Onlayn kitobni bepul oʻqish  Glass Manufacture

muallif Walter Rosenhain

The “Westminster” Series

GLASS MANUFACTURE

GLASS
MANUFACTURE

BY
WALTER ROSENHAIN B.A. B.C.E.

SUPERINTENDENT OF THE DEPARTMENT OF METALLURGY AND
METALLURGICAL CHEMISTRY AT THE NATIONAL
PHYSICAL LABORATORY

NEW YORK
D. VAN NOSTRAND COMPANY
23 MURRAY AND 27 WARREN STREETS
1908

BRADBURY, AGNEW, & CO. LD., PRINTERS,
LONDON AND TONBRIDGE.

PREFACE

The present volume on Glass Manufacture has been written chiefly for the benefit of those who are users of glass, and therefore makes no claim to be an adequate guide or help to those engaged in glass manufacture itself. For this reason the account of manufacturing processes has been kept as non-technical as possible; no detailed drawings of plant or appliances have been given, and only a few illustrative diagrams have been introduced for the purpose of avoiding lengthy verbal descriptions. In describing each process the object in view has been to give an insight into the rationale of each step, so far as it is known or understood, and thus to indicate the possibilities and limitations of the process and of its resulting products rather than to provide a detailed guide to the technique of the various operations. The practical aim of the book has further been safeguarded by the fact that the processes described in these pages are, with the exception of those described as obsolete, to the author’s definite knowledge, in commercial use at the present time. For this reason many apparently ingenious and beautiful processes described in earlier books on glass have not been mentioned here, since the author could find no trace of their employment beyond the records of the various patents involved. On the other hand the reader must be warned to bear in mind that the peculiar conditions of the glass manufacturing industry have led to the practice on the part of manufacturers of keeping their processes as secret as possible, so that the task of the author who would give an accurate account of the best modern processes used in any given department of the industry is beset with great difficulties. The author has endeavoured to steer the best course open to him under these circumstances, and he would appeal to the paucity of glass literature in the English language as evidence of the difficulty to which he refers.

In addition to these difficulties, which arise largely from considerations of a commercial nature, the writer of a book on glass is further confronted with technical difficulties of no inconsiderable order. As already indicated, the aim of the present author has been to describe processes from the point of view of principles and methods rather than as mere rule-of-thumb descriptions of manufacturing manipulations, but in doing this he is met at every turn by the fact that from the scientific side the greater part of the field of glass manufacture is a “terra incognita.” In making this statement the labours of many eminent scientific workers are by no means forgotten, but the entire field is so large and beset with such great experimental difficulties that even the labours of a list of investigators that includes the names of Fraunhofer and Faraday, Stokes, Hopkinson, Abbé and Schott, have resulted in little more than an accumulation of empirical data which, while they have been productive of great direct practical results, have left the science of glass still in a very elementary condition. To take two examples in illustration of this fact we may mention the question of the connection between chemical composition and any of the physical properties of glass, such as refraction and dispersion of light, and on the more mechanical side the question why all processes, such as rolling or moulding, which involve the contact of hot glass with metal result in a roughening of the glass surface. The former question has been studied by several of the investigators named above, Schott and Abbé having particularly devoted an enormous amount of labour and money to the study of the question with results which have proved disappointing from the scientific point of view. By prolonged experimenting and the employment of a costly system of trial and error an important series of novel and useful glasses has been produced by these workers, but no law by whose aid the optical properties of a glass of given chemical composition could be predicted has yet been discovered, and as a summary of the known facts only the vaguest general principles are available for the guidance of those who wish to produce glasses of definite properties. The same applies in a similar degree to most of the other properties of glass, with the exception, perhaps, of density and thermal expansion; attempts to generalise from the known data of a limited number of glasses generally meet with unqualified failure. The conclusion which one is forced to admit is that the fundamental principles underlying the nature and constitution of glasses have yet to be discovered. A study of the other question mentioned above as an example of the limitations of our knowledge leads to the same conclusion; an almost endless succession of inventors have busied themselves with devices for overcoming the roughening action of rollers and moulds upon glass, but without any real success. A long list of other examples of the same kind could be given, our knowledge of the physical and chemical principles underlying many of the phenomena met with in glass manufacture being deplorably deficient. It will thus be seen that to write a truly scientific account of glass manufacture is at the present time impossible, and the reader is asked to bear this in mind if he should find the chemical or physical explanations given in this book less frequent or less adequate than could be desired.

Having dwelt somewhat emphatically on the limitations of our present scientific knowledge as applied to glass manufacture, it is perhaps scarcely necessary at the present time to emphasise the fact that this state of affairs should act as the strongest incentive to further investigation of the whole subject. The difficulty, however, lies in the fact that such investigation can scarcely be carried on by voluntary workers in ordinary laboratories, but must be undertaken with the active help of glass manufacturers at their works. Glass is essentially a substance that cannot be satisfactorily handled in small quantities, particularly so far as all the phenomena connected with its production and manipulation while hot are concerned; the influences of containing vessels, of furnace gases and of rapid cooling are all enormously exaggerated if ounces instead of hundredweights or tons of glass are used for experimental purposes, and these influences and others of the same nature vitally affect all the results of small-scale laboratory operations. The progress of our scientific knowledge of glass—and the consequent development of the glass industry from its present state where rule-of-thumb and “practical experience” still hold excessive sway—lies in the hands of those concerned in the industry itself. It must be admitted that to undertake such work involves the expenditure of much time and money on the part of a manufacturer, while the field is so large and the problems so complicated that any adequate return cannot be promised for the immediate future; on the other hand the very size of the field and the difficulty of the problems offers the promise of the greatest ultimate reward; a really important scientific discovery in connection with glass would be certain to bring in its train industrial developments whose limits it is impossible to foresee. The industrial success of the glass-works of Schott in Jena is often quoted as a brilliant example of commercial success resulting from purely scientific investigations in this actual field; an example of still greater magnitude is furnished by the success of the aniline dye works of Germany which are built up on purely scientific achievements. The glass industry as a whole, supplying some of the absolute necessaries of modern life, should be capable of offering the greatest rewards to success, and the example of other industries has shown that ultimate success is bound to reward properly-conducted and perseverant scientific research. Nowhere is this more urgently needed than in the whole field of glass manufacture.

The author is indebted to Mr. W. C. Hancock for valuable assistance in the reading of proofs and various suggestions in connection with the contents of this book.

TABLE OF CONTENTS

 

PAGE

Preface v

CHAPTER I.

THE PHYSICAL AND CHEMICAL PROPERTIES OF GLASS.

Definition of the term “Glass”—Amorphous structure the common feature of all vitreous bodies—Glass a congealed fluid—Glasses not definite chemical compounds but complex solutions—Range of chemical composition available for glass-making—Considerations governing chemical composition—Influence of composition on physical properties—Chemical stability of glass—Permanence of glass surfaces—Action of water, acids, and alkalies on glass—Action of light on glass

p. 1

CHAPTER II.

THE PHYSICAL PROPERTIES OF GLASS.

Mechanical properties: tensile strength, crushing strength, elasticity, ductility, and hardness—Thermal properties of glass: thermal endurance, coefficient of expansion, thermal conductivity—Thermometer glass—Electrical properties of glass—Transparency and colour of glass

p. 18

CHAPTER III.

THE RAW MATERIALS OF GLASS MANUFACTURE.

General considerations—Chemical purity, moisture, and physical condition, constancy of quality—Sources of silica, sand and sandstone—Felspar—Sources of alkali: Soda ash (carbonate of soda), salt-cake (sulphate of soda), pearl ash (carbonate of potash)—Alkali nitrates—Natural minerals containing alkalies—Sources of other bases: Lime, chalk, limestone, slaked lime—Gypsum (sulphate of lime)—Barium compounds—Magnesia and zinc—Lead oxide, red lead—Aluminium, manganese, arsenic—Carbon—Coke, charcoal, anthracite coal

p. 35 

CHAPTER IV.

CRUCIBLES AND FURNACES FOR THE FUSION OF GLASS.

Fire-clay and silica-brick—Manufacture of glass-melting pots—Drying and first heating of pots—Blocks for tank and other furnaces—Uses of silica brick—Furnaces—Coal-fired and gas-fired furnaces—Gas producers—Regenerative furnaces, principles and construction of Siemens’ furnaces—Recuperative furnaces—General arrangements of modern tank furnaces—Relative advantages of tank and pot furnaces

p. 54

CHAPTER V.

THE PROCESS OF FUSION.

Mixing of raw materials by hand and by machinery—The charging operation—Chemical reactions during melting of carbonate mixtures, and of sulphate mixtures—Influence of carbon on the reactions—The fining process

p. 73

CHAPTER VI.

PROCESSES USED IN THE WORKING OF GLASS.

Ladling, gathering, and casting—Limitations of ladling—Ladling used for rolled glass, gathering for blown glass—Rolling of glass—Blowing processes and operations—Use of moulds—Pressing—Moulding

p. 84

CHAPTER VII.

BOTTLE GLASS.

Raw materials—Furnaces—Predominance of tank furnaces—Process of blowing bottles by hand—Gathering, marvering, blowing—Use of fire-clay and metal moulds—Formation of neck—Improved appliances, moulds and tools—Manufacture of bottles by machinery—The “Boucher” bottle-blowing machine—Annealing of bottles—Large bottles, carboys—Aids to the blower—Sievert’s process—Large shallow vessels, bath-tubs

p. 95

CHAPTER VIII.

BLOWN AND PRESSED GLASS.

Raw materials—Bohemian glass and flint glass—Gathering and blowing—Chair work—Hand work—Production of tumblers by hand—Application of coloured glass to blown articles—Use of moulds as aids to blowing—Roughening effect of moulds—Fire-polishing by reheating—Use of compressed air—Pressed glass—Moulds and presses—Capacity and limitations of pressing process

p. 108

CHAPTER IX.

ROLLED OR PLATE GLASS.

Rolled plate glass—Furnaces—Raw materials—Process of ladling—The rolling table—Annealing—Cutting and sorting—Patterns on rolled plate—“Figured” rolled plate—Machine used for double-rolling—Polished plate—Raw materials—Casting from melting pots—Special casting pots—The rolling table—Importance of flatness—Annealing kilns—Grinding and polishing processes—Machines used for grinding and polishing—Method of holding the glass—Abrasives and polishing materials—Theory of the polishing process—Limiting sizes of polished plate—Homogeneity of polished plate—Uses of plate glass—Bent polished plate—Mirrors—Bevelling, process and machines—Wired plate glass, rolled and polished—Difficulties and limitations—Advantages of wired glass

p. 122

CHAPTER X.

SHEET AND CROWN GLASS.

Comparison of sheet with polished plate—Raw materials for sheet—Furnaces: various forms of tank furnaces—Blowing process—Gathering, forming the gathering on blocks, forming the shoulder of the cylinder, blowing the cylinder, opening the end of the cylinder, detaching cylinder from pipe—Cutting off the “cap”—Splitting the cylinder—Flattening and annealing—Cutting and sorting sheet-glass—Defects of sheet-glass—Variations of the process—Attempts to produce “sheet” glass by rolling—Sievert’s process—Direct drawing processes—The American process for drawing cylinders—Fourcault’s processes—Difficulties and limitations—Crown glass—The blowing process—Limitations

p. 149

CHAPTER XI.

COLOURED GLASSES.

Definition of coloured glass—Physical causes of colour—Colouring substances: copper, silver, gold, carbon, tin, arsenic, sulphur, chromium, uranium, fluorine, manganese, iron, nickel, cobalt—Range and depths of tints available—Intensely coloured glasses—The process of “flashing”—Character of “flashed” glass—Colours produced on glass by painting: use of coloured “glazes” as paints—Ancient stained glass and modern glass—Technical uses of coloured glass, photography, railway and marine signals

p. 178

CHAPTER XII.

OPTICAL GLASS.

Nature and properties of optical glass—Homogeneity—Formation and removal of striæ in solutions and in glass—Transparency and colour—Absorption of light in “decolourised” glasses—Refraction and dispersion—Definitions—Refractive index, dispersion, medium dispersion, the quantity ν—Specification of optical properties in terms of certain spectrum lines—Table of typical optical glasses and their optical constants—Crown and flint glasses—Relation between refraction and dispersion in the older and newer glasses—Work of Abbé and Schott—Applications of the new glasses—Non-proportionality of dispersion in different types of glass—Resulting imperfections of achromatism—The relative partial dispersions of glasses—Pairs of glasses giving perfect achromatism not yet fully available—Constants of Schott’s telescope crown and flint—Narrow range of optical glasses, consequent limitations in lens design—Causes of these narrow limits—Possible directions of extension—Chemical stability of optical glasses—Double refraction in optical glass arising from imperfect annealing

p. 205

CHAPTER XIII.

OPTICAL GLASS.

The manufacture of optical glass—Raw materials—Mixing—Furnaces and crucibles—Kilns for heating pots—Transfer of pots from kiln to melting furnace—Introduction of cullet and raw materials—The fining process, difficulties and limitations—The stirring process—The final cooling of the glass—Rough sorting of the glass fragments—Moulding and final annealing of the moulded glass—Grinding and polishing of plates and discs for examination; smallness of yield obtained—Difficulty of obtaining large blocks of perfect glass

p. 223

CHAPTER XIV.

MISCELLANEOUS PRODUCTS.

Glass tubing—Gathering and drawing of ordinary tubes—Special varieties of tube—Combustion tubes—Tubes of vitreous silica—Varieties of vitreous silica—Transparent, glass-like silica ware—Great cost of production—Translucent “milky” silica ware produced electrically—Great thermal endurance of vitreous silica—Sensitiveness to chemical action of all basic substances at high temperatures—Glass rod and fibre—Glass wool—Quartz fibres—Glass beads—Artificial gems—Use of very dense flint glass coloured to imitate precious stones—Means of distinguishing imitations—Precious stones produced by artificial means—Chilled glass—Great strength and fragility of chilled glass—Rupert’s drops—Manufacture of “tempered” glass by Siemens—De La Bastie’s process—Massive glass, used for house construction and paving blocks—Water-glass (silicate of soda or potash), manufacture in tank furnaces—Glass for lighthouse lenses and searchlight mirrors—Production by casting glass in iron moulds—Sizes and types of lenses and prisms produced

p. 238 Appendix

—Bibliography of Glass Manufacture

p. 253

GLASS MANUFACTURE

CHAPTER I.
THE PHYSICAL AND CHEMICAL PROPERTIES OF GLASS.

Although the term “glass” denotes a group of bodies which possess in common a number of well-defined and characteristic properties, it is difficult to frame a satisfactory definition of the term itself. Thus while the property of transparency is at once suggested by the word “glass,” there are a number of true glasses which are not transparent, and some of which are not even translucent. Hardness and brittleness also are properties more or less characteristic of glasses, yet very wide differences are to be found in this respect also, and bodies, both harder and more fragile than glass, are to be found among minerals and metals. Perhaps the only really universal property of glasses is that of possessing an amorphous structure, so that vitreous bodies as a whole may be regarded as typical of “structureless” solids. All bodies, whether liquid or solid, must possess an ultimate structure, be it atomic, molecular or electronic in character, but the structure here referred to is not that of individual molecules but rather the manner of grouping or aggregation of molecules.

In the great majority of mineral or inorganic bodies the molecules in the solid phase are arranged in a definite grouping and the body is said to have a crystalline structure; evidences of this structure are generally visible to the unaided eye or can be revealed by the microscope. Vitreous bodies on the other hand are characterised by the entire absence of such a structure, and the mechanical, optical and chemical behaviour of such bodies is consistent only with the assumption that their molecules possess the same arrangement, or rather lack of arrangement, that is found in liquids.

The intimate resemblance between vitreous bodies and true liquids is further emphasised when it is realised that true liquids can in many instances pass into the vitreous state without undergoing any critical change or exhibiting any discontinuity of behaviour, such as is exhibited during the freezing of a crystalline body. In the latter class of substances the passage from the liquid to the crystalline state takes place at one definite temperature, and the change is accompanied by a considerable evolution of heat, so that the cooling of the mass is temporarily arrested. In the case of glasses, on the other hand, the passage from the liquid to the apparently solid condition is gradual and perfectly continuous, no evolution of heat or retardation of cooling being observed even by the aid of the most delicate instruments. We are thus justified in speaking of glasses as “congealed liquids,” the process of congealing in this case involving no change of structure, no re-arrangement of the molecules, but simply implies a gradual stiffening of the liquid until the viscosity becomes so great that the body behaves like a solid. It is, however, just this power of becoming exceedingly stiff or viscous when cooled down to ordinary temperatures that renders the existence of vitreous bodies possible. All glasses are capable of undergoing the change to the crystalline state when kept for a sufficient time at a suitable temperature. The process which then takes place is known as “devitrification,” and sometimes gives rise to serious manufacturing difficulties.

Molten glass may be regarded as a mutual solution of a number of chemical substances—usually silicates and borates. When cooled in the ordinary way these bodies remain mutually dissolved, and ordinary glass is thus simply a congealed solution. The dissolved substances have, however, natural freezing-points of their own, and if the molten mass be kept for any length of time at a temperature a little below one of these freezing-points, that particular substance will begin to solidify separately in the form of crystals. The facility with which this will occur depends upon the properties of the ingredients and upon the proportions in which they are present in the glass. In some cases this devitrification sets in so readily that it can scarcely be prevented at all, while in other cases the glass must be maintained at the proper temperature for hours before crystallisation can be induced to set in. In either of these cases, provided that the glass is cooled sufficiently rapidly to prevent crystallisation, the sequence of events during the subsequent cooling of the mass is this: as the temperature falls further and further below the natural freezing-point of one or other of the dissolved bodies, the tendency of that body to crystallise out at first rapidly increases; as the temperature falls, however, the resistance which the liquid presents to the motion of the molecules increases at a still greater rate, so that two opposing forces are at work, one of them an increasing tendency towards crystallisation, the other a still more rapidly increasing resistance to any change. There is thus for every glass a certain critical range of temperature during which the greatest tendency exists for the crystallising forces to overcome the internal resistance; through this range the glass must be cooled at a relatively rapid rate if devitrification is to be avoided; at lower temperatures the crystallising forces require increasingly longer periods of time to produce any sensible effect, until, as the ordinary temperature is approached, the forces of internal resistance entirely prevent all tendency to crystallisation.

The phenomena just described in reality constitute the natural limit to the range of bodies which can be obtained in the vitreous state: as we approach this limit the glass requires more and more rapid cooling through the critical range of temperature, and is thus more and more liable to devitrify during the manufacturing processes, until finally the limit is set when no industrially feasible rapidity of cooling suffices to retain the mass in the vitreous state.

While the range of bodies that can be obtained in the vitreous state is very large, only a comparatively small number of substances are ordinarily incorporated in industrial glasses. With the exception of certain special glasses used for scientific purposes, such as the construction of optical lenses, thermometers and vessels intended to resist unusual treatment, all industrial glasses are of the nature of mixed silicates of a few bases, viz., the alkalies, sodium and potassium, the alkaline earths, calcium, magnesium, strontium, and barium, the oxides of iron and aluminium (generally present in minor quantities), and lead oxide. The manner in which these various elements enter into combination and solution with one another has been much investigated, and the more general conclusions have been anticipated in what has been said above. It is abundantly evident that glasses are not definite chemical compounds, but rather solutions, in varying proportions, of a series of definite compounds in one another. In many cases the actual constitution of industrial glasses is so complex as, for the present at all events, to baffle adequate chemical expression.

One of the factors that limit the range of possible compositions of glasses has already been indicated, and two others must now be discussed. For industrial purposes, the cost and rarity of the ingredients becomes a vital bar at a certain stage; thus the use of such elements as lithium, thallium, etc., is prohibitively costly. In another direction the glass-maker is very effectively restrained by the limitations of his furnaces as regards temperature. The presence of excessive proportions of silica, lime, alumina, etc., tends to raise the temperature required for the free fusion of the glass, and when this temperature seriously exceeds 1600° C., the manufacture of the glass in ordinary furnaces becomes impossible. Thus pure silica can be converted into a glass possessing very valuable properties, but the requisite temperature cannot be attained in regenerative gas-fired furnaces such as are ordinarily used by glass manufacturers. The production of this glass has accordingly been carried on upon a small scale only by means of laboratory furnaces heated by oxy-acetylene flames, while latterly a less perfect variety of silica glass-ware has been produced on a large scale by the aid of electric furnaces. Such methods are, however, obviously limited to very special products commanding special prices.

A further limitation in the choice of chemical components is placed upon the manufacturer by the actual chemical behaviour of the glass both during manufacture and in use. As regards chemical behaviour during manufacture, it must be borne in mind that, although glasses are of the nature of solutions rather than of compounds, yet these solutions tend towards a state of saturation; thus a glass rich in silica and deficient in bases will readily dissolve any basic materials with which it may come in contact, while, on the other hand, a glass rich in bases and poor in acid constituents such as silica, boric acid or alumina, will readily absorb acid bodies from its surroundings. During the process of melting, glass is universally contained in fire-clay vessels. These are chosen, as regards their own chemical composition, so as to offer to the molten glass a few of those materials in which the glass itself is deficient; yet a limit arises in this respect also, since glasses very rich in bases, such as the very dense lead and barium glass made for optical purposes, rapidly attack any fire-clay with which they may come in contact. The finished glass also betrays its chemical composition by its chemical behaviour towards the atmospheric agents, such as moisture and carbonic acid, with which it comes in contact; glasses containing an excessive proportion of alkali, for example, are found to be seriously hygroscopic and to undergo rapid decomposition, especially in a damp atmosphere.

Within the limits set by these considerations, the glass manufacturer chooses the chemical composition of his glass according to the purpose for which it is intended; for most industrial products the cheapest and most accessible raw materials that will yield a glass of the requisite appearance are employed, while for special purposes the dependence of physical properties upon chemical composition is utilised, as far as possible, in order to attain a glass specially suited to the particular requirements in question. Thus the flint and barium glasses used for table and ornamental ware derive from the dense and strongly refracting oxides of lead and barium their properties of brilliancy and weight. The fusibility and softness imparted to the glass by the presence of these bases further adapts it to its purpose by facilitating the complicated manipulations to which the glass must be subjected in the manufacturing processes.

Taking our next example at almost the opposite extreme, the hardest “combustion tubing,” which is intended to resist a red heat without appreciable softening, is manufactured by reducing the basic contents of the glass to the lowest possible degree, especially minimising the alkali content, and using the most refractory bases available, such as lime, magnesia, and alumina in the highest possible proportions. Such glass is, of course, difficult to melt, and special furnaces are required for its production, but on the other hand this material meets requirements which ordinary soda-lime or flint glass tubing could never approach. Another instance of these refractory glasses is to be found in the Jena special thermometer glasses and in the French (Tonnelot) “Verre dur”; the best of these glasses show little or no plasticity at temperatures approaching 500° C., and have thus rendered possible a considerable extension of the range of the mercury thermometer. Further modification of chemical composition has resulted in the production of glasses which are far less subject to those gradual changes which occur in ordinary glass when used for the manufacture of thermometers—changes which vitiated the accuracy of most early thermometers. A still more extensive adaptation of chemical composition to the attainment of desired physical properties has been reached primarily as a result of the labours of Schott and Abbé, in the case of optical glasses. The work of these men, and the developments which have followed from it, both at the works founded by them at Jena and elsewhere, have so profoundly modified our knowledge of the range of possibilities embraced by the class of vitreous bodies, that it is not at all easy at the present time to realise the former narrow and restricted meaning of the term “glass.” The subject of the dependence of the optical properties of glass upon chemical composition will be referred to in detail in Chapter XII. on “Optical Glass,” but the outline of the influence of composition on properties here given could not be closed without some reference to this pioneer work of the German investigators.

The chemical behaviour of glass surfaces, to which we have already referred, is of the utmost importance to all users of glass. The relatively neutral chemical behaviour of glass is, in fact, one of its most useful properties, and, next to its transparency, most frequently the governing factor in its employment for various purposes. Thus the entire use of glass for table-ware depends primarily upon the fact that it does not appreciably affect the composition and flavour of edible solids or liquids with which it is brought into contact—a property which is only very partially shared even by the noble metals. Again, the use of glass windows in places exposed to the weather would not be feasible if window-glass were appreciably attacked by the action of water or of the gases of the atmosphere. For these general purposes, it is true, most ordinary glasses are adequately resistant, but this degree of perfection in this respect is only the outcome of the centuries of experience which the practical glass-maker has behind him in the manufacture and behaviour of such glass. When, however, a higher degree of chemical resistance is required for special purposes, as for instance when glass is called upon to resist exposure to hot, damp climates, or is intended to contain corrosive liquids, the rules which are an adequate guide to the glass-maker in meeting ordinary requirements are no longer sufficient, particularly when the glass is expected to meet other stringent requirements as well. It has, in fact, frequently happened that a glass-maker, in striving to improve the colour or quality of his glass, as regards freedom from defects, brilliancy of surface, etc., has spoilt the chemical durability of his products. The reason lies in the fact, long known in general terms, that an increased alkali content reduces the chemical resistance of glass, while at the same time such an increase of alkali is the readiest means whereby the glass-maker can improve his glass in other respects by making it more fusible and easier to work in every way.

This subject of the chemical stability of glass surfaces attracted much attention during the later part of last century, and careful investigations on the subject were carried out, particularly at the German Reichsanstalt (Imperial Physical Laboratory) at Charlottenburg. Here also the labours of Schott and Abbé proved helpful, until at the present time such glass as that used by the Jena firm in the production of laboratory ware, and certain other special glasses of that kind, are fitted to meet the most stringent requirements.

Leaving aside the inferior glasses, containing, generally, more than 15 per cent. of alkali, the behaviour of glass surfaces to the principal chemical agents may be summed up in the following statements. Pure water attacks all glass to a greater or lesser extent; in the best glasses the prolonged action of cold water merely extracts a minute trace of alkalies, but in less perfect kinds the extraction of alkali is considerable on prolonged exposure even in the cold, and becomes rapidly more serious if the temperature is raised. Superheated water, i.e., water under steam pressure, becomes an active corroding agent, and the best glasses can only resist its action for a limited time. For the gauge-glass tubes of steam boilers working at the high pressures, which are customary at the present time, specially durable glasses are required and can be obtained, although many of the gauge-tubes ordinarily sold are quite unfit for the purpose, both from the present point of view and from that of strength and “thermal endurance.”

In certain classes of glass, the action of water, especially when hot, is not entirely confined to the surface, some water penetrating into the mass of the glass to an appreciable depth. The exact mechanism of this action is not known, but the writer inclines to the view that it arises from a partial hydration of some of the silica or silicates present in the glass. If such glasses be dried in the ordinary way and subsequently heated, the surface will be riddled with minute cracks, some glass may even flake off, and the whole surface will be dulled. As such penetrating action sometimes takes place—in the poorer kinds of glass—by the action of atmospheric moisture when the glass is merely stored in a damp place, it is often mistaken for “devitrification.” This latter action, however, is not known to occur at the ordinary temperature, although glass when heated in a flame frequently shows the phenomenon; it is, however, entirely distinct from the surface “corrosion” just described. Water containing alkaline substances in solution acts upon all glasses in a relatively rapid manner; it acts by first abstracting silica from the glass, the alkali and lime being dissolved or mechanically removed at a later stage. Water containing acid bodies in solution—i.e., dilute acid—on the other hand acts upon most varieties of glass decidedly less energetically than even pure water, and much less vigorously than alkaline solutions; this peculiar behaviour probably depends upon the tendency of acids to prevent the hydration of silica, this substance being thereby enabled to act as a barrier to the solvent action of the water upon the alkaline constituents of the glass. The better varieties of glass are also practically impervious to the action of strong acids, although certain of these, such as phosphoric and hydrofluoric, exert a rapid action on all kinds of glass. Only certain special glasses, containing an excessive proportion of basic constituents and of such substances as boric or phosphoric acid, are capable of being completely decomposed by the action of strong acids, such as hydrochloric or nitric, the bases entering into combination with the acids, while the silicic and other acids are liberated.

In connection with the action of acids upon glass, mention should be made of certain special actions that are of practical importance. The dissolving action of hydrofluoric acid upon glass is, of course, well known. It is used in practice both in the liquid and gaseous form, and also in that of compounds from which it is readily liberated (such as ammonium or sodium fluoride), for the purpose of “etching” glass, and also in decomposing glass for purposes of chemical analysis. Next in importance ranks the action of carbonic acid gas upon glass, especially in the presence of moisture. The action in question is probably indirect in character; the moisture of the air, condensing upon the surface of the glass, first exerts its dissolving action, and thus draws from the glass a certain quantity of alkali, which almost certainly at first goes into solution as alkali hydrate (potassium or sodium hydroxide); this alkaline solution, however, rapidly absorbs carbonic acid from the air, and the carbonate of the alkali is formed. If the glass dries, this carbonate forms a coating of minute crystals on the surface of the glass, giving it a dull, dimmed appearance; this, however, only occurs ordinarily with soda glasses, since the carbonate of potassium is too hygroscopic to remain in the dry solid state in any ordinary atmosphere. Potash glasses are, as such, no more stable chemically than soda glasses, but they are for the reason just given less liable to exhibit a dim surface. If the dimming process, in the case of a soda glass, has not gone too far, the brightness of the surface of the glass may be practically restored by washing it with water, in which the minute crystals of carbonate of soda readily dissolve, while separated silica is removed mechanically. An attempt made to clean the same dimmed surface by dry wiping would only result in finally ruining the surface, since the small sharp crystals of carbonate of soda would be rubbed about over the surface, scratching it in all directions.

The dimming process in the case of the less resistant glasses is not only confined to the formation of alkaline carbonates; the films of alkaline solution which are formed on the surface of glass form a ready breeding-ground for certain forms of bacteria and fungi, whose growth occurs partly at the expense of the glass itself; the precise nature of these actions has not been fully studied, but there can be little doubt that silicate minerals—and glass is to be reckoned among these—are subject to bacterial decomposition, a well-known example in another direction being the “maturing” of clays by storage in the dark, the change in the clay being accompanied by an evolution of ammonia gas. In the case of glass it has been shown that specks of organic dust falling upon a surface give rise to local decomposition. In this connection it is interesting to note the effect of the presence of a small proportion of boric acid in some glasses. The presence of this ingredient in small proportions is known to render the glass more resistant to atmospheric agencies, and more especially to render it less sensitive to the effects of organic dust particles lying upon the surface. It has been suggested—probably rightly—that the boric acid, entering into solution in the film of surface moisture, exerts its well-known antiseptic properties, thus protecting the glass from bacterial and fungoid activity.

The durability of glass under the action of atmospheric agents is a matter of such importance that numerous efforts have been made to establish a satisfactory test whereby this property of a given glass may be ascertained without actually awaiting the results of experience obtained by actual use under unfavourable conditions. One of the earliest of the tests proposed consisted in exposing surfaces of the glass to the vapour of hydrochloric acid. For this purpose some strong hydrochloric acid is placed in a glass or porcelain basin, and strips of the glass to be tested are placed across the top of the basin, the whole being covered with a bell-jar. After several days the glass is examined, and as a rule the less stable glasses show a dull, dimmed surface as compared with the more stable ones. A more satisfactory form of test depends upon the fact that aqueous ether solutions react readily with the less stable kinds of glass; if a suitable dye, such as iod-eosin, be dissolved in the water-ether solution, then the effect upon the less stable glasses when immersed in the solution is the formation of a strongly adherent pink film. The density or depth of colour of this film may be regarded as measuring the stability of the glass; the best kinds of glass remain practically free from coloured film even on prolonged exposure. A test of a somewhat different kind is one devised in its original form by Dr. Zschimmer, of the Jena glass works; this depends upon the fact that the disintegrating action of moist air can be very much accelerated if both the moisture and the temperature of the air surrounding the glass be considerably increased. For this purpose the samples of glass are exposed to a current of air saturated with moisture at a temperature of about 80° C. in a specially arranged incubator for one or more days, means being provided for securing a constant stream of moist air during the whole time. On examining the glass surfaces after this exposure—any wiping or other cleaning of the surfaces being avoided—various qualities of glass are found to show widely varying appearances. The best and most stable glasses remain entirely unaffected; less stable kinds show small specks, which merge into a generally dulled surface in unstable kinds. There is no doubt that this test gives a sharp classification of glasses, but it yet remains to be proved that this classification agrees with their true relative durability in practice; the writer is inclined to doubt whether this is really the case, since certain glasses that have proved very satisfactory in this respect in practical use all over the world were classed among the less stable kinds by this test.

Before leaving the subject of the chemical behaviour of glass, a reference should be made to the changes which glass undergoes when acted upon by light and other radiations. Under the influence of prolonged exposure to strong light, particularly to sunlight, and still more so to ultra-violet light, or the light of the sun at high altitudes, practically all kinds of glass undergo changes which generally take the form of changes of colour. Glasses containing manganese especially are apt to assume a purple or brown tinge under such circumstances, although the powerful action of radium radiations is capable of producing similar discoloration in glasses free from manganese. Apart from these latter effects, of which very little is known as yet, there can be no doubt that the action of light brings about chemical changes within the glass, but it is by no means easy to ascertain the true nature of these changes, although they most probably consist in a transfer of oxygen from one to another of the oxides present in the glass. Although it has not been definitely proved, it seems very unlikely that the glass either loses or gains in any constituent during these changes. Good examples of the changes undergone by glass under the action of sunlight are frequently found in skylights, where the oldest panes sometimes show a decided purple tint which they did not possess when first put in place. The glass spheres of the instruments used for obtaining records of the duration of sunshine at meteorological stations also show signs of the changes due to light—the glass of these spheres when new has a light greenish tint, but after prolonged use the colour changes to a decided yellow. The coloured glass in stained-glass windows also shows signs of having undergone changes of tint in consequence of prolonged exposure to light; glass removed from ancient windows usually shows a deeper tint in those portions which have been protected from the direct action of light by the leading in which the glass was set, and it is at least an open question whether the beauty of ancient glass may not be, in part, due to the mellowing effect of light upon some of the tints of the design. This photo-sensitiveness of glass is also of some importance in connection with the manufacture of photographic plates. It has been found that if the glass plate of a strongly-developed negative be cleaned, a decided trace of the former image is retained by the glass, and this image is apt to re-appear as a “ghost” if the same glass be again coated with sensitive emulsion and again exposed and developed. The best makers of plates recognise this fact and do not re-coat glass that has once been used for the production of a negative.

CHAPTER II.
THE PHYSICAL PROPERTIES OF GLASS.

The Mechanical Properties of Glass are of considerable importance in many directions. Although glass is rarely used in such a manner that it is directly called upon to sustain serious mechanical stresses, the ordinary uses of glass in the glazing of large windows and skylights depend upon the strength of the material to a very considerable extent. Thus in the handling of plate-glass in the largest sheets, the mechanical strength of the plates must be relied upon to a considerable extent, and it is this factor which really limits the size of plate that can be safely handled and installed. The same limitation applies to sheet-glass also, for, although its lighter weight renders it less liable to break under its own weight, its thinner section renders it much more liable to accidental fracture. In special cases, also, the mechanical strength of glass must be relied upon to a considerable extent. Gauge tubes of high-pressure boilers, port-hole glasses in ships, the glass prisms inserted in pavement lights, and the glass bricks which have found some use in France, as well as champagne bottles and mineral water bottles and syphons, are all examples of uses in which glass is exposed to direct stresses. It is, therefore, a little surprising that while the mechanical properties of metals, timbers, and all manner of other materials have been studied in the fullest possible manner, those of glass have received very little attention, at all events so far as published data go. One reason for this state of affairs is probably to be found in the fact that it is by no means easy to determine the strength of so brittle and hard a body as glass. As a consequence even the scanty data available can only be regarded as first approximations. The following data are only intended to give an idea of the general order of strength to be looked for in glass:—

Tensile strength:

From

1 to 4

tons per sq. in.

(Trautwine).

”

¹

/

₃

to 1¼

”

” ”

(Henrivaux).

”

2 to 5½

”

” ”

(Winkelmann and Schott).

”

5 to 6

”

” ”

(Kowalski).

Crushing strength:

From

9 to 16

tons per sq. in.

(Trautwine).

”

3 to 8

”

” ”

(Winkelmann and Schott).

”

20 to 27

”

” ”

(Kowalski).

Of the above figures the experiments of Winkelmann and Schott are probably by far the most reliable, but these refer to a series of special Jena glasses, selected with a view to determining the influence of chemical composition on mechanical properties, and, unfortunately, this series does not include glasses at all closely resembling those ordinarily used for practical purposes. The attempt to connect tensile and crushing strength with chemical composition was also only very partially successful; but the results serve to show that the chemical composition has a profound influence on the mechanical strength of glass, so that by systematic research it would probably be possible to produce glasses of considerably greater mechanical strength than those at present known. It must be noted in this connection that the mechanical properties of glass depend to a very considerable extent upon the rate of cooling which the specimen in question has undergone. It is well known that by rapid cooling, or quenching, the hardness of glass can be considerably increased; such treatment also increases the strength both as against tension and compression, and numerous processes have been put forward for the purpose of utilising these effects in practice. Unfortunately the “hardened” glass thus obtained is extremely sensitive to minute scratches, and flies to pieces as soon as the surface is broken, and the great internal stress which always exists in such glass is thereby relieved. All these peculiarities are, of course, dependent as to their degree upon the rapidity with which the glass has been cooled, and the aim of inventors in this field has been to devise a rapid cooling process which should strike the happy mean between the increased strength and the induced brittleness resulting from quenching. Thus processes for “tempering” glass by cooling it in a blast of steam or in a bath of hot oil or grease have been brought forward; but, although some such glass is manufactured, no very extensive practical application has resulted.

Elasticity and Ductility of Glass.—In a series of glasses investigated by Winkelmann and Schott, the modulus of elasticity (Young’s Modulus) varied from 3,500 to 5,100 tons per sq. in., the value being largely dependent upon the chemical composition of the glass. Measurable ductility has not been observed in glass under ordinary conditions except in the case of champagne bottles under test by internal hydraulic pressure; in these tests it was found that a permanent increase of volume of a few tenths of a cubic centimetre could be obtained by the application of an internal pressure just short of that required to burst the bottle—pressure of the order of 18 to 30 atmospheres being involved. This small permanent set has been ascribed to incipient fissuring of the glass, and this explanation is probably correct. On the other hand, it is in the writer’s opinion very probable that glass is capable of decided flow under the prolonged action of relatively small forces; the behaviour of large discs of worked optical glass suggests some such action, but the view as yet lacks full experimental confirmation.

The Hardness of glass is a property of some importance in most of the applications of glass. The durability of glass objects which are exposed to handling or to periodical cleaning must largely depend upon the power of the glass to resist scratching; this applies to such objects as plate-glass windows and mirrors, spectacle and other lenses, and in a minor degree to table-ware. On the other hand, the exact definition and means of measuring hardness are not yet satisfactorily settled. Experimenters have found it very difficult to measure the direct resistance to scratching, since it is found, for example, that two glasses of very different hardness are yet capable of decidedly scratching each other under suitable conditions. Resort has therefore been had to other methods of measuring hardness; the method which, from the experimental point of view, is, perhaps, the most satisfactory, depends upon principles laid down by Hertz and elaborated experimentally by Auerbach. This depends upon measuring the size of the circular area of contact produced when a spherical lens is pressed against a flat plate of the same glass with a known pressure. Auerbach himself found some difficulty in deciding the exact connection between the “indentation modulus” thus determined and the actual hardness of the glass. This method is, therefore, of theoretical interest rather than of use in testing glasses for hardness. A test of a more practical kind consists in exposing specimens of the glasses to be tested to abrasion against a revolving disc of cast-iron fed with emery or other abrasive, and to measure the loss of weight which results from a given amount of abrading action under a known contact pressure. If a number of specimens of different glasses are exposed to this test at one time, a very good comparison of their power of resisting abrasion can be obtained. It is not quite certain that this test measures the actual “hardness” of the glass, but it affords some information as to its power of resisting abrasion, and for many purposes this power is the important factor.

Hardness being, as indicated above, a somewhat indefinite term, it is not possible to give any precise statement as to the influence of chemical composition upon the hardness of glass. In general terms it may be said that glasses rich in silica and lime will be found to be hard, while glasses rich in alkali, lead or barium, are likely to be soft. It must, however, be borne in mind that rapid cooling, or even the lack of careful annealing, will produce a very great increase of hardness in even the softest glasses. The actual behaviour of a given specimen of glass will, therefore, depend at least as much upon the nature of the processes which it has undergone as upon its chemical composition.

The Thermal Properties of Glass, although not of such general importance as the mechanical properties, are yet of considerable interest in a large number of the practical uses to which glass is constantly applied. Perhaps the most important of these properties is that known as thermal endurance, which measures the amount of sudden heating or cooling to which glass may be exposed without risk of fracture; the chimneys employed in connection with incandescent gas burners, boiler gauge glasses, laboratory vessels, and even table and domestic utensils are all exposed at times to sudden changes of temperature, and in many cases the value of the glass in question depends principally upon its power of undergoing such treatment without breakage. The property of “thermal endurance” itself depends upon a considerable number of more or less independent factors, and their influence will be readily understood if we follow the manner in which sudden change of temperature produces stress and, sometimes, fracture in glass objects. If we suppose a hot liquid to be poured into a cold vessel, the first effect upon the material of the vessel will be to raise the temperature of the inner surface. Under the influence of this rise of temperature the material of this inner layer expands, or endeavours to expand, being restrained by the resistance of the central and outer layers of material which are still cold; the result of this contest is, that while the inner layer is thrown into a state of compression, the outer and central layers are thrown into a state of tension. Accordingly, if the tension so produced is sufficiently great, the outer layers fracture under tension and the whole vessel is shattered by the propagation of the crack thus initiated. From this description of the process it will be seen that a high coefficient of expansion and a low modulus of elasticity will both favour fracture, while high tensile strength will tend to prevent it. The thermal conductivity of the glass will also affect the result, because the intensity of the tensile stress set up in the colder layers of glass will depend upon the temperature gradient which exists in the glass; thus if glass were a good conductor of heat it would never be possible to set up a sufficient difference of temperature between adjacent layers to produce fracture; for the same reason, vessels of very thin glass are less apt to break under temperature changes than those having thick walls, since the greatest difference of temperature that can be set up between the inner and outer layers of a thin-walled vessel can never be very considerable. It also follows from these considerations, that if a cold glass vessel be simultaneously heated or cooled from both sides, it can be safely exposed to a much more sudden change of temperature than it could withstand if heated from one side alone; on the other hand, when very thick masses of glass have to be heated, this must be done very gradually, as a considerable time will necessarily elapse before an increment of temperature applied to the outside will penetrate to the centre of the mass. It should also be noted here, that in addition to the thermal conductivity of the glass, its heat capacity or specific heat also enters into this question, since heat will obviously penetrate more slowly through a glass whose own rise of temperature absorbs a greater quantity of heat. It will thus be seen that “thermal endurance” is a somewhat complicated property, depending upon the factors named above, viz.: coefficient of expansion, thermal conductivity, specific heat, Young’s modulus of elasticity, and tensile strength.

The coefficient of thermal expansion varies considerably in different glasses, and we can here only state the limiting values between which these coefficients usually lie; these are 37 × 10^-7 as the lower, and 122 × 10^-7 as the upper limit. These figures express the cubical expansion of the glass per degree Centigrade, the corresponding figures for steel and brass respectively being about 360 × 10^-7 and 648 × 10^-7 respectively. It should be noted that vitreous bodies of extremely low expansibility are obtainable by the suitable choice of ingredients, but in some cases these “glasses” are white opaque bodies, and in all cases they present great difficulty in manufacture, owing to the fact that alkalies and lime must be avoided in their composition.

Quite apart from the question of thermal endurance, the expansive properties of glass are of some importance. Thus when several kinds of glass have to be united, as, for example, in the process of producing “flashed” coloured glass, it is essential that their coefficients of expansion should be as nearly as possible the same; otherwise considerable stresses will be set up when the glasses, which have been joined at a red heat, are allowed to cool. On the other hand, this mutual stressing of two glasses owing to differences in their thermal expansion has been utilised for the production of tubes and other glass objects possessing special strength. If a tube be drawn out of glass consisting of two layers, one considerably more expansible than the other, and the cooling process be rightly conducted, it is possible to produce a tube in which both the inner and outer layers of glass are under a considerable compressive stress. Not only is glass, as we have seen above, enormously stronger as against compression than it is against tension, but glass under compressive stress behaves as though it were a much tougher material, being less liable to injury by scratches or blows. Moreover, if a tube in this condition be heated and then exposed to sudden cooling, the first effect of the application of cold will be a contraction of the surface layers, resulting in a relief of the initial condition of compression. These tubes are, therefore, remarkably indifferent to sudden cooling, although they are naturally more sensitive to sudden heating. In this respect they differ entirely from ordinary glass, which is considerably more sensitive to sudden cooling than to sudden heating, particularly when the heat or cold is applied to all the surfaces of the object at the same time. The special tubes made of two layers of glass above referred to are manufactured by the Jena Glass Works for special purposes, among which boiler gauge glasses are the most important. It should be also mentioned here that the remarkable thermal endurance of vitrified silica, which can be raised to a red heat and then immersed in cold water without risk of breakage, is chiefly due to its very low coefficient of expansion.

In another direction the expansive properties of glass are of importance wherever glass is rigidly attached to metal. At the present time this is done in several industrial products, such as incandescent electric lamps and “wired” plate glass. In certain varieties of incandescent lamps, metallic wires are sealed into the glass bulbs, and the only metal available for this purpose, at all events until recently, has been platinum, whose coefficient of expansion is low as compared with most metals, and whose freedom from oxidation when heated to the necessary temperature makes it easy to produce a clean joint between glass and metal. More recently the use of certain varieties of nickel steel has been patented for this purpose, since it is possible to obtain nickel steel alloys of almost any desired coefficient of expansion from that of the alloy known as “invar,” having a negligibly small expansion compared with that of ordinary steel. By choosing a suitable member of this series a metal could be obtained whose coefficient of expansion corresponds exactly with that of the glass to which it is to be united. The oxidation of the nickel steel when heated to the temperature necessary for effecting its union with the glass presented serious difficulties to the production of a tight joint, and several devices for avoiding this oxidation have been patented. In the incandescent electric lamp, although the joint between glass and metal is required to be perfectly air-tight, the two bodies are only attached to one another over a very short length. In wired plate glass, however, an entire layer of wire netting is interposed between two layers of glass, the wire being inserted during the process of rolling. Here a certain amount of oxidation of the wire is not of any serious importance, as it only appears to give rise to a few bubbles, whose presence does not interfere with the strength and usefulness of the glass; but any considerable difference of coefficient of expansion will produce the most serious results on account of the great lengths of glass and metal that are attached to each other. This factor has been neglected by some manufacturers, with the result that much of the wired glass of commerce is liable to crack spontaneously some time after it has left the manufacturer’s hands, while there is also much loss by breakage during the process of manufacture.

Thermal expansion is a vital factor in yet another of the uses of glass. Our ordinary instrument for measuring temperature—the mercury thermometer—is very considerably affected by the expansive behaviour of glass. When a mercury thermometer is warmed the mercury column rises in the stem because the mercury expands upon warming to a greater extent than the glass vessel, bulb and stem, in which it is contained. The subject of the graduations and corrections of the mercury glass thermometer is a very large one and somewhat outside the scope of the present volume; but attention should be drawn in this place to the peculiarities of the behaviour of glass that have been discovered in this connection. One of these is that when first blown the bulb of a thermometer takes a very considerable time to acquire its final volume, the result being, that if a freshly made thermometer is graduated, after some time the zero of the instrument will be found considerably changed, generally in a direction which indicates that the volume of the bulb has slightly increased. By a special annealing or “ageing” process this change can be completed in a comparatively short time before the instrument is graduated. There is, however, a further peculiarity which is prominent in some thermometers, although very greatly reduced in the best modern glasses. This becomes apparent in a decided change of zero whenever the thermometer has been exposed for any length of time to a high temperature, the zero gradually returning more or less to its original position in the course of time. With thermometers made of glasses liable to these aberrations, the reading for a given temperature depended largely upon the immediate past history of the instrument; but, thanks to the Jena Works, thermometer glasses are now available which are almost entirely free from this defect. In this connection the curious fact has been observed that glass containing both the alkalies (potash and soda) shows these thermal effects much more markedly than a glass containing one of the alkalies only.

The thermal conductivity of glass, except in so far as it affects the thermal endurance, is not a matter of any great direct practical importance, although the fact that glass is always a comparatively poor conductor of heat is utilised in many of its applications, as, for example, the construction of conservatories and hot-houses, although even in that case the opacity of glass to thermal radiations of long wave-lengths is of more importance than its low thermal conductivity. Similar statements apply, in a still more marked degree, to the subject of the specific heat of glass.

The electrical properties of glass are of much greater practical importance, glass being frequently used in electrical appliances as an insulating medium. The insulating properties of glass, as well as the property known as the specific inductive capacity, vary greatly according to the chemical composition of the material. Generally speaking, the harder glasses, i.e., those richest in silica and lime, are the best insulators, while soft glasses, rich in lead or alkali, are much poorer in this respect. In practice, particularly when the glass insulator is exposed to even a moderately damp atmosphere, the nature of the glass affects the resulting insulation or absence of insulation, in another way. Almost all varieties of glass have the property of condensing upon their surfaces a decided film or layer of moisture from the atmosphere, and, as we have seen above, glasses differ very considerably in the degree to which they display this hygroscopic tendency. The softer glasses are much more hygroscopic than the hard ones, and the resulting film of surface moisture serves to lessen or even to break down the insulating power of the glass, the electricity leaking away along the film of moisture. In the case of appliances for static electricity, where very high voltages have to be dealt with, an endeavour is sometimes made to avoid this leakage by varnishing the surface of the glass with shellac or other similar substance, and this proves a satisfactory remedy up to a certain point. Quite recently a variety of glass has been brought forward which is peculiar in having a comparatively low electrical resistance, so that for certain purposes it can be used as an electric conductor. Although interesting in itself, this glass is not very likely to prove useful even for the limited number of applications that could be found for an electrically conducting glass, since it is very rich in alkali, and is, therefore, likely to be unstable chemically, even under the action of atmospheric agencies alone.

The most valuable and in many ways the most interesting of the properties of glass—its transparency—has not been dealt with as yet, and all mention of this subject has been postponed to the end of the present chapter, because the whole subject of the optical properties of glass will be dealt with more fully in the chapter on optical glass (Chap. XII.), so that a very brief reference only need be made to the matter here.

There can be no doubt that, in most of its practical applications, transparency is the fundamental and essential property which leads to the employment of glass in the place of either stronger or cheaper materials. By transparency, in this sense, we wish to include mere translucence also, since very frequently it is as necessary to avoid undisturbed visibility as it is to secure the admission of light. It is indeed hard to find any use to which glass is extensively put into which the function of transmitting light does not very largely enter. Almost the only such example of use is the modern application of opal glass to the covering of walls, and the use—not as yet widely extended—of pressed glass blocks as bricks and paving stones; in these cases it is the hardness and smoothness of surface that gives to the vitreous body its superiority over other materials, but apart from these special cases, the fact remains that well over 95 per cent. of the glass used in the world is employed for purposes where transmission of light is essential to the attainment of the desired result, either from the point of view of utility or from that of beauty. It is interesting to note that the power of transmitting light is not shared by many solid bodies. Some colloidal organic bodies, such as gelatine and celluloid, possess the property to a degree comparable with glass, while certain mineral crystals, such as quartz and fluor-spar, may even surpass the finest glass in this respect; while some of the other optical properties of glass are greatly exceeded by such natural substances as the diamond and the ruby. But the very brevity of this list is in itself striking, because it must be borne in mind that transparency by no means constitutes the only common characteristic of vitreous bodies.

Although the transparency of glass is so valuable and indeed so essential a property of that substance, it must be remembered that no kind of glass is perfectly transparent. Quite apart from the fact that of the light that falls upon a glass surface, however perfectly polished, a considerable proportion is turned back by reflection at the surface of entry and again by reflection at the surface of exit from the glass, a certain proportion of light is absorbed during its passage through the glass itself, and the transmitted beam is correspondingly weakened. In the purest and best glasses this absorption is so small that in any moderate thickness very delicate instruments are required to show that there has been any loss of light at all; but even the best glass, when examined through a thickness of 20 in. or more, always shows the effects of the absorption of light quite unmistakably. In fact, not only does all glass absorb light, but it does this to a different degree according to the colour of the light, so that in passing through the glass a beam of white light becomes weakened in one of its constituent colours more than in the others, with the result that the emergent light is slightly coloured. Thus the purest and whitest of glasses, when examined in very thick pieces, always show a decided blue or green tint, although this tint is quite invisible on looking through a few inches of the glass. The ordinary glass of commerce, however, is far removed from even this approach to perfect transparency. The best plate glass shows a slight greenish-blue tint, which is just perceptible to the trained eye when a single sheet of moderate thickness is laid down upon a piece of white paper. When a sheet of this glass is viewed edgewise, in such a way that the light reaching the eye has traversed a considerable thickness, the greenish-blue tint of the glass becomes more apparent. By holding strips of various kinds of glass, cut to an equal length, close together and comparing the colour exhibited by their ends, a means of comparing the colours of apparently “white” glasses is readily obtained. It will be found that different specimens of glass differ most markedly in this respect. Sheet glass is, as a rule, decidedly deeper in colour than polished plate, but rolled plate is as a rule much greener—the colour of this glass can, in fact, in most cases be seen quite plainly in looking through or at the sheets in the ordinary way.

The question of how far the colour of glass affects the value of the light which it transmits depends for its answer upon the purpose to which the lighted space is to be put. Where delicate comparisons of colour are to be made, or other delicate work involving the use of the colour sense is to be carried on, it is essential that all colouration of the entering daylight should be avoided, and the use of the most colourless glass obtainable will be desirable. Again, in photographic studios it is important to secure a glass which shall absorb as small a proportion of the chemically active rays contained in daylight as possible, and special glasses for this purpose are available. Although for the present the price of these special glasses may prove prohibitive for the glazing of studio lights, their use is found highly advantageous where artificial light is to be used to the best advantage. On the other hand, for every-day purposes, the slight tinge of colour introduced into the light by the colour of ordinary sheet and plate glass, or even of greenish rolled plate glass, has no deleterious effect whatever, the majority of persons being entirely unconscious of its presence. The transmission of light by glass, its absorption, refraction, dispersion, etc., are, however, best grouped together as the “optical” properties of glass, and under that heading they will receive a fuller treatment in connection with the subject of the manufacture of glass for optical purposes.

CHAPTER III.
THE RAW MATERIALS OF GLASS MANUFACTURE.

The choice of raw materials for all branches of glass manufacture is a matter of vital importance. As a rule all “fixed” bodies that are once introduced into the glass-melting pot or furnace appear in the finished glass, while volatile or combustible bodies are more or less completely eliminated during the process of fusion. Thus while the chemical manufacturer can purify his products by filtration, crystallisation or some other process of separation, the glass-maker must eliminate all undesirable ingredients before they are permitted to enter the furnace, and the stringency of this condition is increased by the fact that the transparency of glass makes the detection of defects of colour or quality exceedingly easy. For the production of the best varieties of glass, therefore, an exacting standard of purity is applied to the substances used as raw materials. As the quality of the product decreases, so also do the demands upon the purity of raw materials, until finally for the manufacture of common green bottles, even such very heterogeneous substances as basaltic rock and the miscellaneous residues of broken, defective and half-melted glass forming the refuse of other glassworks may be utilised more or less satisfactorily.

For the best kinds of glass the most desirable quality in raw materials is thus as near an approach to purity as possible under commercial conditions, and next to that, as great a constancy of composition as possible. For instance, the quantity of moisture contained in a ton of sand appreciably affects the resulting composition of the glass, and if the sand cannot be obtained perfectly dry, it should at least contain a constant proportion of moisture, otherwise it becomes necessary to determine, by chemical tests, the percentage of moisture in the sand that is used from day to day, and to adjust the quantity used in accordance with the results of these tests, a proceeding which, of course, materially complicates the whole process. In other cases, variable composition is not so readily allowed for, and uncontrollable variations in the composition of the glass result—at times the quality falls off unaccountably, or the glass refuses to melt freely at the usual temperature. The systematic employment of chemical analysis in the supervision of both the raw materials and of various products will frequently enable the manufacturer to trace the causes of such undesirable occurrences; but however necessary such control undoubtedly is, it cannot entirely compensate for the use of raw materials liable to too great a variation in composition or physical character. For not only the chemical composition, but also the physical condition and properties of the material are of importance in glass manufacture. Thus it is essential that materials to be used for glass-melting should be obtainable in a reasonably fine state of division, and in this connection it must be remembered that both exceedingly hard bodies and soft plastic substances can only be ground with very great difficulty. Further, where a substance occurs naturally as a powder, this powder should be of uniform and not too fine a grain, more especially if it belongs to the class of refractory rather than of fluxing ingredients. In that case the presence of coarser grains will result in their presence in the undissolved state in the finished glass, unless excessive heat and duration of “founding” be employed to permit of their dissolution. This applies chiefly to siliceous and calcareous ingredients, but hardened nodules of salt-cake may behave in a similar manner.

A further consideration in the choice of raw materials is facility of storage. Thus limestone in the shape of large lumps of stone which are only ground to powder as required, is readily stored, and undergoes no deleterious change even if exposed to the weather; on the other hand, sulphate of soda (salt-cake), if stored even in moderately dry places, rapidly agglomerates into hard masses, at the same time absorbing a certain percentage of moisture. Such properties are not always to be avoided, salt-cake for example being at the present time an indispensable ingredient in many kinds of glass-making, but the value of a substance is in some cases materially lessened by such causes.

The raw materials ordinarily employed in glass-making may be grouped into the following classes:—

(1) Sources of silica.
(2) Sources of alkalies.
(3) Sources of bases other than alkalies.

(1) Sources of Silica.—The principal source of silica is sand. This substance occurs in nature in geological deposits, often of very considerable area and depth. These deposits of sand have always been formed by the disintegration of a siliceous rock, and the fragments so formed have been sifted and transported by the agency of water, being finally deposited by a river either in the sea (marine deposits) or in lakes (lacustrine deposits), while the action of the water, either during transport or after deposition, has frequently worn the individual particles into the shape of rounded grains.

In consequence of this origin, the chemical composition of sand varies very greatly with the nature of the rock whose denudation gave rise to the deposit. Where rocks very rich in silica, or even consisting of nearly pure silica, have been thus denuded, the resulting sand is often very pure, deposits containing up to 99·9 per cent. silica being known. More frequently, however, the sand contains fragments of more or less decomposed felspar, which introduce alumina, iron and alkalies into its composition. Finally, “sands” of all ranges of composition from the pure varieties just referred to down to the clay marls, very rich in iron and alumina, are known.

For the best varieties of glass, viz., optical glass, flint glass and the whitest sheet-glass, as well as for the best Bohemian glass, a very pure variety of sand is required, preferably containing less than 0·05 per cent. of iron, and not more than 0·05 per cent. of other impurities such as alumina, lime or alkali. As a matter of fact, sands containing so little iron rarely contain any other impurity except alumina in measurable quantities. The best-known deposit of such sand in Europe is that at Fontainebleau near Paris, but equally good sand is found at Lippe in Germany, whence sand is delivered commercially with a guaranteed silica content of 99·98 per cent. Sand of excellent quality, although not quite so good as the above, is obtained at Hohenbocka in Germany (Saxony) and at a few other places in Europe. In England no deposit of sand of such purity is at present being exploited.

Next in order of value to these exceedingly pure sands, come the glass-making sands of Belgium, notably of Epinal. These usually contain from 0·2 to 0·3 per cent. of iron and rather more alumina, but they are used very largely for the manufacture of sheet and plate-glass. When the standard of quality is further relaxed, a large number of sand deposits become available, and the manufacturers of each district avail themselves of more or less local supplies; thus in England the sands of Leighton in Bedfordshire and of Lynn on the East Coast, are largely used. Finally, for the manufacture of the cheapest class of bottles, sands containing up to 2 per cent. of iron and a considerable proportion of other substances are employed.

Silica, in various states of purity, occurs in nature in a number of other forms than that of sand. By far the commonest of these is that of more or less compact sedimentary rock, known as “sandstone.” As far as chemical composition is concerned, some of these stones are admirably suited for making the best kinds of glass, although as a rule a stone is not so homogeneous as the material of a good sand-bed. The stone has the further disadvantage that it requires to be crushed to powder before it can be used for glass-making, and the crushed product is generally a mixture of grains of all sizes ranging from a fine dust to the largest size of grain passed by the sieves attached to the crushing machine. The presence of the very fine particles is a distinct objection from the glass-maker’s point of view, so that it would probably be necessary to wash the sand so as to remove this dust—a process that in itself adds to the cost of the crushed stone and at the same time leads to the loss of a serious percentage of the material. Objections of the same kind apply, but with still greater force, to the use of powdered quartz or flint as sources of silica for the glass-maker; further, these materials are exceedingly hard and therefore difficult to crush, so that the price of the materials is prohibitive for glass-making purposes. The use of ground quartz and flint is therefore confined to the ceramic industries in which these substances serve as sources of silica for both bodies and glazes; in former times, however, ground flint was extensively used in the manufacture of the best kinds of glass, as the still surviving name of “flint glass” testifies.

Minerals of the felspar class, consisting essentially of silicates of alumina and one or more of the alkalies, are extensively used in glass-making and should be mentioned here, since their high silica-content (up to 70 per cent.) constitutes an effective source of silica. As a source of this substance, however, most felspars would be far too expensive, and their use is due to their content of alumina and alkali.

(2) Sources of Alkali.—Originally the alkaline constituents of glass were derived from the ashes of plants and of seaweed or “kelp”; in both cases the alkali was obtained in the form of carbonate and was ordinarily used in a very impure form; at the present time, however, the original source of alkali for industrial purposes is found in the natural deposits and other sources of the chlorides of sodium and potassium. At the present time it is not yet industrially possible to introduce the alkalies into glass mixtures in the natural form of chlorides. The principal difficulty in doing this arises from the fact that the chlorides are volatile at the temperature of glass-melting furnaces and are only acted upon by hot silica in the presence of water vapour. Introduced into an ordinary glass furnace, therefore, these salts would be driven off as vapour before they could combine with the other ingredients in the desired form of double silicates.

Alkalies are, therefore, introduced into the glass mixture in less volatile and more readily attackable forms. Of these the carbonate is historically the earlier, while the sulphate is at the present time industrially by far the more important. The Carbonate of Soda, or soda ash, which is used in the production of some special glasses, and is an ingredient of English flint glasses, is produced by either of two well-known chemical processes. One of these is the “black ash,” or “Le Blanc” process, in which the chloride is first converted into sulphate by the direct action of sulphuric acid, and the sulphate thus formed is converted into the carbonate by calcination with a mixture of calcium carbonate and coal. The sodium carbonate thus formed is separated by solution and subsequent evaporation. A purer form of sodium carbonate can be obtained with great regularity by the “ammonia soda” process, in which a solution of sodium chloride is acted upon by ammonia and carbonic acid under pressure. Soda ash produced by this process is now supplied regularly for glass-making purposes in a state of great purity and constancy of composition. It is upon these qualities that the great advantages of this substance depend, since its relatively high cost precludes its use except for special kinds of glass, and for these purposes the qualities named are of great value.

For most purposes of glass-making, such as the production of sheet and plate-glass of all kinds, the alkali is introduced in the form of salt-cake—i.e., sulphate of soda. This product is obtained as the result of the first step of the Le Blanc process of alkali manufacture—i.e., by the action of sulphuric acid on sodium chloride; salt-cake is thus a relatively crude product, and its use is due to the fact that it is by far the cheapest source of alkali available for glass-making. There are, however, certain disadvantages connected with its use. The chief of these is the fact that silica cannot decompose salt-cake without the aid of a reducing agent; such a reducing agent is partly supplied by the flame-gases in the atmosphere of the furnace, but in addition to these a certain proportion of carbon, in the form of coke, charcoal or anthracite coal must be added to all glass mixtures containing salt-cake. The use of a slightly incorrect quantity of carbon for this purpose leads to disastrous results, while even under the best conditions it is not easy to remove all traces of sulphur compounds from glass made in this way. A further risk of trouble arises in connection with salt-cake from the fact that it is never entirely free from more or less deleterious impurities. According to the exact manner in which it has been prepared, the substance always contains a small excess either of undecomposed sodium chloride or of free sulphuric acid, or the latter may be present in the form of sulphate of lime. A good salt-cake, however, should contain at least 97 per cent. of anhydrous sodium sulphate, and not more than 1·0 per cent. of either sodium chloride or sulphuric acid. While pure sodium sulphate is readily soluble in water, ordinary salt-cake always leaves an insoluble residue, consisting frequently of minute particles of clay or other material derived from the lining of the furnace in which it was prepared, or from the tools with which it was handled; and these impurities are liable to become deleterious to the glass if present in any quantity. The insoluble residue should not exceed 0·5 per cent. in amount, and in the best salt-cake is generally under 0·2 per cent.

Salt-cake possesses certain other properties that make it somewhat troublesome to deal with as a glass-making material. Thus, on prolonged exposure, particularly to moist air, the powdered salt-cake absorbs moisture from the atmosphere and undergoes partial conversion into the crystalline form of “Glauber’s Salt,” a process which results in the formation of exceedingly hard masses. Ground salt-cake, therefore, cannot be stored for any length of time without incurring the necessity of regrinding, and this accretive action even comes into play when mixtures of glass-making materials, containing salt-cake as one ingredient, are stored. In practice, therefore, salt-cake can only be ground as it is wanted, and its physical properties make it difficult to grind it at all fine, while the dust arising from this process is peculiarly irritating, although not seriously injurious to health.

Potash is utilised in glass-making almost entirely in the form of carbonate, generally called “pearl-ash.” Originally derived from the ashes of wood and other land plants, this substance is now manufactured by processes similar to those described in the case of soda, the raw material being potassium chloride derived from natural deposits such as those at Stassfurth. The pearl-ash thus commercially obtainable is a fairly pure substance, but its use is complicated by the fact that it is strongly hygroscopic and rapidly absorbs water from the atmosphere. Where it is desired to produce potash glasses of constant composition, frequent analytical determinations of the moisture contents of the pearl-ash are necessary, and the composition of the glass mixture requires adjustment in accordance with the results of these determinations.

The alkalies are also introduced into glass in the form of nitrates (potassium nitrate, or saltpetre, and sodium nitrate, or nitre); but although these substances act as sources of alkali in the glass, they are employed essentially for the sake of their oxygen contents. Such oxidising agents are not, of course, added to glass mixtures containing sulphates and carbon, but are employed to purify the mixtures containing alkali carbonates, and more especially to oxidise the flint glasses. Since these substances are only introduced into glass in small quantities their extreme purity is not of such great importance to the glass-maker, and the ordinary “refined” qualities of both nitrates are found amply pure enough to answer the highest requirements.

A certain number of natural minerals which contain an appreciable quantity of alkali are sometimes utilised as raw materials for glass manufacture. The most important of these are the minerals of the felspar class already referred to. These, however, contain a considerable proportion of alumina, while all but the purest varieties also contain more or less considerable quantities of iron. Some glass-makers regard alumina as an undesirable constituent, while others take the opposite view, and upon this view their use of felspathic minerals will depend. For the cheaper varieties of glass, however, such as bottle glass, felspathic minerals and rocks, such as granite and basalt, are freely used as raw materials. Another mineral in which both alkali and alumina are found is cryolite. This mineral is a double fluoride of soda and alumina, whose properties are particularly valuable in the production of opal and opalescent glasses. As a mere source of alkali, however, cryolite is much too expensive.

(3) Sources of Bases other than Alkalies.—The most important of these are lime and lead oxide, the former being required for the production of all varieties of plate and sheet-glass, as well as for bottles and a large proportion of pressed and blown glass, while lead is an essential ingredient of all flint glass. The only other base having any considerable commercial importance in connection with glass-making is barium oxide, while oxide of zinc, magnesia, and a few other substances are used in the manufacture of special glasses for scientific, optical or technical purposes, where glass of special properties is required. The metallic oxides which are used for the production of coloured glass are, of course, also basic bodies. These will be treated in connection with coloured glasses, with the exception of manganese dioxide, which is used in large quantities in the manufacture of many ordinary “white” glasses.

Calcium Oxide (lime) is generally introduced into glass mixtures in the form of either the carbonate or the hydrated oxide (slaked lime). The carbonate may be derived either from natural sources, or it may be of chemical origin, while the hydrate is always obtained by the calcination of the carbonate, followed by “slaking” the lime thus produced. Natural calcium carbonate occurs in great quantities in the form of chalk and limestone rocks. Both varieties are used for glass-making. Chalk is a soft friable material which is apt to clog during the grinding operations, particularly as the natural product is generally somewhat moist. As regards the greater part of its mass, chalk is often found in a state of great purity, but it is frequently contaminated by the presence of scattered masses of flint. Chemically this impurity is not very objectionable to the glass-maker, since it merely introduces a small proportion of silica whose presence need scarcely be allowed for in laying down the mixture. On the other hand, if any fragments of flint remain in the mixture when put into the furnace, they prove very refractory, and are apt to be found as opaque enclosures in the finished glass. Natural limestone can also be obtained in great purity in many parts of the world. It is generally a hard and rather brittle rock that can be readily ground to powder of the requisite degree of fineness. Flint concretions are not so frequently found in this material, but, on the other hand, it is often contaminated with magnesia and iron. The former ingredient, when present in small quantities, tends to make the glass hard and viscous, so that limestone of the lowest possible magnesia content should be used, especially for the harder kinds of glass, such as plate and sheet-glass, etc. The iron contents of the limestone used must also be low where a white glass is required; but since a smaller quantity of limestone is used for a given weight of glass produced than the quantity of sand used for the same purpose, the presence of a somewhat higher percentage of iron is permissible in the limestone as compared with the sand; for the better varieties of glass, however, the iron should not exceed 0·3 per cent. of the limestone.

Slaked lime is sometimes used as the source of lime for special glasses where the process of manufacture renders it desirable to avoid the evolution of carbonic acid gas which takes place when the carbonate is heated and attacked by silica. When slaked lime is used only the water vapour of the hydrate is driven off, and this occurs at a much lower temperature. For the production of slaked lime, an adequately pure form of limestone, preferably in the form of large lumps, is burnt in a kiln until the carbonic acid is entirely driven off; after cooling, the lime so formed is slaked by hand. The product so obtained is, however, apt to vary both as regards contents of moisture and carbonic acid, which latter is readily absorbed from the atmosphere; the use of this material, therefore, requires frequent analytical determinations of the lime contents and corresponding adjustments of the mixture if constant results are required.

It is possible to introduce lime into glass mixtures in the form of gypsum or calcium sulphate, but the decomposition of this compound, like that of sodium sulphate, requires the intervention of a reducing agent such as carbon, and the difficulties arising from this source in connection with the use of salt-cake are still further increased in the case of the calcium compound. Since limestones of considerable purity are more or less plentiful in many districts, the commercial value of calcium sulphate for glass-making is probably slight.

The Compounds of Barium may best be dealt with at this stage, since they are chemically so closely allied to the compounds of lime just described. Barium occurs in nature in considerable quantities in the minerals known as barytes (heavy spar) and witherite respectively. The former is essentially sulphate of barium, while the latter is a carbonate of barium. The use of the sulphate meets with the same objection here as in the case of calcium sulphate discussed above, except that the barium compound is much more easily reduced and decomposed than the lime compound. The natural mineral witherite is used to a considerable extent in the production of barium glasses, and these have been found capable of replacing lead glasses for certain purposes. On the other hand, for the best kinds of barium glasses, viz., those required for optical purposes, the element is introduced in the form of artificially prepared salts. Of these the most important is the carbonate, commercially described as “precipitated carbonate of barium”; this precipitated compound, however, does not ordinarily correspond to the chemically pure substance, but contains more or less considerable quantities of sulphur compounds. The question whether these impurities are or are not objectionable can only be determined for each particular case, since much depends upon the special character of the glass to be produced. Both the nitrate and the hydrate of barium are commercially available, but they are very costly ingredients for use in the production of even the most expensive kinds of glass; these substances are, however, obtainable in a state of considerable purity, although the hydrate has the inconvenient property of rapidly absorbing carbonic acid from the atmosphere, thus becoming converted into the carbonate.

Magnesia is another glass-forming base that is closely related, chemically, to calcium and barium. This element is usually introduced into glass mixtures in the form of either the carbonate or the oxide. The carbonate occurs in nature in a more or less pure state in the form of magnesite, and by calcination, the oxide is obtained. The natural mineral and its product are, of course, by far the cheapest sources of magnesia, but as the element is only used in comparatively small quantities, the artificial precipitated carbonate or calcined magnesia are frequently preferred. Magnesia is only introduced intentionally in notable quantities in special glasses where the properties it confers are of special value; in ordinary lime glasses this element, as has already been mentioned, is to be regarded as an undesirable impurity.

Zinc oxide lies, chemically, between the bases already discussed on the one hand, and lead oxide on the other. This element is only introduced into special optical glasses, a special “zinc crown” having found some application. Chemically prepared zinc oxide is almost the only form in which the element is used, but the very volatile character of this substance must be borne in mind when it is introduced into glass mixtures.

Lead is one of the most widely-used ingredients of glass; the glasses containing this substance in notable quantity are all characterised to a greater or less degree by similar properties, such as considerable density and high refractive power, and are classed together under the name “flint glasses.” Lead is now almost universally introduced into glass mixtures in the form of red lead, although the other oxides of lead might be employed almost equally well. Red lead is a mixture of two oxides of lead (PbO and Pb₂O₃) in approximately such proportions as to correspond to the formula Pb₃O₄. It is prepared by the roasting of metallic lead in suitable furnaces, where the molten lead is exposed to currents of hot air. The product is obtainable in considerable purity, very small proportions of silica, derived from the furnace bed, and of iron derived from the tools with which the lead is handled, being the principal foreign substances found in good red lead. Silver would be an objectionable impurity, but owing to the modern perfect methods of de-silvering lead, that element is rarely found in lead products. Analytical control of red lead as used in the glass mixtures, and consequent adjustments of the mixture, are, however, necessary where exact constancy in the glass produced is desired. The reason for this necessity lies in the fact that the oxygen content, and therefore the lead-oxide (PbO) content, varies decidedly from batch to batch, while the material as actually delivered and used frequently contains notable proportions of moisture.

A word should perhaps be said here as to methods of handling red lead on account of the injurious effects which the inhalation of lead dust produces upon the workmen exposed to it. For glass-making purposes it is not feasible to adopt the method adopted by potters of first “fritting” the lead and thus rendering it comparatively insoluble and innocuous; even if this were done, the difficulty would only be moved one step further back, and would have to be overcome by those who undertook the preparation of the frit. The proper solution of the problem, in the writer’s opinion, is to be found in properly preventing the formation of lead dust, or at all events in protecting the workmen from the risk of inhaling it. Where only small quantities of lead glass are made, and therefore only small quantities of lead are handled and mixed at a time, it is no doubt sufficient to provide the workmen engaged on this task with some efficient form of respirator to be worn during the whole of the time that they are engaged on such work, and to take the further precautions necessary—by way of cleanliness and the provision of proper mess-rooms—to avoid any risk of lead dust either directly or indirectly contaminating their food. Where, however, large quantities of flint-glass are made every day, it is possible and proper to make more perfect arrangements for the mechanical handling and mixing of the lead with the other ingredients by the provision of suitable mixing and transporting machinery, so arranged as to be dust-tight. It is only fair to state, however, that partly under their own initiative, partly under pressure from the authorities, glass makers in this country are complying with these requirements in an adequate manner.

Aluminium.—There are several varieties of glass into which alumina enters in notable quantities, the principal examples being certain optical and many opal glasses, while most ordinary glasses contain this substance in greater or less degree. In the latter, the alumina is derived by the inevitable processes of solution, from the fire-clay vessels or walls within which the molten glass is contained, while in some cases the element is intentionally introduced in small proportions (about 2 per cent. to 3 per cent. of Al₂O₃) by the use of felspar as an ingredient of the mixture. Where larger proportions of alumina are required, the substance is introduced in the form of the hydrate, which is obtainable commercially in a state of almost chemical purity, but of course at a correspondingly high cost. In opal glasses alumina is derived partly or wholly from felspars, or in some cases from the use of the mineral cryolite. This is a double fluoride of aluminium and sodium which is found in great natural masses, chiefly in Greenland. Owing to the high price of this mineral, however, artificial substitutes of nearly identical composition and properties have been introduced and are used successfully in the glass and enamelling industries.

Manganese.—Although the oxides of this element really belong to the class of colouring compounds, they are so widely used in the manufacture of ordinary “white” glasses that it is desirable to deal with them here. The element manganese is most usually introduced into glass mixtures in the form of the per-oxide (MnO₂), although the lower oxide (Mn₃O₄) can also be used. The material ordinarily used is the natural manganese ore, mined chiefly in Russia; the purest forms of this ore consist almost entirely of the per-oxide, but “brown” ores, containing more or less of the lower oxide, are also used with success. These ores always contain small amounts of iron and silica, but provided the iron is not present in any considerable quantity, the value of the ore is measured by the percentage of manganese which it contains. The colouring and “decolourising” action of manganese will be discussed in a later chapter. Certain other substances, which have been suggested as either substitutes for, or improvements upon, manganese for this purpose need only be mentioned here, viz., nickel, selenium and gold.

Arsenic is another substance frequently introduced into “white” glass mixtures. This element is universally introduced in the form of the white arsenic of commerce (i.e., arsenious acid, As₂O₃) which is obtained in a pure form by a process of sublimation. Owing to the very poisonous nature of this material, special precautions must be taken in its use for glass-making purposes to avoid all risk of poisoning.

Carbon.—As has already been indicated, an admixture of carbon in some suitable form is essential in the case of certain glass mixtures. The carbon for this purpose may be used in the form of either charcoal, coke, or anthracite coal. Of these, charcoal is undoubtedly the purest form of carbon, but it is excessively expensive in this country. Coke varies very much in quality according to the coal from which it has been produced, but it always contains notable proportions of ash rich in iron, and also some sulphur. Anthracite coal can be obtained in a very pure form, containing considerably less ash than that found in most kinds of coke, and this is therefore probably the most convenient form of carbon for this purpose.

CHAPTER IV.
CRUCIBLES AND FURNACES FOR THE FUSION OF GLASS.

For the successful production of substances which are formed by a process of fusion, the use of refractory materials of a proper kind is of great importance. In the production of glass the double difficulty has to be overcome of finding substances capable of being formed into furnaces and crucibles which shall not only resist the softening and melting action of the furnace heat for long periods of time, but shall also resist the dissolving action of the molten glass itself. The refractory materials employed in connection with glass-making thus fall into two distinct groups, members of one group being those which meet both of the above requirements and can therefore be used in positions exposed to direct contact with molten glass, while members of the second group are materials which resist the action of the heat and flame gases but cannot resist the dissolving effect of the glass itself; these, of course, can only be placed where molten glass is not liable to touch them. We shall deal with the former group first.

Those portions of glass-melting plant which come into contact with molten glass are almost universally made of some form of fire-clay. To discuss in detail the composition and properties of the varieties of fire-clay best suited to this purpose would exceed the entire limits of this book, so that only a few leading principles can be stated. Taking first the clays intended for the production of crucibles or “pots,” we find that for the purposes of the production of such objects the prepared clay must possess a certain degree of plasticity while damp and a considerable degree of strength when dried. The dried and burnt material must be so refractory as to resist the high temperatures used in glass-melting without undergoing fusion or even serious softening. Clays of various composition and physical nature also differ very widely in their power of resisting the chemical attack of molten glass; all clays are more or less dissolved under these circumstances, but not only the rate, but also the manner, of dissolution is of importance, so that frequently a clay which dissolves rapidly but uniformly is preferred to one which dissolves more slowly but in such an irregular manner as to throw off particles of undissolved material which contaminate the glass in the form of opaque enclosures or “stones.” It is also to be noted that the best results in this direction can only be obtained by careful adaptation of the clay employed to the particular kind of glass which is to be melted in the crucibles in question. In England this question has not received the amount of attention it deserves, but in Germany and America the available fire-clays of the country have been systematically studied and exploited. As a result the glass-maker has at his disposal a large selection of materials of accurately known physical and chemical properties. By carefully correlating these with the performance of his “pots” in the furnaces, the manufacturer is able to select the most suitable material, and is, moreover, in a position to know in what direction to look for improvement or for replacement if the supply of a satisfactory brand should cease.

We may now follow briefly the process of manufacture of a fire-clay pot or crucible. The size and shape of the crucible will depend upon the particular purpose for which it is intended. Crucibles varying in capacity from 4 cwt. to 2½ tons of glass are used for various kinds of glass, but the more usual sizes lie between 30 in. and 50 in. in diameter. For many kinds of glass the shape of the pot is simply that of an open basin, circular or oval in plan and larger in diameter at the brim than at the base (Fig. 1), but for the production of flint glass, and of other glasses which are to be protected from contact with the flame and gases of the furnace, so-called “covered” pots are used. In these the basin—here of a more nearly cylindrical shape—is covered over by a dome, and access is allowed only by a relatively small hooded opening (Fig. 2). Covered pots are built up on wooden moulds, which are made collapsible, and are removed before the drying of the pot is begun.

Fig. 1.—Open “pot” or crucible for glass melting.

Fig. 2.—Covered pot for glass melting, as used for flint glass and optical glass.

The material for pot-making is first prepared with great care. The proper variety of clay having been selected, it is ground to a fine powder in suitable mills and carefully sieved; with this fine clay powder is mixed, in accurately determined proportions, a quantity of crushed burnt fire-clay. In some works this burnt material is obtained by simply grinding up fragments of old used pots, but the better practice is to burn specially-selected fire-clay separately for this purpose. The quantity of such burnt material added to the mixture depends upon the chemical nature and especially on the plasticity of the virgin clay employed; with so-called “fat” or very plastic clays up to 50 per cent. of burnt material is added, but with the leaner clays, such as those of the Stourbridge district in England, very much smaller proportions are used. The object of this addition of burnt material is to facilitate the safe drying of the finished pots and to diminish—by dilution—the total amount of contraction which takes place both when plastic clay is allowed to dry, and further when the dry mass is subsequently burnt; the burnt material or “chamotte,” having already undergone these shrinking processes, acts both as a neutral diluent and also as a skeleton strengthening the whole mass and reducing the tendency to form cracks.

The virgin clay and chamotte having been intimately mixed, the whole mass is “wet up” by the addition of a proper proportion of water and prolonged and vigorous kneading, usually in a suitable pug mill. The mass leaves this mill as a fairly stiff, plastic dough, but the full toughness and plasticity of such clay mixtures can only be developed by prolonged storage of the damp mass. In the next stage of the process, the plastic clay is passed to the “pot maker” in the form of thick rolls, and with these he gradually builds up the pots or crucibles from day to day, allowing the lowest parts to dry sufficiently to enable them to bear the weight of the upper parts without giving way. The building of large pots in this way occupies several weeks, and during this time the premature drying of any part of the pot must be carefully avoided. After the completion of the pot, drying is allowed to take place, slowly at first, but more vigorously after a time when the risk of cracking is smaller; when it is taken into use, the pot is usually many months old and is thoroughly air-dry. The clay, however, is still hydrated, i.e., contains chemically combined water, and this is only expelled during the early stages of the burning process. This process is carried out in smaller furnaces or kilns placed near the melting furnaces. In these the pot or pots are exposed to a very gradually increasing temperature, until a bright red heat is finally attained. This is a delicate process in which great care is required to secure gradual and uniform heating, especially during the earlier stages, otherwise the pots are apt to crack and become useless. Finally, when a bright red heat has been maintained for at least a day, the pots are ready to be placed in the furnace, and this is ordinarily done while both pots and furnace are at a red heat, the pots never being allowed to cool down again once they have been burnt.

Fire-clay is also used in the manufacture of bricks and blocks of various sizes required for the construction of glass-melting furnaces. Here fire-clay is only used in positions where contact with molten glass is expected, as in the walls of the basin or tank proper in “tank” furnaces, or at a level below that of the pot or crucible in pot furnaces; in the latter position leakage of glass from broken pots or overflow being liable to result in an accumulation of molten glass on the floors or walls of the furnace and passages. The fire-bricks used in these latter positions are usually of a much poorer quality of fire-clay than that used for the manufacture of pots, and this is justified in so far as certain of the requirements that apply to crucibles do not apply here—but on the other hand the use of more refractory bricks would result in a longer life for the furnace. Such bricks, it should be noted, are not laid in mortar when used for furnace construction, but are set in a thin paste of fire-clay in water, and these joints are kept as thin as possible. The part of the furnace known as the “siege” (French “siège”), i.e., the floor of the furnace upon which the pots are placed, is usually built of very large blocks of fire-clay, made of coarse materials calculated to give great strength. At or near the points where the flame enters the furnace, these blocks rapidly wear away, partly by melting but chiefly by a process of abrasion, for it seems that a rapidly moving flame has an abrading action of a very marked kind.

The actual tanks or basins which contain the molten glass in tank furnaces are also built of large blocks of fire-clay, but these are made of the best procurable materials, and should receive at least as much care in every respect as crucibles; it is true that their shape and size gives them greater strength, but on the other hand these blocks are expected to resist the contact of molten glass for very much longer periods of time than the average crucible. To understand the requirements for tank-blocks it is necessary to anticipate the next section to the extent of stating that in tank furnaces the glass is contained, during melting, refining and working, in a basin built up of large blocks. These blocks are not cemented together in any way, but are built up “dry” and are supported on the outside by a system of iron bars and rods. The molten glass penetrates between the blocks to a certain extent, but as the outside of all such blocks is intentionally kept as cold as possible the glass rapidly stiffens as it penetrates further into these interstices, and this stiffened glass effectually binds the blocks together and prevents all leakage. It will thus be seen that the blocks are exposed to the full heat of the furnace and to the corroding action of the glass on the inner side, but are kept cold on the outer side. As this state of affairs tends to produce cracks, these blocks are necessarily made of rather coarse material. On the other hand, the material of a block never gets so hot as the wall of a crucible, which is heated from both sides, so that extreme refractoriness is not so essential.

It is impossible, within the limits of this chapter, to go into the details of the choice of materials for tank-blocks; it is a subject upon which no finally satisfactory conclusion has yet been reached, and what has been said above will suffice to show the nature of the considerations upon which such choice must be based.

We now turn to the second class of refractory materials used in the construction of glass-melting furnaces, viz., those which are so placed as not to come into contact with molten glass. Here mechanical strength and refractoriness are almost the only considerations, but in the roof-vaults or “crowns” of tank furnaces and also of furnaces in which glass is melted in open pots, there is the further consideration that the material of the bricks used shall not contain notable quantities of any colouring oxide, since small flakes, etc., are apt to drop down into the molten glass, and would thus be liable to cause serious discolouration. Such a material as chrome-ore brick is therefore excluded. As a matter of fact, some form of “silica brick” is in universal use. Bricks of this material, otherwise known as “Dinas bricks” from the place of their first origin, in Wales, consist of about 98 per cent. of silica (SiO₂). Pure silica cannot be baked or burnt into coherent bricks entirely by itself, since it possesses neither plasticity when wet nor any binding power when burnt, but an admixture of about 2 per cent. of lime and a little alumina makes it possible first to mould the bricks when wet and then to burn them so as to form fairly strong, coherent blocks. These are of amply adequate refractoriness for the highest temperatures that can be attained in industrial gas-fired furnaces, and their mechanical strength is sufficient to make it possible to build vaults of considerable span, but on the other hand this material requires very gradual heating and constant watching while the temperature is rising or falling to any considerable extent; the reason for this difficulty lies in the fact that silica bricks swell very markedly during heating, so that unless a vault built of this material is given room to spread somewhat, it will rise seriously and may even break up completely. This risk is avoided by gradually slackening the tie-bolts that hold the vault together, and correspondingly “taking up the slack” as the vault cools when the furnace is let out. Sudden local heat also has a disastrous effect on this material, producing serious flaking. For positions where intense heat is to be borne, and at the same time mechanical strength is required, silica brick is a most valuable material, but owing to its chemical composition it is rapidly attacked by molten glass or by any material containing a notable proportion of basic constituents, so that the silica bricks can only be employed out of contact with glass.

We now turn to consider, very briefly, the general design and arrangement of some typical glass-melting furnaces. The oldest and simplest form of furnace is, in effect, simply a box built of fire-brick, in the centre of which stands the crucible, while a fire of wood or coal is placed upon either side. To attain any great degree of heat by such means, however, the size of the box or chamber and especially of the grates in which the fires are maintained must be properly proportioned both to the dimensions of the crucible and to each other. The grates are generally wide and deep, while draught is provided by means of a tall conical chimney which stands over the entire chamber and communicates with it by a number of small openings. In a more refined furnace, the chamber itself is double, and the flame, after playing around the crucible in the inside of the chamber, is made to pass through the space between the outer and inner chamber before passing to the chimney or cone. We need not give any greater attention to these primitive furnaces, since they are practically obsolete at the present time. In modern furnaces the process of combustion is carried on in two distinct stages; the first stage takes place in a subsidiary appliance known as a “gas producer,” where part of the heat which the fuel is capable of generating is utilised for the production of a combustible gas; this gas passes into the furnace proper, either direct, while it is still hot from the producer, or after being conveyed some distance, when it is again heated up by the waste heat of the furnace. In either case the gas is hot when it enters the furnace proper, and there it meets a current of air, also heated by the aid of the waste heat of the furnace. Hot gas and hot air burn rapidly and completely, and if properly proportioned yield exceedingly high temperatures. Seeing that in this process a part of the heat of combustion yielded by the fuel is generated in a subsidiary appliance and is thus lost to the furnace, it appears at first sight somewhat surprising that this system of firing is very considerably more efficient than the old “direct” system where the whole of the fuel is burnt in the furnace itself. But the advantage arises from the fact that in the newer system the fuel is handled in the gaseous form. This has the advantage, first and most important, that the heat escaping from the furnace in the hot products of combustion (chimney gases) can be transferred to the incoming unburnt gas and air and can thus be returned to the furnace. The manner in which this is accomplished will be considered below, but it may be noted here that in some furnaces the escaping products of combustion are so thoroughly cooled that they are unable to produce an effective draught in the chimney of the furnace. Another advantage of the use of gaseous fuel is the fact that complete combustion can be obtained without the use of so great excess of air, such as is required when solid fuels are to be burnt completely. For this reason much higher temperatures can be readily obtained with gaseous fuel, while the pre-heating of both gas and air also facilitates the attainment of high temperatures; further, the great facility with which the flow of either gas or air can be regulated by means of suitable valves, makes it possible to secure much greater regularity in the working of the furnaces. Finally, in modern gas-producers, the amount of sensible heat generated and therefore lost to the furnace, is kept very low, the greater part of the heat set free by the partial combustion of coal in the producer being absorbed by the decomposition of a corresponding quantity of steam into hydrogen and carbonic oxide gas. The gas as it leaves one of these producers is not very hot, and the percentage of heat lost in this way is therefore much smaller than in the older forms of gas-producer.

It is again impossible, within the limits of this chapter, to enter into the details of construction and working of gas-producers. We must content ourselves with saying that most modern producers are of the form of a tower in which a thick bed of fuel is partially burnt and partly gasified under the action of a blast of air mixed with steam. The chemical actions that take place are complicated, but the final result is the production of a gas containing from 2 to 8 or 10 per cent. of carbonic acid, 10 to 20 per cent. of hydrogen, 8 to 25 per cent. of carbonic oxide (CO), 1 to 3 per cent. methane (CH₄), and 45 to 60 per cent. of nitrogen, with varying quantities of moisture, tarry matter, and ammonia. In good producer gas, the combustible constituents (hydrogen, carbonic oxide and methane) should total from 30 to 48 per cent. of the whole by volume, but the exact composition to be expected depends very much on the type of producer and the class of fuel used. Some producers are capable of dealing with exceedingly low-grade fuels, and the gas which they yield can still be utilised for obtaining the highest temperatures—a proceeding that would have been impossible if it had been attempted to burn these fuels directly in the furnace.

Fig. 3.—Diagram of the arrangements of a regenerative furnace.

The gas on leaving the producer passes along fire-brick flues or passages to the furnace proper; the path which it is now caused to take varies somewhat according to the arrangement of the furnace in question. Modern gas-fired furnaces usually belong to one of two distinct types according to the manner in which the heat of the escaping products of combustion is utilised for heating the incoming gas and air; these two types are known as the “regenerative” and the “recuperative” respectively. In regenerative furnaces the hot products of combustion, after leaving the furnace chamber proper, and before reaching the chimney, pass through chambers which are loosely stacked with fire-bricks; these chambers absorb the heat of the escaping gases, and thus rapidly become hot. As soon as a sufficiently high temperature is attained in these chambers or “regenerators,” the path of the gas-currents is altered; the escaping products of combustion are made to pass through, and thus to heat a second set of regenerating chambers, while the incoming gas and air are drawn through the heated regenerator chambers before entering the furnace chamber proper. The incoming gas and air are thus heated, absorbing in turn the heat stored in the brickwork of the regenerators. It is evident that two sets of such regenerators are sufficient, the one set undergoing the heating process at the hands of the escaping products of combustion, while the other set is giving up its heat to the incoming gas and air; when this process has gone far enough, it is only necessary to interchange the two sets of chambers, by the operation of suitable valves, and this series of alternations may be continued indefinitely. The arrangement is shown diagrammatically in Fig. 3.

In recuperative furnaces the same principle is utilised in a somewhat different manner; the outgoing products of combustion pass through tubular channels formed in fire-clay blocks, while the ingoing gas and air pass around the outside of these same blocks; the heat of the outgoing gases is thus transferred to the incoming gases by the process of conduction through the fire-clay walls of the recuperator tubes. The relative merits of the two systems have been hotly contested; the regenerative system has the advantage of avoiding all reliance on the heat conductivity of fire-clay, while it also avoids the somewhat complicated special tubular blocks required for the other system; on the other hand, the recuperative system avoids the necessity for all “reversing” valves and their regular periodical working, while it also occupies somewhat less space. Temperatures sufficiently high for all glass-melting purposes can be attained by both means.

In both systems of furnace, heated gas and heated air are admitted to the furnace by separate fire-brick flues or passages, air and gas being allowed to mix just before they enter the furnace chamber proper. The economy and efficiency of the furnace depend to a very great extent upon the manner in which this mixing is accomplished. Rapid and complete mixing of air and gas results in an intensely hot, but short and local flame, while slower mixing tends to lengthen the flame and spread the heat through the entire furnace chamber; on the other hand if the mixing of gas and air is too slow, combustion may not have been completed in the short time occupied by the gases in passing through the furnace, and combustion may either continue in the outflow flues and regenerators, or it may be prevented by the narrowness of these passages, and unburnt gases may pass to the chimney. When the openings or “ports” are properly proportioned, and the draught of the chimney is properly regulated, combustion should be just complete as the gases leave the furnace chamber, and under these circumstances small tongues of keen flame will escape from every opening in the furnace; large smoky flames issuing from a gas-fired furnace indicate incomplete combustion.

Fig. 4.—Sectional diagram of a regenerative pot furnace working with covered pots.

As has already been indicated, glass is melted either in pots or crucibles of various shapes and sizes, or in open tank furnaces. The general arrangement of a pot furnace working with closed or “covered” crucibles is shown in Fig. 4. In this particular furnace, the “ports” or apertures by which the gas and air enter the furnace chamber, are placed in the floor of the chamber, but these apertures are often placed in the side or end walls, or even in a central column, the object being in all cases to heat all the pots as uniformly as possible and to avoid any intense local heating, which would merely endanger the particular crucible exposed to it, without greatly aiding the real work of the furnace. In pot furnaces, however, in which the more refractory kinds of glass are to be melted, it is generally considered desirable that the flame should be made to play about the pots in such a way as to heat the lower parts of the pots most strongly. In connection with the question of the uniformity of heat distribution in a gas-fired furnace it must further be borne in mind that in the case of regenerative furnaces the direction of the flame is reversed every time the valves are thrown over, and in practice this is done about once every half-hour; this proceeding, of course, tends very much to equalise the temperature of the two sides of the furnace. In recuperative furnaces, on the other hand, the direction of the flame is not changed, and for that reason a flame returning upon itself, usually called a horse-shoe flame, is often employed; this is obtained by placing the entry and exit ports side by side at one end of the furnace; the impetus of the flame gases and their rapid expansion during combustion carry the flame out across the furnace, while the chimney draught ultimately sucks it back to the exit ports, the shape of the flame being shown in Fig. 5.

Fig. 5.—Diagram of a furnace with “horse-shoe” flame.

In general arrangement, a tank furnace for glass melting resembles an open-hearth steel furnace. The tank or basin, as already indicated, is built up of a number of large fire-clay blocks, forming a bath varying in depth from 20 in. to 42 in. according to the design of the furnace and the kind of glass to be melted in it. The ports for entry of gas and air and for exit of the products of combustion are in most modern furnaces placed in the side walls of the furnace just above the level of the glass, the whole being covered by a vault built of silica brick. Figs. 6 and 7 show the general arrangement of a simple form of tank-furnace such as that used in the manufacture of rolled plate glass. The furnace indicated in the diagram is intended for regenerative working with alternating directions of flame; in recuperative furnaces the horse-shoe flame is always used in tanks, while this arrangement of ports is sometimes adopted for regenerative tanks also, particularly in the manufacture of bottles. For the production of sheet glass, tank furnaces are generally sub-divided into two compartments and are also provided with various constrictions intended to arrest impurities and to allow only clear glass to pass, but as regards the arrangement of flues and ports there is a very general similarity between various furnaces of this type.

Fig. 6.—Longitudinal sectional diagram of tank furnace.

Fig. 7.—Transverse sectional diagram of tank furnace, showing regenerators and gas and air passages.

It is beyond the scope of this book to discuss the relative merits of tank and pot melting furnaces; wherever the former can be made to produce glass of adequate quality for the purpose desired, the great economy of the tank furnace inevitably carries all before it, so that bottle glass, for example, is now made exclusively in tanks, and the same applies also to rolled plate of the ordinary kind, and to the great majority of sheet glass. On the other hand, where special qualities of glass are required in relatively small quantities, or where the requirements as to quality are very extreme, the pot furnace remains indispensable. Optical glass and coloured glasses are examples of this kind, although some tinted glasses are used in sufficient quantity to justify the use of small tank furnaces for their production. The causes of the greater economy of the tank furnace are numerous, and complicated by the detailed requirements of each particular manufacture, but the most important factors in the question may be summed up thus:—

(1) The tank furnace utilises the heat of the flame more efficiently, as the glass is exposed to the heat in a basin whose surface covers the entire area of the furnace, while in a pot furnace there is much vacant, unused space.

(2) The tank furnace permits of continuous working, the raw materials being introduced at one end while the glass is being withdrawn and worked at the other end. There are thus no idle periods, and each part of the furnace remains at or near the same temperature during the whole time that a furnace is alight. For a given size of plant, therefore, a tank furnace yields a much larger output, with a relatively smaller fuel consumption.

(3) The tank furnace obviates the need for pots or crucibles, which are not only costly and troublesome to produce, but are liable to premature failure and require periodical renewal, which involves a serious loss of time for the furnace.

(4) Finally, the molten glass in a tank furnace can be always maintained at or near one constant level and is, therefore, always convenient for withdrawal by means of the gatherer’s pipe or the ladle.

In pot furnaces, on the other hand, the composition of the glass can be more accurately regulated, and the molten glass itself can be more effectively protected from contamination either by matter dropping into it or by the action of the furnace gases, while in pots it is also possible to effectually melt together materials which, in the open basin of a tank, could not be kept together long enough to combine.

CHAPTER V.
THE PROCESS OF FUSION.

It has already been indicated that, for glass-making purposes, the raw materials are required in a state of reasonably fine division. The exact degree of fineness required depends very much upon the nature of the ingredient in question, the general rule being that the more refractory and chemically resistant materials require to be most finely ground, while substances which melt and react readily, such as soda ash and salt-cake, do not require very fine grinding.

Assuming that the materials are available in a suitable state of fineness, the first step in the process of glass melting consists in securing their admixture in the proper proportions. This may be done by hand entirely, by hand aided by some machinery, or entirely automatically. The process of hand mixing is only available for relatively small quantities of material and requires very careful supervision if inadequate mixing is to be avoided. In most cases the actual weighing out is done by hand, while the mixing is done by machinery. In this process the separate ingredients are weighed out from barrows or skips and are tipped into a large hopper whence each batch, as soon as it is completed, passes into the mixing chamber of the mixing machine. This may consist of nothing more than a cylindrical chamber in which steel arms revolve and stir up the contents, but more modern appliances take the form of rotating barrels or cylinders, set up on an inclined axis and provided with suitable shelves and baffles; in these the materials are very thoroughly shaken over and mixed. Where hand mixing is adopted, the various ingredients of each batch are thrown into a large bin and are there turned over several times with shovels, the entire material being ultimately sieved through a wire sieve of suitable mesh. In all cases the resulting mixture should be perfectly uniform in colour and texture, and analyses of different samples should show only small variations. With the mixture thus prepared the “cullet” or broken glass which is to be re-melted is now incorporated; ideally this should also be uniformly distributed, but this is rarely attempted in practice on the large scale.

The next step in the process is the introduction of the mixture into the furnace. In the case of tank furnaces this is a simple matter, since in these the temperature is kept as nearly constant as possible, and raw materials may, therefore, be introduced at almost any time, the amount introduced being so regulated as to keep the level of the molten glass or “metal” as nearly constant as possible. The actual introduction is managed by means of a large opening or door at what is known as the “melting end” of the furnace. Normally this opening is covered by a large fire-brick block suspended by a chain running over pulleys and counterbalanced by a counterpoise weight. When charging is to begin, this block is raised and the opening is uncovered. The raw materials are then introduced either by hand, by the aid of long-handled shovels, or they are first filled into a long scoop moved by mechanical means forward into the furnace, where it is given a half-turn, which empties the contents out, and is then rapidly withdrawn.

This charging process may be repeated every half-hour, or larger quantities may be introduced once every four hours, according to the practice that may be adopted at any particular furnace.

In the case of pot furnaces the charging process is not so simple. Here the first charge of raw materials has to be introduced into a pot which has been almost entirely emptied during the working-out process, and the temperature of the furnace has also fallen very considerably during this time. Before new material is introduced, the heat of the furnace must first be adequately restored. If this is not done, the fusion of the glass takes an abnormal course and very imperfect results arise. Further, the quantity of material introduced at one time must be carefully adjusted to the capacity of the pot. During the earlier stages of fusion most glass mixtures form large masses of foam, and if the crucible has been too heavily charged this foam overflows, with the result that valuable material is lost and the floor and passages of the furnace are clogged with glass. A certain amount of overflow, as well as leakage from defective crucibles, is, however, unavoidable, and for this purpose every pot furnace is provided with a chamber so placed that the glass will flow into it and so be prevented from finding its way into the regenerators or other parts where its presence would hinder the working of the furnace. These receptacles or “pockets” must, however, be periodically cleared of their contents from outside, and this constitutes one of the most irksome operations connected with glass manufacture. Owing to the occurrence of foaming and to the fact that the raw materials occupy much more space than the glass formed from them, it is necessary to fill the pot with fresh batches of raw materials several times, the quantity which can be introduced decreasing each time. The number of times that this must be done depends upon the particular circumstances, but from four to eight “fillings” are commonly used for various kinds of glass and size of pot. The precise stage at which a fresh batch of raw materials should be introduced is another matter requiring careful attention. For some purposes it is necessary to wait until the previous batch is completely melted, while in other cases raw material may be added whilst some of the previous batch is still floating on the surface of the glass in the pot.

We have now to consider the chemical reactions which take place in the mixture of raw materials that are introduced into the hot furnace. The exact course of these reactions is not known in very great detail, as this could only be ascertained by an elaborate research on the nature of the intermediate products that result under various circumstances. A research of this kind would throw much light on the whole of the melting processes but is in itself so difficult that it has not yet been carried out at all fully. We can therefore only give an account of the chemical changes from our knowledge of the end-results and of a few intermediate products that are known. To take the simplest case, we may consider a mixture consisting of sand, carbonate of lime and carbonate of soda mixed in suitable proportions. In such a case we know that the mere action of heat alone will produce two changes—the carbonate of soda will melt and the carbonate of lime will lose its carbonic acid and be “burnt” or converted into caustic lime. The first stage of the fusion process thus probably results in a mass consisting of sand grains and grains of carbonate of lime undergoing decomposition, all cemented together by molten carbonate of soda. This mass will be full of bubbles, some derived from the air enclosed between the grains of the original mixture and thus trapped by the melting mass, and others formed by the carbonic acid which is being driven off in the form of gas by the decomposition of the carbonate of lime. At the temperature of the furnace, however, silica has the properties of a strong acid, and not only attacks the carbonate of lime much in the same manner as, for instance, hydrochloric acid would do in the cold, but the silica also attacks the carbonate of soda, which heat alone can scarcely decompose. The exact order in which these reactions take place will depend upon the temperature of the furnace and the degree of mixing attained in the preparation of the raw materials. Although in the long run the final result will probably be the same as regards purely chemical constitution, much of the technical success of the process must depend upon the exact sequence of the changes involved, as this must govern the number and size of the bubbles that are formed in the glass and the fluidity of the mass from which these bubbles have to free themselves. In the present state of our knowledge, however, we can only say that the final result is the complete expulsion of all carbonic acid from the compounds present (although it may remain entangled in the glass in the form of bubbles) and the formation of silicates of both lime and soda which remain in the finished glass in a state partly of mutual chemical combination, partly of mutual solution.

The description of the process of fusion just given applies, with slight modifications, to the melting of ordinary flint-glass mixtures as well as to lime glasses, with the one modification that the carbonate of lime of the lime-soda glass is replaced by red-lead, and the gas evolved by the decomposition of the red-lead is oxygen in place of the carbonic acid evolved from the decomposition of the carbonate of lime. In the case of both lime and flint glasses, however, certain other substances besides those mentioned are usually introduced in small quantities. Although these substances do not very materially affect the end-products of the chemical reactions, they very materially affect the intermediate stages, and thus serve the purpose for which they are introduced by affecting the course of the chemical changes in a favourable manner. The substances usually employed for this purpose are arsenic and nitrate of either soda or potash. The manner in which the arsenic acts is very obscure and cannot be discussed in detail here; the chief factors in its action are, however, its volatility and its power of either absorbing oxygen or parting with it according to circumstances. The action of the nitrates is chiefly dependent upon the oxygen which they yield on decomposition by heat. This oxygen is in some cases stored up by other ingredients of the mixture and only given off at a much later stage, when the evolution of this gas assists in the removal of the last small bubbles of inert air or carbonic acid gas still left in the glass. The oxidising action of the nitrates, however, serves chiefly for the destruction of organic matter and the full oxidation of any iron present; both processes which tend to improve the colour of the glass, while in the case of flint glasses the presence of these oxidising additions is necessary to avoid all risk of reduction of lead, since this would result in the complete blackening of the glass.

A much more complicated set of reactions occur when the alkali of a soda-lime glass is introduced either partly or wholly in the form of sulphate of soda (salt-cake). We have already pointed out that the unaided action of heat and of silica is not sufficient to bring about the rapid decomposition of sulphate of soda which is required for successful glass manufacture, and that the intervention of reducing agents is required. For this purpose a certain amount of carbon in the form of coke, charcoal or anthracite coal, is introduced into all salt-cake mixtures, but the reducing gases of the furnace atmosphere also play an important part in the reactions that take place. Here again it is not possible to give anything but an incomplete account of what takes place. The rationale of the whole process lies, no doubt, in the fact that sulphite of soda (Na₂SO₃) is much more readily decomposed by the action of hot silica than the sulphate (Na₂SO₄) itself, so that the essential action of the reducing agents consists in robbing the sulphate of part of its oxygen, thus reducing it to the condition of sulphite and rendering it accessible to the attack of silicic acid. But if we attempt to express such a reaction in the usual manner by a chemical equation from which the quantity of carbon required to effect the reduction in question can be calculated, we find that the amount of carbon required in practice is very considerably less than that given by this theory; it follows therefore that either this very large amount of reducing action must be ascribed to the furnace gases, or that the actual reactions are not strictly of the kind we have described. Both explanations are probably partly correct, and in practice the amount of carbon to be used in a given mixture and furnace can only be found by actual trial, in which the manufacturer is, of course, guided by the results obtained with other furnaces of a similar type. The end-product of the reactions is again a mixture of silicates, but a certain amount of undecomposed sulphate is always found in such glasses, while gaseous oxides of sulphur escape from these furnaces in considerable quantity. Under exceptional circumstances the glass may even contain sulphides of soda or of lime, and sometimes even suspended carbon, but these are abnormal constituents and result in the serious discolouration of the glass.

It is obvious that to a mixture containing carbon as a reducing agent such oxidising materials as nitrates cannot be added, but small quantities of arsenic and of manganese dioxide are added because their other properties are sufficiently valuable to outweigh their disadvantages as oxidising agents.

Having now briefly considered the process of fusion proper, we pass to the second stage in the melting of glass. In a properly conducted glass-furnace, when the last trace of undecomposed raw materials has disappeared, we find the glass as a transparent mass throughout which gas bubbles are thickly disseminated. For the majority of purposes it is necessary to free the glass as perfectly as possible from these bubbles before it is worked into its final form. This freeing or “fining” process is carried out by further and more intense heating of the molten glass, which is thereby rendered more fluid and allows the bubbles to disengage themselves by rising to the surface. This occurs much more readily when the bubbles are large; very minute bubbles, in fact, show no inclination to rise through the fluid mass. The glass-maker accordingly compounds his mixtures of raw materials in such a way as to yield large bubbles, or, failing that, he adds to the molten mass some substance that evolves a great many large bubbles, and these in their upward course through the glass sweep the small ones away with them. The added substance may be an inorganic volatile body, such as arsenic, or more frequently some vegetable substance containing much moisture is introduced into the glass. The most usual method is to place a potato in the crook of a forked iron rod and then to dip the rod with the attached potato into the molten glass; the heat at once begins to drive off the moisture and to decompose the potato, so that there is a violent ebullition of the whole mass. This “boiling up” process assists the fining considerably and also serves to mix the whole contents of the pot very thoroughly, but it has some attendant disadvantages, such as the introduction of oxide of iron into the glass from the rod which is used in the operation, while the contaminated material adhering to the walls of the pot itself is dragged off and mixed with the rest of the glass by the violent stirring action that takes place. It is, of course, further obvious that this process can only be usefully applied to glass melted in pots, since the bulk of the molten glass in a tank furnace could not be reached at all in this manner. Mixtures that are to be melted in tanks must therefore be capable of freeing themselves of their enclosed bubbles without such outside aid. In a tank, in fact, the whole melting process proceeds on somewhat different lines, since the temperature of the furnace is never intentionally varied, while on the other hand the melting glass travels down the furnace into regions whose temperature can be regulated to favour the various stages of the process that take place in each part of the furnace. On the whole, however, it is an undoubted fact that while the running of a pot furnace can be varied, within wide limits, to suit the requirements of whatever mixture it is desired to melt, in the case of tank furnaces the mixture must be closely adjusted to the requirements of the furnace, whose general “run” cannot be very readily altered.

The completion of the “fining” process is generally determined by taking samples of the glass out of the pot or tank and examining them for enclosed bubbles. Such samples may be obtained in a variety of ways, the most usual method being to dip a flat iron rod just below the surface of the glass and to lift it out vertically upwards, thus retaining on the flat surface of the rod some of the glass that lay there at the moment when the rod was immersed. These test samples or “proofs” are examined very carefully, and if no trace of bubbles can be observed the glass is generally regarded as “fine,” but it is by no means certain that the absence of bubbles from such a small sample will prove that the whole mass is free; that, however, is a point where the melter’s experience enables him to judge how far he may rely upon the indications given by the “proofs.” When the glass is “fine” it frequently happens that the surface of the molten mass is contaminated by specks of foreign matter floating on the glass; for the purpose of removing these, the surface of all glass is skimmed before work is begun upon it. This is done by removing the surface skin of glass by means of suitably shaped iron rods, upon which small masses of molten glass are first “gathered.” Finally, it only remains to reduce the temperature of the glass from that of the melting and fining process to the much lower temperature at which the various methods of working the glass are carried out. In pot furnaces this is accomplished by lowering the temperature of the entire furnace, while in tank furnaces the fine glass flows into the working chamber of the tank which is always kept at the working temperature.

CHAPTER VI.
PROCESSES USED IN THE WORKING OF GLASS.

In the previous chapter we have followed in outline the process of fusion and fining of glass, leaving the molten material ready for working up into the final shape. Up to that point the process is very similar in all kinds of glass, although the furnaces, pots and utensils employed vary considerably, as do also the temperatures to which the materials are heated at various stages. The working processes, however, differ entirely from one class of product to another, as obviously the process employed for the production of a sheet of plate-glass can have little in common with that used in the manufacture of a wine-glass. On the other hand, the modes of working hot glass are not so numerous as the products that are produced, so that we find very similar appliances and manipulation recurring in various branches of the industry. For that reason we propose to deal here with the principal methods of manipulating glass, leaving the details of each method as applied to special purposes to be discussed in connection with the special product in question.

The first stage in the working of all glass is the removal of a suitable quantity of molten glass from the furnace. Practically only three methods are available, viz., ladling, pouring and gathering. If we think of a familiar substance of physical properties somewhat resembling those of glass, we may take thick treacle and suppose it confined in a jar or bottle; there are three obvious ways of extracting it from the bottle: we may ladle it out with a spoon, or we may pour it out by tilting the whole bottle, or we may dip a spoon or fork into the thick liquid, slowly draw it out and turn it round as we do so, thus bringing out on the spoon or fork a round adherent mass or “gathering” of treacle. In the case of molten glass, the process of ladling is by far the simplest, but it has certain very decided limitations and disadvantages. These arise from the fact that a ladle cannot be introduced into molten glass without contaminating the whole mass of glass, at any rate with numerous air bubbles. The metal of the ladle carries with it a considerable amount of closely adherent air which is partially detached while in contact with the hot glass, so that both the contents of the ladle and the glass remaining in the furnace are contaminated. These bubbles might perhaps be avoided if hot ladles were used, but in that case the glass would adhere to the surface of the metal, and each ladle would require laborious cleaning after each time that it was used. In practice, therefore, ladling is only used for the production of those classes of glass where the presence of a certain number of air-bells is not injurious, and the ladles are kept cold by immersion in water after each time of use. The use of the cold ladle has, however, the further disadvantage that a certain quantity of the glass withdrawn in the ladle is very considerably chilled by contact with the cold metal, and is thus too stiff to undergo the further processes satisfactorily—this chilled glass has, therefore, to be rejected from each ladleful; this not only involves loss of glass, but also necessitates the separation of this spoilt glass from the rest.

The general process of rolling requires little treatment here. Two essentially different processes are used; in one the glass is thrown on a flat table and rolled out by a moving roller passing along the table; in the other the glass passes between two moving rollers, and the sheet so formed is received on a moving table or slab. The former mode of rolling is used for the production of the ordinary rolled plate glass; if the surface of both table and roller is smooth, the glass also has a comparatively smooth surface, but the surface is far from being level or free from irregularities. It has been found that it is quite impossible to prevent these irregularities, which appear to arise from the buckling of the glass against the iron surfaces with which it comes into contact; when rolled, the glass is too stiff to recover its true, smooth surface under the influence of surface tension, so that it retains all the marks of roller and table—nor can the roller be made perfectly smooth, since in that case it appears to slip over the glass and does not roll it out properly. All efforts, therefore, to produce a glass having a true and smooth surface by direct rolling have failed, and are likely to fail, so long as tables and rollers are made of materials similar to those now in use. The process of rolling on a stationary table is, however, used for the manufacture of plate-glass; but here the slab as rolled has still the rough, uneven surface similar to that of ordinary “rolled plate,” and this is removed and replaced by a true polished surface by the mechanical processes of grinding and polishing. The second mode of rolling, i.e., with two or more “stationary” rollers and a moving table, is used for the production of rolled plate having special surface features or patterns; the variety of rolled glass known as “figured rolled plate,” having a deeply imprinted pattern, is produced in this way. This method requires much more complicated mechanical appliances, some of which are still protected by patent rights.

Ladling being thus limited to the production of inferior kinds of glass, the better varieties are dependent upon either gathering or pouring. The former process is limited as regards the quantity of glass that can be dealt with in one piece, although surprisingly large quantities can be gathered upon a single pipe; the great masses of glass, however, that are required for the production of modern polished plate could not be handled in this way, and the method of pouring is accordingly adopted. For this purpose either the pots in which the glass has been originally melted, or others specially designed for this purpose, and into which the molten glass has been transferred, are removed bodily from the furnace by the aid of powerful mechanical appliances; they are then carried by overhead cranes to the place where the glass is to be rolled into the form of a plate, and there the pot is tilted and the molten glass is allowed to run out and to form a pool on the rolling table, the passage of the great roller ultimately rolling the pool out into a sheet much as dough is rolled out with a rolling-pin. This process is obviously only possible with pots or crucibles of a suitable size, and is, moreover, very destructive to these pots, since they are exposed to such great variations of temperature. In the case of tank furnaces, numerous devices have been patented for allowing the glass to flow out over a sill or weir of suitable size, ready to be rolled or drawn into the form of sheets or slabs; but none of these devices have, so far as the writer is aware, found their way into practice; the reason for this probably lies in the fact that it is not easy to find a material which will present a smooth face to the outflowing glass, such materials as fire-clay leading to contamination from detached fragments, while chilled metal leads to local chilling of the glass. Unless, therefore, the various processes of drawing glass into sheets direct from the furnace undergo very material improvement, the laborious process of gathering is likely to retain its importance even in the production of such large objects as sheets of window glass.

In its essence the process of gathering consists in introducing into the glass a heated iron rod or tube to which a small quantity of glass is allowed to adhere; rod and glass are removed from the furnace together, and the small adherent ball of glass is allowed to cool so far as to become stiff enough to carry its own weight. The rod with its adherent ball is then again dipped into the glass, where a fresh layer of glass attaches itself to the ball already on the rod. The whole is again withdrawn, allowed to cool down, and then dipped into the molten glass again to gather a fresh quantity. This cycle of operations is repeated until the desired quantity of glass is attached to the rod or tube. These operations, particularly when weights of thirty or forty pounds of glass have to be gathered, require the exercise of a great deal of skill and care; the introduction of the gathering into the molten glass is each time liable to produce air-bells which would spoil the whole mass of glass or would contaminate the contents of the crucible, while subsequently the mass of hot glass adhering to the rod or pipe tends to run down and even to drop off entirely if not properly checked by suitable rotation of the pipe. Further, the manual labour and exposure to heat involved for the operator all tend to increase the cost of such work. Mechanical aids have been invented, and some of these are in actual use, but they are chiefly confined to mechanism for relieving the operator of the great weight of the gathering in its later stages.

Just as ladling is nearly always preliminary to rolling, so gathering is usually the preliminary to some blowing process, although the blowing is often combined with and sometimes replaced by the mechanical pressing of the glass. Where the glass is to be blown, the gathering is always made on a glass-maker’s pipe. This is an iron tube from 4 to 6 ft. long, provided at one end with a wooden casing to serve as a handle, and with a suitably arranged mouthpiece for blowing. The shape of the lower or “butt” end of the pipe depends upon the character and size of the objects to be blown; for small articles the pipe must be narrow and light, but for heavy sheet-glass the butt of the pipe is extended into a conical mass whose base is from 2 to 3 in. in diameter. The bore of the pipe at both ends also depends upon the class of work for which it is intended. The first stage of all blowing processes consists in the formation of a hollow sphere by blowing into the pipe, the pressure of the breath being as a rule sufficient to cause the gradual distension of the hot mass of glass. From this rudimentary hollow sphere the various shapes of blown articles are then evolved by a series of manipulations which vary very widely in different branches of manufacture. They generally consist, however, in gradually changing the shape of the mass of glass by the pressure either of hand tools or of specially prepared moulds or blocks against which the glass is held or turned, either with or without simultaneous blowing into the pipe. The extent to which the aid of such moulds and blocks is invoked varies continuously from the production of the hand-made vase or glass to the moulded bottle; in the former, practically only hand tools, whose shape bears no direct resemblance to that of the finished article, are employed, while in the latter the elongated hollow mass of glass is placed inside a mould, and internal air-pressure is used to press the glass into contact with the mould from which the shape of the finished bottle is thus directly derived.

The art of the blower further takes the fullest advantages of the peculiar physical properties of glass while in the heated viscous condition, the material being made to flow under the action of gravity and centrifugal forces, as well as under the pressure of the breath, the glass being held aloft, twirled or swung about to ensure the production of the various shapes required. For the great majority of such purposes the unaided manipulations of the operator are sufficient, but various mechanical aids are used to facilitate the more laborious stages of the work, while for the simpler forms that are required in very great numbers, such as bottles, the whole of the operations are now carried out by automatic machines. Of the more usual mechanical aids at the disposal of the glass-blower, we have already mentioned hand-tools, blocks, and moulds of various kinds. Next in importance to these is the use of compressed air for blowing large or heavy articles; the pressure available by the human breath is very limited, and the volume of air that can be thus delivered is not very large, while the constant use of the lungs for such a purpose is trying for the workman. In many works, therefore, air under pressure is supplied to the benches or stages where the blowing is done, and the blowers’ pipes can be coupled to this air-supply by means of flexible connections when required. The principal difficulty lies in the correct regulation of the air-pressure for each special purpose; but this difficulty has been overcome by the use of delicate valves under the control of each blower, who can thus regulate the pressure to his own exact requirements. Such a system, of course, requires some little practice on the part of the men using it, but when they have become accustomed to the working of the plant the results achieved are decidedly better and more regular than those obtained by mouth blowing. Besides the use of compressed air supplied in the way just indicated, several other devices are in use to aid the blower in producing the requisite pressure in the interior of the hollow bodies he is producing. The simplest of all these consists in utilising the expansive force of the air enclosed in the hollow body when that body is exposed to heat. Thus, for instance, in blowing a cylinder of sheet-glass, if the blower holds his thumb over the aperture of his pipe, and brings the closed end of the cylinder near the hot “blowing hole,” the heat which softens that end of the glass will also act upon the enclosed air, and will very rapidly produce such an expansive effect as to burst open the softened end of the cylinder, and this means of opening the closed ends of the cylinder is frequently employed in practice. It is, of course, obvious that any other expansive fluid might be employed in a similar manner, and in some blowing processes it has long been the practice to introduce a small quantity of water into the interior of the hollow body, when the rapid expansion of the steam produced thereby is utilised for the purpose of generating the requisite internal pressure. This use of the expansive force of steam generated by the heat of the hot glass body has received great development at the hands of Sievert in Germany, whose process is described in Chapter VII.

Whatever mechanical aids are employed to facilitate the various stages of the process, all glass blowing involves a series of operations requiring considerable skill, while the whole manner of dealing with the glass is essentially extravagant of material, except perhaps in the production of bottles or flasks having narrow mouths. The reason for this latter statement lies in the fact that by blowing it is only possible to produce closed or nearly closed hollow bodies or vessels; thus a blown wine-glass or tumbler is formed with a hood or dome closing in the open top of the glass, and this hood or dome has subsequently to be removed by subsidiary processes, such as cutting off by the aid of strong local heat or by grinding, and the cut edge has to be provided with a smooth finish. In the case of comparatively small articles like glasses the loss involved from this cause is not so very great, but were large flat bowls or dishes to be produced by blowing, the loss in the dome or covering would be very serious. This difficulty is entirely avoided by the process of pressing glass. We have already indicated the manner in which moulds are used for the production of the desired shape in the case of bottles, etc., but in these cases, where the final object is to be a hollow vessel, the glass is readily forced into contact with the mould by means of internal air—or steam—pressure; in the process to which we are now referring, however, the hot glass is forced into contact with the external mould by means of an internal plunger which is forced downward with considerable force. By this means, flat or shallow bodies can be produced without the preliminary formation of a completely closed vessel, while it is obvious that by the use of suitable moulds, complicated and elaborate shapes can be produced. It is true, of course, that pressed articles do not show the same smooth and brilliant surface which is characteristic of the fire-polish of blown articles, while the facility with which elaborate surface ornamentation can be applied by this process has not tended to artistic refinement in design, but the great majority of cheap and useful glass articles of domestic use have been made available by the development of the pressing industry.

In the ordinary course, pressed glass is produced direct from the molten material, which is introduced into the presses either by gathering or by means of ladles, but for some special purposes glass is brought into its final shape by mechanical pressure after having first been allowed to solidify and having then been specially re-heated to undergo the pressing or moulding process. This is principally done in the case of the best kinds of optical glass, where the molten glass is first allowed to cool in the actual crucible and is then broken up into lumps of a suitable size, from which the more defective portions can be rejected, the more perfect portions only being heated up again in special kilns and then forced to take the desired shape by being pressed—sometimes with hand tools only and sometimes by the aid of powerful presses—into moulds of the required shape. Small lenses, however, for which the requirements of quality are not so high are sometimes pressed direct from small gatherings taken from the molten glass in the crucible.