FIG. A. AND 85 - B. - - VERTICAL CRACKING-OFF MACHINE 87 - - FOUR STAGES IN CROWN GLASS MAKING (A, B, 90 - C, D) - - SIX STAGES IN SHEET GLASS MAKING (A, B, 91 - C, D, E, F) - - MACHINE FOR SMOOTHING BOTTOMS OF 110 - TUMBLERS - - GLASS ENGRAVING 113 - - ------------------------------------------------------------------------- - - - GLASS AND GLASS - MANUFACTURE - - ------- - - - - - CHAPTER I - - HISTORY - - -The discovery of making glass is attributed to the early Phoenicians. -Pliny relates that certain mariners who had a cargo of soda salt, having -landed on the banks of a river in Palestine, started a fire to cook -their food, and, not finding any stones to rest their pots on, they -placed under them some lumps of the soda from their cargo. They found -that the heat of their fire had melted the soda and fused it with the -sand of the river bank, producing a transparent glass. The natives in -the vicinity where this discovery was made in process of time carried on -the practice of fusing sand with soda and other materials to make glass, -until they succeeded in improving and bringing the art to a high degree -of excellence. Discoveries amongst the ruins of Pompeii and Herculaneum -present some first-rate examples of the skill attained by the ancients -in glassmaking: glass was found to have been used there, admitting light -into dwellings in the form of window glass. - -The ancient Egyptians have left us many distinct proofs that glassmaking -was practised in Egypt. At the same time, the glazing of pottery was -also carried out, proving that they knew the mode of mixing, fusing, and -melting the proper ingredients for glassmaking. Among the tombs of -Thebes many specimens of glass and glazed pottery beads have been found, -which suggests a date about 3,500 years ago. - -From the Egyptians, the Greeks and Romans acquired the art of -glassmaking, which in Nero’s time was so highly developed that clear -crystal glasses were produced in the form of drinking cups and goblets, -which superseded the use of gold cups and were much prized by the -Emperor in those days. - -Many specimens of old Roman glass discovered have been preserved in the -British Museum, and, although many valuable pieces have been lost by -disintegration and collapse due to the influence of years of exposure, -there still remain some very fine examples which show that the Romans -were highly skilled in glassmaking. One of the finest examples of the -work of the ancient Romans in glassmaking is the Portland Vase, which -was unearthed near Rome. This is an ornamented vase showing white opaque -figures upon a dark blue background. The white opal appears to have been -originally cased all over the blue and the beautiful figures carved out -in cameo fashion, with astonishing patience and skill upon the part of -the operator. - -The Venetians and Muranians followed the Romans in the art, and examples -of old Venetian glassware show rare skill and ingenuity. To the -Venetians belongs the honour of first making glass at a cost to allow of -its being more generally used, and they also introduced the art of -making window glass and drinking vessels into this country. Jacob -Verzelina, a Venetian, introduced such glassmaking into England, working -at a factory in Crutched Friars, London, between 1550 and 1557, where he -made window glass, afterwards carrying on similar work in other places -about the country until his death in 1606. - -Not until 1619 were glass works started in the neighbourhood of -Stourbridge. There we find some remains of a factory worked by Tyzack -about that date in making window glass in the village of Oldswinford. -That Stourbridge should have been selected as one of the early centres -for glassmaking is probably due to the presence in that locality of the -so necessary and important to glass manufacturers in building their -furnaces and pots, and the coal used for maintaining the fires for -melting their glass. - -Stourbridge was known for a long time before this as a centre for the -mines producing , and eventually this clay was adopted for making -glass-house pots; now many other sources are available for these -fire-clays. Much of the antiquity of the glassmaking of England is -hidden in the neighbourhood of Stourbridge, and the writer has himself -found a few antique specimens of old green devitrified window glass -embedded in the subsoil of some fields near Oldswinford, probably relics -of the Huguenots, who practised and extended the art of glassmaking in -that district. Other important centres for glassmaking now are York, -London, Manchester, Edinburgh, Newcastle, and Birmingham; but, although -glassmaking has reached a high degree of excellence in this country, -there is nothing yet comparable with the extensive factories which exist -abroad. The conservatism of many English manufacturers, and the adverse -influence of the Glass Makers’ Society, considerably restrict the -progress of this trade compared with the broad and progressive manner in -which it is carried on abroad.[1] - -Footnote 1: - - _See_ article “Trade Unionism,” in last chapter. - - ------------------------------------------------------------------------- - - - - - CHAPTER II - - THE CHEMISTRY OF GLASS-MAKING AND THE MATERIALS USED - - -The term “glass,” in a general sense, is applied to the hard, brittle, -non-crystalline, transparent, opaque or translucent vitreous substance -which results from fusing silica with active mineral solvents or fluxes, -such as the alkalies, earthy bases, or metallic oxides. Silica exists in -great abundance, in a free natural state, in the form of flints, quartz, -and sand; and in the latter form it is now most generally used for -glassmaking. When sand alkali and lead oxide are heated together to a -high temperature, the sand is dissolved by the solvent action of the -fused alkali and lead oxide until the whole becomes a molten mass of -glass. The solvent action of the alkalies, soda potash or lead oxide, is -very energetic whilst being heated, and the mass boils with evolution of -gases until, at last, the solution, becoming complete, settles down to a -clear quiescent molten liquid metal, which is quite soft and malleable, -after the nature of treacle. In this condition it is ready for working. -The time and temperature necessary for melting such mixtures vary -according to the proportions and composition of the ingredients. - -=Silica=, combined with alumina and other oxides, is freely distributed -in nature in the form of clays, granites, and feldspars, which are also -available for use in glassmaking. Originally glass was made by using -crushed and ground flint stones as the source for the silica: hence is -derived the old name of “flint” glass; but now the large extensive -deposits of white sand present a much more convenient and less expensive -source, and sand has become universally used. Fine white sand is -obtained from Fontainebleau, near Paris; other sources are Lippe, Lynn, -Aylesbury, Isle of Wight, Holland, and Belgium.[2] These are the sources -preferred by crystal glass manufacturers and makers of fine quality -glass, such as chemical ware pressed glass, tube, cane, and medical -bottles, on account of their greater purity. The commoner varieties of -sand from Reigate and Bagshot and even red sand are being used in the -manufacture of the lower grades of glass such as beer bottles and jam -jars, where a greater latitude in the chemical impurities present is -permissible. Only the best and purest silica sands are used for making -cut crystal and optical glasses. In these trades the sand is always -cleaned by washing it in water to clear it from any salt, chalk, or -other impurities which may possibly be present. The sand, after washing, -is heated to redness, or “burnt,” in order to burn off any organic or -vegetable matter, and when cold it is sifted through a fine screen to -take out any coarse grains or lumps. In this prepared state, the sand is -ready for weighing out into the proportions desired for mixing with the -other materials, and is stored for use in covered wooden compartments -situated in or near the mixing rooms, along with the other materials -which may be used in the glass mixtures. - -Footnote 2: - - _See_ “British Glass Sands” (Boswell), “British Glassmaking Sands” - (Peddle); papers read at the third meeting, Society of Glass - Technology, Sheffield, for further information. - -The alkalies, potash or soda, or a mixture of both, are commonly used in -making glass in the form either of carbonates, sulphates, or nitrates. -The soda and potash silicates form very fusible glasses, but they are -not permanent, being soluble in water; therefore they cannot be used -alone. In making glassware for domestic use, other bases, such as lead -oxide, barium, or lime, have to be added to form more insoluble -combinations with the silica or sand. - -=Carbonate of Potash= or =Pearlash=, which before the war was imported -into this country by glass makers from Stassfurt, is much prized by -crystal glass makers on account of the colourless silicate it forms when -fused with the best white sand. It is now very expensive and difficult -to get, and is less used on this account. Potash carbonate is very -hygroscopic and absorbs much moisture from the air; therefore it is -necessary to keep it within sealed chests while in store. - -Potash and soda each have an influence upon the colour of the resulting -glasses in which they are respectively used. The potash silicate gives -better and clearer glasses than the soda silicate. - -=Carbonate of Soda=, or =Soda Ash=, is now more generally used. Being a -less expensive form of alkali, it constitutes a base in most of the -commoner varieties of glassware. Carbonate of soda is manufactured in -England from common salt, of which there are large deposits in the -Midlands. This common salt, or chloride of sodium, is treated chemically -and converted into the carbonate, in which form it is supplied to the -glass manufacturers as soda ash. - -=Sulphate of Soda= (=Salt Cake=) is the form of alkali used in window -and bottle glassmaking. In mixtures containing sulphate of soda it is -necessary to use a small proportion of carbon in some form, such as -charcoal or coal, in order to assist the decomposition of the salt and -the formation of the sodium silicate. Sulphate of soda is used in this -class of glassware on account of its cheapness. Glasses made from -sulphate of soda mixtures are not so clear and colourless as those in -which the source of alkali is potash or soda carbonate. On this account, -the best crystal glasses cannot be made from sulphate of soda. - -=Potash Nitrate= (=Saltpetre=) is used in glass mixtures to oxidise the -molten metal and improve the colour of the glass. In fusing it -disengages oxygen gas, which purifies the glass while melting, and -assists the decolorizers in their action by keeping up an oxidising -condition within the molten mass. - -=Sodium Nitrate=, or =Chili Nitre=, is the corresponding soda salt to -potash nitre. It is much cheaper, but less pure; it has a similar but -not nearly so powerful an oxidising action in the glass as potash nitre. -It is exported from Chili, where it exists naturally in a crude state as -“Caliche,” from which the nitrate is refined by recrystallisation. - -=Boric Acid= acts as an acid in glass, as does silicic acid. It renders -glass more fusible and brilliant; it has a searching action upon the -colourising properties of certain metallic oxides when they are -dissolved in the glass. It is an expensive ingredient, but is -considerably used in optical and special chemical glassware in replacing -a portion of the silicates ordinarily used and forming borates. It -cannot be used in large amounts, as an excess produces glass of a less -stable nature. - -=Borax=, or =Borate of Soda=, consists of boric acid combined with soda. -It is a very useful glassmaking material and is an active fluxing agent. -If used in excess in glass mixtures it causes considerable ebullition, -or boiling of the metal. In moderate proportions it is used in the -manufacture of enamels for glass, as it helps to dissolve the colorific -oxides and diffuse the colouring throughout the enamel mass. - -=Tincal=, and =Borate of Lime=, are other forms in which borates may be -introduced into glass. - -=Carbonate of Lime=, =Limespar=, =Limestone=, =Paris White=, or -=Whitening= are all forms of =Calcium Carbonate=. It is an earthy base -and is added to the simple alkaline silicates and borates to form -insoluble combinations or double silicates of soda and lime. By the use -of lime, glasses are rendered more permanent and unchangeable when in -use. Lime forms a very powerful flux at high temperatures. The quantity -used must be carefully regulated according to the proportion of other -bases present; otherwise an inferior or less stable glass may be -produced. In excess it causes glass to assume a devitrified state. - -=Dolomite= is a _Magnesium Limestone_, and is a natural stone which is -available for use in making glass in tank furnaces. - -=Fluorspar=, or =Fluoride of Lime=, is used in giving opacity and -translucency to glass. It can only be used in small amounts, as the -presence of any large proportion attacks the clay of the pots, causing -serious damage by the sharp cutting chemical action due to the evolution -of fluorine gas. - -=Phosphate of Lime= is another material which produces opacity and -translucency, but does not seriously attack the pots. Bone ash is a form -of phosphate of lime, and is procured by calcining bones until all -organic matter is consumed. - -=Carbonate of Barium=, or =Witherite=, is a very heavy, white powder, -and is a form of earthy base available for use in glassmaking. It can be -used to replace lime, with similar results. By replacing other elements -in the glass which are of lower density, barium can be used to increase -the density of glass. Like lime it is a very powerful flux in glass at -high temperatures. It gives increased brilliancy and little coloration. -For this reason it is very useful in the manufacture of pressed -glassware, giving a glass which leaves the moulds with better gloss than -is found to be the case with lime glasses. - -=Magnesia= and =Strontia= are other bases which are less used in -glassmaking. - -=Zinc Oxide= is a base used in the manufacture of many optical glasses. -With boric acid it gives silicates of a low coefficient of expansion and -special optical values. Used with cryolite, it forms a very dense opal -suitable for pressed ware. It is rather more expensive than the other -bases used. - -=Cryolite= is a natural opacifying ingredient used in making opal -glasses. It consists of a combination of the fluorides of aluminium and -sodium, and is one of the most active fluxes known to glass and enamel -makers. Its cutting chemical attack on the pots is very intensive. It is -imported from Greenland. An artificially manufactured form of cryolite -is known, which is a little cheaper than the natural variety and gives -similar results in opacifying glass. - -=Alumina.= This is sometimes present to a small extent in glass makers’ -sands. As such it is not a dangerous impurity. It exists in combination -with silica and potash to a large extent in feldspars, china clays, and -granites. Alumina, when used, has a decided influence upon the viscosity -and permanency of glass. In large proportions it noticeably diminishes -the fusibility of glass, and makes it more or less translucent. Owing to -the refractory nature of alumina it is with difficulty that it can be -diffused in alkaline silicates, borates, or lead silicates; consequently -any considerable proportion present in glass may cause cords or striae, -which are objectionable defects in the glass. - -=Oxide of Lead.= _Red Lead_, or _Minium_, is much used in the -manufacture of enamels, table glassware, and heavy optical glass. It -gives great brilliancy and density to all glasses in which it is used, -but if used in excess the glass is attacked readily by mineral acids and -becomes unstable. Red lead is a powerful flux, even at low temperatures, -and forms the chief base in making best crystal ware and enamels. The -red oxide of lead used by glass manufacturers is a mixture of the -monoxide and peroxide. Glass manufacturers, in buying red lead, should -realise that it is the peroxide present which is the active oxidising -agent, and that at least 27 per cent. should be present. A dull, dark -red oxide shows a low percentage of peroxide; a bright orange red a high -percentage. Impure red oxides of lead may be adulterated with barytes, -finely divided metallic lead, or added water. Such impure varieties -should be avoided. The red oxide of lead is preferred to the other -oxides and forms of lead for glassmaking, on account of its greater -oxidising action, which is desirable in producing crystal glassware. - -=Tin Oxide= and =Antimony Oxide= are used as opacifiers. When used they -generally remain suspended in a finely divided form in the glass. Used -in small quantities they have a favourable influence in the development -of ruby-coloured glasses. - -=Manganese=, =Arsenic=, and =Nickel Oxides= are used in glassmaking as -“decolorizers,” which will be treated in a later chapter. - -=Cullet.= In all glasses a proportion of “cullet,” or broken glass -scrap, is used. This cullet is usually of the same composition as the -glass mixture or “batch.” The use of cullet facilitates the melting, and -assists in giving homogeneity to the resultant glass by breaking up the -cords and striae which tend to develop in most glasses. - -In the commoner varieties of bottle glass =Basalt= and other igneous -rocks are crushed and used. These are naturally occurring silicates -containing lime, alumina, alkalies, iron, and other elements in varying -proportions. They are used more on account of their cheapness, and -produce dark, dirty-coloured glasses, which in the case of common -bottles are not objected to. In some instances iron, manganese or carbon -is added to produce black bottle glass. - -Of the various silicates used in glassmaking, the silicate of alumina is -the most refractory. The silicates of lime and barium are rather -refractory, but under a strong heat and in the presence of other -silicates they can be readily formed. The silicates of the alkalies, -lead, and many of the other metals are formed at much lower -temperatures. In the case of the silicate of iron, manganese, or copper, -a strong affinity is shown between the metal and the silica, and a black -or dark-coloured slag with a very low melting point is formed. Such -slags are very active in corroding the masonry and pots of the furnace. - -No single silicate is entirely free from colour. Each gives a slight -distinctive coloration, the lead silicate being yellowish and the soda -silicate greenish, but by the judicious mixture of different silicates -and the use of decolorizers, such as manganese, nickel, etc., compound -silicates are obtained, giving less perceptible colours or crystal -effects. In optical glassmaking the use of the ordinary decolorizers is -not permissible, and the purity of the materials used becomes the most -important factor. - -The raw mixture of the various materials used in making glass is termed -a “batch.” The mixing is usually done by hand, but in many cases -mechanical batch mixers are used. If the mixing is done by hand, the -materials are first weighed out in their correct proportions by means of -a platform weighing-machine. As they are weighed out, one by one, they -are introduced into a rectangular wooden arbour or box, large enough to -hold the whole unit weight of the batch and allow of its being mixed and -turned from side to side. The batch is then sieved, and all the coarse -materials reduced or crushed to a size not coarser than granulated -sugar. By sieving and turning the batch several times a thorough mixture -of the ingredients is obtained. A few ounces of manganese dioxide are -then added, according to the unit weight of the batch weighed out, and -the proportion of decolorizer necessary; which varies according to the -heat of the furnace and the amount of the impurities present. - -The whole batch is then put into barrels and conveyed to the glass -house, where the furnace is situated. Here it is tipped into another -arbour or box in a convenient position near to the melting pot, and, a -proportional quantity of “cullet” being added, the mixture is then ready -for filling into the pots. The stopper of the pot mouth is taken away -and placed aside, and a man shovels the mixture or batch into the hot -pot until it is full. He then replaces the stopper, and, after a few -hours, when the first filling has melted and subsided, another filling -of batch into the pot takes place until it becomes full of glass metal -in its molten state. The batch melts with considerable ebullition, owing -to the chemical reactions taking place under the heat of the furnace, -giving off at the same time large quantities of gas. By the evolution of -these gases the batch shrinks in volume so that it becomes necessary to -fill a pot more than once with the batch before it becomes full of -molten metal. The capacity of the pots varies between 250 and 1,200 -kilogrammes, according to the type of glass and nature of the goods -made. - -Much care is required in mixing and sieving batches containing lead and -other poisonous ingredients, to prevent the inhalation of the dust by -the mixer. Therefore, where such materials are used, exhaust fans and -ventilating ducts should be provided and fitted in the mixing rooms. A -proper respirator should be worn by the mixer in charge to prevent any -absorption into his system of the poisonous dust. Cases of poisoning are -not unknown, but these are due to gross carelessness. A small regular -weekly dose of Epsom salts should be taken by the mixers who have to -prepare lead batches. This salt tends to remove any lead salts absorbed -in the system by converting them into insoluble lead sulphate. - ------------------------------------------------------------------------- - - - CHEMICAL FORMULAE AND MOLECULAR WEIGHTS. - - ───────────────┬───────────────┬───── - │ │_Molecular - _Materials._ │ _Formulae._ │Weight._ - ───────────────┼───────────────┼───── - Alumina │Al_{2}O_{3} │ 102 - Antimony Oxide │Sb_{2}O_{3} │ 287 - Arsenic │As_{2}O_{3} │ 197 - Bismuth Oxide │Bi_{2}O_{3} │ 468 - Boric Acid │H_{3}BO_{3} │ 62 - Borax │Na_{2}B_{4}O_{7}10H_{2}O│ 382 - Calcined Borax │Na_{2}B_{4}O_{7}│ 202 - Calcined Potash│K_{2}CO_{3} │ 138 - Carbon │C │ 12 - Carbonate of │BaCO_{3} │ 197 - Barium │ │ - Carbonate of │MgCO_{3} │ 84 - Magnesia │ │ - China Clay │2SiO_{2}Al_{2}O_{3}2H_{2}O│ 258 - Chrome Oxide │Cr_{2}O_{3} │ 153 - Cobalt Oxide │Co_{2}O_{3} │ 105 - Copper Oxide │Cu_{2}O │ 143 - (Red) │ │ - Copper Oxide │CuO │ 79 - (Black) │ │ - Cryolite │6NaFAl_{2}F_{6}│ 210 - Dolomite │CaOMgO2CO_{2} │ 184 - Fluorspar │CaF_{2} │ 78 - Gold Chloride │AuCl_{3}2H_{2}O│ 339 - Iron Oxide │Fe_{2}O_{3} │ 160 - Lime │CaO │ 56 - Lime Spar │CaCO_{3} │ 100 - Manganese Oxide│MnO_{2} │ 87 - Nickel Oxide │NiO_{2} │ 75 - Nitrate of Soda│NaNO_{3} │ 85 - Phosphate of │Ca_{3}(PO_{4})_{2}│ 310 - Lime │ │ - Potash │K_{2}CO_{3}(2H_{2}O)│ 174 - Carbonate │ │ - Potash Felspar │6SiO_{2}Al_{2}O_{3}K_{2}O│ 556 - Red Lead │Pb_{3}O_{4} │ 683 - Saltpetre │KNO_{3} │ 101 - Sand │SiO_{2} │ 60 - Soda Carbonate │Na_{2}CO_{3} │ 106 - Sodium Fluoride│NaF_{3} │ 61 - Sulphate of │Na_{2}SO_{4} │ 142 - Soda │ │ - Tin Oxide │SnO_{2} │ 150 - Uranium Oxide │UO_{2} │ 272 - Zinc Oxide │ZnO │ 81 - ───────────────┴───────────────┴───── - - ------------------------------------------------------------------------- - - - - - CHAPTER III - - THE CHEMICAL AND PHYSICAL PROPERTIES OF GLASS - - -The main essential and peculiar property of glass is its transparency. -When subjected to a gradually increasing temperature, glass becomes -softened, and whilst hot it is plastic, ductile, and malleable, in which -state it can be cut, welded, drawn, or pressed. A thread of glass can be -drawn so thin and fine that it can be twisted and bent to a remarkable -extent, showing that glass is flexible. - -The above properties shown by glass while softened under heat permit it -to be shaped and formed by a variety of methods, so that in the -manufacture of the different kinds of glass we find goods pressed, -blown, drawn, moulded, rolled and cast from the hot metal. Upon cooling, -the form given to them is retained permanently. - -Another property of glass is its conchoidal fracture and liability to -crack under any sudden change of temperature. Advantage is taken of this -peculiarity in dividing or cracking apart glass when necessary, during -the stages of the manufacture of any glass article. - - -[Illustration: - - _By permission of_ - _Melin & Co._ - HORIZONTAL CRACKING-OFF MACHINE -] - -If a glass worker, in making an article of glass, desires to detach or -cut apart certain sections, he applies a cold wet substance, such as an -iron file wetted with water, to any portion of the hot glass, which -causes it to fracture at the point of contact with the cold metal, and a -slight jar is then sufficient to break the two portions apart. This -method of chilling heated glassware to divide it is applied in the -mechanical process of cutting up the long cylindrical tubes of glass -into short sections for use as miners’ safety lamp chimneys. Wherever it -is desired to cut them through, a narrow section or line round the -cylinder is first heated by a sharp, hot pencil of flame projected from -a burner against the rotating cylindrical tube of glass at equidistant -short sections, and the divisions chilled by contact with a cold, steel -point, or the heated area may be gently scratched with a diamond point, -when a clean, sharp fracture results exactly where the chill or scratch -has been applied and spreads round the whole circumference in a circle, -giving neat, clean-cut divisions. In cutting narrow tube and cane, the -fracture caused in the structure of the glass by scratching its surface -with a steel file or diamond is sufficient to cause it to break apart -without the application of heat. - -A piece of hot glass will weld on to another piece of hot glass of -similar composition. The glass maker uses this method of welding for -sticking handles on to jugs, etc., during the process of making table -glassware. - -The density of glass varies according to its composition. Certain -classes of lead and thallium glass for optical work are of very high -density. The specific gravities of such glasses may vary from 3·0 to -well over 4·0. In soda-lime glasses the density is less and approaches -2·4. Ordinary crystal glass approximates to a specific gravity of 3·1. - -The elasticity and thermal coefficient of expansion of glass can be -regulated within normal limits. Glasses are now manufactured which can -be perfectly sealed to copper, iron, nickel, and platinum wires. - -Glass, if kept heated for any length of time at a temperature just short -of its softening or deformation point, becomes devitrified and loses its -transparency, becoming opaque and crystalline. In this state it has much -of the nature of vitreous porcelain and is totally different to -manipulate, being tough and viscid on further heating. This devitrified -state may occur during glassmaking, where the metal is allowed to remain -in the pot or tank furnace for a considerable time under low -temperature. Small stars or crystals first develop throughout the glass -and continue to grow until it becomes a stony, white, opaque, vitreous -mass. “Réaumur’s Porcelain” is a glass in a devitrified state, and is -used for pestles and mortars, devitrified glass being less brittle than -ordinary glass and similar to vitrified porcelain. - -Glass can be toughened to an extent which is surprising. Bastie’s -process consists of plunging the finished glass article whilst hot into -a bath of boiling oil, which toughens the glass so much as to make it -extremely hard and resistant to shocks, losing most of its brittle -nature. Strong plates of glass are produced by a process of toughening -under pressure. These plates of glass are used for ship porthole lights -and in positions where great strength is required. Toughened or hardened -glass is of great value in the production of miner’s lamp glasses and -steam-gauge tubing. Glass, when hardened, is difficult to cut even with -the diamond, and difficulty is experienced in finding suitable means to -cut it into shapes to suit commercial requirements. - -“_Prince Rupert drops_,” or tears, exhibit the state in which unannealed -glass physically exists. These are made as a curiosity by dropping a -small quantity of hot metal from the gathering-iron into a bath of water -and then taking the pear-shaped drops out quickly. These pear-shaped -drops of glass will stand a hard blow on the head or thicker portion -without breaking, but, if the tail is pinched off or broken, the whole -mass crumbles and falls to powder. This well illustrates the latent -stresses or strains apparently in a state of tension and thrust within -the structure of unannealed glass. - -Glass is not a good conductor of heat. This accounts for the necessity -of slow cooling or annealing glassware, and also applies when re-heating -glass, which must be done slowly and evenly to allow time for the -conduction of the heat through the mass gradually. Glass is a -non-conductor of electricity, and is used to a considerable extent in -the electrical trades for insulation purposes. Most glasses are attacked -slightly, but not readily, by water and dilute mineral acids. Continued -exposure to a moist, humid atmosphere causes slight superficial -decomposition, according to the stability and chemical composition of -the glass. Old antique specimens of glass show the superficial -decomposition caused by long continuous exposure to atmospheric -moisture. Many antique specimens have been known to collapse instantly -upon being unearthed. The first change in antique glass is exhibited by -a slight iridescence forming on the surface, gradually increasing -towards opacity afterward disintegration sets in, until it finally -collapses or crumbles to powder. Glasses high in lead are readily -attacked by the acid vapours met with in the atmosphere, but the harder -soda-lime glasses are more resistant. An excess of boric acid, soda, or -potash also renders glass subject to disintegration and decay. - -Hydrofluoric acid attacks all silicate glasses, liberating silicon -fluoride. Use is made of this acid reaction in decorating glasswares in -“Etching,” by exposing the surface of glass to the fumes of hydrofluoric -acid gas in some form. - -The most permanent glasses are those containing the highest proportion -of silica in solution, but the available heat necessary to decompose -such highly silicious mixtures is limited by the present known -refractory materials which can be procured for constructing the -furnaces. Quartz glassware is a highly silicious glass. It is now made -and used in the manufacture of special chemical apparatus and laboratory -ware such as crucibles, muffles, etc., which have to withstand severe -physical and chemical tests. This quartz glass possesses remarkable -features in its low coefficient of expansion and resistance to heat -changes. It is highly refractory. Articles made of this glass can be -heated to red heat and plunged directly into cold water several times -without fracturing. Several varieties of quartz glass are now -manufactured, and a new field for investigation is presented in applying -the features and properties of this glass for use in chemical processes. - -In a purely physical sense glass is a supercooled liquid, the silicates -are only in mutual solution with each other, and they appear to be -constantly changing. Glass cannot be described as a homogeneous or -definite chemical compound. Many of the after effects and changes which -occur in glass, and the formation of crystals in the devitrification of -glass tend to prove the above assertion. The colour changes which take -place when ruby and opalescent glass is re-heated, and even the change -in colour of glass going through the lehr, cannot be explained unless in -the above sense of viewing these remarkable changes. Glasses with an -excess of lime in their composition are more subject to devitrification -than lead glasses or those of moderate lime content constructed from -more complex formulas. The presence of a small proportion of alumina in -glass prevents this tendency to devitrification and ensures permanency. -Those glasses which have the highest silica content, and which have been -produced at the highest temperatures, show the greatest stability in -use. Bohemian glasses of this type contain as much as 75 per cent. -silica, and are produced in gas-fired regenerative or recuperative -furnaces, where the heat approaches 1,500° Centigrade. Such glass is -much sought after for enamelling on, being harder and less easily -softened by the muffle heat firing on the enamels used. Taking two -corresponding glasses of the same basicity, or proportion of silicic -acid to the bases present, those formulae which have the greater -complexity of bases produce the more fusible glasses. A multiple of -bases constituting a more active flux than a single base content, it -follows that a compound mixture of silicates fuses or melts at a lower -temperature than the respective simple silicates would. These facts are -useful in constructing commercial formulae for glasses. - -Glasses containing lead oxide as an ingredient are subject to reduction -when exposed to flames of a carbonaceous nature. The carbon partially -reduces the lead oxide to its metallic state, forming a black deposit. -On this account, lead glasses cannot be used in blow-pipe working with -the ease with which soda-lime glasses can be worked, without reduction -taking place. English crystal glass, which contains a high percentage of -lead, is usually melted in hooded or covered pots to prevent the -carbonaceous flames of the furnace reducing the lead and otherwise -destroying the clearness of the glassware. Soda-lime glass and others -without the presence of lead can be melted in open pots without any fear -of reduction. Modern gas-fired recuperative furnaces, in which more -complete combustion of the carbon takes place, can now be used for -melting lead glasses in open pots, thus presenting a great saving in the -fuel required to melt and produce such glass, besides permitting the use -of a cheaper form of pot. This cannot be done with the ordinary English -coal-fired furnaces. - -Advantage is taken of the reducing action of the coal-gas flame when -producing lustre and iridescent glassware. A small proportion of easily -reducible metal, such as silver or bismuth, is introduced into the glass -and first melted under oxidising conditions. It is then reduced in -after-working by flaming, which deposits the metal in a thin sheen upon -the surface of the glass, where it comes in contact with the reducing -flames. An example of this effect is shown in Tiffany lustre ware, in -which silver chloride is used and reduced within the glass, giving a -pretty coloured iridescence on the surface, due to the reflection of -light from the particles of metal deposited under the surface. - -“Aventurine” is a form of glass in which copper and iron oxides are -introduced under reducing conditions during melting. The glass is then -allowed to cool slowly. The metallic copper tends to separate out in -small spangled crystals, which give a pretty sparkling effect. The use -of strong reducing agents with very slow annealing is necessary to -produce this effect. Copper and gold ruby-coloured glass presents other -instances of partial precipitation of the metal by reduction within the -glass. According to the extent of reduction, the glass ranges in colour -from yellow, ruby, to brown. - -The manganese silicate is readily affected by oxidising or reducing -conditions, the purple colour being present under oxidising influences -and a greenish-grey colour under reducing conditions. In using manganese -as a decolorizer, the glass maker may have added too much of it to his -glass, in which case it shows too prominent a purple colour. To destroy -this excess of colour he pushes either a little strip of green willow -wood or a clean potato to the bottom of the pot of metal. The reducing -action of the carbonaceous gas involved takes out the excess of purple -colour by partially reducing the manganese present to a colourless -state. - -The colour of glass is gradually affected in course of time by sunlight. -This change in colour is often noticeable in old windows, the glass -having developed a yellowish green tint in course of time from the -action of the solar rays. - -Glass which has been incompletely fused or not sufficiently melted to -give a complete solution of the materials present is in a weakened state -of cohesion and is liable to disintegration. The presence of -undecomposed sulphates, chlorides, or borates in the glass also tends to -early disintegration. A continual exudation and crystallisation of salt -takes place upon the surface until the glass wholly disintegrates away -to a white powdered salt. - -Glass is a poor conductor of heat. When a piece of glass has been -expanded under the influence of heat, and is rapidly cooled, the -superficial outer portions become intensely strained and contracted upon -the interior portions, which retain the heat longer. Under these -conditions of cooling, glass is apt to “fly,” or collapse and fall to -pieces, owing to the outer portions giving way under the great strain. -These stresses or strains are relieved in the process of annealing, -under which they are gradually eased by a slow and regular cooling from -the heated condition. Certain glasses, the composition of which shows -considerable differences in the density of the respective bases present, -are more subject to this defect than those in which the bases are of -more even density and homogeneous in character. Such glasses should be -“de-graded” and re-melted in order more thoroughly to diffuse and -distribute the denser portions throughout the mass. In de-grading glass, -the hot glass is ladled out and quenched in cold water, dried, and -re-used as “cullet.” - - ------------------------------------------------------------------------- - - - - - CHAPTER IV - - THE COMPOSITION OF THE DIFFERENT KINDS OF GLASS - - -The composition of glasses may be simple, compound, or complex, -according to the number of bases or acids which may be present in the -mixture. - -=The Simple types of glass= are exhibited in the soda silicate, potash -silicate, and lead silicate. The two former silicates are of most -industrial value. - -=Soda Silicate= is made from a fusion of 100 parts of sand with 50 parts -of soda carbonate and 5 parts of charcoal. The charcoal is added to -facilitate the decomposition. The fused mass when cool is transparent -and of a pale, bluish, sea-green colour. Upon boiling it in water it -dissolves and gives a thick viscid solution called “Water Glass.” This -is extensively used in the various arts and manufactures. Textile fabric -and woodwork saturated with this solution and dried are rendered -fireproof. In the manufacture of artificial stone it forms, with lime -and other basic oxides, very stable cements. Mixed with silicious or -ganister it forms the well-known fire cements for repairing the cracks -in retorts, muffles, etc. Water glass is also used in soap, and colour -making, and for preserving eggs. - -=Potash Silicate= is less used, being more expensive. It is produced -from a fusion of 100 parts sand, 60 parts potash carbonate, and 6 parts -charcoal. - -=Lead Silicate= is composed of 100 parts sand and 66 parts of red lead -fused together. This silicate is mostly used in the manufacture of soft -enamels and artificial gems, and goes under the names of “Rocaili flux,” -“strass metal,” and “diamond paste.” - -There is another form of soluble glass which is a combination of the -soda and potash silicates. This is really a double silicate and may be -produced by fusing sand 100 parts, soda carbonate 25 parts, potash -carbonate 30 parts, and 6 parts of charcoal. This silicate is used in -soap making. Soluble glass can also be formed by using sulphate of soda -as the alkali. In this case, a larger proportion of the alkaline salt -has to be used, also a larger amount of carbon, in order to complete the -decomposition of the sulphate. A mixture of sand 100 parts, saltcake 70 -parts, and carbon 16 parts would produce sodium silicate. The boron -silicate and borate of alumina are two other forms of soluble glass used -in their simple states. - -=The Compound Glasses= may be flint or crystal glass, soda-lime glass, -Bohemian glass, pressed glass, and sheet glass. These are the general -type of glasses used in the manufacture of domestic glasswares. - -=Crystal Glass=, which is a silicate of lead and potash, is made from -best sand 100 parts, red lead 66 parts, potash carbonate 33 parts, -cullet 50 parts, to which a small proportion of potash nitre, arsenic, -and manganese dioxide is added. The bulk of English cut-glass table ware -and fancy goods are made from this type of glass. It gives very -brilliant and colourless results, more especially when cut and polished. -A second-rate quality of crystal glass for table ware may consist of a -silicate of lead and soda, as follows: sand 100 parts, red lead 66 -parts, soda carbonate 25 parts, cullet 50 parts; with small proportions -of Chili nitre, arsenic, and manganese. - -=Bohemian Glass= is made from sand 100 parts, potash carbonate 35 parts, -lime carbonate 15 parts, cullet 50 parts; with small proportions of -potash nitre, arsenic, and manganese dioxide. This type of glass is used -mostly by continental manufacturers for chemical ware, table and mirror -glass. It is a hard, brilliant, and stable glass, very suitable for -enamelled glassware. It is a silicate of potash and lime. - -=Pressed Glass= consists of sand 100 parts, soda carbonate 50 parts, -barium carbonate 15 parts, cullet 50 parts; together with soda nitre, -arsenic, manganese, and cobalt. This is used by manufacturers of pressed -glass table ware or moulded ware. It is a silicate of soda and barium, -the barium having a direct influence in giving a good surface to the -pressed goods. - -=Crown Glass= consists of a silicate of soda and lime; sand 100 parts, -soda carbonate 36 parts, lime carbonate 24 parts, soda sulphate 12 -parts, cullet 50 parts; with traces of manganese and cobalt. This glass -is used for making sheet window glass by the crown, disc, and cylinder -methods. - -=Plate Glass= is a silicate of soda and lime; sand 100 parts, soda -sulphate 55 parts, limestone 30 parts, coal or anthracite 5 parts; with -traces of nickel oxide, cobalt, or antimony oxide. This is used for cast -plate glass, rolled plate, cathedral glass, window and mirror glass. - -=The Complex Glasses= may be described as those in which more than three -bases are introduced, and constitute such types of glasswares as -bottles, thermometer tubes, chemical ware, etc. - -=Common Bottle Glass= may be described as an example of complex -formulae. Common bottle glass, or tank metal, is made from a silicate of -soda, alumina, lime, magnesia, and iron, as follows: Common sand, -containing iron and alumina, 100 parts; greenstone or basalt (a silicate -of alumina, iron, lime, magnesia, and potash), 25 parts; dolomite -limestone (magnesia and lime), 30 parts; sulphate of soda, 35 parts; -carbon, 5 parts. Felspathic granites may be also used in such glasses. - -Bottle glasses require intense heat to melt, and are usually dark in -colour when made from igneous rocks, owing to the large amount of -colorific oxides present in such materials. These dark colours are not -objected to in bottles for stout, wine, and beer. - -It will be noticed these formulae cover a long range, from the best -table glass to the commonest dark bottle glass. Besides these, opal, -opalescent, and fancy glasses are made, in which either arsenic, tin, -alumina, antimony, zinc or barium oxides or borates phosphates and -fluorides may enter into the compositions. - -Glass makers’ recipes vary considerably in the proportions of the -various materials used, according to the locality and the type of -furnace used. Generally, it will be found that, where a gas-fired -furnace is in use, a larger proportion of sand can be used and a cheaper -metal produced. - -The metals produced in covered pots are usually softer and contain more -lead and fluxes than those produced in open pots. In using open pots the -heat of the furnace has direct access to the surface of the metal -therein. In the case of covered pots, the heat has to be conducted -through the cover of the pot, which retards the heat to a certain -extent. On this account, softer mixtures are used in covered pots. - - ------------------------------------------------------------------------- - - - - - CHAPTER V - - COLOURED GLASS AND ARTIFICIAL GEMS - - -In colouring glass, either or several of the following colorific oxides -may be used. They are added to the batch before fusion. Varying -proportions are added, according to the depth of the colour desired. -Occasionally the colour is influenced by the nature and composition of -the rest of the batch. In some instances several colouring oxides are -used. In this way many delicate tints are obtained; in fact, there are -but few colours that cannot be produced in glass. - -=For Green Glasses= the following oxides may be used: Chromium oxide, 2 -to 6 per cent. of the batch; black oxide of copper, ·5 to 3 per cent.; -red iron oxide, ·5 to 1 per cent.; or a mixture of two or three of the -above oxides in less proportions. Salts of chromium, copper, or iron may -be used as the carbonates, sulphates, and chromates. - -=For Blue Glasses=, cobalt oxide, ·1 to 1 per cent. of the batch; zaffre -blue or smalts, 1 to 3 per cent.; nickel oxide, 2 to 4 per cent.; iron -oxide, 1 to 2 per cent.; black oxide of copper, 2 per cent. - -=For Violet and Purple=, manganese oxide, 2 to 4 per cent. of the batch. - -=For Rubies=, red oxide of copper, gold chloride, purple of cassius, -antimony oxysulphide, selenium metal in small proportions. - -=For Yellows=, uranium yellow, 4 to 6 per cent. of the batch; potassium -antimoniate, 10 per cent.; carbon, 6 per cent.; sulphur, 5 per cent.; -ferric oxide, 2 to 4 per cent. Silver nitrate and cadmium sulphide are -also used. - -=Black Glass= is obtained from mixtures of cobalt oxide, nickel oxide, -iron oxide, platinum and iridium. Many very dark or black bottle glasses -are obtained by using basalt, iron ores, or greenstone in a powdered -form, added to the batch ingredients. - -=White Glasses= or =Opal= are obtained by using phosphate of lime, talc, -cryolite, alumina, zinc oxide, calcium fluoride, either singly or in -double replacements of the bases present in the glass batches. - -Many of the colouring oxides give distinctive colours to glass of -different compositions; also the resulting colours may vary with the -same colouring ingredient, according to reducing or oxidising meltings. -Thus, in a batch of reducing composition, red copper oxide gives ruby -glass, but in oxidising compositions the colour given is green or -bluish-green. Iron oxide in an oxidising batch gives a yellow. In the -reducing batch it gives bluish or green results. Manganese is similarly -affected. - -Many colouring oxides give more brilliant tints with glasses made from -the silicates of potash and lime than if used in glasses composed from -silicates of lead and soda. For many colours the lead glasses are -preferred. In colouring the batches, the colouring oxides must be -intimately mixed with the batch materials before fusion, more especially -in the preparation of the pale tints, where only small quantities of -colouring are necessary. It is a well-known fact that careful mixings -give good meltings, for then the materials are more evenly distributed -and uniformly attacked during the melting. Careful and exact weighings -are necessary when using colorific oxides, and a pot is kept for each -respective colour melted, so that the different colours and crystal -glasses do not get contaminated with each other. When open pots are used -for colours, the colour pots should be kept together in one section of -the furnace, so that whilst melting, especially during the boiling up of -the batches, the colours do not splash over into the other pots -containing crystal metal. - -As a rule, smaller pots are used for coloured glass; generally they are -only a third of the size of crystal melting pots. When this is so, they -are set together under one arch of the furnace, and the workman informed -which pots contain the respective colours. All colour cuttings and -scraps should be kept separate from other cullet for re-use. Coloured -glasses are expensive, and no waste of glass should be permitted. - -=Artificial Gems.= In the manufacture of the glasses for imitation -“paste” jewels, every effort is made to procure pure materials and -colorific oxides. The base for making artificial gems is a very heavy -lead crystal glass termed “=Strass paste=,” which gives great brilliancy -and refraction. The composition of such a paste would be: Best white -sand 100 parts, pure red oxide of lead 150 parts, dry potash carbonate -30 parts. These should be thoroughly well melted until clear and free -from seed, and the molten mass ladled out of the pot and quenched in -cold water, or “de-graded.” This assists in making the paste -homogeneous. After repeated melting and de-grading, the paste or cullet -is collected, dried, and crushed for use in making the coloured pastes. -Usually, this strass metal is melted in small, white porcelain crucible -pots holding about 5 to 10 kilogrammes of the metal and heated in a -properly regulated gas and air injector furnace. The coloured paste is -kept in fusion for a whole day, after which it is slowly cooled and -annealed within the pot, and the gems cut from the lumps of glass thus -obtained. The following are some of the compositions used in the -preparation of the respective gems. - -=Opal.= Powdered strass paste, 1,000 parts; white calcium phosphate, 200 -parts; uranium yellow, 5 parts; pure manganese oxide, 3 parts; antimony -oxide, 8 parts. - -=Ruby.= Powdered strass paste, 1,000 parts; purple of cassius, 1 part; -white oxide of tin, 5 parts; antimony oxide, 10 parts. - -=Beryl.= Powdered strass, 1,000 parts; antimony oxysulphide, 10 parts; -cobalt oxide, ·25 parts. - -=Amethyst.= Powdered strass glass, 1,000 parts; purest manganese oxide, -8 parts; pure cobalt oxide, 2 parts. - -=Emerald.= Powdered strass glass, 1,000 parts; green chrome oxide, 1 -part; black copper oxide, 8 parts. - -=Sapphire.= Powdered strass glass, 1,000 parts; pure cobalt oxide, 15 -parts. - -=Topaz.= Powdered strass glass, 1,000 parts; antimony oxide, 50 parts; -uranium yellow, 10 parts. - -=Garnet.= Powdered strass glass, 1,000 parts; antimony oxysulphide, 100 -parts; gold chloride in solution, 1 part; pure manganese oxide, 4 parts. - -=Turquoise.= Powdered strass glass, 1,000 parts; cobalt oxide, ·5 parts; -black copper oxide, 10 parts; white opal glass, made with tin oxide, 200 -parts. - -After suitable pieces of glass of the requisite tints are obtained, they -are cut and ground on a Lapidary’s wheel, then polished, engraved, and -set as gems. - -=Artificial Pearls= are now cleverly made in glass. A tube of the -requisite size made of translucent or opal glass is cut into small -sections, which are heated on a tray to softening point whilst set in a -rotatory movement. As the heat increases they gradually melt in and seal -at the openings, when they are removed from the tray and sorted. - - ------------------------------------------------------------------------- - - - - - CHAPTER VI - - DECOLORIZERS - - -Decolorizers are the agents employed by the glass maker to neutralise or -subdue the objectionable tints given by the colouring action of small -traces of iron oxide, which exists as an impurity present in the -materials used or otherwise become accidentally admixed during the -process of the manufacture of glassware. - -The small additions of manganese dioxide, arsenic, nitre, nickel oxide, -selenium, antimony, oxide, etc., to glass batches may be considered as -decolorizers. The most commonly used of these materials is manganese -dioxide, so the action of this material will be explained. Every glass -maker finds that one or other of the raw materials he uses may contain -impurities. It is seldom that glass makers’ sand can be obtained that -does not contain traces of iron oxide present as an impurity. Again, the -cullet collected from the glass house often contains iron scale or rust -from the blowing-irons, which firmly adheres to the glass and gets -admixed with the batch for re-melting. The presence of even very small -traces of iron in glass becomes evident as a pale sea-green tint when -viewed through any thickness of metal. The chemical action of the glass -upon the walls of the pot is continually dissolving a minute quantity of -iron from the and diffusing it throughout the metal, giving it a -tendency to the pale-green tint. - -To subdue or neutralise this objectionable tint in the glass, the glass -maker uses certain metallic oxides which give delicate counter-tints. -Only those glasses which are made from the purest materials can be -decolorized to become sufficiently clear to use in making the best table -glassware. In optical glassware, where the use of manganese is not -permissible, the greatest care has to be taken in the selection and -testing of the materials to be used. If manganese oxide be used in -making optical glass, although the eye may not be sensitive enough to -observe the actual color absorption, glass is produced in which the -solar rays are obstructed, and much less light is transmitted by the -glass when used as an optical lens or prism. Therefore the optician -avails himself of those glasses which have not been decolorized as being -more satisfactory for his purpose, as more light is transmitted by such -glasses. - -Apart from the pale sea-green tint given to glass by the presence of -small traces of iron, certain of the silicates themselves produce -natural colors. The soda silicate present in soda-lime metal tends to -give a pale bluish-green tint when viewed through any thickness of -glass. The lead silicate has a yellowish hue. Each of these influences -has to be counteracted if clear crystal glass is desired. The -decolorization of glass by manganese dioxide depends upon the purple -tint it gives to glass. This purple color, being complementary to the -pale green color given by the presence of iron, serves and acts as a -counter-tint, and by the absorption of the green light, a less -perceptible coloring is produced. In the case of the decolorization of -glass, we get the red and blue of the purple subduing the blue and -yellow or green tint given by the iron. But certain other factors are -necessary. The purple color from manganese oxide is given only to glass -in the presence of oxidizing agents; and the absence of sufficient -oxidising agents in the glass batch, the purple manganese colour is -unstable and its action as a counter-tint is lost. Therefore, the glass -maker uses strong oxidising agents in his glass mixtures for crystal -effects, usually in the form of potassium nitrate and red lead, which -liberate oxygen. Whilst undergoing decomposition in the glass melt, the -presence of this free oxygen keeps the manganese used in a higher state -of oxidation, and gives the necessary purple coloration. It is also -evident that, if the glass melting in the pot is kept at a high -temperature for any considerable length of time, this period of -oxidation cannot last, and, after all the free oxygen gas has been -evolved, any further heating tends to turn the glass greenish again or -of poor colour, by the conversion of the manganese into the lower state -of oxidation in which the purple colour is not evident. If by chance the -glass maker has added too much manganese to the glass, and the purple -colour becomes too evident, he resorts to the use of a small amount of -carbonaceous reducing agent, such as a piece of charred wood or potato, -which he plunges or pushes to the bottom of the pot by means of a forked -iron rod or pole, where it vaporises, giving off moisture and -carbonaceous gases which reduce the manganese purple colour to a lower -oxidised colourless state, and in a very short time the excess of purple -colour has disappeared and the glass appears colourless. - -Much of the success of crystal glassmaking depends upon the proper -adjustment of the decolorizers used and obtaining the best colourless -effect. The quality of the manganese is important; only pure manganese -dioxide should be used. In many cases the mineral ore, pyrolusite, is -used on account of its cheapness. This is objectionable, as much iron -may be present in the ore, when its use as a remedy is worse than the -defect. The necessity of taking advantage of the services of a -consultant chemist here becomes apparent, for, if glass manufacturers -would only have their different consignments of materials examined and -tested from time to time, many of the disappointments and difficulties -experienced by them at present would be obviated. A considerable saving -in the cost of batch materials can be made by the judicious selection of -more suitable qualities in preference to inferior or adulterated -varieties. In many cases, a chemist can substitute for certain of the -expensive batch materials other cheaper materials introducing the same -elements at less expense, and still retain the same quality in the glass -produced. - - ------------------------------------------------------------------------- - - - - - CHAPTER VII - - THE REFRACTORY MATERIALS USED - - -Of the greatest importance to the glass manufacturer are the refractory -materials upon which the life of his furnace and pots depends. A few -notes giving a description of them and dealing with the manufacture of -the fire-resisting blocks used in building the furnaces will be of -interest. - -The chief and most generally used of such materials are the goods. The -best known deposits of fire-clays in this country are those in the -Midlands, Stourbridge, Leeds, and Glasgow districts. In each of these -districts the mining of fire-clays and the manufacture of fire-resisting -goods for furnace work forms an important industry. The theoretical -composition of a true would be a double silicate of alumina, and in this -pure state it would be of a very refractory nature. But, naturally, -fire-clays show the presence of other bases, such as iron, lime, -magnesia, titanium, and alkalies, which, if present to any appreciable -extent, lower the degree of resistance to heat or refractoriness of the -clay. These other bases may be considered as impurities or natural -fluxing agents. The characteristics of a highly refractory clay suitable -for glass manufacturers’ requirements would be: (_a_) that such a clay -should show no signs of softening at the highest heat of the furnace; -(_b_) a squatting point not below Cone 31 or 1690° Centigrade; (_c_) a -high alumina content not below 30 per cent.; (_d_) the greatest freedom -from impurities; (_e_) a fine-grained texture; and (_f_) a high degree -of plasticity. These are the qualities most essential for glass house -work. The figures given by the chemical analyses of good fire-clays -would probably fall within the following limits— - - - Silica 49% to 65% - - Alumina 48% to 31% - - Ferric Oxide 0·5% to - 1·5% - - Titanium Oxide nil to - 1·5% - - Lime nil to - 0·5% - - Magnesia nil to ·2% - - Total Potash and 0·5% to - Soda 1·8% - - -Clays of higher silica content than 70 per cent. would not be considered -suitable as pot-clays owing to the case in which glass attacks silicious -clays. It is important that chemical analyses of fire-clays should be -compared with results obtained from the analysis of fired or burnt -samples, or they should be recalculated to allow of such comparison, so -as to exclude the figures for the hygroscopic and chemically combined -water of the clays. - -The writer gives the following particulars of a very suitable for glass -house pot-making. It is plastic and highly refractory, and is now being -considerably used by the trade. The clay is supplied by Mansfield Bros., -Church Gresley. The figures are from a report made by Mr. J. W. Mellor, -D.Sc., of the County Laboratory, Stoke-on-Trent, and are as follows— - - - Raw Fire-clay Dried at 109° Cent. - - Silica 46·45 per - cent. - - Titanic Oxide 2·65 per cent. - - Alumina 35·32 per - cent. - - Ferric Oxide 1·31 per cent. - - Manganese Oxide — - - Magnesia 0·09 per cent. - - Lime 0·41 per cent. - - Potash 1·08 per cent. - - Soda ·76 per cent. - - Loss when 12·14 per - calcined over cent. - 109° Cent - - The melting point is given as equal to Seger Cone 33 - or 1730° Centigrade. - - -The physical properties of fire-clays vary as well as their chemical -properties. The analysis alone of a is not always sufficient indication -as to its ultimate behavior when in use. Many physical tests have to be -carried out before a clay can be proved satisfactory for a particular -purpose, and much information can be gained by engaging the services of -a specialist upon refractory materials to carry out petrographic, -pyrochemical, and physical tests, and report upon the suitability of the -material for its specific purpose. Fire-clays should be plastic, and -this plasticity should be developed to its utmost to increase the -binding properties of the clay when used. To develop the plasticity, -fire-clays should be weathered or exposed in thin layers to the action -of atmospheric influences. The heat of the sun and the action of frosts -and rain have a direct influence in breaking up the clay and developing -its better properties. The use of new unweathered clay is the cause of -much trouble to the glass manufacturer who makes his own pots and -furnace goods, and on this account he should insist upon having his -clays weathered for some time before use, so as to have them thoroughly -matured. Before fire-clays are weathered or used for important work they -should undergo a process of selection and cleaning. When first raised -from the mines all foreign and inferior portions, carbonaceous matter, -vegetation, iron pyrites, and stones are removed. The best and cleanest -portions of the are sorted out and removed to the weathering beds, where -the lumps are broken down to small pieces about the size of an egg, and -left to mature and season by weathering. - -This is then spread out in a layer about 2 ft. deep, and, after a period -of exposure to the action of the weather, the heap is turned by men -shoveling the clay from one side to the other. The clay, under the -continued action of the wind, frost, and rain, disintegrates and slacks -down until it is reduced to a mild, fine-grained mass, which condition -shows it to be well seasoned and ready for use. Fire-clays vary in this -respect: some clays season quickly in the course of a few months, others -take years to develop their proper nature. The former may be classed as -mild fire-clays, the latter as strong fire-clays. - -After weathering, the clay is carted or conveyed to the clay-grinding -plant, where it is stored under cover until it is dry enough to be -ground on the clay-mill. Here the clay is fed into a revolving pan, and -crushed under heavy iron runners, and, after passing through -perforations in the bottom of the pan, it is elevated on to screens -which sieve the clay to a requisite degree of fineness. It is then -admixed with a large proportion of ground-burnt and the mixture is -tempered with water until it forms a plastic mass of dough, which is -conveyed to the workshops where the furnace blocks or pots are to be -made. These making and drying shops have false or double floors, under -which steam or heated air is passed using pipes or flues below the -floors, giving the steady and uniform heat which is necessary to dry the -goods as they are made. Heavy goods should on no account be hurried in -drying, lest trouble should occur through the goods cracking or warping. - -In making the blocks for the furnaces the workman takes a portion of the -prepared clay and tramps the plastic mass into a wooden frame, or mold, -the shape and size of the block required, with due allowance made for -shrinkage. The blocks are made on the warm floor, which is of cement or -overlaid with quarries. When the mold is filled the surplus clay is cut -off and the wooden frame is lifted up, leaving the clay block on the -floor. The empty mold is then cleaned and refilled. The blocks are left -until they attain considerable stiffness from the evaporation of the -water present by the heat of the room. They are then dressed and cut to -the final shape desired, after which they are further dried until they -become quite hard and white. When thoroughly dry the blocks are removed -from the drying sheds to the kiln for burning. - -In burning thick and heavy blocks much care and vigilance is required in -expelling the chemically combined water present in the clay, and, as the -temperature rises and approaches red heat, the rate of heating should be -retarded to allow proper oxidation to take place throughout the -structure of the blocks, and prevent black cores from being formed. In -all fire-clays, besides the mechanically admixed water used in preparing -the clay to a plastic mass, which is mostly driven off whilst in the -drying shed, there exists water in a chemically combined state. This the -combined water is not expelled below 250° Centigrade, and is tenaciously -held by many varieties of mild fire-clays. Due care has to be exercised -in dehydrating goods made from such clays; therefore the man in charge -of the burning regulates his fires, keeping the kiln at a moderate heat -for some time to allow this chemically combined water to be properly and -completely expelled. This dehydration stage in burning clay goods occurs -between the temperatures of 300° and 650° Centigrade. - -After the dehydration stage of burning is completed, the fireman raises -the temperature within the kiln to a dull red heat, when the next stage -in the process of burning begins. This is the oxidation period, during -which any organic carbonaceous matter present in the clay is expelled. -During this stage in burning, goods require extended time, to allow for -the heated air to permeate and get to the interior portions of the -blocks and oxidize the cores; otherwise the blocks are badly burnt. - -After the oxidation stage is completed, the fireman raises the heat -quickly until he obtains a high temperature, sufficient to eliminate and -complete the shrinkage of the goods. When this heat is sufficient to -complete the fire-shrinkage, the kiln is finished and is allowed to cool -down. The blocks, when cold, are then withdrawn and delivered to the -furnace builder. - -For the erection of the furnaces several grades of blocks are used, -according to the conditions and nature of the heat they have to resist. -In the presence of reducing agents, fuel ash, or glass, goods vary -greatly as to their suitability. So the local conditions to which they -are to be subjected whilst under heat should be first ascertained, and -the mixtures for the blocks adapted accordingly. So many differences -exist in the pyrochemical and physical properties of clays that their -misuse is often apt to occur if the conditions under which they are to -be used are not properly understood and allowed for. A may show a the -high degree of refractoriness under a fusion test, and yet be less -suitable for a specific purpose than one of less refractoriness showing -better physical properties and of the more suitable chemical -constitution. The size of grain in both the burnt clay and raw clay used -in the mixtures for making glasshouse furnace blocks is of the greatest -importance. In many cases it is necessary to grade the ground-burnt -material used, so that the proportion of coarse grains to the fine flour -can be regulated to suit requirements. The burnt clay used in making the -furnace blocks should be hard and well burnt, to prevent any -after-shrinkage of the goods when they are used in the furnace. -Fire-clay goods for glass house furnaces should not be burnt at a lower -temperature than Cone 12, and in the construction of gas-fired furnaces -and tanks, burning the blocks at a higher temperature, Cone 14 would -give much better results. - -On the Continent the glass manufacturers usually grind and mix their -fire-clays, with the result that they know exactly what they are using -in making their pots and furnace goods, and they are not then dependent -upon outside firms to carry out their wishes. English glass -manufacturers usually buy their clays ready mixed, and as often as not -have perforce to take the mixtures offered by the clay firms. -Unfortunately, in Great Britain many of the firms who supply the -refractory requirements of the glass trade are exceptionally backward in -applying technical knowledge to their trade; consequently, progress is -somewhat retarded in the glass trade as far as the refractory materials -are concerned. So obstinate is this ignorance of science that quite -recently one well-known firm replied to an inquiry for samples of -fire-clays to be sent for important research work then being undertaken -upon the resources of the country, stating “that, as their clay product -was perfect, and research work was quite unnecessary.” It often turns -out that their conservatism is simply a cloak to hide ignorance, as it -is quite evident to any technicist that there is ample scope for -improvement in the present goods on the market, and such an open -opportunity for a scientific investigation into the nature of their -fire-clays, however well known they may be, should be welcomed with -delight, and every facility and assistance offered for research chemists -to improve their material, and apply tests with the object of developing -the best properties of such refractories for special purposes. - - ------------------------------------------------------------------------- - - - - - CHAPTER VIII - - GLASS HOUSE FURNACES - - -The pots within which the raw materials are melted are set within a -strongly heated chamber called the glass furnace. The old circular type -of English furnace usually contains either six, ten, or twelve pots, and -will be described first. The pots stand in a circle upon a form of hob -called the “siege,” which constitutes the floor of the furnace. In the -centre of this chamber and below the level of the siege is the “eye” of -the furnace through which the flames come from the furnace fire below. -The burning fuel is contained in a circular or cylindrical-shaped -fire-box, about 4 ft. deep and 5 ft. in diameter, and is supported by a -number of strong iron bars across the bottom of the fire-box. Passing -under the fire-box, and across the whole width of the glass furnace, -there is an underground tunnel called the “cave,” each end of which is -exposed to the outside air, which is drawn in through the caves by the -draught of the chimney cone above the fires. These caves are of -sufficient height and width to allow the fireman, or “tizeur,” as he is -called, to attend to the stirring of the furnace fires from time to -time. Using a long hooked bar of iron, he rakes out the dead ashes and -clinkers, as they are formed, and stirs the fire through the bars by -prodding the fuel with a long poker. The coal is fed upon the furnace -fire through a narrow mouth situated in the glass house leading into a -chute which runs under the siege, from the glass house floor level -towards the fire-box of the furnace. The fuel is pushed down this chute -and falls into the fire-box and is fed at intervals of the half to -three-quarters of an hour, according to the heat desired and the draught -allowed. - - -[Illustration: - - INTERIOR OF ENGLISH TYPE OF GLASS-MELTING FURNACE -] - - -Above the siege and over the pots is a covering called the crown of the -furnace, which is supported by fire-brick pillars. This is built of the -most refractory material possible to be obtained, as the hottest flames -from the furnace fires beat against this crown and are reverberated -downwards upon the surrounding pots. The flames, continuing their -course, pass between the pots into small openings or flues leading from -the siege floor and passing upwards through the pillars which are -situated between each pair of pots, they then escape from little -chimneys leading into the outer dome or conical-shaped structure so -familiar to outsiders. This outer truncated cone-shaped structure -constitutes the main chimney of the furnace. The furnace chamber -containing the pots is constructed entirely within this cone. The blocks -are carefully shaped, neatly fitted, and cemented together with a mortar -made of fine, plastic, raw ground mixed to thin paste with water. The -presence of any molten glass which escapes from a cracked pot, and the -fluxing action of the fuel ashes, cause severe corrosion of the blocks -forming the siege and fire-box, and these necessarily have to be made of -extra thickness in order to extend the life of the furnace. When the -furnace crown or siege becomes badly corroded away, the furnace has to -be put out for repair; so generally an auxiliary furnace is kept at -hand, in order that it may be started and the workmen transferred from -one furnace to the other whilst the repairs are being done. - - -[Illustration: - - EXTERIOR VIEW OF ENGLISH GLASS-MELTING FURNACE - Pot Trolley in foreground -] - - -The action of the glass upon the siege of the furnace is very active, -and any leakage quickly destroys the blocks, leaving fissures which -gradually increase in size until the blocks are eaten right through. -Consequently, every care is taken to preserve the pots from losing -metal. If by chance any pot develops a crack through which the metal -leaks into the furnace, the glass working is ceased at that particular -pot, and every endeavour is made to ladle out what remains of the metal, -and so prevent any more running on to the siege and causing further -mischief. The metal is ladled out of the pot by means of thick, heavy, -iron spoons, with which the hot metal is scooped out of the pot and -dropped into a large cauldron containing water. This is very exhausting -work, but there is worse trouble still if the metal is allowed to -continue to run through the crack in the pot and over the siege into the -eye of the furnace, for it then fluxes with the ashes of the fuel, -causing them to form into a big mass of conglomerate, which, lying in -the fire, interferes with the draught and combustion of the fuel within -the furnace, and before the furnace can be got to work properly again -has to be cut away, piece by piece, through the firebars whilst hot, -until it is all removed. At the sign of any glass running down into the -fires and through the bars, the tizeur hurries up to give the word that -a pot is leaking in the furnace, and when the pot is isolated the work -of ladling the hot metal out into water begins in earnest. A pot which -has cracked and leaks is useless for any further work of melting glass, -and at a convenient time it has to be withdrawn from the furnace and a -new pot must be substituted. Glass-melting pots form a very expensive -item in the glass manufacturer’s costs; consequently, every care is -taken to prevent the pots within the furnace from getting chilled by -inadvertently allowing the fires to burn too low or allowing cold air to -rush through the bars, through unskilful clinkering and inattention to -the furnace fires. Sometimes these furnaces are fitted with a Frisbie -Feeder. This is a mechanical firing arrangement fitted underneath the -furnace bars, by which the fuel is fed upwards into the furnace box, so -that all smoke given off by the fuel baitings has to travel through the -hot fuel above, and thereby is more completely consumed, giving better -combustion than when the black fuel is thrown on the top of the hot bed -of fuel. A mechanically operated piston pushes up small charges of fuel -from within a cylindrical-shaped box, which works on a swivel backwards -and forwards as the fuel is fed into it. - -In the old type of English furnace containing twelve pots, each 38 in. -diameter and holding about 15 cwts. of metal, the furnace would be -capable of melting 7 to 8 tons of glass a week, taking 40 tons of best -fuel. The more up-to-date glass-melting furnaces are constructed upon a -much better principle than the coal-fired old English type of furnace -just described. These are usually producer gas-fired and give more -economy and greater convenience in every way. - - -[Illustration: - - FIG. A - SIEMENS SIEGBERT TYPE OF REGENERATIVE GLASS-MELTING FURNACE -] - - -In these better types of modern furnaces some form of regeneration or -recuperation of the waste heat is usually adopted. These furnaces are -much smaller and more compact; being gas-fired, they give much higher -temperatures, more complete combustion of the fuel, greater ease in -regulation, cleaner conditions, and far greater production than the -older types of English furnaces. Considering the reasonable initial cost -that the latest types of these modern furnaces can be built for, it -appears incredible that so many of the old out-of-date English furnaces -remain in use in this country. - - - -[Illustration: - - FIG. B - SIEMENS SIEGBERT TYPE OF REGENERATIVE GLASS-MELTING FURNACE -] - - - -As examples of the types of regenerative and recuperative furnaces, a -description will be given of the Siemens Siegbert Gas-fired Regenerative -Furnace and the Hermansen Recuperative Furnace for glass-melting, which -are extensively used on the Continent and are giving remarkably good -results. - -[Illustration: - - FIG. C - SIEMENS SIEGBERT TYPE OF REGENERATIVE GLASS-MELTING FURNACE -] - - - -In the Siemens Siegbert type, the furnace may be a rectangular or an -oval-shaped chamber, approximately 18 ft. by 9 ft., the crown of which -is about 4 ft. 6 in. high. No outer cone-shaped dome exists, and the -pots within the chamber are arranged much closer together and -practically touching each other round the furnace. The furnace chamber -is heated by a mixture of producer gas and heated air, the gas being -generated in an independent gas producer situated outside the glass -house and some little distance away from the furnace. At either end of -the furnace, beneath the floor of the siege, are two blocks of -regenerators. These are deep rectangular chambers containing an open -lateral arrangement of fire-brick chequers, through which the air or -products of combustion pass on their way to or from the furnace. -Port-holes are situated directly above these regenerators which lead the -gases through the floor or siege into the furnace chamber. The draught -is induced by a tall stack, which draws the gas from the gas producers -through a duplicate arrangement of flues to the port-holes at one end of -the furnace, where it is mixed with the air which has been drawn and -heated in its passage through the regenerator beneath. This gaseous -mixture, whilst in combustion, is drawn across the furnace chamber to -the other end of the furnace. The flames playing across the tops of the -pots on either side pass down through the port-holes and regenerator at -the opposite end. The hot gases or products of combustion, in passing -through the lateral channels of this regenerator, leave behind their -heat by the absorptive or conductive capacity of the fire-brick chequers -through which the hot gases have passed on their way to the stack. The -direction of the current is reversed at intervals of half an hour or -less by using an arrangement of valves situated in the gas and air -flues, so that the currents are made to travel on the contrary -direction, the air necessary for combustion then being drawn through the -hot block of regenerators which was previously heated by the exit gases. -On its way through these lateral channels the air becomes intensely -heated, and, when it is admixed with the coal gas at the porthole, this -pre-heated air accelerates the combustion and calorific intensity of the -gaseous mixture. The direction of the current is continually being -reversed at the interval of half an hour or less by the manipulation of -the valves, so long as the high temperature is desired. - -In practice, however, the regenerators are only used whilst the batch -materials are being melted during the night, and by morning, when the -metal is melted and “plain,” the heat is brought back, or retarded, by -using the gas from the gas producers and cool atmospheric air under -natural draught, instead of the regenerated hot air. This cooler -mixture, naturally not being so active in combustion, maintains just -sufficient temperature for working the metal out during the day. Later -in the day, when the pots are emptied and refilled with batch, the -regenerators are re-connected and the founding proceeds again through -the night, and the metal is again got ready for the workmen coming in -next morning. - -It will be seen that this method of melting and working out the metal -does away with night work, the furnace man alone remaining in charge -during the night. All firing is done outside the glass furnace room, -which is well lighted, clean, and free from coal dust, totally different -conditions from those existing in many English glass houses of to-day. - -A Siemens Siegbert furnace taking ten open crucible pots, and filled -each day, turns out 15 to 18 tons of metal a week. The crucibles are -about 30 in. in diameter and have a capacity of 5-1/2 cwts. of metal -each. The amount of fuel consumed is about 18 tons a week. This type of -furnace costs about £1,600 to £2,000 to build. In the miter’s opinion, a -disadvantage of this furnace is that, during the reversing in the -direction of the fire gases, the greatest heat is suddenly brought to -bear on the cooler pots, resulting in a short life for the pots. The -temperature of the incoming air is not so constant as with the -recuperative type of furnace; however, with proper control, these -defects may be obviated to some extent. - - -[Illustration: - - A MODERN GLASS HOUSE - The Hermansen Continuous Recuperative Glass-melting - Furnace in foreground (Twelve Covered Pot Type). -] - - -By the kindness of Messrs. Hermansen, the patentees, I am permitted to -illustrate their Recuperative Glass-melting Furnace, eight pot type. - - - -[Illustration: - - HERMANSEN GLASS HOUSE FURNACE (EIGHT POT TYPE) -] - - - -[Illustration: - - Sectional Elevation. - A - HERMANSEN’S CONTINUOUS RECUPERATIVE GLASS-MELTING FURNACE - _P._ Producer. - _B._ Burner. - _G.P._ Glass Pocket. -] - - - -[Illustration: - - B - HERMANSEN FURNACE - Cross Section through Gas Producer. - _P._ Gas Producer. - _R._ Recuperators. -] - - -The Hermansen furnace, like the Siemens furnace, is producer gas-fired. -The gas producer is built within the body of the furnace, (=P=) below -the glass house floor. On either side of this gas producer the -recuperators are situated. These are constructed by an arrangement of -tubes, designed to give two distinct continuous channels, the one -horizontal and the other vertical. The vertical channels are connected -with the atmosphere and supply the air necessary for combustion. The -horizontal channels (=R=) are the flues through which the hot waste -products of combustion are continually being drawn from the furnace by -the stack. It will be evident that, the horizontal channels being -intermediate to the vertical tubes, the waste heat is continually being -absorbed by the air travelling inwards. In other words, the air is -pre-heated by passing through flues which are surrounded by the hot -waste gases. Therefore, in this type of furnace there is no necessity -for reversing the currents to procure the necessary pre-heated air for -combustion, and the regulation of the furnace heat becomes a simple -matter of controlling the draught by means of the dampers provided in -the main flue. In this type of furnace the glass is melted nightly; open -or covered pots may be used, the capacity of which varies between 5 and -12 cwts., according to the class of glassware manufactured. The furnace -is designed in four, six, and eight pot types, and several are now -working in this country. These Hermansen furnaces are capable of -producing 20 tons of metal, with a fuel consumption of 16 tons. - - -[Illustration: - - C - PLAN OF HERMANSEN’S FURNACE - (Eight Pot Type) -] - - -The Hermansen Continuous Recuperative Furnace is the most efficient -furnace known to the writer. It is easier to control than the -regenerative types. Being compact, it takes up little space and is easy -to repair, and its life well surpasses other types. The initial outlay -and cost of erection varies from £850 to £1,200. The combustion in this -type of furnace is so perfect that it is used with open crucible pots -for melting lead crystal glasses. On the Continent this furnace is in -general use for all types of glassware, and, from the amount of glass it -will melt, its efficiency is greater than the regenerative type. - -=Tank Furnaces= are at present used for the melting of the commoner and -cheaper types of glass. They are so constructed as to contain a single -rectangular-shaped compartment, or tank, about 18 in. to 2 ft. deep, and -from 30 to 100 ft. long. The bed and retaining walls of this tank are -constructed of specially selected fireclay blocks; no pots are used. -Tank furnaces are simple and melt the glass economically, but the metal -produced is not nearly so good a quality as pot metal. - -Tank furnaces are chiefly used for making the cheaper glasswares, such -as wine, stout, and beer bottles, gum bottles, ink-pots, sauce bottles, -and like goods, where a large production is essential. Improvements are -continually taking place in the design of this type of furnace, and much -finer and clearer metals are being produced. It is quite probable that -in the future tanks will be preferred for making cast plate and sheet -window glass, as a larger body of metal is held by them when compared -with pot furnaces. Like the Siemens and Hermansen furnaces, they are -gas-fired, but the port-holes by which the gas and air are introduced -and the products of combustion are withdrawn from the melting chamber, -are situated on either side, above the level of the metal, whilst the -glass blowers work at one end of the furnace. The melting and working of -the metal is continuous. The tank is divided by a shallow bridge, which -is partially submerged and situated midway between the two ends of the -furnace, dividing it into two sections, respectively the melting and -working compartments. This bridge keeps back all unmolten material and -allows only that portion which is melted to travel forward to the -working compartment. The tank is crowned or arched over, and at the -working end openings are provided to enable the glass workers to gather -the metal from within. Small rings, or syphons, are used, which, -floating on the metal, serves further to refine the glass as it is -gradually used. The batch mixture is filled through a convenient opening -near to the port-holes. Tank furnaces vary in capacity. Some have been -constructed to give an output of 300 tons of glass a week. This pace can -only be kept up with the aid of automatic bottle-making machinery; in -which case hand labor is practically eliminated. - -Liquid fuel or oil-fired glass furnaces have not proved a success, being -very costly in repairs on account of the local heating effects of the -flames issuing from the burners vaporizing the oil. - -Electric furnaces for glass-melting have been tried with partial -success. These are expensive in maintenance compared with their -efficiency in producing glass. - - ------------------------------------------------------------------------- - - - - - CHAPTER IX - - GLASS-MELTING POTS AND THEIR MANUFACTURE - - -Glass house pots are large hollow vessels made of refractory in which -the glass manufacturer melts the materials of which his glass is -composed, and which retain the molten metal whilst in a state of fusion -for the workmen’s use. In the case of the lead crystal glass, the -materials, whilst being melted, require protection from the flames, -smoke, and fuel ash present in the old English types of furnace -chambers, which would otherwise reduce the lead present to a metallic -state and spoil the glass; therefore, such glasses are melted in covered -or hooded pots and thus protected from the direct action of the flames. -Consideration has to be given to the extra amount of heat required from -the furnace to find its way through the hood of the pot. For crown plate -and chemical glassware, the metal is usually melted in open or uncovered -pots. In this case the fusion is facilitated by allowing the heat of the -furnace to come into direct contact with the materials within the pots. - -Pots which are covered or hooded have an opening cut out in the front, -in a position just above the level of the molten metal. Through this -opening the workman gathers the hot metal. In the case of open pots, the -crucible is set in a similar position within the furnace, but the -working hole or mouth is built to form part of the construction of the -furnace in front of the crucible. - -Good pots are of the greatest importance to the glass manufacturer, and -upon their life much of the success of glassmaking depends. They have -necessarily to resist the corrosive action of the raw materials and -molten glass within, and, at the same time, withstand the very intense -heat of the furnace without giving way under the great weight of the -glass within them. Should a pot of metal give way whilst in the furnace, -the loss is considerable and very serious, for not only has the metal -been wasted, but much of it has flooded the floor of the furnace and -siege, and, finding its way into the fire-box, attacks the furnace -walls, fusing and melting with the fuel ash, checking the draught, and -causing endless trouble. - -Glass house pots are very difficult and expensive to manufacture, and -upon an average each pot has cost £10 by the time it is set within the -furnace; therefore every care is taken to extend their life by procuring -the best possible materials for their manufacture. - -Only the best selected pot-clays available are used, and every endeavour -is made to keep them clean and free from foreign contamination. Only the -best portions of the seam are taken for this purpose, and a considerable -amount of diligence and stringent precaution is taken to procure the -best qualities. As the clay is raised from the mine, clay pickers look -over the lumps and select out the best portions. A foreman of long -experience is stationed at the head of the mine, and it is his duty to -supervise the clay pickers and see that every care is exercised to guard -against any unfortunate results which would naturally attend any -indiscriminate or indifferent selection. The best portions having been -selected and placed aside, the lumps are scraped on the surface to -remove any dirt, and broken into pieces about the size of an egg, which -are again carefully examined on all sides and cleaned from foreign -matter such as pyrites or bluish parts. If this is carefully done, and -the clays analysed and tested from time to time, a good pot-clay is -obtained. - -The clay for burning is treated similarly and dried. It is then burnt to -a very high temperature and taken to the mill to be ground to the -necessary fineness of grain. All pot-clays are well seasoned and -weathered before use. They are first ground to a very fine flour and -then mixed with a ground-burnt clay, or “chamotte.” The proportion of -raw clay to burnt varies with most manufacturers but depends very much -upon the plasticity or binding property of the raw pot-clay used. The -burnt clay is preferable if ground to a size about 1 to 1-1/2 mm., being -sieved to take out any coarser particles. Some clays are more plastic -than others, so the proportions in the pot-clay mixtures may vary from -six parts of burnt clay to five of raw, down to one part of burnt clay -to three of raw clay. The proportions are reckoned by volume, not by -weight. The mixture is sieved into a trough and mixed with water to form -a stiff paste, and removed into a large tank, where it is allowed to -soak for some time. It is then well-tempered by treading with the bare -feet until the whole mass becomes plastic and tough. The clay mass is -turned and trodden several times, in order thoroughly to consolidate the -clay particles. Many efforts have been made to do this work -mechanically, but without success. The fact remains, and experience has -proved that, in the process of treading, the clay is more consolidated -than by any mechanical method of preparation. The tempered and toughened -clay is then allowed to sour and mature for a few weeks before use. It -is then ready for the pot maker to begin the work of building the pots. - -The room in which the pots are to be made is kept evenly warm using a -series of hot water circulating pipes arranged around the outer walls. -Usually a temperature of between 60 to 70° Fahr. is maintained. - -Double doors are provided at the entrance, with a porch, so as to -prevent sudden inrushes of cold air and prevent draughts in the -pot-making room. All unauthorised persons are prohibited entrance, and -only those who work therein are allowed free access. They are made -responsible for keeping the place clean, as well as looking after the -clay and taking care of the pots whilst they are being made. - -The usual shape of a pot is of round section, 38 in. in diameter and 42 -in. high, but many other shapes and sizes are used, according to the -class of goods being manufactured. Thus, for colours, a very much -smaller pot, less than one-third this size, is used, three of them, -taking the position of one large pot, being set within one arch. For -sheet and optical glass, a covered pot with a very large mouth or -working opening is used. - -In some instances, as in the Hermansen furnace, the pots are oval or -egg-shaped. These are used on account of their larger capacity in -relation to the space occupied in the furnace. Other pots have an -interior division, which has a syphonic refining action upon the glass; -such pots permit of continuous melting and working, instead of the -intermittent process adopted when the regular or common shape is used. -For plate glass, open crucible or bowl-shaped pots are used. - -In regard to the manner in which the pots are made, and their subsequent -treatment in annealing, the utmost care and control is necessary. In -making the pots, the pot maker begins by making the pot bottom first, -working the plastic clay paste into rolls about the size of a large -sausage. He takes these rolls and applies them one after another in a -circular form upon a round level board, the size of the bottom of the -pot. This board is supported on a low table. As he applies each roll, he -presses them together to exclude all air spaces between them, and -continuously work the rolls on the top of each other in circles, until -he gets a circular flat slab of clay in thickness about 4 in. and the -width of the pot bottom. He then has the necessary thickness and size of -the pot bottom formed as a clay slab, which is smoothed and leveled over -the face with a knife or straight piece of wood. The slab of clay is -then reversed upon another board, covered with a strong hurden cloth and -a layer of ground-burnt clay, which prevents the clay from sticking to -the board, and facilitates drying of the pots. - -The first board is then removed, and the pot maker begins to build the -sides or walls of the pot upon the circular clay slab by working the -clay in rolls around the circumference of the slab to a thickness of 3 -in., which gives the thickness of the pot walls. As he works and presses -on each roll with his right hand, he supports the inside of the curve -with his left hand, and presses roll after roll around the circumference -of the slab of clay, increasing the height of the walls until he attains -a height of about 6 in. The height of this wall is increased by about 6 -in. every other day or so; these time intervals allow each section built -to stiffen a little before beginning upon the next section. - -The workman passes from one pot bottom to another, building up these -sections until he builds each to a height of about 30 in., when he -places within each pot a clay ring about 18 in. in diameter, which he -has previously made.[3] After placing these rings within the pots, the -pot maker begins to form the hood or dome of the pot by working on the -clay rolls, and at the same time drawing the sides inwards towards the -middle, lessening the thickness of the walls and gradually diminishing -the open space until it is covered and sealed in. Whilst the clay is -still soft, the mouth or working opening is worked on and cut out of the -dome, and the whole finished and smoothed by means of wooden tools. - -Footnote 3: - - These rings, floating on the metal, are used by the glass makers to - keep back the scum of the glass away from the middle portion from - which he gathers. - -The pots are now completed and are left to dry gradually at a moderate -heat, which is increased a little at the end of a few months in order to -thoroughly dry them. They are then removed from the boards and are ready -for the furnace. - -Crucible pots are made in a similar way, except that at the height of -about 27 to 30 in. the pot maker finishes off the top edge of the walls -and leaves it in that form to be dried. - -Many efforts have been made to manufacture pots by other methods. One -which has been tried with a fair amount of success is to cast the whole -pot or portions thereof by using a plaster case mould and pouring in -liquid clay slip. Another method which has been tried is to press the -form by means of a hydraulic press and mould. Other mechanical -contrivances have been used, but few of them have given such -satisfactory results as the hand-made pots. - - - MIXTURE FOR POT-CLAY - - _By - volume._ - - (=Base=) Fine ground strong 5 parts - Fire-clay - - (=Binder=) Fine ground mild Plastic 4 parts - Fire-clay - - (=Grog=) Ground burnt Chamotte 2 parts - - (=Grog=) Ground selected Potsherds 1/2 part - - -The fusion point of the mixture should not be less than Cone 32, or -1710° Centigrade. - -Strong fire-clays are those coarser and harder grained, and are usually -more silicious and less plastic than the mild fire-clays. Mild -fire-clays are very fine-grained, plastic, and easily weathered clays. -They act as the binder portion in fixing the burnt grog used in -pot-clays. - -The raw clays should be ground very fine and separately from the burnt -clays. The ground burnt should be crushed from hard and well-burnt -fire-clays, and should pass a sieve of ten meshes to the linear inch. - -The mineralogical composition of the fire-clays for making pots is -important. The presence of pyrites renders fire-clays unsuitable as -pot-clays. Some indication as to the subsequent behavior of a can be -obtained by submitting it to a petrographic examination, and the usual -pyrochemical and physical tests carried out in testing refractory -materials. In this country, Stourbridge pot-clays are chiefly used for -pot-making, and so conservative are the majority of glass manufacturers -that they will not use other clays, although, in the writer’s opinion, -many better clays exist in Great Britain, and have now been introduced -and used successfully by some firms for pot-making. - -Ground potsherds are selected pieces of old broken pots, cleaned from -any adhering glass. These selected pieces are crushed and ground in a -similar way to the burnt clay, and sieved to the same degree of fineness -before use. - -=Plumbago= glass house pots are sometimes used. These are made from -mixtures of graphite, or plumbago, and raw . They are very refractory -and withstand the attack of very basic glasses, where such have to be -manufactured. - -Pot rings are made by taking a long roll of clay about 3 in. in -thickness and shaping it around a circular frame. The two ends are -joined and finished smoothly, the frame took away, and the ring dried. A -ring is placed in each pot. - -Stoppers are the lids used to close the mouth of covered pots whilst the -metal is being melted. These are made in plaster case molds by pressing -a bat of clay into the desired shape and releasing the outer case by -turning the whole upside down upon a board and lifting off the mold. An -indentation is made in the middle, forming a small hole. An iron rod can -there be inserted, by which the stopper can be lifted away from the pot -mouth whilst hot. Stoppers are burnt before use and are made in various -sizes to fit the mouths of different pots. - -It is always advisable for the glass manufacturer to make his pots and -prepare his clay, as he then knows exactly what he is using, and he is -not dependent upon outside firms for his pots as he has them ready at -hand when needed. The conveyance of pots from one district to another by -rail or road is always accompanied by considerable risk, as the -vibrations are given they in such journeys often cause mischief. As they -are very heavy and fragile, their loading and unloading into the wagons -is often attended with a mishap. As often as not, latent strains are -caused, which only develop when the pot is put in the furnace. - -=Annealing and Setting the Pots in the Furnace.= The pots, when made and -dried, being of raw clay have to be carefully annealed before they can -be introduced into the hot furnace. In doing this, the pot is removed -from the drying rooms and placed within a small auxiliary furnace called -a pot arch, which is constructed purposely to anneal them and get them -hot before placing them in the glassmaking furnace. The pot is moved by -picking it up on a long three-pronged iron trolley, made purposely to -lift and move them about. The pot is set within the pot arch, resting -upon two or three rows of fire-bricks, which allows the trolley to be -removed and brought away, leaving the pot in a raised position in the -pot arch. The doors of the pot arch are then closed and sealed with a -stiff clay paste or mortar, and slow fires started which gradually heat -the pot, until at the end of a week it is got to a white heat, and the -pot is ready to be removed and set within the furnace for melting the -glass. - -At a convenient time, arrangements are made for setting the pot. All -other work about the glass house has to cease, as all hands are required -to help in the strenuous and arduous work. The old pot in the furnace, -which has done work for several months, has to be withdrawn from the -furnace and the new pot from the pot arch has to take its place. We see -gangs of men here and there. Some are pulling down the wall of bricks -from the front of the old pot, making an opening in readiness to remove -it. Another gang of men advance with long, heavy, strong iron crowbars, -sharpened at the points, with which by heavy blows and levering they -endeavour to loosen the old pot from the floor of the siege, to which it -has become firmly cemented by the heat and any leakage of glass which -may have taken place. Eventually, by their combined exertions, they -succeed in loosening the pot, and then, levering it up, they place the -low iron pot trolley under it and drag it out of the furnace, whence it -is taken away and thrown aside. - -The old pot having been removed from the furnace, the glowing heat -radiates more intensely than ever into the faces of the men at work, who -endure it in relays whilst they work clearing away the old bricks and -preparing the siege for the new setting. When this is done, a gang of -men open the pot arch doors, and, placing the iron trolley under the new -pot, convey it to the opening in the glass furnace from which the old -pot has been removed. Facing the terrific heat, they struggle to push -the new pot into its place in the furnace, with the aid of crowbars, and -working in relays, in turn face the heat till at last it is got into -position. Naturally, everything has to be done in a hurry, so that the -new pot may not be chilled before it is got into the furnace by being -exposed too long to the outside air. The whole work proves very -exhausting to the men, as there is little protection from the heat. -After the pot is set in its place, the trolley is brought away and the -wall of bricks rebuilt up in front of the pot to protect it, clay being -daubed over the exterior of the brick wall to prevent any inrushes of -air, which would cause the pot to crack by finding a way through the -joints in the brickwork. - -The furnace, during these operations, is driven and worked to its full -capacity, so as to allow for the very considerable loss of heat which -takes place whilst the opening is being made and the pots removed. - -The above is a description of the usual method of pot setting. In more -modern and up-to-date works a travelling chain screen is used. This -screen is like a curtain of loose chains, which is adjusted to hang in -front of the open arch of the furnace and protects the workmen from the -fierce heat. At the same time it permits the workmen to see and carry -out the work of pot setting with greater ease and convenience. In using -this screen arrangement whilst setting, the pot is pushed through the -chain screen, which closes upon it after it has passed through. The -workmen are thus enabled to get closer to their work by manipulating the -crowbars through the screen as the heat is not radiated full upon them. - -The newly set pot is allowed to stand empty in the furnace for a day or -two to regain heat before it is filled with batch. It is first glazed on -the inside by a workman taking a gathering of glass from another pot and -plastering or covering the inside all round with the hot metal, which -flows down and glazes the surface of the pot, giving it a certain amount -of protection from the attack of the raw batch materials which are to be -introduced later. - -The founder, or glass melter, now takes charge of the pot, and he brings -up the mixture of batch and cullet and shovels it into the empty pot -until it is filled well above the mouth or level of the opening. The -heat of the furnace melts the batch, and after several hours it becomes -liquid and shrinks in volume so that probably only two-thirds of the -height or capacity of the pot is occupied. The pot is then again filled -with more batch materials until it is full of molten metal up to the -level of the mouth of the pot. - -The furnace is kept going at its full heat until the founder, drawing a -small portion of the glass on the end of an iron rod, examines it and -finds that it is melted clear and free from seeds or bubbles of gas. -When clear, the metal is “plain,” and at this stage is in a very liquid, -fluid, and watery state, too liquid to be easily gathered. It is, -therefore, allowed to cool off by removing the stopper down and leaving -the mouth of the pot open, until the glass becomes more viscid, or of a -stiffer nature. The glass is then skimmed by dragging off any scum -present on the surface, which is due to undecomposed salts that may have -risen during the melting. - -The metal is now ready for the glass blowers to begin work. Upon looking -into the pot, the ring will now be noticed floating on the surface of -the glass. This ring keeps back from its interior any further scum that -may arise whilst work is in progress. The glass blower always gathers -from within this ring, where the metal is cleanest; and from time to -time the metal within the ring is skimmed in order to keep that portion -in the best condition. When the greater part of the metal within the pot -has been gathered or worked out, the heat of the furnace is raised again -and fresh batch materials filled and the process repeated. - -The time taken to melt the glass depends upon the heat of the furnace. A -gas-fired furnace will melt the batches in eight hours, but the old type -of English furnace takes much longer, usually two to three days. - - ------------------------------------------------------------------------- - - - - - CHAPTER X - - LEHRS AND ANNEALING - - -Owing to the peculiar structure of glass, and its liability to fly or -collapse when exposed to sudden changes of temperature, a process of -annealing becomes necessary in order to produce a more equal -distribution of the tensions throughout the structure of the glass; -otherwise, glassware of any thickness would be in such a state of -tension as to be extremely liable to fracture when passing through any -sudden change in the atmospheric temperature, especially in frosty -weather. In this state it is useless or dangerous for general purposes. -On this account most glasswares undergo a form of annealing at some time -during the process of their manufacture. And in the case of certain -goods, such as table glass, lamp glasses, optical glass, etc., special -care and time are devoted to this process of annealing. Often in the -case of improperly annealed glass, instances are known where its -unhomogeneous structure has suddenly given way as the result of -derangements set up by internal tension. Friction, or rough handling -whilst cleaning, at the ordinary temperature of the atmosphere, is -sufficient to cause a rupture. Therefore annealing cannot be too -carefully attended to. - -For annealing the glass manufacturer uses a lehr, which is an arched -tunnel with a fully exposed opening at the exit end and partially closed -at the entrance end, where the goods are introduced. The lehr is heated -at the entrance end to a temperature of about 350° Cent., which -temperature is gradually diminished towards the exit end, which is quite -cool. The hot end, or entrance, should be constantly at a temperature -just short of the actual deformation or softening point of the glass -introduced; usually the entrance is in a position near, or convenient -to, the glass furnace around which the glass blowers make the goods. - -In old-fashioned works coal-fired lehrs are used, but they are very -unsatisfactory and difficult to regulate. The heat of the lehrs in -modern works is maintained and regulated by a series of gas burners -situated under the floor of the tunnel or lehr. Along this floor are -placed iron trays linked up with each other to form a continually -travelling track, which gradually moves towards the cold end of the -lehr; these trays are operated by a mechanical jack and gears. As each -tray of goods comes out of the cooler end of the lehr, they are taken -off and conveyed to the warehouses for cleaning and packing, and the -empty tray is sent back to the entrance end to be linked up and refilled -again with fresh goods. - -These tunnels, or lehrs, are about 40 ft. long, and as the glasswares -travel through on the trays they are subjected to the gradually -diminishing heat, until they are ultimately removed at the cooler end in -an annealed condition, in which state they are less liable to fracture -in use. The time occupied in travelling through the lehr is usually -about three days. But this period varies according to the nature of the -ware being manufactured. In special glasses, and in the annealing of -optical glass, the glass may undergo a process of annealing that takes -as long as ten days, and in other cases, where the glassware is made -very thin, no annealing at all is necessary. Usually the thicker and -heavier articles require the longest time in annealing. Table glass -which is made thick and heavy for cutting or decoration requires a -little more care and time in the lehr than ordinary plain glassware, as -the abrasive action of cutting quickly develops any latent strains and -causes fracture. - -In some works, especially on the Continent, several small -externally-heated kilns are used for annealing, in which the hot -glassware, as it is made, is packed in tiers; when full, these kilns are -closed up and then allowed to cool of their own accord; after which they -are opened and the goods taken out to the warehouse. This is an -intermittent process of annealing, and is quite satisfactory for certain -classes of goods, such as lamp shades, which are usually of equal -thickness throughout their form. - -The travelling or continuous form of lehr admits goods of more unequal -thickness in form and variety. Thus, wine-glasses, jugs, and bowls may -be annealed together with less risk of malformation in their shape than -would be present if they were annealed together in kilns. The -manufacturer can, by suitably arranging the temperature of the gas -burners, give more heat to one side of the lehr than to the other. He -then places the heavier goods on the hotter side and reserves the other -for lighter goods, such as wines, etc. They then travel down together -side by side under the most suitable conditions for the annealing of -each class. - -Many physical changes take place in the glass passing through the lehr. -One remarkable effect is the slight change in colour which occurs in -glass decolorized with manganese. It is noticed that the glass becomes a -greener tint in passing through the lehr when the decolorization is just -on the margin of efficiency. - -The state in which the structure of glass exists when quickly cooled and -the action of annealing might be explained. When glass is quickly -cooled, being a bad conductor of heat, insufficient time is allowed for -the middle or interior portions of the glasswares to settle down and -assume their normal state of solidification. The outer portion, or -crust, will first cool and contract with an enormous strain upon the hot -interior. This difference in the state of tension between the outer and -interior portions gives a want of uniformity, and stresses of tension -and thrust are developed, which cause the whole to collapse with the -slightest external scratch or heat change. In annealing, this strained -or forced condition in the structure of the glass is relieved by -subjecting the glass to a pre-heating, and gradually diminishing the -temperature, allowing a sufficient time for the different layers -mutually to adjust themselves to their comparative normal positions, and -thus relieve the strains within the mass. Much depends upon the -pre-heating temperature and the rate at which the diminution of the -temperature takes place. If this is properly provided for, the best -results are obtained in the stability of the resulting glass. The -presence of any stress can be determined by using a polariscope. - -The average British glass manufacturer has little knowledge of the value -of a polariscope, or stress viewer, in ascertaining the physical state -of his glasswares, and until he adopts its use there is little prospect -of an improvement in his annealing methods. Much faulty annealed glass -is being turned out which the glass manufacturer is not aware of, and -which could be avoided by the intelligent use of such a simple -instrument, which detects badly annealed glass at once by the aid of -crossed nicols and a selenite plate. - -Owing to the unequal densities of the various silicates present in the -heavy lead and barium glasses, they are more subject to striation and -require more careful annealing than the soda-lime glasses, in which the -silicates present are of more equal density. However, much depends upon -the proper “founding” and melting of such glasses. The use of a larger -proportion of cullet assists in breaking up striation. The presence of -striae or cords in glass disqualifies it for most purposes, as it is -usually found that, apart from their defective appearance, they tend to -produce stresses within the glass. - -Transparency, brilliancy, stability, and homogeneity are important -factors in producing perfect glassware, and the proper development of -these distinguishing properties requires considerable skill on the part -of the glass manufacturer, alike from a technical, physical, and -practical standpoint. - - ------------------------------------------------------------------------- - - - - - CHAPTER XI - - THE MANIPULATION OF GLASS - GLASS MAKERS’ TOOLS AND MACHINES - - -The tools used by the glass blowers are few and simple. The greater part -of the crude form is produced by blowing out the hot glass into a -spherical or pear-shaped bulb and regulating the size and thickness by -gathering more or less material. The tools are mainly employed in -finishing and shaping this bulb into the desired form, such as shearing, -forming the neck spout, crimpling, and sticking on the handles to the -various shapes made. - -According to the type of the goods manufactured, different manipulative -methods in forming the articles are adopted in various works. - -The best English table glassware is mostly hand-made blown ware, -generally entirely executed by the handicraft of the workman without the -aid of moulds to form any part of the articles, and a considerable -amount of skill and practice is necessary before the workman is -competent enough to shape a number of articles exactly to the form of -his model. It is astonishing to notice the skill and precision with -which a workman produces wine-glasses one after another, so uniform that -one cannot trace any dissimilarity between them. - -A second class, or cheaper form, of tableware is made by blowing the -sphere or bulb of hot glass within a mould, to give some part, or the -whole form, of the desired article. If only a portion of the intended -shape is thus formed by the mould, it is afterwards finished by hand -with tools. This is the general continental method of working, and has -only been partially adopted by this country for making tableware. Where -a number of articles of one shape have to be produced, this is by far -the most economical method. Glass tumblers, honey pots, and rose bowls -illustrate this class of ware. - -Another class of tableware produced by a method of pressing the form is -known as “Pressed glassware.” The hot metal is gathered from the pot and -a portion cut off, and allowed to fall into an iron mould fixed within a -lever press, which carries a plunger fitting within the mould formed to -shape the interior and exterior, with the thickness of the glass as the -intermediate space between them. As the hot glass is introduced, the -workman brings down the lever arm and the plunger presses the hot metal -to shape. The plunger is then released and the mould reversed, turning -out the pressed form of glass, which is then carried away to be -fire-polished or further manipulated with tools before it goes to the -lehr. The case or mould portion is made in two halves, to facilitate the -removal of the hot glass after being pressed. Pressed glass tableware -can be recognised by the presence of seams, showing these divisions of -the mould. Many exquisite designs imitating cut-glass tableware are -executed in pressed glassware. The moulds are a very expensive item, as -there is much tool work in cutting the patterns and refacing them after -prolonged use. In making pressed goods, an oily, carbonaceous liquid is -used to give the moulds some protection and prevent the oxidation of the -iron. This liquid is from time to time applied, as the work of pressing -proceeds, by mopping the interior of the mould with a mop dipped in the -preparation. - -Another process in glassmaking is that of bottle-making by automatic -machinery, in which the glass worker does little but gather the -requisite quantity of glass from the pot and place it into the revolving -clips of a bottle-making machine, which does the work of formation, by -the aid of compressed air delivered from a supply main. This is largely -of American introduction, and is the method adopted in making common -bottles. In some cases the bottle neck may be finished by a hand tool -after a mould has done its part of forming the bottle. Modern machines -have been perfected to do the whole work of gathering the metal, forming -the shape, and completing the bottle; a number of arms travelling round -a track carry the mould forms, which alternately dip into water to keep -them cool, open to receive the hot metal, close, deliver a requisite -pressure of air to extend the hot glass within the mould, and then -deliver the bottle on to a travelling belt, which takes them to be -annealed. - -In the manufacture of bottles by machines, hand labour is practically -eliminated as far as the actual making of the bottle is concerned. The -bottle-making industry is undergoing great changes by the introduction -of such machinery. In some plants a ten-armed machine will produce -automatically 120 gross of 16 oz. bottles in twenty-four hours, at an -average cost of 1s. 6d. a gross. - -Owen’s Bottle-making Machines are of this type. Such machines produce -700 bottles an hour, according to their size and the number of arms -fitted to the machine. - -As an illustration of a less complicated bottle-making machine, “The -Harlington” may be described. - -This machine consists principally of a table, on which is arranged on -the left-hand side a parrison mould, and on the right-hand side a column -with a revolving table carrying two finishing moulds. - - -[Illustration: - - _By permission of_ _Melin & Co._ - “THE HARLINGTON” BOTTLE-MAKING MACHINE -] - - -Below the table, near the parrison mould, is arranged an air cylinder, -through which a piston runs, operated by a hand lever. On the upper part -of the column, on which revolves the table with the two finishing -moulds, is also arranged an air cylinder operated by a hand lever. - -The method of working is now as follows— - -A gatherer puts the metal into the parrison mould into which it is -sucked by moving the left-hand lever. Through this operation the head of -the bottle is formed and finished. By reversing the lever, air enters -the parrison, thus blowing the same out to the height of the parrison -mould. The parrison mould is now opened and the parrison hanging in the -head-mould held by the tongues is placed under the blowing cylinder -above the open finishing mould. Now the latter is closed, and by moving -the lever, the bottle is blown and finished. Whilst this last operation -is being effected by a boy, the table is revolved and the previously -finished bottle is taken out and another parrison is made ready to be -handled in the described way. This machine produces 200 bottles per -hour. - -=The Glass Blower’s Tools.= The glass maker’s chief tool is the -blow-iron. This is a tube of iron 1/2 to 1-1/4 in. wide and about 4 to 5 -ft. long, one end of which is shaped or drawn in so as to be convenient -for holding to the lips, and the other end is slightly thickened into a -pear-shaped form, on which the hot metal is gathered. - -In making crystal tableware the workman manipulates the glass he has -gathered on this blow-iron by marvering it on a marver. This is a heavy -slab of iron with a polished face about 1 ft. by 1 ft. 6 in., and 1 in. -thick, supported on a low table. Sometimes this marver may be a block of -wood with hollows of definite forms, in which the workman rotates the -hot glass he has gathered to regulate the form and thickness of the -metal to suit his work before beginning to blow it out into a hollow -bulb. - -The pontil is a solid rod of iron of similar length and thickness to the -blow-iron. By gathering a little wad of hot glass on the pontil and -sticking it against the end of the bulb attached to the blow-iron, the -workman can detach the bulb from the blow-iron and hold it by the pontil -to which it has been transferred, and which enables him to work on the -other end or opening in the bulb which is exposed in detaching it from -the blow-iron. - - -[Illustration: - - GLASS WORKER’S CHAIR -] - - -After re-heating the glass, he may shear it with his scissors or shears, -open it out with his pucellas, crimple it with his tongs, measure and -caliper it, or shape it to a template. - -Whilst he is doing such operations he sits in a glass worker’s chair. -This chair has two long extending arms, which are slightly inclined, and -along which he rolls his blow-iron or pontil, with the glass article -attached, working upon the rotating form, turning the iron with one -hand, whilst he uses his tools with the other hand, to shape or cut the -glass to its requisite form whilst it is hot, soft, and malleable. - -The shears are like an ordinary pair of scissors, and are used for -cutting the hot glass, or shearing off the tops of bowls and wines to -their proper height. - -The pucellas is a steel, spring-handled tool in the form of tongs, which -the workman uses to widen, extend, or reduce the open forms of glass by -bringing pressure upon the grips of the tool whilst applying it to the -hot glass. - -The glass maker also uses another form of spring tool in taking hold of -hot glass or pinching hot glass to form. These are the tongs. - -The battledore, or palette, is a flat board of wood with a handle, used -for flattening and trueing the bottoms of jugs or decanters, etc. - -The chest knife is a flat bar of iron, usually an old file, used for -knocking off the waste glass remaining on the blow-irons and pontils -after use. A chest or iron box is kept for collecting such waste glass -for further use. A pair of compasses, calipers, and a foot rule complete -the glass maker’s outfit of tools. - -=Making a Wine-glass.= The manipulations in the manufacture of a -wine-glass will now be described. A common mule wine-glass is formed -from three distinct pieces of glass: (_a_) the bowl; (_b_) the leg; -(_c_) the foot. - -A wine “shop,” or “chair,” consists of three men; a “workman,” whose -main work consists of finishing the wine-glass; a “servitor,” who forms -or shapes the bulb; a “foot maker,” who gathers and marvers the glass; -and a boy who carries away and cleans the blow-irons. - -The “footmaker” of the “chair” gathers on the end of a blowing-iron -sufficient glass to form a bowl. This is then shaped on a marver until -the required shape is obtained. The footmaker then blows this out to a -hollow bulb similar in size to the pattern to which he is working. When -the bulb leaves the footmaker it is the shape of the bowl of the -wine-glass. - -This is then handed over to the servitor, who drops a small piece of hot -glass on to the end of the bulb, and heats the whole by holding it in -the furnace. This serves to make the joint of the two pieces perfect. -The servitor next proceeds to draw out the leg from the small piece of -glass at the end of the bulb, leaving a button of glass at the end of -the leg. The servitor then dips the end of the leg into the molten glass -within the pot and gathers on sufficient glass to form a foot. He -spreads this portion of the glass out to the required shape and size -with a pair of wooden clappers, with which he squeezes the hot glass to -form the foot. - -The servitor has now done his part of the work, and the glass is handed -to the workman. It is then cracked off, and the foot caught by a spring -clip arrangement attached to a pontil, called a “gadget.” The workman -now re-heats or melts the top edge of the glass by holding it within the -furnace, and when it is hot he cuts off the surplus glass with a pair of -shears. A line is chalked on at the correct distance from the foot, and -guides the workman in cutting the glass to the proper height. He then -melts the top again and opens it out with his spring tool to the -required shape, after which the glass is taken to the annealing lehr by -the boy, to be annealed. - -Other forms of wine-glasses are made, and various methods are adopted, -according to the district and class of workmen. - -For instance, the method of making the above common mule wine-glass -varies in different districts. Instead of gathering the metal for the -foot upon the leg of the glass, the workman may drop a piece of hot -glass, which has been gathered by the servitor, on to the button at the -end of the leg, and by means of a pair of wood clappers spread the hot -glass to form the foot. - -In another method of making a wine-glass, the stem or leg is drawn out -from the body of the bulb by pinching down a knob at the end of the -glass. The servitor draws the leg out of this knob and knocks off the -extreme end. Meanwhile, the footmaker has been preparing a foot, -gathering a small portion of metal on a blow-iron and blowing it out and -shaping it into a double globule. The end globule forms the foot and the -second merely acts as a support. The footmaker takes these globules, and -the servitor sticks them on to the drawn stem of the wine whilst it is -hot; the blow-iron holding the globules is knocked away, leaving them -adhering to the leg of the wine-glass. The footmaker then knocks off the -second globule at the line between the two and, re-heating the bulb at -the foot of the glass, opens and widens the edges out. The glass then -goes to the workman to be finished in the same way as the common mule -wine-glass. - -Many articles of glassware are formed with the aid of moulds. Take as an -illustration the manufacture of tumblers and honey pots. A quantity of -glass is gathered on the blow-iron, marvered, and blown out into an -elongated bulb, which is introduced into a mould divided in two halves, -which open or shut by hinges, a handle being fixed on either half to -facilitate the operation. The interior of the mould is made to the shape -of the article, and as the bulb of hot glass is introduced it is shut, -and the workman blows down his blow-iron and extends the glass until it -expands and fills the space within the mould, giving the complete form -of the article with a surplus of metal just where the blow-iron is -attached to the glass at the top. These tops are then cut off and -finished, either by the workman re-heating the article by attaching the -bottom to a pontil and shearing off the top edges, or the glass is -annealed in its unfinished state and the top surplus portion cut off by -an automatic machine specially constructed for cracking off such goods. - - -[Illustration: - - GLASSWARE BLOWN IN MOULDS SHOWING PORTIONS CRACKED OFF - (_a_) Tumbler. - (_b_) Honey Pot. -] - - -Such machines consist of a set of revolving tables upon which the glass -articles are centred, and each in turn revolves in front of a thin, -pointed, hot jet of gas flame, which impinges on the glass at the height -at which the glass is to be cracked off. After one or two revolutions in -front of this hot pencil of flame, it is removed, and, by applying a -cold steel point so adjusted as to touch the part where the jet has -heated the glass, a chill is imparted which causes the upper portion of -the glass to crack away in a clear, sharp line round the glass. This top -portion of surplus glass is thrown aside and returned to the furnace for -re-melting as cullet. - -The tumbler or honey pot is then conveyed to another machine which -fire-polishes the edges to a smooth finish. - -This machine consists of a circular revolving frame carrying small -supports, which themselves rotate on their own centres. Upon each -support an article is placed to be fire-polished and the frame carries -them round, and they travel into another section of the machine, passing -under a hooded chamber, which is heated by a fierce jet of flame. The -jet of flame, which is localised on to the top edges of the tumblers or -other goods passing through the hood, gives just sufficient heat to melt -and round off the sharp edges of the glassware where they have been -cracked off by the previous machines. By using these machines in this -way labour is considerably economised, and as many as 300 or more -articles an hour can be cracked off and fire-polished with unskilled -labour. - -These machines are extensively adopted in the manufacture of electric -light bulbs, shades, lamp chimneys, and tumblers. - -Moulds are usually opened, shut, and dipped by boys, but in up-to-date -glass works an automatic machine called a “Mechanical Boy” is used. With -this machine, the mould is operated at the desire of the workman and not -at the desire of the boy. The output is considerably expedited by the -use of these automatic devices for opening and shutting the moulds. - -It is obvious that whatever the shape of the mould, or whatever the -design within the case, the glass takes the impression and retains it in -after working. In this way, square sections, fluted indentations, or -raised bosses can be formed with facility and regularity. - - -[Illustration: - - _By permission of_ _Melin & Co._ - VERTICAL CRACKING-OFF MACHINE -] - - -The Glass Workers’ Union consider that the introduction of machinery -deprives men of their independence and right to work, but as yet the -glass blowers have been always fully occupied with useful work about the -factories in which such machines have been introduced, so it cannot be -said that they have been forced to be idle. - -The advantages possessed by these automatic machines in their larger -output at so much less cost compared with hand labour is the great -factor in inducing their adoption; and in these days of progress and -competition such machines enable the glass manufacturers to cope with -the increasing demand and go far towards bringing a factory up to date -and making it well equipped. - -Manufacturers should certainly turn their attention to these mechanical -methods, as their use is quite general on the Continent and in America, -and by their use the metal can be worked out of the pots or tanks much -more quickly, increasing considerably the turnout or capacity of the -furnace against the fuel consumption. Much of the glassware imported -into this country is composed of such articles as would have been -manipulated by machines, and, unless a similar method of manufacturing -them is adopted here, we cannot hope to compete with other countries in -supplying our own needs. In the writer’s opinion, it is mainly due to -the adoption of machinery for producing glassware that the continental -people have been enabled to undersell us in our own market, and English -manufacturers could produce at a much cheaper rate if they would only -adopt similar methods of manufacture and the gas-fired furnaces as used -abroad. - - ------------------------------------------------------------------------- - - - - - CHAPTER XII - - CROWN, SHEET, AND PLATE GLASS - - -The glass used in windows may be either crown, sheet, or plate. - -=Crown Glass= is made in the form of circular flat discs about 4 ft. in -diameter. The workman, by repeated gatherings, collects sufficient glass -on the end of his blow-iron until he has a mass approximately 10 or 14 -lb. in weight, which he marvers into a pear-shaped lump by rotating the -hot glass in the hollow of a wooden block. He then blows the glass into -a spherical bulb (_a_), which, by quick rotation, is widened and assumes -a mushroom shape (_b_). Another workman attaches a pontil to the outer -centre of this bulb by welding it on with a small portion of hot metal. - -The blow-iron is then detached by wetting and chilling the glass near to -the blow pipe, which breaks away, leaving an opening in the bulb where -it has become detached (_c_). - -This is then carried to an auxiliary heated furnace, which has a wide -opening emitting great heat, and by resting the pontil upon a convenient -support and rotating it quickly the action of centrifugal force and heat -causes the glass to spread out at the opening, which becomes larger and -larger until the glass finally opens out into a flat circular disc of -fairly even thickness throughout, with the pontil still at the centre, -forming a bullion point or slight swelling, due to the knob of glass -used in affixing it (_d_). - -Next, the workman, keeping the disc in rotation, brings it away from the -furnace and allows the metal to stiffen and set by cooling, when it is -carried to the annealing oven and detached from the pontil. The discs -are then stacked up for annealing. When annealed, these are afterwards -cut across in sections or squares of convenient size by using a glass -cutter’s diamond. - - -[Illustration: - - FOUR STAGES IN CROWN GLASS-MAKING -] - - -It is evident that the centre portion, containing the bullion point or -bull’s eye, is useless for plain window glazing, but occasionally these -are sought after by glass decorators for use in coloured leaded lights -for door panels, etc. - -=Sheet Glass= is made in the form of thin, walled, hollow cylinders of -glass, which are split along their length and round the cap and then -opened out by heat and allowed to uncurl until each sheet lies out flat. -The workman gathers a sufficiency of glass upon his blow-iron by -repeated gatherings, and marvers it into a ball about as big as one’s -head. This is blown out (_a_) and widened by rotating the blow-iron -until he gets a mushroom shape (_b_), with a heavier bulk of glass at -the extremity than at the sides. - - -[Illustration: - - SIX STAGES IN SHEET GLASS-MAKING -] - - -This extra thickness of glass at the extremity of the bulb tends to -lengthen the bulb of glass as he swings it in a pendulum fashion, and by -blowing and swinging it alternately he gets an extended form (_c_). - -To permit the workman to swing the mass of glass out conveniently to the -full length of the intended cylinder, a long, narrow pit or trench is -provided below the floor level, and by standing alongside this trench -the workman is enabled to swing the glass within the trench at arm’s -length until the requisite length and width of cylinder are obtained. -This work requires a high degree of skill and strength. The shape of the -cylinder of glass is now as shown on page 91 (_d_). - -The extremity of this cylinder is now re-heated and opened with the aid -of a spring tool with charred wooden prongs, until the opening is -enlarged and drawn out to the same diameter as it is throughout the -cylinder. It is now in the form of an open-ended cylinder (_e_). - -The cap of the cylinder at the blow-iron end is now cracked off. A -thread of hot glass is wrapped round the shoulder near the cap, and the -line chilled by using a curved, hook-shaped rod of iron. Whilst the cap -is being cracked off, the cylinder is allowed to rest supported by a -wooden cradle. - -The cylinder is now open at both ends (_f_) and is taken to the -flattening kiln or furnace. This kiln has a level, smooth floor, heated -from below, upon which the cylinders are flattened out. Placing the -cylinder on the floor in front of him, the workman places along the -inside length of the cylinder a long red-hot iron rod touching the -glass, and then chills the line with a touch from a cold iron rod. This -causes a split to take place along the whole length of the cylinder. As -these cylinders are split open, they are removed to a hotter zone within -the flattening kiln, where the heat causes the cylinder to uncurl and -gradually flatten out. - -As the sheet becomes flat the workman levels it out with a flat block of -charred wood called a polisher. This is attached to a long handle, and -is rubbed over the face of the sheet of glass. The weight of the wooden -block is just sufficient to smooth out any creases and assists in -levelling out any irregularities of the surface. It is essential that -the floor upon which the glass is resting should be perfectly smooth and -level, and uniformly heated. As each sheet is levelled, it is removed to -the annealing oven and afterwards stacked up until cool, after which the -rectangular sheets are cut up to the various sizes required for window -panes. - -It is evident that the crown glass method gives more waste in cutting -up, and does not provide such large sheets as the cylinder method. On -the other hand, cylinder glass always shows a certain amount of waviness -on the surface, and is not so brilliant as crown glass. The better -surface of crown glass no doubt is due to the fire-polishing it receives -when being expanded out into the disc. It appears to be somewhat -difficult to get a perfectly smooth level face to cylinder glass by -using the wooden polisher. - -=Plate Glass= is used as mirror glass and in glazing shop windows and -showcases. It may vary between 1/4 and 3/4 in. in thickness, and is more -expensive to produce than crown or cylinder glass. - -In the manufacture of best plate glass, the materials are melted in open -crucible-shaped pots of varying sizes; sometimes, in making large, heavy -plate, their capacity reaches 25 cwts. of metal. When the metal is plain -and clear from seeds it is either ladled out into smaller crucible pots -for casting, or, as in the case of casting large sheets, the whole -crucible of metal is lifted bodily out of the furnace by means of a -crane, and, after being skimmed, is conveyed by an overhead travelling -derrick to the casting table. - -This table is a level iron bench the size of the plate to be cast, the -face of which consists of thick sheets of iron plate riveted together to -form a level top; along the whole length of each side of this table is a -raised flange of a height sufficient to give the thickness of the plate -of glass to be cast: resting on these two outer edges a long, heavy -metal roller runs, covering the full width of the table. The crucible of -hot metal is brought to a convenient position and the contents poured -out on the table in front of the metal rollers. These rollers then -travel along and squeeze or roll out the hot metal over the surface of -the table to the thickness regulated by the side pieces, which also -prevent the metal from flowing over the sides. The empty crucible is -then conveyed back to the furnace for refilling. - -The cast plate of glass is then trimmed from any excess of glass at the -ends, and when set and stiff is lifted at one end slightly and pushed -forward into a conveniently situated annealing oven, where it is -re-heated and subjected to a gradually diminishing temperature to anneal -it. The plate of glass, as delivered from the annealing oven, shows -surfaces somewhat rough, wavy, and uneven, from the marks left by the -table and the roller, and it has to be ground and polished level and -smooth on both sides. This is done by fixing one face of the glass plate -in a plaster of Paris bedding and setting it within a mechanical -grinding machine. - -This machine carries several revolving arms, to which are attached other -smaller plates of glass. These are used as the rubbers, a slurry or -paste of sharp sand and water, or abrasive powder, being interposed -between the two. The revolving circular motion of the arms causes a -grinding action between the two plates, which wears down any -irregularities and gives a more even face. After this, finer grades of -abrasive materials are employed, and, finally, polishing powder, until -the face of the glass plate is polished smooth and level. The large -plate of glass is then reversed and the process of grinding resumed on -the other side. - -Much care is necessary in handling these large plates, and every -attention is necessary and devoted to get the largest pieces of plate -without defects. All portions showing defects have to be cut away, and, -consequently, reduce the size of the plate when finished. - -In another method of making plate glass the molten metal is fed between -two or more parallel rollers, which are spaced apart to the thickness of -the glass required (about 1/4 in.). These rollers squeeze the glass out -to a uniform thickness. A roughly decorated surface is sometimes given -to this glass intentionally, by the metal rollers being indented with -some form of set star pattern. This glass is not ground or polished, and -is sold under the name of muffled or cathedral glass. It is mostly used -for roof lighting, where the transparency may be somewhat obscured. - -Wired glass, or strengthened plate, is formed by embedding in the soft -glass, whilst being rolled, a network of metallic wire of special -composition to suit the temper of the glass. This wire is fed from a -separate roller into the space between the parallel rolls as the hot -metal is fed in from either side. It is necessary that the wire should -be made from a metallic alloy which is not easily oxidised. Another -method of strengthening plate glass consists in sealing together two -plates with an intersecting film of celluloid. - -A decorated coloured rolled plate is made for use in leaded lights by -mixing portions of several differently coloured glasses together in a -small pot and slightly agitating the contents so as to intermix the -respective colours. When the glass is rolled out, a pretty agate or -marbled effect is obtained, due to the distributed coloured glasses -becoming intermixed. As a rule, these glasses are more or less -opalescent, and are only used for decorative purposes, church lights, -etc. - - ------------------------------------------------------------------------- - - - - - CHAPTER XIII - - TUBE, CANE, AND CHEMICAL GLASSWARE - - -Laboratory and chemical glassware consists of thin blown ware in the -form of flasks, beakers, test tubes, etc., used in chemical operations. -Most of these goods are blown in hinged moulds mechanically or -automatically operated by the worker. The lips and flanges of the necks -are neatly formed afterwards by re-heating and working the edge to a -form allowing them to pour cleanly, and prevent any fluid contained -therein from running down the sides of the flask or beaker whilst in -use. The heavier glassware, in the form of desiccators, measuring -cylinders, specimen jars, and three-necked bottles, are made by -handwork. Chemical apparatus has necessarily to be made from a permanent -stable highly refractory glass, so as to resist the solvent actions of -mineral acids, alkaline solutions, and boiling water, as well as sudden -changes in temperature. - -The manufacture of tube and cane glass for various purposes forms a -large and extensive portion of the glass trade. Considerable quantities -of tube and cane glass in various sizes are used by lamp workers in the -manufacture of certain forms of chemical apparatus and filling electric -light bulbs. By re-heating glass tube and working before a blow-pipe -flame, the various forms of test tubes, pipettes, burettes, soda-lime -U-tubes, and condensers are made. Generally, for chemical apparatus two -classes of tube are made, one a soft soda tube, and the other hard -combustion tubing. Particular care has to be devoted to the grading and -sorting of the various sizes. The bore of the tube, the thickness of the -walls, and the outside width have all to be checked and the lengths -classed accordingly. - -In the manufacture of tubing, unless the glass is of large size or great -thickness, it is not annealed, and shows a case-hardened condition which -materially increases the strength of the tube to resist internal -pressure, as is the case with boiler gauge tubing. In the manufacture of -apparatus from tube and cane, care must be taken that the various pieces -used in welding together the different portions of the apparatus should -be of the same temper and composition, and supplied from one source, so -that they may join and work perfectly together. - -The lamp worker or glass blower should take care to get his supplies -from a reliable source, so that the glass pieces will be adapted to work -together. Trouble occurs when odd tubings from various makers are worked -together. The same applies to fancy glass working, where various -coloured canes are worked into ornaments. Reputable firms can always -supply from stock such colours and tubing properly adapted for their -specific purposes, and they take every precaution to see that the -various colours join and work together. Supplies of glass rod can be had -that will join on to platinum, nickel, iron, or copper wire with sound -joints. - -In making cane glass, the workman gathers sufficient metal upon a -pontil: for thin cane he would gather less than for heavy thick cane. -After gathering, he marvers the metal into the form of a solid cylinder. -Meanwhile, an assistant gathers a little metal on a post or pontil with -a flattened end. The metal he has gathered has covered the flat end of -the post, and he holds this in readiness for the workman, who is now -re-heating the cylinder of glass at the pot mouth. As the cylinder of -glass becomes soft, he withdraws it and allows the end of the -cylindrical shaped mass of glass to fall gently upon the flat end of the -post, to which it adheres. They then carry the glass between them to a -wooden track or run-way, along which they walk at a smart pace in -opposite directions; stretching out the hot glass between them, it -gradually thins out and rests on the floor. The pace the men separate -apart from each other is regulated according to the thickness of the -cane desired: for very thin cane a smart trot is necessary, but for a -thick cane a slow walk is sufficient. As the glass is drawn out it is -allowed to rest on wooden supports, and when cool is cut up into -convenient lengths by scratching the glass with a steel file. These -lengths are collected and bundled up for sorting and classification. All -portions distorted or over-size are returned as cullet for re-melting -and re-use. - -In tube making, instead of a solid cylinder as in cane making, the -workman, by gathering the glass on a blow-iron and blowing and marvering -it, obtains a thick-walled, hollow, cylindrical form. This is re-heated -and the end stuck to a post and drawn apart as before described in cane -making, forming a tube of a width proportional to the rate the two have -travelled apart in drawing it out, and to the quantity of metal -gathered. In this way the respective sizes and thicknesses are -regulated. A narrow cane or tube may be drawn out for 300 ft., but for a -thick or wide one probably only 30 ft. may be drawn. In making the -larger widths, some method of cooling, or fanning, is adopted, to ensure -uniform size by cooling the hot glass quickly as it is drawn out. It is -evident that, whatever shape is given to the original mass of glass -whilst being marvered, the tube will bear a similar shape in proportion, -either within or outside the glass. In this way, square, triangular, or -oval sections can be produced in both tube and cane. - -The manufacture of white opal, coloured cane, and tube is carried out on -like methods to those used in ordinary cane and tube making. - -We will now describe the manufacture of Filigree. This is rod or tube -containing opal or coloured threads, either straight, spiral, or -interlaced within a transparent glass; these threads follow the whole -length of the cane or tube. - -This curious form of glasswork was originated by the Venetians, who are -exceptionally skilled in producing some elegant and ornamental filigree -decorated glassware. - -The method of producing filigree cane consists of first taking a number -of short lengths of opal or coloured cane previously drawn and cut to -about 6 in. lengths. These are then placed in vertical positions around -the inner circumference of an iron cup mould, which may be about 5 in. -in diameter. The opal strips of cane are supported vertically in small -recesses provided in the rim of the mould at equidistant intervals. A -ball of hot crystal glass is gathered on a pontil and is lowered into -the inside of the mould, the hot metal coming in contact with the opal -strips of glass adheres to them, and upon withdrawing the glass it -brings the opal strips away with it arranged in sections round the -circumference of the ball of glass. This is now re-heated and marvered -until the canes or strips of opal are well embedded in the hot glass. -Then the workman gathers another coating of hot glass over the whole, -marvers it again into a cylindrical form, and then proceeds to draw it -out as described in cane making. - -If a spiral form of lines is desired, the workmen, whilst drawing out -the cane, turn or twist the pontil and post in contrary directions. -These rotations cause the opal veins or threads to assume a spiral or -twisted form within the glass. Various coloured cane may be used in the -above process, and by placing them in alternate positions to the opal -strips within the cup mould some very pretty and curious filigree work -is obtained. These twisted filigree canes are used and manipulated over -again in the process of making the various Venetian goblets and wine -stems. Some fine effects in the application of filigree decoration can -be seen in the specimens of Venetian glassware exhibited in the British -Museum. - -Millefiore work is produced by the workman, first spreading a layer of -an assortment of small coloured glass chips of varying sizes (between -1/8 and 1/4 in. cube) over the face of the marver, and then taking a -gathering of crystal metal on his blow-iron and rolling the ball of hot -glass into the coloured mixture on the marver. The hot glass collects up -a coating of the coloured chippings, and is then re-heated and again -marvered, another gathering of crystal metal is made, which incases the -whole. This is then blown out and worked into some form of ornament, -such as a paper weight, inkpot, or bowl, producing a curious result that -shows blotches of colours embedded within the glass, the effect of which -is increased if a backing of opal glass has been used in the first -gathering: this shows the coloured effect against a white background. - -Spun Glass. Another curious form of glass is the spun glass which is -much employed in making fancy ornaments. Glass can be spun into a thread -so fine and flexible that it can be worked into a fabric like any -textile material. In this way, glass ties can be made by plaiting the -spun glass threads into the required form. Spun glass fibre is used in -making the brushes used for cleaning metals with acids. On account of -its greater resistance to acids than is shown by ordinary cloth, an -endeavour is being made to use spun glass cloth in certain industries as -a commercial application. Spun glass is used for making a form of filter -cloth which is being used successfully in filtering acid residues in -certain chemical processes, and, no doubt, when the elasticity and -strength of the glass threads can be more developed, the scope for its -use in other industrial processes will be increased. - -The method of making spun glass thread consists in melting the end of a -plain or coloured glass rod (which may be square, round, or triangular -in section) in a blow-pipe flame and grasping the end which is melting -with a pair of pincers, drawing it out and affixing it to a wooden drum, -which is turned rapidly away from the glass being heated. The drum may -be 2 or 3 ft. in diameter, and as the glass is continually fed into the -heat it is drawn out into a very thin thread by the rapidly revolving -drum, and coiled up until a sufficient quantity has been obtained. The -thread is then cut across the drum, collected, and used for plaiting or -braiding into the fabric or cloth. - -The iridescence and variety of colours yielded by the refraction of -light between the glass threads gives spun glass its peculiar effect, -very evident in the forms in which it is used in decorating small -ornaments such as forming the tails of glass birds. - -Glass wool is made in a somewhat similar way, and is successfully used -as a non-conductive packing material for insulation from heat. - -Glass frost or snow is made by blowing small gatherings of glass out to -a bursting point. These very thin shells are then crushed and the flakes -collected, and used for such purposes as surfacing sand paper or -decorating Christmas cards, being sieved to the requisite size and -affixed with a siccative to the paper. - -Dolls’ eyes and artificial human eyes are made by well-trained operators -working before a blow-pipe flame and manipulating tube and cane of -delicately coloured tints to form the pupil and shell of the eye, the -veins being pencilled on with thin threads of red-coloured glass. A -considerable amount of skill and adaptation is necessary to do this -class of work, and much depends upon the matching of the coloured cane -glass used to give the natural effects. When properly made, so clever -and natural are these glass imitations of the human eye that it is with -difficulty that the ordinary observer can tell that they are not real. A -skilled worker will make the artificial eye to fit the muscles of the -socket and so move. In this way much ingenuity has been shown in fitting -the eye sockets damaged during the war. - -Aventurine is a golden coloured glass containing minute yellowish -spangles or crystals reflecting upon each other and giving its peculiar -effect. This glass is obtained by the use of an excess of copper with -strong reducing agents in the glass, whereby the copper is partially -reduced within the glass, giving the pretty spangled effect. This glass -is often used in the form of jewel stones, being cut and polished and -fitted in ornaments. The process of making this glass was originated by -the Italians, and for some time it remained a secret with them, and even -now is styled “Italian aventurine.” - -Chrome aventurine is another form, giving a green, spangled effect. This -is got by using an excess of chromium in the presence of reducing -agents. - -The successful production of aventurine depends upon slowly cooling the -molten glass so as to assist crystallisation. - -Mica schist, or flake mica, is used to give another curious effect in -glass. A gathering of some dark-coloured glass is rolled or marvered -upon a thin layer of flaked mica, and then a further gathering or -coating of clear crystal metal is made. The whole is then blown and -formed into some fancy ornament or vase. When finished, the glistening -mica flakes show through against the coloured background, giving a -curious silvery reflection. - - ------------------------------------------------------------------------- - - - - - CHAPTER XIV - - OPTICAL GLASS - - -The manufacture of optical glass forms a very important section of the -glass industry, and presents some of the most difficult problems the -glass maker has to deal with. It is in this section of the glass trade -that applied physical and chemical science becomes of the utmost -importance to the manufacturer. The production of optical glass is -impeded by any defects which become evident in the structure of glass -when examined under a polariscope. The presence of any striae, seeds, or -stresses within the structure of the glass disqualifies it for any -important optical work. It is a difficult matter to get pieces of -optical glass only a few inches in diameter of the right optical -constant and refractive index that are homogeneous enough to allow of -the light rays passing without some dispersion when set up for use. It -becomes necessary, therefore, to achromatise one glass with another in -the form of doublets to correct aberration. A high degree of -transparency and durability is necessary in all optical glasses. - -The persistent evidence of stresses developed in the solidification of -the glass upon cooling, even when the glass is slowly and carefully -annealed, is a most difficult factor to deal with. In annealing optical -glass, the various temperatures and time periods have to be delicately -adjusted and controlled, or big losses result. Even then many efforts -may be made before a suitable piece of glass is obtained, and the costs -keep accumulating with each attempt, and some idea of the amount of -labour involved in the undertaking to produce optical glass at once -becomes evident. The use of decolorizers and impure materials is not -permissible, on account of the absorption and consequent resistance to -the passage of light rays. The annealing, instead of occupying one or -two days, is sometimes extended over a course of ten or fifteen days, in -order gradually to relieve any stress present. The pots in which the -glass is melted may only once be used, as the glass is usually allowed -to cool down gradually and undergo the process of annealing within the -pot. - -The temperature of the furnace is controlled by regulating the draught -by means of dampers in the main flues, arranged to act so as to carry -out the annealing of the glass within the furnace. The regulation of the -temperature within the furnace is of the greatest importance; if too hot -the glass dissolves the clay of the pot, and if retarded too much it -gives difficulty in freeing the metal from seeds, and plaining or fining -the glass properly. Small furnaces containing one or two pots give the -best results. These furnaces are worked on an intermittent process of -first melting the glass and then gradually cooling to anneal the glass -within the pots in mass, the furnace being allowed to die out gradually. -When cool, the pots are broken away from the glass, which is then -cleaved into lumps. Each lump is carefully examined for any defects and -the best pieces selected for re-annealing. These are afterwards ground -to the desired shape in the form either of a lens or prism. The chances -are that not many pieces of perfect glass can be obtained from each pot -of metal, and probably out of a whole pot only a fifth would be suitable -for use after the process of selection and cleaving has taken place. - -In the manufacture of optical glass, batch materials are chosen that do -not differ greatly in specific gravity. Every effort is devoted to -obtain the purest materials possible; the batches are finely ground and -well mixed before melting. The glass-melting pots should be made of the -purest and most refractory obtainable in order to prevent the solution -of any impurities into the glass whilst it is melting. In heating the -pots for melting optical glasses every endeavour is made to heat them -equally all round the top, bottom, and sides, so as to dissolve all -portions of the glass evenly and completely together. At times the -melted glass is stirred with a bent iron rod encased in a porcelain -tube, and the glass agitated in order thoroughly to mix the components -whilst fusing, and keep the composition of the glass as uniform as -possible. After the metal has melted and plained clear from all seed and -cords, the pot of metal is annealed, and when cooled the glass is -extracted in lumps and examined for any defective pieces, which are -rejected. The selected pieces are afterwards ground to the desired shape -and, if necessary, re-annealed. In this process the pots being used only -once, are expensive items, and they considerably increase the cost of -production. - -Before the war the optical glass trade was confined to a few firms in -this country, who supplied only a fraction of our needs. We have been -dependent mostly upon continental supplies of optical glass, and it is -only quite recently that Government state assistance has been -forthcoming in giving scientific aid to manufacturers by investigating -and reorganising this section of the glass industry. It is to be hoped -that this state assistance will continue, and that the optical branch of -the glass trade will be perfected to such an extent that we may in -future be independent, and produce for ourselves all the optical glass -requirements of our navy and army. It is to be regretted that this -industry did not receive state assistance before the war. If it had, we -should certainly have been better prepared and equipped than was the -case at the start of the Great War. - - ------------------------------------------------------------------------- - - - - - CHAPTER XV - - DECORATED GLASSWARE - - -Certain methods of decorating glass are carried out whilst the glass is -being made by the workmen. Other methods consist in decorating the glass -after it has been made, such as cutting, fluting, etching, engraving, -and enamelling. In another form of decoration the method consists of a -combination of two or more of the above processes. The crystal glass may -be cased over with a thin covering of coloured glass by the glass -worker, and this outer coloured casing cut through by the glass cutters, -exposing and showing through the colourless crystal underneath with very -effective results. - -A small portion of coloured glass, such as citron green, topaz, blue, or -ruby metal is gathered from the pot by an assistant, and the workman, -gathering a ball of crystal glass on his blow-iron, allows a portion of -the coloured metal held by the assistant to fall or drop upon the ball -of crystal. Upon blowing the whole out, the coloured metal is spread as -a thin casing upon the outside of the bulb of crystal. This bulb is then -worked into a wine-glass or other article, which, after annealing, is -sent to the glass cutter, who decorates the outer surface by cutting the -glass on his wheel. The colourless glass then shows through against the -coloured surface where it has been cut to the pattern, the colour -standing out in relief. - -In another form of decoration, the workman allows small pear-shaped -tears or drops of coloured glass to fall upon the outer surface of a -bowl or vase, in equidistant positions round the circumference of the -article, By placing and working the coloured glass into position in this -way, some pretty artistic results are obtained, dependent upon the skill -and artistic taste of the workmen. - -In another method of decoration, certain coloured glasses are used, the -composition of which causes them to turn opalescent upon re-heating the -glass to a dull red heat. The re-heating of the tops of crimpled flower -vases made from such glass gives pretty results, showing a gradual -fading opalescence, extending from the top edges to a few inches down -the vase, into a clear coloured glass at the foot of the stand. A -similar effect, without the opalescence, is obtained by the workman -gathering a small piece of coloured glass on the tip of his blow-iron, -and then taking a further gathering of clear crystal metal. The whole is -then blown out and worked into a vase or wine-glass, thus obtaining a -coloration denser at the top edges, where the vase or wine-glass has -been sheared off, and gradually fading away to a colourless glass a few -inches towards the foot, which is clear crystal. - -There are also certain compositions which, when worked into a vase, and -re-heated on the edges, strike or turn to a colour such as pale blue or -ruby. These are self-coloured glasses, in which the colouring remains -latent until the glass is re-heated, like the opalescent glasses. In -these glasses the composition is the more essential factor. - -=Glass cutting= is an effective way of decorating glassware. In using -this method, the crystal glassware is made fairly heavy and strong, so -as to permit of the deep cuttings which refract the light and show up -the prismatic patterns so brilliantly. - - -[Illustration: - - MACHINE FOR SMOOTHING BOTTOMS OF TUMBLERS -] - - -In cutting glassware, the glass cutter works in front of a rotating disc -of iron carried in a frame. This wheel has a bevelled edge upon which a -fine jet of sand and water is allowed to drip from a tundish above. The -abrasive action of the sand cuts into the glass, and the workman, by -holding the glass dish or bowl against the wheel, follows the design or -pattern in diagonal lines across the article. These cuttings are -recrossed, and the intermediate diamond spaces filled in with lightly -cut set patterns, until the whole of the intended design is “roughed” -out over the surface of the glass, after which the glass is taken to -another frame carrying a stone wheel, which is of much finer abrasive -action. This stone wheel smooths the rough cuts done by the previous -wheel. After this the cuts are polished successively on a wood wheel and -brush with polishing powders, until a smooth and polished cut is -obtained. - -As the value of the glass is greatly increased by cutting, only the best -and clearest articles of table glass are so treated. The work of cutting -becomes technical and expensive, according to the richness of the -cutting demanded. The crystal table glass made from lead gives the most -brilliancy in cutting. Soda-lime glasses are found to be hard to cut and -do not give such brilliant and prismatic effects as the glass made from -lead compositions. - -An automatic machine for grinding, smoothing, and polishing the bottoms -of tumblers, etc., “bottoms” or grinds, smooths, and polishes tumblers -at the rate of 2,000 a day. Four vertical revolving wheels are fixed -within a frame, one iron, two stone, and one wood. Over each of these is -a rotating spindle carrying the tumbler so that the bottom of it is -automatically pressed against each vertical wheel in turn. The first -wheel does the roughing, the two next the smoothing, and the fourth the -polishing. These machines are simple and require only unskilled labour -to operate, and go far towards cheapening production. - -=Glass engraving= and intaglio work is a much lighter and more artistic -method of decorating glass than the deep cutting before described. In -these processes the glass is cut or ground to a less extent, and a more -free treatment of design is possible. Floral ornamentation and natural -forms of applied designs can be carried out, and portions may be left -rough or polished, according to the effect of light and shade required. -The workman, whilst engraving, works before a small copper or metal -wheel rotating in a lathe, and uses fine grades of emery or carborundum -powders made into a paste with oil, as the abrasive medium. The frame -turning these wheels is like a lathe, and may be worked by a foot -treadle. The wheels are interchangeable, and an assortment of various -sizes, having different bevelled edges, is kept at hand in a case, from -which the engraver selects the one most suitable for the particular work -to be done. - -Glassware for engraving and intaglio may be made much lighter than that -required for cutting. - -=Etching= is a method of decorating glass by the chemical action of -hydrofluoric acid. This acid in its various combinations attacks glass, -decomposing its surface and giving a dull or semi-matt effect. Only -those portions of glass which constitute the design are exposed to the -acid paste or fumes. The other portions are protected by a covering of -beeswax, which is unaffected by the acid and protects any portions -covered by it. - - -[Illustration: - - GLASS ENGRAVING -] - - -The process carried out is varied in many ways. In some cases pantograph -and etching machines are introduced to give the designs. A warm copper -plate, with the design or ornament engraved thereon, is covered with a -wax paste, and the surplus cleaned off with a palette knife or pad of -felt, leaving the paste in the recesses of the engraving; a piece of -thin tissue paper is laid over the engraved plate and takes an -impression of the design in wax. This tissue is then transferred to the -glass to be decorated, the wax design adheres to the glass, and the -paper is drawn away. A further resist or coating of wax is painted round -the design to protect the rest of the glass, and a paste composition -giving the action of hydrofluoric acid is applied, which after a short -time eats into the exposed portions of glass. After another short -interval, it is washed off, and the wax coating removed by washing the -glass in hot, soapy water. The design then appears in a matt state -against the clear, unattacked glass. - -The mechanical method of etching the design is carried out by first -dipping the whole glass into a bath of hot liquid wax, allowing a thin -coating to set and cool upon the surface of the glass. The article is -then introduced into a machine which has a number of needles, worked by -sliding gears in an eccentric fashion. These needles are adjusted just -to scratch away the thin coating of the wax into a design, and expose -the glass in the form of a decorated scroll or band round the glass. The -glass is then dipped into a vat or bath of dilute hydrofluoric acid for -a few minutes, after which it is removed and washed, and the wax -recovered by heating the glass upon a perforated tray, when it melts and -runs off the glass, and is collected for further use. The article is -then washed and cleaned and shows the scroll or etched portions where -the needle has traced the design. Another effective result is obtained -by etching a design on the back of a plate glass panel. After cleaning -and silvering or gilding the back, the design appears in a matt silver -or gilt lustre upon viewing it from the front of the mirror. - -Glass which has been sand-blasted has a similar appearance to etched -glass, but a rather coarser surface. The portions of the glass plate to -be decorated are exposed to the action of a blast of air, into which -fine, sharp-grained quartz sand is automatically fed. An abrasive -action, due to the force with which the particles of sand are blown -against the glass, takes place, rendering the surface opaque or matt. -This method is generally adopted in printing trade names or badges upon -bottles, etc. A stencil of parchment or lead foil is cut out to form, -and used to protect the glass and resist the abrasion where required. -Rubber gloves are worn by the operator. The work of sand-blasting is -executed within a small enclosed dust-proof chamber fitted with glass -panels. The operator manipulates the glass through openings in the sides -of the chamber. The air blast is supplied by a motor-driven air -compressor and is regulated by a foot pedal. The action is very sharp -and quick, and is a cheap and effective way of badging hotel glassware -and proprietary bottles. - -Glassware may be decorated by being enamelled with coloured enamels. In -this method of decorating, soft, easily-fused, coloured enamels are -used, containing active fluxes such as borates of lime and lead, which -melt at low temperatures. These enamel colours are prepared by being -fused and then ground to fine powders, which are mixed with a siccative -or oil medium, and painted upon the glass. The painted ware is then -heated within a gas or wood-fired enamelling furnace or muffle, until -the painted designs are melted and fused well upon the glass. The glass -is re-annealed in cooling down the muffle. For this form of decoration, -a hard refractory glass is required that will not soften easily under -the heat of the muffle; otherwise the glassware becomes misshapen too -easily under the heat necessary to flux or fuse the enamels properly. - -A form of staining glass is also practised which consists of applying -compositions containing silver salts to portions of the glass and firing -at a low heat. The silver stains the glass a deep yellow. The colour may -be varied by the use of copper salts, when a fine ruby stain is obtained -wherever applied. - -=Iridescent= glassware is produced by several methods. Sometimes a small -proportion of silver and bismuth is added to a coloured glass batch, and -by manipulating the resulting glass in a carbonaceous flame the silver -is partially reduced within the glass, forming a pretty iridescent -reflection on the glassware. By a suitable adjustment of the oxygen -content in the composition of such glasses, the iridescence can be -regulated to such an extent that the slightest flash or reducing -influence gives a beautifully finished lustre over the ware. - -Iridescence can also be formed by re-heating crystal glassware within a -chamber in which salts of tin, barium, aluminium, and strontium are -volatilised. This method produces a superficial iridescence which is not -quite so permanent as the previous process. - -=Glass Silvering.= The silvering of mirrors is carried out by taking a -thoroughly cleaned plate of polished glass and floating one surface in a -solution of silver nitrate, to which a reducing agent is added. The -silver is thereby precipitated or deposited in a thin lustrous film upon -the glass, which causes reflection by the rays of light striking against -the silvered background. - -After silvering, the back of the plate is coated with a protecting paint -or varnish, which dries and preserves the silver deposit and gives it -permanency. - -In the manufacture of fancy ornaments, such as birds, hat pins, and -small animals, various coloured glass cane and tube are worked together -by the operator melting and welding the respective colours together -before a blow-pipe flame, the tails of the birds being formed by sealing -in a fan of spun glass into the body of the bird, which has been blown -out and formed from a piece of tube. Some very curious ornaments are -formed in this way. Glass buttons, pearl, and bead ornaments are formed -by working cane and tube of various coloured compositions before the -blow-pipe, sticking and shaping the various forms on to wire. - -Mosaic glass decoration is used in jewellery in a mural or tessellated -form. In this method small cubical or other shaped cuttings of various -coloured opaque glass are inlaid in mastic cements or pastes to form the -design, the face being afterwards ground and polished smooth, and -mounted or set within the ornament. - -Larger cuttings may be inlaid in cement for pavement or mural -decoration. - - ------------------------------------------------------------------------- - - - - - CHAPTER XVI - - ENGLISH AND FOREIGN METHODS OF GLASS MANUFACTURE COMPARED - - -The continental methods of glassmaking differ so much from the English -methods that a few remarks giving comparisons will be of interest. It is -noticeable that chemical and engineering science is more thoroughly -applied in the manufacture of glassware abroad. Their method of -specialising wherever possible, and the introduction of mechanical and -automatic machines have done much toward increasing their production and -efficiency. - -The flourishing and extensive state of glassmaking abroad is shown by -the size and extent of the glass works, some of which work as many as -forty or fifty furnaces and employ 3,000 to 5,000 hands. Gas-fired -regenerative or recuperative furnaces are more generally used, which -permit higher temperatures, cheaper metal, and greater economy in fuel -and labour. - -The present type of English furnace is very wasteful, and even with good -fuel it is difficult to maintain high temperatures and regularity in -working. Our method of firing, raking, and teasing is very exhausting to -the workmen in attendance. - -In many English glass works, especially those in the Stourbridge -district, it is the practice to fill the pots on a Saturday morning and -take until the following Monday night to melt and plain the glass, no -glassware being made for three days of each week. Starting on Monday -night or Tuesday morning, the glass makers work in six hour shifts day -and night until Friday night or Saturday morning, when the pots are -again filled and the weekly course starts over again. Abroad the pots -are filled nightly and hold just sufficient metal to last out the work -during the day, and are built of a capacity to suit the articles being -made. The disadvantages of our method are obvious when a comparison is -made with the continental method of melting the glass nightly and -working it out daily, especially when the efficiency or output of the -furnaces as compared with their fuel consumption is taken into -consideration. - -Abroad the furnaces are small and compact; they take up less floor -space, yet they are far greater in efficiency. As they are gas-fired, -the combustion is more complete, and by the use of regenerators or -recuperators greater heat is available for melting the glass quickly. -Larger proportions of sand are used in the glass mixtures, which, being -the cheaper component, cheapen the production of their glass wares. - -Owing to the more perfect combustion which takes place within the -chambers of gas-fired furnaces abroad, lead glasses are successfully -melted within open crucible pots. When the heat comes into direct -contact with the batch materials being melted, it does its work quicker -and with less fuel consumption than is the case if it has first to be -conducted through the hood of covered pots which have necessarily to be -used in the old English type of furnace. - -It is particularly noticeable that the glass workers abroad do not spend -so much time upon producing an article as is usual under the English -method of working. By the extensive use of moulds fitted to mechanical -contrivances operated by the foot, their work is expedited and made -simple and easy. - -Technological education in the glass industry abroad is more thorough -and general. The glass workers, not having to work at night, have the -evenings free for recreation and education. It would do much towards -developing the English glass trade if night work for boys could be -abolished. The adoption of the continental system of melting the metal -during the night and working only during the day (by using gas-fired -furnaces) would do much in this direction. One cannot expect the youths -of the glass trade, who have to work during nights, to attend the -evening classes for educating themselves, without a severe strain upon -their constitutions. This fact partially accounts for the repeated -failure to establish technical classes and trade schools in the -glassmaking centres of this country. The conservatism and lack of -support from the glass manufacturers themselves account for much of the -slow progress and development of the trade. As a rule, it will be found -that the manufacturers have everything to gain by the better technical -education of their employees. It is with pleasure we notice that a few -at least are now taking this broader view and giving such schools their -hearty support and financial aid. In the glassmaking centres abroad -there are established state-aided technical and trade schools, where, -for a small nominal fee, the youths of the glass works are trained and -taught the principles of their industry. Apprenticeship in the factories -then becomes unnecessary. - -The working hours abroad are usually sixty hours a week (ten hours a -day), compared with the English forty-four to fifty hours’ week (six -hour shifts). - -The trade unions of the glass workers abroad are more progressive, and -their officials do not interfere with the manufacturers’ endeavours to -increase efficiency and cheapen production by introducing machinery. The -promotion of the workpeople goes by merit, and not by the dictation of -the trade union officials, as is too often the case in this country. -Here, very little sentiment or good-fellowship exists between the glass -workers’ union and the employers, and in its place the rank officialdom -of unionism has become so evident as to be a bar to the progress of the -industry. Instead of assisting the progress of the trade, and mediating -in cases of dispute, the union appears to exist as a buffer of -antagonism between the glass workers and their employers. Many a capable -youth in the glass trade here has been kept back from promotion to a -better position solely by the dictation of the union to which the men -belong. Cases are known where the union have restricted the workman’s -output when he may be working under piece rate. The best inducements may -have been offered him by the employer to increase his output, and, -although the workman may be willing to accept the master’s terms, we -find a union official stepping between them, and fixing the maximum -number of the articles that shall be made in his six hour shift. -Usually, this fixed quantity is got through in four hours, yet the -workman is not allowed to make more than the stipulated number fixed by -the union, or he is fined. Another incredible fact is that the employer -here, when in need of a workman, is not allowed to choose his own men. -He must apply to the union, and the man remaining longest on the -society’s unemployed book is then sent to him. Whatever his inefficiency -may be, the employer is bound to take him; if he employs anyone else, a -strike results. Such action is despotic and shows up the worst features -of trade unionism that can possibly be conceived. The English glass -industry has been repeatedly disorganised by this obstinate attitude of -the glass makers’ union, and a consequence is that the foreigner has -seized the opportunity to step in and increase his market, to the -detriment of our own trade; with this extended market, increased output, -and cheaper production, the foreigner undersells us in our own country. - -It is to be hoped these adverse conditions will soon be remedied and the -English glass industry restored to a more flourishing state by the -prompt and united action of the men and masters, realising the gravity -of the position and acting accordingly. - - ------------------------------------------------------------------------- - - - - - APPENDIX - - - JOURNALS AND BOOKS FOR REFERENCE - -“American Pottery Gazette.” (New York, U.S.A.) - -“Boswell’s Memoir on Sands Suitable for Glassmaking.” (Longmans, Green & -Co., London.) - -“Pottery Gazette.” (Scott Greenwood, London.) - -“Sprechsaal.” (Coburg, Germany.) - -“Painting on Glass and Porcelain.” Hermann. (Scott Greenwood.) - -“Decorated Glass Processes.” (Constable, London.) - -“Jena Glass.” Hovestadt. (Macmillan & Co.) - -“Glass Manufacture.” Rosenhain. - -“Producer Gas-Fired Furnaces.” Ostwald. - -“Glassmaking.” By A. Pellatt. (Bogue, London.) - -“Gas and Coal Dust Firing.” Putsch. (Scott Greenwood.) - -“The Collected Writings of H. Seger.” (Scott Greenwood.) - -“Ceramic Industries.” Vol. I. By Mellor. - -“Modern Brickmaking”; “British Clays, Sands, and Shales”; “Handbook of -Clay Working.” By A. B. Searle. (Griffin & Co.) - -“Glass Blowing.” By Shenstone. - -“Asch’s Silicates of Chemistry and Commerce.” - -“Clays.” By A. B. Searle. (Pitman, London.) - -“Fuel and Refractory Materials.” Sexton. (Mackie & Sons.) - -“Furnaces and Refractories.” Harvard. (McGraw, New York) - - - SOCIETIES’ JOURNALS AND TRANSACTIONS - -“The Society of Glass Technology.” (Sheffield.) - -“The American Ceramic Society.” (Columbus, Ohio, U.S.A.) - -“The English Ceramic Society.” (Stoke-on-Trent, Staffs.) - -“Journal of the Society of Chemical Industry.” (Westminster, London.) - - ------------------------------------------------------------------------- - - - - - INDEX - - - Aberration, 104 - - Acids, action of, on glass, 18, 19 - - Action of glass on , 45 - - Alkali, 23 - - Alumina, 9-11, 20 - - Amethyst, 31 - - Analysis of , 37 - - Ancient glass, 1 - - Annealing glass, 18 - - —— pots, 66 - - Arsenic, 31 - - Artificial eyes, 101 - - —— cements, 24 - - —— pearls, 31 - - Aventurine, 22-102 - - - Barytes, 8-26 - - Basalt, 10 - - Bastie’s Process of hardening glass, 18 - - Batch, 11-13 - - Beads, 31-116 - - Black glass, 29 - - Blowing glass, 80, 82 - - Blow-iron, 80 - - Blue glass, 28 - - Bohemian glass, 25 - - Borates in glass, 7, 8, 9 - - Boric acid, 7 - - Bottle glass, 26, 27 - - Bottle-making, 77, 79 - - Bull’s eye, 90 - - Buttons, 116 - - - Cane, 97 - - Capacity of pots, 51-52 - - —— of tank furnace, 56 - - Carbonate of soda, 6 - - Cements, 24 - - Chain screen, 68 - - Chair, Glassmakers’, 81 - - Chemical properties of glass, 4-15 - - Chemical Formulae, 12 - - Chimneys, Lamp, 16 - - Clays for pots and furnaces, 36 - - Coloured glasses, 28, 29 - - Colour of silicates, 11-22 - - Complex glass, 26 - - Composition of glass, 4-25 - - Compound glasses, 25 - - Conductivity of glass, 23 - - Continental glass, 3, 88, 118 - - Covered pots, 21-27 - - Cracking-off glass, 15, 17, 85 - - Crown glass, 26-89 - - Crucible pots, 21, 27, 64 - - Cullet, 10, 85 - - Cutting glass, 8, 10, 16 - - - Decay in glass, 2 - - Decomposition, 2, 19 - - Decorated glass, 108 - - Decolorants, 32 - - Defects, 9, 23, 34 - - De-grading glass, 23 - - Density, 16 - - Devitrification, 3, 8, 20 - - Discovery of glass, 1 - - Doll’s eyes, 101 - - - Education, Technical, 120 - - Electric furnaces, 58 - - Emerald, 31 - - Enamelling glass, 115 - - English type of furnace, 43 - - Engraving glass, 111 - - Etching, 19-112 - - Expansion, Thermal, 16 - - Eye of furnace, 43 - - Eyes, Artificial, 101 - - - Fancy glass, 116 - - Filigree, 99 - - Fire-clay, 3, 11-36 - - —— analyses, 37 - - - - Fire-clay, blocks, 39, 45 - - ——, Burnt, 39, 41, 61 - - —— crucibles, 64 - - ——, Grinding of, 39 - - ——, Melting point of, 64 - - ——, Mild, 39, 65 - - —— pots, 62 - - ——, Properties of, 36-38, 41 - - —— rings, 65 - - ——, Selection of 38 - - —— stoppers, 66 - - —— Strong, 39, 64 - - ——, Tempering, 39, 61 - - ——, Weathering, 39, 61 - - Flint glass, 4 - - —— stones, 4 - - Fluorspar, 8 - - Foot maker, 82 - - Formulas, 12, 21 - - Frisbie’s Feeder, 47 - - Furnaces, 21, 41, 51, 57 - - Fusibility of glass, 9 - - - Gadget, 28 - - Garnet, 31 - - Gas-fired furnaces, 47, 51, 55 - - Gathering, 76, 77 - - Glass, Afterworkings of, 86 - - ——, Alkalies in, 23 - - ——, Alumina in, 9, 11, 20 - - ——, Ancient, 2 - - ——, Annealing, 71 - - ——, Cane, 97 - - ——, Coloured, 28 - - —— cloth, 101 - - ——, Cut, 109 - - ——, Enamelled, 115 - - ——, Founding of, 69, 74 - - —— furnaces, 21, 41, 51, 56 - - ——, Gauge, 18 - - ——, Grinding of, 94 - - ——, Hardened, 95 - - ——, Homogeneity of, 23 - - —— house pots, 62 - - ——, Moulds for, 77 - - ——, Melting of, 69 - - ——, Plasticity of hot, 4-16 - - ——, Polishing of, 92-94 - - ——, Properties of, 15 - - ——, Process of making, 15, 76 - - ——, Sand-blasted, 114 - - ——, Scum on, 69 - - ——, Seeds in, 105 - - ——, Silvered, 116 - - —— snow, 101 - - ——, Stress in, 74 - - ——, Strengthened, 95 - - ——, Temperature of melting, 20 - - ——, Tube, 96 - - ——, Types of, 15, 25 - - ——, Wired, 95 - - —— wool, 101 - - ——, Yellow, 28 - - Grinding tumblers, 110 - - —— plate glass, 92, 94 - - - Hardened, 18, 23 - - Hermansen’s Furnace, 52, 53 - - History, 1 - - Homogeneity, 23 - - Honey pot making, 85 - - Hydrofluoric acid, 19 - - - Introduction of glassmaking in England, 2 - - Iridescence, 21-101 - - Iron in glass, 32 - - Italian Aventurine, 102 - - - Laboratory glass, 25 - - Ladling glass, 45 - - Lamp glass chimneys, 16 - - Lead glass, 21 - - —— poisoning, 14 - - Lehr, 71 - - Light and glass, 33 - - Lime glass, 25, 26 - - - Machines in glassmaking, 79, 111 - - Mechanical boy, 86 - - Millefiore, 100 - - Moulds, 85 - - - - Opalescent glass, 95-109 - - Opal glass, 29, 31 - - Optical glass, 5, 9, 33, 104 - - Oxidising agents, 7 - - - Pearl ash, 6 - - Pearls, 31, 116 - - Phosphates in glass, 8 - - Polariscope, 74 - - Potash, 6 - - —— glass, 24 - - Pots, 8-13, 27-58 - - ——, Annealing, 66 - - —— cracking, 45, 69 - - —— clays, 37, 64 - - ——, Glazing, 69 - - ——, Making, 62 - - ——, Open, 21, 27, 64 - - ——, Plumbago, 65 - - ——, rings, stoppers, 63-65, 66 - - —— sherds, 65 - - ——, Setting, 67 - - ——, Trolley, 46 - - Plaining glass, 4, 51, 69 - - Plasticity, 11, 16 - - Plate glass, 26, 93 - - Plumbago, 65 - - Pressed glass, 26, 77 - - Pucellas, 82 - - - Quartz glass, 19 - - - Réaumur’s Porcelain, 17 - - Recipes for glass making, 25, 26 - - Recuperative furnaces, 52-54 - - Reduction in glass, 28, 31 - - Regenerative furnaces, 49 - - Rocaille flux, 25 - - Roman glass, 2 - - Ruby glass, 28, 31 - - Rupert drops, 18 - - - Saltpetre, 7 - - Sands, 4 - - Sand-blast, 114 - - Scratching glass, 16 - - Screens for pot setting, 68 - - Seeds in glass, 4, 13, 105 - - Servitor, 83 - - Shearing glass, 81, 83 - - Sheet glass, 91 - - Siemens Furnace, 48 - - Siege of furnace, 43, 44 - - Silica, 4, 5 - - Silicates in glass, 24 - - Silvering glass, 116 - - Simple glasses, 24 - - Soda-lime glass, 21-26 - - Soft glass, 5 - - Soluble glass, 24 - - Spun glass, 15, 100 - - Stirring glass, 105 - - Strengthened glass, 95 - - Stress in glass, 18, 104 - - Striae, 9 - - Sulphates, 5, 25 - - - Table glass, 25, 76, 77 - - Tank glass, 26, 57 - - Technical Education in glass manufacture, 120 - - Temperature of furnaces, 20, 105 - - Thermal expansion of glass, 16 - - Tin oxide in glass, 10 - - Tizeur, 43, 46 - - Tools, 76 - - Topaz, 31 - - Trades Unionism, 86 - - Tube, 26, 98 - - Tumblers, 85, 110 - - Turquoise, 31 - - - Uranium, 28 - - - Varieties of glass, 25, 102 - - Venetian glass, 2 - - Violet glass, 28 - - - Waste glass, 30 - - Waterglass, 24 - - Wine-glass making, 82 - - Window glass, 1 - - Wired glass, 95 - - Working hours, 120 - - - Zinc oxide, 9 - - - - - _Printed by Sir Isaac Pitman & Sons, Ltd., Bath, England_ - - ------------------------------------------------------------------------- - - - - - ● Transcriber’s Notes: - ○ Missing or obscured punctuation was silently corrected. - ○ Typographical errors were silently corrected. - ○ Inconsistent spelling and hyphenation were made consistent only - when a predominant form was found in this book. - ○ Text that was in italics is enclosed by underscores (_italics_); - text that was bold by “equal” signs (=bold=). - ○ The use of a caret (^) before a letter, or letters, shows that the - following letter or letters was intended to be a superscript, as - in S^t Bartholomew or 10^{th} Century. - ○ Superscripts are used to indicate numbers raised to a power. 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