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If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: Glass Manufacture - -Author: Walter Rosenhain - -Release Date: August 5, 2016 [EBook #52724] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK GLASS MANUFACTURE *** - - - - -Produced by Chris Curnow, Charlie Howard, and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - - - - -Transcriber’s Note - - -In the Plain Text version of this eBook, superscripts are indicated by -^{}, subscripts by _{}, italics by _paired underscores_, and boldface -by paired =equals signs=. Underlining in the advertisement at the end -of the book is indicated by _paired underscores_, not distinguished -from italics. - - - - -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 - - [Illustration] - - 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). - ” ⅓ 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 Pb_{2}O_{3}) in approximately such -proportions as to correspond to the formula Pb_{3}O_{4}. 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 Al_{2}O_{3}) 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 (MnO_{2}), although the -lower oxide (Mn_{3}O_{4}) 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, As_{2}O_{3}) -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. - -[Illustration: FIG. 1.--Open “pot” or crucible for glass melting.] - -[Illustration: 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 (SiO_{2}). 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 (CH_{4}), 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. - -[Illustration: 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. - -[Illustration: 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. - -[Illustration: 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. - -[Illustration: FIG. 6.--Longitudinal sectional diagram of tank -furnace.] - -[Illustration: 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 (Na_{2}SO_{3}) is much more -readily decomposed by the action of hot silica than the sulphate -(Na_{2}SO_{4}) 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. - - - - -CHAPTER VII. - -BOTTLE GLASS. - - -Although bottles are in some respects the cheapest and crudest products -that are manufactured of glass, their uses are so innumerable and their -numbers so enormous that their production constitutes a most important -branch of the industry. - -In the choice of raw materials for the production of ordinary bottles -cheapness is necessarily the first consideration. Natural minerals, -bye-products of other industries, and the crudest chemicals are -utilised so long as it is possible by compounding these ingredients -in suitable proportions to obtain a glass whose composition meets -the somewhat crude requirements which bottles are expected to meet. -The most essential of these requirements are that the bottles shall -be strong enough to resist the internal pressure which may come upon -them when used for the storage of fermented or effervescent liquors as -well as the shock of ordinary use, while the glass itself must possess -sufficient chemical resistance to remain unattacked by the more or less -corrosive liquids which it is called upon to contain. Further, from -the point of view of the bottle manufacturer it is desirable that the -glass shall be readily fusible, easily worked, and easily annealed. -In other branches of glass manufacture increased fusibility is often -attained by increasing the alkali contents of the glass, but in bottle -making this is inadmissible, both on account of the prohibitive cost -of alkali and because an increased alkali content renders the glass -more liable to chemical attack. On the other hand, in many varieties -of bottle the _colour_ of the glass is nearly, or quite, immaterial -so that the introduction of relatively large proportions of iron -oxide is permissible. This substance acts as a flux and assists in -the production of a fusible, workable glass containing little alkali. -Such alkali as bottle glass does contain is frequently derived from -felspathic minerals, which generally also contain considerable -proportions of iron. The use of these minerals also introduces -notable proportions of alumina into the glass. In certain classes -of bottles, notably those used for special wines, certain shades of -colour are required--the well-known “Hock bottle” colour being an -example. The presence of iron in the glass tends to the production of -a green or greenish-yellow colour deepening to a black opacity if the -quantity of iron be high. The lighter shades of this green tint may -be “neutralised” by the introduction of manganese into the glass, the -resulting colours ranging from light amber to purple; nickel oxide is -also sometimes used as a colouring material in these glasses. - -In the production of ordinary bottles the continuous tank furnace has -now entirely superseded the old pot furnaces, the character of the -product being in this case particularly suited to this process of -production. The modern bottle-glass tank is generally an oblong basin -having one semi-circular end. The flame is often of the “horse-shoe” -type, the gases both entering and leaving the furnace at the flat -or charging end of the furnace. The raw materials are thrown into -the furnace at the square end of the tank, and the glass flows -uninterruptedly down the furnace to the colder semi-circular end where -the working holes are situated. At these points fire-clay rings are -kept floating on the glass, and from within these the gatherer takes -his gathering, the rings serving to retain the grosser impurities -carried down by the glass. The producing power of such a furnace, even -when the bottles are blown by hand, is very considerable; a furnace -having ten working holes and containing normally about 85 tons of -molten glass will yield some four million bottles per annum, and -furnaces of considerably larger capacity are in use. - -The methods of bottle making are at the present time passing through -what is probably a stage of transition. Up to the middle of last -century the processes in use were little better than those of the -middle ages; the first step of a more modern development of the -industry took the direction of improved tools and implements for -carrying out the old operations. More recently a whole series of -inventions have been put forward with the aim of producing bottles by -entirely different and wholly mechanical processes with the object of -eliminating the uncertain element of skilled labour entirely. While it -must be admitted that some of the earlier of these inventions proved -to be brilliantly ingenious failures, there is little doubt that -here, as in other manufacturing processes, the machine-made article -will ultimately supersede the hand-made product. Even now, mechanical -processes are largely in use both in America and Europe, and at some -recent exhibitions machine-made bottles have been shown which in every -point of quality were superior to the best hand-made goods. - -The first stage in the production of bottles by hand, and also for most -of the machine processes, is that of gathering the requisite quantity -of glass. The bottle-blower’s pipe is between 5 and 6 ft. long, and is -provided with a slightly enlarged end or “nose” upon which the glass is -gathered. Three gatherings are generally sufficient for the production -of ordinary bottles, but for extra large bottles, and especially for -carboys, heavier gatherings are necessary, and for these the gatherer -must go to the furnace four, five, or even six times. When the -requisite quantity of glass has been gathered on the pipe the gathering -is worked and rounded by rolling it either on a flat metal plate or -“marver,” or in a hollowed block made of wood or more rarely of metal; -by this process the glass is formed into a well-rounded, symmetrical -pear-shaped body. The blower now distends the mass gradually by the -pressure of his breath, at the same time swinging the pipe, the effect -of these movements being to draw the bulk of the glass downwards, -leaving a thinner and colder portion having the rudimentary shape of -the neck of the bottle next to the pipe. In the oldest form of the -process the next stage in the production of the bottle is accomplished -by the aid of a cylindrical mould of fire-clay, whose diameter is that -of the external size of the finished bottle. The pear-shaped bulb of -glass is for this purpose re-heated at the melting furnace, and is -then placed inside the fire-clay mould. By vigorous blowing, and a -rapid rotation of the pipe and glass, the bulb is forced to assume -the cylindrical shape of the mould, the glass forming the neck of the -bottle being at this stage of the process too cold and stiff to be -further deformed. The next step is the formation of the concavity found -in the base of wine and beer bottles; this is produced by pushing up -the hot plastic glass that forms the bottom of the bottle as it leaves -the clay mould. This is done by a second workman using an iron rod -known as the “pontil,” upon which a small mass of glass has previously -been gathered. This mass of glass remains attached to the bottom of -the bottle, which is thus for the moment fastened both to the “pontil” -and to the blower’s pipe. The blower, however, immediately detaches -the bottle from the pipe at the point where the neck of the bottle -is intended to end, effecting this by locally chilling the glass--a -process known by the descriptive term of “wetting off.” The unfinished -bottle is now attached to and handled by means of the “pontil.” The -neck is softened by re-heating it over the furnace, and is then moulded -into the desired shape by the aid of specially-shaped tongs. Finally -a thread of glass is wound round the end of the neck to produce the -thickening usually found at that point. The finished bottle, still -attached to the “pontil,” is now carried to the annealing kiln, where -it is placed in position and detached from the “pontil” by a sharp -blow, which severs the glass that had been gathered on the “pontil” -from the bottom of the bottle. - -The process, in the form described above, has been obsolete for -many years, improvements, consisting of appliances for facilitating -the various operations, having been gradually introduced. The most -important of these is the substitution of metal moulds for the -fire-clay moulds of earlier times. These metallic moulds are made to -open and close at will by the action of a pedal, and are designed to -give the entire bottle its final shape, except for the indentation -of the bottom, although this is sometimes produced by a convex -piece placed on the bottom of the mould. In the formation of the -neck thickening, also, important mechanical aids have become almost -universal. These last consist of tongs provided with rollers and -arranged to rotate about an axis that terminates in a tapered spike -which enters the neck of the bottle; by pressing the tongs together so -as to bring the rollers against the outside of the neck and rotating -the whole, the rollers are made to form the neck thickening in an -accurate and rapid manner. - -Important and valuable as these improvements of the ancient process -of bottle-blowing undoubtedly are, they do not touch the main -disadvantages of the process--disadvantages that seriously affect the -economy of the process and the well-being of the workers employed upon -it. It is consequently not surprising that a great number of inventors -have laboured at the problem of the purely mechanical production of -bottles. A large number of patents have accordingly been taken out in -connection with bottle-making machinery. The first of these to attain -any favour was that devised by Ashley, but although great claims were -made for it, its use has not extended. At the present time, however, -there are a number of bottle-works actually at work producing bottles -by mechanical means; one of the most successful of these machines is -that devised by Boucher, of Cognac. The products of this machine, -exhibited in Paris at the exhibition of 1900, were equal, and possibly -superior, to the best hand-made bottles. The Boucher machine, although -by no means entirely automatic, requires no highly-skilled labour -beyond that of a workman whose duty it is to operate the various -levers of the machine at the right instant and in the proper order. - -The details of the machine, as set forth in the patents and other -published descriptions, are somewhat complicated, and vary somewhat -in the different models; the general principle and mode of operation -is, however, the same in all varieties of the machine, and we shall -therefore give a brief account of it here. - -In the Boucher process, the glass is first gathered from the furnace, -but as no blowing-pipes are required, the gathering is done on a light -iron rod, thus saving the gatherer much of the labour of carrying -the heavy pipes. The requisite quantity of the glass so gathered is -then dropped into the first or “measuring” mould of the machine, the -“thread” being cut by hand by the operator. From the measuring mould, -the glass is next caused to pass into the “neck” mould; the glass -flows into this mould, and is further pressed into it by the aid of -compressed air, applied above the free surface of the glass. At this -stage the still liquid glass has the external shape of the neck of -the bottle, but the mass of glass is solid, _i.e._, no cavity has yet -been produced in it. The formation of the cavity is next begun by the -action of a plunger which is driven into the “solid” mass of glass -filling the neck mould, this plunger thus punching out the passage -through the neck of the bottle. As soon as the plunger is withdrawn, -compressed air is admitted into the cavity so formed, and the mass -of glass is at the same time inverted, and that part occupying the -position of what is to be the shoulder of the bottle is allowed to -descend while being blown out by the compressed air. This process of -distension is limited, and the desired shape is imparted to the mass -by bringing towards it a third mould, by contact with which the glass -is considerably stiffened--a row of jets of compressed air, impinging -on the outside of the glass forming the shoulder of the bottle, being -further used to stiffen the glass, once the requisite extension has -been attained. The mass has now a shape very similar to that known -as a “parason” in hand bottle-blowing, and is by this time decidedly -stiff. It is now introduced into the finishing mould and is blown into -perfect contact with the mould by powerful air-pressure, thus attaining -the proper shape of barrel and base; the indentation of the base is, -however, sometimes produced on a separate machine or press. During -all these operations the neck of the bottle, which was the first part -to be formed, has remained firmly held in the neck mould, and all the -movements that have been described are performed by means of levers -actuating movements of this mould as a whole, which, of course, carry -the glass with them. The last movement of the levers, which releases -the bottle from the finishing mould, also opens the neck mould, and -thus leaves the bottle finished and entirely free. - -It will be seen that the process adopted in this machine follows as -closely as possible the various stages of hand blowing, but that the -mechanical movements of the machine replace the laborious and difficult -technique of the blower. One such machine is capable of producing -as many as 120 bottles, each weighing 1¾ lbs., per hour, but this -is accomplished only by having some of the moulds in duplicate and -so arranged as to come into use alternately. The machine itself is -attended by one “moulder,” who operates the levers, and by a youth, -who carries the finished bottles to the annealing kiln, while, of -course, the services of a gatherer are also required. The appearance of -a bottle works equipped with these machines is in striking contrast to -that of a hand-blowing works, where the stages around the working-holes -are crowded with men doing arduous work under very severe conditions of -temperature and atmosphere. Finally, it must be pointed out that the -use of the Boucher machine is by no means confined to the production of -the cheapest kinds of bottles, but that it has shown itself especially -well suited to the production of champagne and other bottles that -are required to withstand a high internal pressure, the machine-made -bottles showing excellent results under pressure tests. The machine -is also used for the production of moulded glass-ware of white glass, -since it can be adapted to the production of any kind of glass vessel -that can be produced by blowing into a mould. - -The annealing of bottles was formerly carried out in large chambers or -kilns of very simple construction, in which the bottles were stacked as -made, the kiln being previously heated to the requisite temperature: -when full, the kiln was closed up in a rough temporary manner and -allowed to cool naturally, thus annealing the bottles stacked within -it. In this branch of glass-making also, however, the continuous -annealing kiln has superseded the older kinds, and continuous kilns are -now almost universal in bottle-making. In these kilns, which consist -of long tunnels, kept hot at one end and having a gradually decreasing -temperature as the other end is approached, the bottles are stacked on -trucks which are slowly drawn through the kiln from the hot to the cold -end. At the cold end the trucks are unloaded and are then returned, by -an outside route, to the charging end, but of course the bottles cannot -be stacked on the truck until it has actually entered the hot end of -the tunnel and acquired the temperature there prevailing. In a slightly -different form of kiln, the bottles are carried down the kiln on a -species of conveyer belt formed of iron plates, but the principle of -all these appliances is similar even when used for very different kinds -of glass. - -In the account of bottle manufacture given above we have referred -almost exclusively to the mode of production of the ordinary bottles -used for the storage of such liquids as wine, beer, spirits, etc., and -we will now deal with some other branches of manufacture closely allied -to these. - -An important branch of glass manufacture is the production of vessels -of large dimensions. Those most closely allied to ordinary bottles are -the vessels known as carboys, used for the storage and transportation -in bulk of chemical liquids, and especially of acids. Formerly these -were blown by hand in a manner closely resembling that used for -ordinary bottles, but the weight of the mass of glass to be handled by -gatherer and blower is very great, while the lung-power of a blower is -not sufficient to produce the great expansion required. Formerly the -only aid available to the blower was the device of injecting into the -hot, hollow glass body, at an early stage of the process, a quantity -of water or alcohol; this liquid was immediately vapourised by the -heat of the glass, and if the blower closed the mouthpiece end of his -pipe by placing his thumb over it, the expansive force of the vapour -so generated served to blow out the glass to the desired extent. More -recently mechanical aids to the production of these large vessels -have become available, first in the shape of mechanical arrangements -for relieving the workmen of the full weight of the glass and pipe -by providing suitable arms upon which the whole can be supported -without interfering with the blower’s freedom of manipulating the pipe -and glass in the desired way; further, a supply of compressed air, -which can be readily connected with the pipe at any desired moment, -facilitates the blowing process. - -A process of producing hollow glass vessels of very large size by -purely mechanical means has, however, been introduced during recent -years by P. Sievert, of Dresden. By the methods of this inventor, -glass vessels of quite unprecedented size--such as bath-tubs freely -accommodating full-grown men--can be produced. For this purpose the -glass is spread out on the surface of a large cast-iron plate, provided -with numerous small holes through which steam or compressed air may be -blown when desired. The slab of viscous glass, when properly spread -over this plate, is clamped down against it all around the outside -edge by means of a suitably-shaped iron collar, which holds the glass -in air-tight contact against the plate beneath. The whole iron plate, -with the slab of glass clamped to it, is now turned over, so that the -glass hangs down under the plate. The glass immediately begins to sag -under its own weight, and is assisted in this tendency by a suitable -blowing of steam or air into the space between the plate and the glass. -In blowing bath-tubs in this way the glass is allowed to distend -downwards until the desired depth is attained, when further distension -is arrested by bringing a flat supporting plate under the glass, which -is pressed against this flat plate by the pressure of the air, thus -forming the flat bottom of the tub. In this process the outline of the -object is determined by the shape of the clamping bars or plate that -fix the edges of the hot glass against the iron plate described above, -and by this means almost any desired shape can be given to objects of -simple form. - -It is obvious that this process can also be employed for blowing a -hollow body into contact with a mould of any desired form and forcing -the hot glass to take the exact shape of the mould; for smaller -bodies, however, the blowing in of separately generated steam is not -required, the heat of the molten glass itself being used to generate -the necessary steam. For this purpose the requisite quantity of glass -is dropped on the surface of a wet slab of asbestos. On this surface -the glass remains floating upon a layer of steam, which is constantly -renewed by the intense heating action of the hot glass on the water -contained in the asbestos below. The moulds used in this process are -provided with a sharp edge or lip, and as soon as the glass has spread -into a slab of sufficient size, the inverted mould is brought down -upon the glass and pressed against it. The sharp lip or edge of the -mould forces the glass into close contact with the asbestos under -it all around the edge of the mould, thereby enclosing the space -existing between the rest of the glass and the wet asbestos. The heat -of the glass continues to generate steam at a rapid rate, but now the -steam can no longer escape from under the glass around the edges, and -therefore blows the glass upwards into the mould, ultimately forcing -the glass into intimate contact with the surface of the mould; when -this is accomplished, the pressure of the steam rises rapidly, and -ultimately lifts the entire mould and glass sufficiently to allow -the excess steam to escape--and this is the sign that the blowing is -complete. The whole process takes only a very few seconds, and is very -successful when applied to suitable glass and used with moulds of -proper shape. It is, of course, obvious that ordinary narrow-mouthed -bottles could not be produced in this way, but wide-mouthed bottles and -jars are made in this manner, although the chief utility of the process -lies in the production of comparatively shallow articles, which are not -of a shape that lends itself to pressing. - - - - -CHAPTER VIII. - -BLOWN AND PRESSED GLASS. - - -In many ways very similar to the processes employed in the production -of bottles are those used in the manufacture of all hollow glass -vessels that are produced by blowing, either with or without the aid -of moulds. Apart from the actual shapes of the articles themselves, -however, the principal difference between bottles and the better -classes of hollow glass-ware lies in the composition and quality of -the glass itself. In this respect all grades of manufacture are to -be met with, from the light-coloured greenish or bluish glass used -for medicine bottles to the most perfectly colourless and brilliant -“crystal” or flint glass. This gradation in the perfection of the -glass represents a corresponding gradation in the care bestowed upon -the choice of raw materials and the various manipulations of melting -the glass. As we have seen, for the commonest kinds of bottles, where -colour and quality are immaterial, all kinds of fusible materials -can be utilised, loamy or ferruginous sands and refuse glass of all -kinds being employed. Where somewhat higher requirements have to be -met, rather purer sands have to be used as sources of silica, while -lime and alkali must be introduced in purer forms, the alkali in the -shape of the cheapest qualities of salt-cake and the lime in that -of lime-stones reasonably free from iron and magnesia. Finally, for -the best qualities of glass the purest sand obtainable is used, being -often specially washed to remove all loamy matter, while the alkali -is introduced in the form of carbonate, a chemical product which in -its better qualities is practically free from injurious impurities. -In these high-class products two very distinct kinds of glass are -met with. One class, of which the Bohemian “crystal” is the highest -example, is chemically of the nature of an alkali-lime silicate, the -alkali in the case of the Bohemian glass being potash; the other -variety of glass contains no lime, its place being taken by lead, -typical of this class being English flint glass. In some varieties of -glass, lead is also replaced, partially or entirely, by barium, but -this material is chiefly used for the manufacture of pressed glass. - -The higher grades of quality in glass, which thus require increased -refinement in the raw materials, also demand increased refinement -in the furnaces and appliances employed in their melting. The -tank-furnace, which holds the field in bottle manufacture, is -scarcely met with in the production of the better qualities of hollow -glass-ware; medicine bottles and other articles of moderate quality -might be produced in tanks, but the quantity of glass required for -such purposes is seldom large enough to justify such large plant. For -the best qualities of colourless glass-ware, however, the tank-furnace -could not be used on account of the fact that both as regards colour -and freedom from defects, the product of a tank-furnace is never equal -to the best product of pot-furnaces. For flint-glass, indeed, _covered_ -pots or crucibles must be used in order to adequately protect the -molten glass from the reducing action of the furnace gases and from -contamination by dust. The materials of which the pots are constructed -are also chosen with a view to avoiding all risk of introducing -colouring or otherwise injurious impurities from that source. - -In all processes for the production of hollow glass-ware, the glass or -“metal” is taken from the pot by the process of gathering which has -already been described; where blown articles are to be produced, as -distinct from pressed goods, the initial stage is always the formation -of a small hollow globe or bulb at the end of the glass-blower’s pipe. -The subsequent manipulations depend upon the nature of the article to -be produced. The article may either be made entirely by hand work, or -rather “chair” work, as it is usually called, or the manipulations -may be facilitated and the product cheapened--while its character is, -of course, also modified--by the aid of moulds, which are used to -bring the object to its proper shape and to impress upon it certain -decorative mouldings or markings. As we have already seen, ordinary -bottles are now always blown with the aid of moulds, and the same -applies to medicine bottles, lamp chimneys, and the bulbs for electric -light; in connection with lamp-chimneys it should be noted that they -are blown in moulds in the form of cylindrical bottles with a flat -bottom and a domed top, the ends being subsequently cut off. - -Many of the cheaper varieties of tumblers and glasses are also blown in -moulds, but they can be, and sometimes are, produced by hand, and as -their manufacture is typical of that of all hand-blown hollow ware, we -shall now describe it in some detail as an example of this class of -work. - -The implements used by the glass-blower and his assistants for this -work are few and simple. The largest item is the glass-blower’s bench -or chair, which is simply a rough wooden bench provided with two -projecting side-rails or arms. When finishing a piece of work the -blower sits on this bench, and the pipe lies across the two rails -in front of him in such a position that by rolling it backwards and -forwards along the rails he can readily keep the pipe in gentle -rotation. In addition to the ordinary blower’s pipe and a “pontil” -or rod for attaching small quantities of glass whereby the piece in -hand can be held, the only other tools used by the blower are a number -of shears and pincers of various shapes which serve for cutting off, -pressing in, and distending the glass as required, a flat board and -a stone or metal plat or “marver” being also used for the purpose of -moulding the glass. - -As already indicated, the first step in the production of such an -object as a tumbler consists in gathering a suitable quantity of glass -on the pipe and blowing it into a small bulb. This bulb is blown out -to the proper size and is then elongated by gently swinging the pipe. -The next step is the flattening of the lower end of the bulb by gently -pressing it on the “marver” or flat plate provided for such purposes; -in this way the flat bottom of the glass is formed, and the bulb now -has the shape of the finished glass, but remains attached to the pipe -by a shoulder and neck. The earliest practice was to separate the -tumbler from the pipe at such a point as to leave the tumbler of the -correct length, the remaining operation consisting in holding the -glass, first fixed to a pontil for the purpose, into the furnace so -as to heat the broken edge; this edge was thereby rounded off, and the -brim of the glass could be widened or otherwise shaped by rotating -the glass or pressing it in or out by the aid of pieces of wood. In -modern practice, however, this is not usual, the glass being separated -from the pipe well above the shoulder and annealed in this shape. -Subsequently the glass is finished in a trimming room or workshop by -being cut off at the desired point and having the rough edge rounded -off by the aid of a blowpipe flame. The cutting-off operation is -carried out in a great variety of ways, the most usual being by the -action of heat applied locally and suddenly, either by the aid of -specially-shaped flat blowpipe flames or by an electrically-heated -wire. Machines for carrying out this operation, as well as the -subsequent rounding of the edge automatically, are in use, but the -latter process is sometimes replaced by slightly grinding and polishing -the edges. - -[Illustration: FIG. 8.--Sectional diagram of the evolution of a -tumbler.] - -The evolution of an ordinary tumbler, as just described, and as -illustrated diagrammatically in Fig. 8, is typical of the whole process -of hollow-glass blowing, but of course the number of operations, as -well as the care and skill involved in each step, increases rapidly as -the form of the vessel becomes more complex; in the highest class of -work a very considerable element of artistic taste and judgment on the -part of the operative also becomes essential, for, although the form -of the object as well as the choice of colour and ornamentation are -chosen by the designer, the blower has to translate the drawing of the -designer into glass, and although his skill enables him to attain a -considerable degree of fidelity in his rendering, many details remain -at his own option, and the proper management of these is no small -factor in the success of the whole work. - -In this connection mention should perhaps be made of the application of -colour and other decorations to this kind of glass. A very considerable -range of effects of this kind is now available to the glass-worker. -In the first place the body of the glass used for the production of -the articles in question may be coloured by the addition of suitable -colouring materials to the molten glass or raw materials, as explained -in Chapter XI., but this procedure has very obvious limitations; where -the article is built up of glass from several gatherings--as, for -example, is the case in an ordinary wine-glass, where the bowl, leg and -foot are each made of separate gatherings--it is possible to use glass -of different colours for these different parts, and this is commonly -done in the production of wine glasses having ruby or green bowls and -white legs and feet. A further modification in the application of -colour is obtainable by taking up two or more gatherings on the same -pipe and superposing a large gathering of white glass on a smaller -one of coloured glass; this is analogous to the process of “flashing” -sheet glass, described in Chapter X. and this process lends itself to a -variety of manipulations resulting in the distribution of the coloured -layer of glass in almost any desired manner over the object in hand. -The principal objection to this process, however, lies in the fact that -pots of molten glass of all the colours desired must be kept available -to the blower at the same time, and this is not easily arranged for in -any reasonably economical manner. For this reason, and also because the -manipulations are simpler, coloured glass intended for application to -blown glass-ware is generally used in the form of short rods previously -prepared; these rods are suitably heated, and the coloured glass can -then be applied to the article in hand at any desired place and in -as small or large a quantity as required. If the two glasses thus -brought into contact are properly related to one another as regards -chemical composition and physical properties, they blend very readily -and perfectly, and the result is quite as good as could be obtained by -using the coloured glass in the molten condition. Other decorations, -such as gilding or other metallic lustres and also various kinds of -iridescence, are produced upon the finished glass. Metallic lustres -are obtained by placing upon the surface of the glass, and slightly -fusing into it a layer of particles of the actual metal. In some cases -this is done by rolling the glass vessel, while still hot, in a mass of -metallic foil of the kind desired, when a sufficient quantity readily -adheres; in other cases the metal is applied in the form of a flux or -glaze containing a large proportion of an easily-reduced compound of -the metal, and this is afterwards reduced to the metallic state by the -action of heat, sometimes aided by that of smoke or other reducing -gases. An iridescent surface is produced upon certain varieties of -glass by the corrosive action of acid vapours; in fact, in localities -where the atmosphere is tainted with sulphur fumes it is quite usual -to see an iridescent lustre on the surface of ordinary window glass. -There are, of course, numerous other means of decorating blown and -other glass, such as cutting, engraving, etching, silvering, etc., but -it would lie beyond the scope of the present volume to deal with these, -since they are outside the field of actual glass manufacture. - -In the production of hollow glass-ware by hand, the glass-blower avails -himself to the full of the property so characteristic of glass of -assuming a pasty or viscous condition when suitably heated; by raising -or lowering the temperature of his material, the blower can at will -render it stiffer or more fluid; by blowing he can distend it, draw it -out by the aid of gravity or centrifugal action, or he can mould it -with the aid of rods and tongs of suitable shape, while at times he -allows it to fall or festoon under its own weight while held aloft. -With all these manipulations at his disposal, the skilful operative -is able to work the glass to his will and to fashion objects of great -variety and beauty, but it should be noted that objects produced by -hand in this way will bear the mark of the processes employed in their -production in the fact that they do not possess the extreme regularity -of size and shape which are associated with machine-made articles; -there is a certain natural variability in the exact shape of curves and -festoons that is foreign to the products of mechanical processes. For -some purposes this variability is a disadvantage, while to some minds -it appears as a defect, and methods have been devised for facilitating -the production of strictly uniform glass-ware by the use of moulds as -an aid to the work of the glass-blower. While undoubtedly reducing the -value and beauty of the ware from the purely artistic standpoint, these -aids to hand-work have rendered possible an immense expansion of the -entire industry, since, with the use of moulds, presentable glass-ware -can be produced by hands far less skilled than those required for pure -hand-work. - -In the description given above of bottle-blowing by hand we have -already seen an example of the use of moulds in aiding the blower to -form his object to the desired size and shape. Much more complicated -and decorative objects can, however, be produced by the use of moulds. -Such objects as globes and shades for gas, oil and electric lamps, -when of a light substance and suitable shape, are usually produced by -blowing bulbs of glass into moulds, where they acquire the general -shape as well as the detailed decorated surface configuration which -they afterwards present. Here again the body remains a closed vessel, -and is only opened and trimmed to the final shape at the end of the -operation when all the blowing and moulding have been done. Articles -blown in this way very frequently show “mould marks,” since the -contact of the hot glass with the relatively cold surface of the mould -results in a certain crinkling or roughening of the glass, much as in -the process of rolling. This effect can be minimised by dressing the -interior surfaces of the moulds with suitable greasy dressings, whose -chief property should be that they do not stick to the hot glass and -leave little or no residue when gradually burnt away in the mould; -the proper care of the moulds and their maintenance is in fact the -first essential to successful manufacture in this as well as in the -pressed-glass industry. Even under the most favourable conditions, -however, the surface of glass blown into moulds is not so good as -that of hand-blown articles which have never come into contact with -cold materials, and therefore retain undiminished the natural “fire -polish” which glass possesses when allowed to cool freely from the -molten state. An effort at producing a similar brilliance of surface -on moulded and pressed articles is often made by exposing them, after -they have attained their final form, to the heat of a furnace to such -an extent as to soften the surfaces and allow the glass to re-solidify -under the undisturbed influence of surface-tension much as it would do -in solidifying freely in the first place. Unfortunately this process -cannot be carried out without more or less softening the entire -article, so that skilful manipulation is required to prevent serious -deformation of the object, while a certain amount of rounding off in -all sharp corners and angles cannot be avoided. - -The air-pressure required to bring the whole of the surfaces of a large -and possibly complicated piece of glass into contact with the surfaces -of the mould is sometimes very considerable, and the lung-power of the -blower is often insufficient for the purpose; in many works, therefore, -compressed air is supplied for the purpose, arrangements being employed -whereby the operative can quickly connect the mouthpiece of his pipe -with the air-main, while he can accurately control the pressure by -means of a suitable valve. The Sievert process of moulding by the aid -of steam pressure has already been described. - -Although the evolution of the industry scarcely followed this path, -it is not a large step to pass from a process in which air-pressure -is used to drive viscous glass into contact with a mould to a process -in which the pressure of the air is replaced by the pressure of a -suitably-shaped solid plunger, and this is essentially the widely-used -process of glass pressing. In the first instance this mode of -manufacture is obviously applicable to solid or flat and shallow -articles which could not be conveniently evolved from the spherical -bulb which stands as embryo of all blown glass; at first sight it -would seem in fact as though the process must be limited to articles -of such a shape that a plunger can readily enter and leave the concave -portions. By the ingenious device, however, of pressing two halves of -a closed or nearly closed vessel simultaneously in two adjacent moulds -and then pressing the two halves together while still hot enough to -unite, it has been made possible to produce by the press alone such -objects as water-jugs, for example, into which a plunger could not -possibly be introduced when finished. The process of pressing being -a purely mechanical one and requiring no very elaborate plant and -little skilled labour, has placed upon the market a host of cheap and -extremely useful articles, thus serving to widen very considerably -the useful applications of glass. On the other hand, the process has -been and is still used to some extent for the production of articles -intended to imitate the products of other processes such as hand-blown -and cut glass, with the result that a great deal of glass has been -produced which cannot possibly be classed as beautiful and much of -which can lay as little claim to utility. - -The essential feature of the process of glass pressing consists, as -already indicated, in forcing a layer of glass into contact with a -mould by the pressure of a mechanically actuated plunger. For this -purpose a suitable mould and plunger as well as a press for holding the -former and actuating the latter are required. The moulds are generally -made of a special quality of close-grained cast-iron, and they are -kept trimmed and dressed in much the same manner as the moulds used -for blowing (except that the latter are sometimes made of wood). For -the purpose of facilitating the removal of the finished article, the -moulds are generally made in several pieces which fit into one another -and can be separated by means of hinges. A very important point about -these moulds is that the various pieces should fit accurately into -one another, since otherwise a minute “fin” of glass will be forced -into every interstice, and the traces of these fins will always remain -visible on the finished article; the very perfect fit required to -entirely prevent the formation of such fins is, of course, scarcely -attainable in practice except in the case of new moulds, so that the -traces of fins are generally to be found on all pressed articles, and -serve as a ready means of identifying these products when an attempt -is made to imitate better classes of glass-ware by their means. The -presses used in this process are generally of the hand-lever type; -power presses could no doubt be used, but it is contended that the -hand-press has a very great advantage in allowing the operator to judge -by touch when sufficient pressure has been exerted, and this is an -important consideration, since an excessive pressure would either force -the glass out of the mould altogether or would be liable to burst or -injure the mould seriously. The actual presses consist of vertical -guides and levers for controlling the movement of the plunger and a -table for holding the moulds, and in some cases a system of cranks and -levers for opening and closing the moulds. The process of pressing is -exceedingly simple. The proper quantity of glass is gathered from the -pot on a solid rod and dropped into the mould. The thread of glass -which remains between the glass in the mould and that remaining on -the iron is cut off with a pair of shears, and then the plunger is -lowered into the mould and allowed to remain there until the glass -has stiffened sufficiently to retain its shape, when the plunger is -withdrawn. In this proceeding it will be seen that the glass is forced -into intimate contact with the relatively cold surfaces of mould and -plunger, and while undergoing this treatment the glass must remain -sufficiently plastic to readily adapt itself to the configuration of -the mould. It is therefore not surprising to find that the pressing -process can only be used successfully with glass of a kind specially -adapted for it. Certain varieties of flint glass and some barium -glasses are used for this purpose, but the greater quantity of pressed -glass, particularly as produced on the Continent, is made of a -lime-alkali silicate containing considerable quantities of both soda -and potash and relatively little lime; while sufficiently resistant for -most purposes, this glass is particularly soft and adaptable while in -the viscous condition. - -The deleterious effect produced upon glass surfaces when brought into -contact with relatively cold metal has already been referred to above, -and it only remains to add that this is the principal difficulty with -which the glass-pressing process has to contend. It is overcome to -some extent by the aid of the reheating process described above; but -this is only a partial remedy, and in the majority of pressed glass -products the surface is “covered” as far as possible by the application -of relief decorations such as grooves, spirals, and ribbings. An -attempt is sometimes made to imitate the appearance of cut glass, -but the rounding of the angles during the reheating process destroys -the sharpness of the effect and allows of the ready detection of the -imitation, while the cheapness of the decoration when applied in the -mould has frequently led manufacturers to grossly over-decorate, and, -therefore, destroy all claim to beauty in their wares. - - - - -CHAPTER IX. - -ROLLED OR PLATE-GLASS. - - -In the present chapter we propose to deal with all those processes -of glass manufacture in which the first stage consists in converting -the glass into a slab or plate by some process of rolling. We have -already considered the general character of the rolling process, and -have seen that, although hot, viscous glass lends itself readily to -being rolled into sheets or slabs, these cannot be turned out with -a smooth, flat surface. In practice the surface of rolled glass is -always more or less dimmed by contact with the minute irregularities -of table or roller, and larger irregularities of the surface arise -from the buckling that occurs at a great many places in the sheet. -These limitations govern the varieties of glass that can be produced by -processes that involve rolling, and have led to the somewhat curious -result that both the cheapest and roughest, as well as the best and -most expensive kinds of flat glass, are produced by rolling processes. -Ordinary rough “rolled plate,” such as that used in the skylights of -workshops and of railway stations, is the extreme on the one hand, -while polished plate-glass represents the other end of the scale. The -apparent paradox is, however, solved when it is noted that in the -production of polished plate-glass the character of the surface of -the glass as it leaves the rollers is of very minor importance, since -it is entirely obliterated by the subsequent processes of grinding, -smoothing, and polishing. Intermediate between the rough “rolled” and -the “polished” plate-glass we have a variety of glasses in which the -appearance of the rolled surface is hidden or disguised to a greater or -lesser extent by the application of a pattern that is impressed upon -the glass during the rolling process; thus we have rolled plate having -a ribbed or lozenge-patterned surface, or the well-known variety of -“figured rolled” plate, sometimes known as “Muranese,” whose elaborate -and deeply-imprinted patterns give a very brilliant effect. - -Rolled plate-glass being practically the roughest and cheapest form of -glazing, is principally employed where appearance is not considered, -and its chief requirement is, therefore, cheapness, although both the -colour and quality of the glass are of importance as affecting the -quantity and character of the light which it admits to the building -where the glass is used. On the ground of cheapness it will be obvious -from what we have said above (Chapter IV.), that such glass can -only be produced economically in large tank furnaces, and these are -universally used for this purpose. The requirements as regards freedom -from enclosed foreign bodies of small size and of enclosed air-bells -are not very high in such glass, and, therefore, tanks of very simple -form are generally used. No refinements for regulating the temperature -of various parts of the furnace in order to ensure perfect fining of -the glass are required, and the furnace generally consists simply of an -oblong chamber or tank, at one end of which the raw materials are fed -in, while the glass is withdrawn by means of ladles from one or two -suitable apertures at the other end. For economical working, however, -the furnace must be capable of working at a high temperature, because -a cheap glass mixture is necessarily somewhat infusible, at all events -where colour is considered. This will be obvious if we remember that -the fusibility of a glass depends upon its alkali contents, and alkali -is the most expensive constituent of such glasses. - -The actual raw materials used in the production of rolled plate-glass -are sand, limestone and salt-cake, with the requisite addition of -carbon and of fluxing and purifying materials. The selection of these -materials is made with a view to the greatest purity and constancy -of composition which is available within the strictly-set limits of -price which the low value of the finished product entails. These -materials are handled in very large quantities, outputs of from 60 to -150 tons of finished glass per week from a single furnace being by no -means uncommon; mechanical means of handling the raw materials and of -charging them into the furnace are therefore adopted wherever possible. - -The glass is withdrawn from the furnace by means of large iron ladles. -These ladles are used of varying sizes in such a way as to contain the -proper amount of glass to roll to the various sizes of sheets required. -The sizes used are sometimes very large, and ladles holding as much -as 180 to 200 lbs. of glass are used. These ladles, when filled with -glass, are not carried by hand, but are suspended from slings attached -to trolleys that run on an overhead rail. The ladler, whose body is -protected by a felt apron and his face by a mask having view-holes -glazed with green glass, takes the empty ladle from a water-trough, in -which it has been cooled, carries it to the slightly inclined gangway -that leads up to the opening in the front of the furnace, and there -introduces the ladle into the molten glass, giving it a half-turn so -as to fill it with a “solid” mass of glass. By giving the ladle two or -three rapid upward jerks, the operator then detaches the glass in the -ladle as far as possible from the sheets and threads of glass which -would otherwise follow its withdrawal; then the part of the handle of -the ladle near the bowl is placed in the hook attached to the overhead -trolley, and by bearing his weight on the other end of the handle, the -workman draws the whole ladle up from the molten bath in the furnace -and out through the working aperture. This operation only takes a -few seconds to perform, but during this time the ladler is exposed -to great heat, as a more or less intense flame generally issues from -the working aperture, whence it is drawn upward under the hood of the -furnace. From the furnace opening, the ladler, generally aided by a -boy, runs the full ladle to the rolling table and there empties the -ladle upon the table just in front of the roller. In doing this, two -distinctly different methods are employed. In one, only the perfectly -fluid portion of the glass is poured out of the ladle by gradually -tilting it, the chilled glass next to the walls of the ladle being -retained there and ultimately returned to the furnace while still hot. -In the other method, the chilling of the glass is minimised as far as -possible, and the entire contents of the ladle are emptied upon the -rolling table by the ladler, who turns the entire ladle over with a -rapid jerk which is so arranged as to throw the coldest part of the -glass well away from the rest. When the sheet is subsequently rolled -this chilled portion is readily recognised by its darker colour, and -since it lies entirely at one end of the sheet it is detached before -the sheet goes any further. Neither method appears to present any -preponderating advantage. - -[Illustration: FIG. 9.--Rolling table for rolled plate-glass.] - -The rolling table used in the manufacture of rolled plate is -essentially a cast-iron slab of sufficient size to accommodate the -largest sheet which is to be rolled; over this slab moves a massive -iron roller which may be actuated either by hand or by mechanical -power--the latter, however, being now almost universal. The thickness -of the sheet to be rolled is regulated by means of slips of iron placed -at the sides of the table in such a way as to prevent the roller from -descending any further towards the surface of the table: so long as -the layer of glass is thicker than these slips, the entire weight of -the roller comes upon the soft glass and presses it down, but as -soon as the required thickness is attained, the weight of the roller -is taken by the iron slips and the glass is not further reduced in -thickness. The width of the sheet is regulated by means of a pair of -iron guides, formed to fit the forward face of the roller and the -surface of the table, in the manner indicated in Fig. 9. The roller, -as it moves forward, pushes these guides before it, and the glass is -confined between them. When the roller has passed over the glass, the -sheet is left on the iron table in a red-hot, soft condition, and it -must be allowed to cool and harden to a certain extent before it can -be safely moved. In this interval, the chilled portion--if any--is -partially severed by an incision made in the sheet by means of a long -iron implement somewhat like a large knife, and then the sheet is -loosened from the bed of the table by passing under it, with a smooth -rapid stroke, a flat-bladed iron tool. The sheet is next removed to -the annealing kiln or “lear,” being first drawn on to a stone slab and -thence pushed into the mouth of the kiln. At this stage the chilled -portion of the sheet is completely severed by a blow which causes the -glass to break along the incision previously made. - -The rolled-plate annealing kiln is essentially a long, low tunnel, -kept hot at one end, where the freshly-rolled sheets are introduced, -and cold at the other end, the temperature decreasing uniformly down -the length of the tunnel. The sheets pass down this tunnel at a -slow rate, and are thus gradually cooled and annealed sufficiently -to undergo the necessary operations of cutting, etc. Although thus -simple in principle, the proper design and working of these “lears” is -by no means simple or easy, since success depends upon the correct -adjustment of temperatures throughout the length of the tunnel and a -proper rate of movement of the sheets, while the manner of handling and -supporting the sheets is vital to their remaining flat and unbroken. -The actual movement of the sheets is effected by a system of moving -grids which run longitudinally down the tunnel. The sheets ordinarily -lie flat upon the stone slabs that form the floor of the tunnel, and -the grids are lowered into recesses cut to receive them. At regular -intervals the iron grid bars are raised just sufficiently to lift the -sheets from the bed of the kiln, and are then moved longitudinally a -short distance, carrying the sheets forward with them and immediately -afterwards again depositing them on the stone bed. The grids return to -their former position while lowered into their recesses below the level -of the kiln bed. - -When they emerge from the annealing kiln or “lear” the sheets of rolled -plate-glass are carried to the cutting and sorting room. Here the -sheets are trimmed and cut to size. The edges of the sheets as they -leave the rolling table are somewhat irregular, and sometimes a little -“beaded,” while the ends are always very irregular. Ends and edges are -therefore cut square or “trimmed” by the aid of the cutting diamond. -For this purpose the sheet is laid upon a flat table, the smoothest -side of the sheet being placed upwards, and long cuts are taken with -a diamond--good diamonds of adequate size and skilful operators being -necessary to ensure good cutting on such thick glass over long lengths. -Strips of glass six or eight feet long and half an inch wide are -frequently detached in the course of this operation, and the final -separation is aided by slight tapping of the underside of the glass -just below the cut and--if necessary--by breaking the strip off by the -aid of suitable tongs. - -No very elaborate “sorting” of rolled plate glass is required, except -perhaps that the shade of colour in the glass may vary slightly from -time to time, and it is generally preferable to keep to one shade of -glass in filling any particular order. Apart from this, the rolled -plate cutter has merely to cut out gross defects which would interfere -too seriously with the usefulness of the glass. As we have already -indicated, air-bells and minute enclosures of opaque matter are not -objectionable in this kind of glass, but large pieces of opaque -material must generally be cut out and rejected, not only because -they are too unsightly to pass even for rough glazing purposes, but -also because they entail a considerable risk of spontaneous cracking -of the glass--in fact, visible cracks are nearly always seen around -large “stones,” as these inclusions are called. These may arise from -various causes, such as incomplete melting of the raw materials, or the -contamination of the raw materials with infusible impurities, but the -most fruitful source of trouble in this direction lies in the crumbling -of the furnace lining, which introduces small lumps of partially melted -fire-clay into the glass. In a rolled plate tank furnace which is -properly constructed and worked, the percentage of sheets which have to -be cut up on account of such enclosures should be very small, at all -events until the furnace is old, when the linings naturally show an -increasing tendency to disintegrate. - -Returning now to the rolling process, it is readily seen that a very -slight modification will result in the production of rolled plate-glass -having a pattern impressed upon one surface; this modification -consists in engraving upon the cast-iron plate of the rolling table in -intaglio any pattern that is to appear upon the glass in relief. As -a matter of fact only very simple patterns are produced in this way, -such as close parallel longitudinal ribbing and a lozenge-pattern, the -reason probably being that the cost of cutting an elaborate pattern -over the large area of the bed-plate of one of these tables would be -very considerable. Further, as these tables and their bed-plates are so -very heavy, they are not readily interchanged or left standing idle, so -that only patterns required in very great quantity could be profitably -produced in this way. These disadvantages are, however, largely -overcome by the double-rolling machine. In this machine, into whose -rather elaborate details we cannot enter here, the glass is rolled -out into a sheet of the desired size and thickness by being passed -between two rollers revolving about stationary axes, the finished sheet -emerging over another roller, and passing on to a stone slab that -moves forward at the same rate as the sheet is fed down upon it. In -this machine a pattern can be readily imprinted upon the soft sheet -as it passes over the last roller by means of a fourth roller, upon -which the pattern is engraved; this is pressed down upon the sheet, -and leaves upon it a clear, sharp and deep impress of its pattern. -The general arrangement of the rollers in this machine is shown in -the diagram of Fig. 10, which represents the sectional elevation of -the appliance. After leaving the rolling machine, the course of the -“figured rolled plate” produced in this manner is exactly similar to -that of ordinary rolled plate, except that as a somewhat softer kind of -glass is generally used for “figured,” the temperature of the annealing -kilns requires somewhat different adjustment. The cutting of the glass -also requires rather more care, and it should be noted that such glass -can only be cut with a diamond on the smooth side; the side upon which -the pattern has been impressed in relief cannot be materially affected -by a diamond. This is one reason why it is not feasible to produce such -glass with a pattern on both sides. - -[Illustration: FIG. 10.--Sectional diagram of machine for rolling -“figured rolled” plate-glass.] - -Figured rolled glass, being essentially of an ornamental or decorative -nature, is generally produced in either brilliantly white glass or in -special tints and colours, and the mixtures used for attaining these -are, of course, the trade property of the various manufacturers; the -whiteness of the glass, however, is only obtainable by the use of very -pure and, therefore, expensive materials. As regards the coloured -plate-glasses, a general account of the principles underlying the -production of coloured glass will be found in Chapter XI. - -The manufacture of polished plate-glass really stands somewhat by -itself, almost the only feature which it has in common with the -branches of manufacture just described being the initial rolling -process. - -The raw materials for the production of plate-glass are chosen with -the greatest possible care to ensure purity and regularity; owing to -the very considerable thickness of glass which is sometimes employed -in plate, and also to the linear dimensions of the sheets which allow -of numerous internal reflections, the colour of the glass would become -unpleasantly obtrusive if the shade were at all pronounced. The actual -raw materials used vary somewhat from one works to another; but, as -a rule, they consist of sand, limestone, and salt-cake, with some -soda-ash and the usual additions of fluxing and purifying material such -as arsenic, manganese, etc. The glass is generally melted in pots, and -extreme care is required to ensure perfect melting and fining, since -very minute defects are readily visible in this glass when finished, -and, of course, detract most seriously from its value. - -The method of transferring the glass from the melting-pot to the -rolling table differs somewhat in different works. In many cases the -melting-pots themselves are taken bodily from the furnace and emptied -upon the bed-plate of the rolling machine, while in other cases the -glass is first transferred to smaller “casting” pots, where it has to -be heated again until it has freed itself from the bubbles enclosed -during the transference, and then these smaller pots are used for -pouring the glass upon the rolling slab. The advantage of the latter -more complicated method lies, no doubt, in the fact that the large -melting-pots, which have to bear the brunt of the heat and chemical -action during the early stages of melting, are not exposed to the great -additional strain of being taken from the hot furnace and exposed -for some time to the cold outside air. Apart from the mechanical -risks of fracture, this treatment exposes the pots to grave risks of -breakage from unequal expansion and contraction on account of the great -differences of temperature involved. Where smaller special casting-pots -are used, these are not exposed to such prolonged heat in the furnace, -and are never exposed to the chemical action of the raw materials, so -that these subsidiary pots may perhaps be made of a material better -adapted to withstand sudden changes of temperature than the high-class -fire-clay which must be used in the construction of melting pots. On -the other hand, the transference of the glass from the melting to -the casting-pots involves a laborious operation of ladling and the -refining of the glass, with its attendant expenditure of time and fuel. -Finally, the production of plate-glass in tank furnaces could only be -attempted by the aid of such casting-pots in which the glass would have -to undergo a second fining after being ladled from the tank, and this -would materially lessen the economy of the tank for this purpose, while -it is by no means an easy matter to produce in tank furnaces qualities -of glass equal as regards colour and purity to the best products of the -pot furnace. - -The withdrawal of the pots containing the molten glass from the furnace -is now universally carried out by powerful machinery. The pots are -provided on their outer surface with projections by which they can be -held in suitably-shaped tongs or cradles. A part of the furnace wall, -which is constructed each time in a temporary manner, is broken down; -the pot is raised from the bed or “siege” of the furnace by the aid -of levers, and is then bodily lifted out by means of a powerful fork. -The pot is then lifted and carried by means of cranes until it is in -position above the rolling table; there the pot is tilted and the glass -poured out in a steady stream upon the table, care being taken to avoid -the inclusion of air-bells in the mass during the process of pouring. -When empty, the pot is returned to the furnace as rapidly as possible, -the glass being meanwhile rolled out into a slab by the machine. -Except for the greater size and weight of both table and roller, the -plate-glass rolling table is similar to that already described in -connection with rolled plate. Of course, since the glass is poured -direct from the pot, there is no chilled glass to be removed. Further, -owing to the large size of sheets frequently required, the bed of the -rolling table cannot be made of a single slab of cast-iron, a number of -carefully jointed plates being, in fact, preferable, as they are less -liable to warp under the action of the hot glass. - -In arranging the whole of the rolling plant, the chief consideration -to be kept in mind is that it is necessary to produce a flat sheet of -glass of as nearly as possible equal thickness all over. The final -thickness of the whole slab when ground and polished into a sheet -of plate-glass must necessarily be slightly less than that of the -thinnest part of the rough rolled sheet. If, therefore, there are any -considerable variations of thickness, the result will be that in some -parts of the sheet a considerable thickness of glass will have to be -removed during the grinding process. This will arise to a still more -serious extent if the sheet as a whole should be bent or warped so -as to depart materially from flatness. The two cases are illustrated -diagrammatically in Fig. 11, which shows sectional views of the sheets -before and after grinding on an exaggerated scale. - -[Illustration: FIG. 11.--Sectional diagram illustrating waste of glass -in grinding curved or irregular plate.] - -While it is evident that careful design of the rolling table will avoid -all tendency to the formation of sheets of such undesirable form, it -is a much more difficult matter to avoid all distortion of the sheet -during the annealing process and while the sheet is being moved from -the rolling table to the annealing kiln. Owing to the great size of -the slabs of glass to be dealt with, and still more to the stringent -requirement of flatness, the continuous annealing kiln, in which the -glass travels slowly down a tunnel from the hot to the cold end, has -not been adopted for the annealing of plate-glass, and a form of -annealing kiln is still used for that glass which is similar in its -mode of operation to the old-fashioned kilns that were used for other -kinds of glass before the continuous kiln was introduced. These kilns -simply consist of chambers in which the hot glass is sealed up and -allowed to cool slowly and uniformly during a more or less protracted -period. In the case of plate-glass, the slabs are laid flat on the -stone bed of the kiln. This stone bed is built up of carefully dressed -stone, or blocks of fire-brick bedded in sand in such a way that they -can expand freely laterally without causing any tendency for the floor -to buckle upwards as it would do if the blocks were set firmly against -one another. The whole chamber is previously heated to the requisite -temperature at which the glass still shows a very slight plasticity. -The hot glass slabs from the rolling table are laid upon the bed -of this kiln, several being usually placed side by side in the one -chamber, and the slabs in the course of the first few hours settle down -to the contour of the bed of the kiln, from which shape and position -they are never disturbed until they are removed when quite cold. In -modern practice the cooling of a kiln is allowed to occupy from four -to five days; even this rate of cooling is only permissible if care -is taken to provide for the even cooling of all parts of the kiln, -and for this purpose special air-passages are built into the walls of -the chamber and beneath the bed upon which the glass rests, and air -circulation is admitted to these in such a way as to allow the whole of -the kiln to cool down at the same rate; in the absence of such special -arrangements, the upper parts of the kiln would probably cool much more -rapidly than the base, so that the glass would be much warmer on its -under than on its upper surface. - -When the slabs of plate-glass are removed from the annealing kilns -they very closely resemble sheets of rolled plate in appearance, -and they are quite sufficiently transparent to allow of examination -and the rejection of the more grossly defective portions; the more -minute defects, of course, can only be detected after the sheets have -been polished, but this preliminary examination saves the laborious -polishing of much useless glass. - -The process of grinding and polishing plate-glass consists of three -principal stages. In the first stage the surfaces of the glass are -ground so as to be as perfectly flat and parallel as possible; in order -to effect this object as rapidly as possible, a coarse abrasive is -used which leaves the glass with a rough grey surface. In the second -stage, that of smoothing, these rough grey surfaces are ground down -with several grades of successively finer abrasive until finally an -exceedingly smooth grey surface is left. In the third and final stage, -the smooth grey surface is converted into the brilliant polished -surface with which we are familiar by the action of a polishing medium. - -Originally the various stages of the grinding and polishing processes -were carried out by hand, but a whole series of ingenious machines -has been produced for effecting the same purpose more rapidly and -more perfectly than hand-labour could ever do. We cannot hope to give -any detailed account of the various systems of grinding and polishing -machines which are even now in use, but must content ourselves with -a survey of some of the more important considerations governing the -design and construction of such machinery. - -In the first place, before vigorous mechanical work can be applied to -the surface of a plate of glass, that plate must be firmly fixed in a -definite position relatively to the rest of the machinery, and such -firm fixing of a plate of glass is by no means readily attained, since -the plate must be supported over its whole area if local fracture is to -be avoided. While the surface of the plate is in the uneven condition -in which it leaves the rolling table, such a firm setting of the glass -can only be attained by bedding it in plaster, and this must be done in -such a manner as to avoid the formation of air-bubbles between plaster -and glass; if bubbles are allowed to form, they constitute places where -the glass is unsupported. During the grinding and polishing processes -these unsupported places yield to the heavy pressure that comes upon -them, and irregularities in the finished polished surfaces result. -The most perfect adhesion between glass and plaster is attained by -spreading the paste of plaster on the up-turned surface of the slab of -glass and lowering the iron bed-plate of the grinding table down upon -it, the bed-plate with the adhering slab of glass being afterwards -turned over and brought into position in the grinding machine. When one -side of the glass has been polished, it is generally found sufficient -to lay the slab down on a bed of damp cloth, to which it adheres very -firmly, although sliding is entirely prevented by a few blocks fixed to -the table in such a way as to abut against the edges of the sheet. In -many works, however, the glass is set in plaster for the grinding and -polishing of the second side as well as of the first. - -The process of grinding and polishing is still regarded in many -plate-glass works as consisting of three distinct processes, known -as rough grinding, smoothing and polishing respectively. Formerly -these three stages of the process were carried out separately; at -first by hand, and later by three different machines. In the most -modern practice, however, the rough and smooth grinding are done on -the same machine, the only change required being the substitution of -a finer grade of abrasive at each step for the coarser grade used in -the previous stage. For the polishing process, however, the rubbing -implements themselves must be of a different kind, for while the -grinding and smoothing is generally done by means of cast-iron rubbers -moving over the glass, the polishing is done with felt pads. The table -of the machine, to which the glass under treatment is attached, is -therefore made movable, and when the grinding and smoothing processes -are complete, the table with its attached glass is moved so as to come -beneath a superstructure carrying the polishing rubbers, and the whole -is then elevated so as to allow the rubbers to bear on the glass. - -The earliest forms of grinding machines gave a reciprocal motion to -the table which carries the glass, or the grinding rubbers were moved -backward and forward over the stationary table. Rotary machines, -however, were introduced and rapidly asserted their superiority, until, -at the present time, practically all plate-glass is ground on rotating -tables, some of these attaining a diameter of over 30 ft. The grinding -“rubbers” consist of heavy iron slabs, or of wood boxes shod with iron, -but of much smaller diameter than the grinding table. The rubbers -themselves are rotary, being caused to rotate either by the frictional -drive of the rotating table below them, or by the action of independent -driving mechanism, but the design of the motions must be so arranged -that the relative motion of rubber and glass shall be approximately the -same at all parts of the glass sheets, otherwise curved instead of -plane surfaces would be formed. This condition can be met by placing -the axes of the rubbers at suitable points on the diameter of the -table. The abrasive is fed on to the glass in the form of a thin paste, -and when each grade or “course” has done the work required of it, the -whole table is washed down thoroughly with water and then the next -finer grade is applied. The function of the first or coarsest grade is -simply to remove the surface irregularities and to form a rough but -plane surface. The abrasive ordinarily employed is sharp sand, but -only comparatively light pressure can be applied, especially at the -beginning of this stage, since at that period the weight of the rubber -is at times borne by relatively small areas of glass that project here -and there above the general level of the slab. As these are ground -away, the rubbers take a larger and more uniform bearing, and greater -pressure can be applied. The subsequent courses of finer abrasives -are only required to remove the coarse pittings left in the surface -by the action of the first rough grinding sand; the finer abrasive -replaces the deep pits of the former grade by shallower pits, and this -is carried on in a number of steps until a very smooth “grey” surface -is attained and the smoothing process is complete. The revolving table -or “platform” is now detached from the driving mechanism, and moved -along suitably placed rails on wheels provided for that purpose, until -it stands below the polishing mechanism. Here it is attached to a -fresh driving mechanism, and it is then either raised so as to bring -the glass into contact with the felt-covered polishing rubbers, or -the latter are lowered down upon the glass. The polishing rubbers are -large felt-covered slabs of wood or iron which are pressed against -the glass with considerable force; their movement is very similar to -that of the grinding rubbers, but in place of an abrasive they are -supplied with a thin paste of rouge and water. The time required for -the polishing process depends upon the perfection of the smoothing -that has been attained; in favourable cases two or three hours are -sufficient to convert the “grey” surface into a perfectly polished one; -where, however, somewhat deeper pits have been left in the glass, the -time required for polishing may be much longer, and the polish attained -will not be so perfect. The mode of action of a polishing medium such -as rouge is now recognised to be totally different in character from -that of even the finest abrasive; the grains of the abrasive act by -their hardness and the sharpness of their edges, chipping away tiny -particles of the glass, so that the glass steadily loses weight during -the grinding and smoothing processes. During the polishing process, -however, there is little or no further loss of weight, the glass -forming the hills or highest parts of the minutely pitted surface being -dragged or smeared over the surface in such a way as to gradually fill -up the pits and hollows. The part played by the polishing medium is -probably partly chemical and partly physical, but it results, together -with the pressure of the rubber, in giving to the surface molecules -of the glass a certain amount of freedom of movement, similar to that -of the molecules of a viscid liquid; the surface layers of glass -are thus enabled to “flow” under the action of the polisher and to -smooth out the surface to the beautiful level smoothness which is so -characteristic of the surfaces of liquids at rest. This explanation -of the polishing process enables us to understand why the proper -consistency of the polishing paste, as well as the proper adjustment -of the speed and pressure of the rubbers, plays such an important part -in successful polishing; it also serves to explain the well-known fact -that rapid polishing only takes place when the glass surface has begun -to be perceptibly heated by the friction spent upon it. - -It has been estimated that, on the average, slabs of plate-glass lose -one-third of their original weight in the grinding and polishing -processes, and it is obvious that the erosion of this great weight of -glass must absorb a great amount of mechanical energy, while the cost -of the plant and upkeep is proportionately great. Every factor that -tends to diminish either the total weight of glass to be removed per -square yard of finished plate, or reduces the cost of removal, must -be of the utmost importance in this manufacture. The flatness of the -plates as they leave the annealing kiln has already been referred -to, and the reason why the processes of grinding and polishing have -formed the subject for innumerable patents will now be apparent. The -very large expansion of the use of plate-glass in modern building -construction, together with the steady reduction in the prices of -plate, are evidence of the success that has attended the efforts of -inventors and manufacturers in this direction. - -At the present time, plate-glass is manufactured in very large sheets, -measuring up to 26 ft. in length by 14 ft. in width, and in thickness -varying from 3/16th of an inch up to 1½ in., or more, for special -purposes. At the same time the quality of the glass is far higher -to-day than it was at earlier times. This high quality chiefly results -from more careful choice of raw materials and greater freedom from -the defects arising during the melting and refining processes, while a -rigid process of inspection is applied to the glass as it comes from -the polishing machines. For this purpose the sheets are examined in -a darkened room by the aid of a lamp placed in such a way that its -oblique rays reveal every minute imperfection of the glass; these -imperfections are marked with chalk, and the plate is subsequently cut -up so as to avoid the defects that have thus been detected. - -Perhaps the most remarkable fact about the quality of modern -plate-glass is its relatively high degree of homogeneity. Glass, as we -have seen in Chapter I., is not a chemically homogeneous substance, -but rather a mixture of a number of substances of different density -and viscosity. Wherever this mixture is not sufficiently intimate, the -presence of diverse constituents becomes apparent in the form of striæ, -arising from the refraction or bending of light-rays as they pass from -one medium into another of different density. Except in glass that has -undergone elaborate stirring processes, such striæ are never absent, -but the skill of the glass-maker consists in making them as few and as -minute as possible, and causing them to assume directions and positions -in which they shall be as inconspicuous as possible. In plate-glass -this is generally secured in a very perfect manner, and to ordinary -observation no striæ are visible when a piece of plate-glass is looked -at in the ordinary way, _i.e._, through its smallest thickness; if the -same piece of glass be looked at transversely, the edges having first -been polished in such a way as to render this possible, the glass will -be seen to be full of striæ, generally running in fine lines parallel -with the polished surfaces of the glass. This uniform direction of -the striæ is partly derived from the fact that the glass has been -caused to flow in this direction by the action of the roller when first -formed into a slab, but this process would not obliterate any serious -inequalities of density which might exist in the glass as it leaves the -pot, so that successful results are only attainable if great care is -taken to secure the greatest possible homogeneity in the glass during -the melting process. - -At the present time probably the greater bulk of plate-glass is used -for the purpose of glazing windows of various kinds, principally the -show windows of shops, etc. As used for this purpose the glass is -finished when polished and cut to size. The only further manipulation -that is sometimes required is that of bending the glass to some -desired curvature, examples of bent plate-glass window-panes being -very frequently seen. This bending is carried out on the finished -glass, _i.e._, after it has been polished; the glass is carefully -heated in a special furnace until softened, and is then gently made to -lie against a stone or metal mould which has been provided with the -desired curvature. It is obvious that during this operation there are -great risks of spoiling the glass; roughening of the surface by contact -with irregular surfaces on either the mould, the floor of the kiln, or -the implements used in handling the glass, can only be avoided by the -exercise of much skill and care, while all dust must also be excluded -since any particles settling on the surface of the hot glass would be -“burnt in,” and could not afterwards be detached. Small defects can, -of course, be subsequently removed by local hand-polishing, and this -operation is nearly always resorted to where polished glass has to -undergo fire-treatment for the purpose of bending. - -In addition to its use for glazing in the ordinary sense, plate-glass -is employed for a number of purposes; the most important and frequent -of these is in the construction of the better varieties of mirrors. -For this purpose the glass is frequently bevelled at the edges, and -sometimes a certain amount of cutting is also introduced on the face of -the mirror. Bevelling is carried out on special grinding and polishing -machines, and a great variety of these are in use at the present time. -The process consists in grinding off the corners of the sheet of glass -and replacing the rough perpendicular edge left by the cutting diamond -by a smooth polished slope running down from the front surface to the -lower edge at an angle of from 45 to 60 degrees. Since only relatively -small quantities of glass have to be removed, small grinding rubbers -only are used, and in some of the latest machines these take the form -of rapidly-revolving emery or carborundum wheels. These grinding wheels -have proved so successful in grinding even the hardest metals that it -is surprising to find their use in the glass industry almost entirely -restricted to the “cutting” of the better kinds of flint and “crystal” -glass for table ware or other ornamental purposes. The reason probably -lies in the fact that the use of such grinding wheels results in the -generation of a very considerable amount of local heat, this effect -being intensified on account of the low heat-conducting power of glass. -If a piece of glass be held even lightly against a rapidly-revolving -emery wheel it will be seen that the part in contact with the wheel -is visibly red-hot. This local heating is liable to lead to chipping -and cracking of the glass, and these troubles are those actually -experienced when emery or carborundum grinding is attempted on larger -pieces of glass. In the case of at least one modern bevel-grinding -machine, however, it is claimed that the injurious effects of local -heating are avoided by carrying out the entire operation under water. - -For the purpose of use in mirrors, plate-glass is frequently silvered, -and this process is carried on so extensively that it has come to -constitute an entire industry which has no essential connection -with glass manufacture itself; for that reason we do not propose to -enter on the subject here, only adding that the nature and quality -of the glass itself considerably affects the ease and success of the -various silvering processes. Ordinary plate-glass, of course, takes -the various silvering coatings very easily and uniformly, but there -are numerous kinds of glass to which this does not apply, although -there are probably few varieties of glass which are sufficiently -stable for practical use, and to which a silvering coating cannot be -satisfactorily applied, provided that the most suitable process be -chosen in each case. - -While there is little if any use for coloured glass in the form of -polished plate, entirely opaque plate-glass, coloured both black -and white, is used for certain purposes. Thus, glass fascias over -shop-fronts, the counters and shelves of some shops, and even -tombstones are sometimes made of black or white polished plate. From -the point of view of glass manufacture, however, these varieties only -differ from ordinary plate-glass in respect of certain additions to -the raw materials, resulting in the production of the white or black -opacity. The subsequent treatment of the glass is identical with that -of ordinary plate-glass, except that these opaque varieties are rarely -required to be polished on both sides, so that the operations are -simplified to that extent. - -Certain limitations to the use of all kinds of plate-glass, whether -rough-rolled, figured or polished, were formerly set by the fact -that under the influence of fire, partitions of glass were liable to -crack, splinter and fall to pieces, thus causing damage beyond their -own destruction and leaving a free passage for the propagation of -the fire. To overcome these disadvantages, glass manufacturers have -been led to introduce a network or meshing of wire into the body of -such glass. Provided that the glass and wire can be made so as to -unite properly, then the properties of such reinforced or “wired” -glass should be extremely valuable. In the event of breakage from any -cause, such as fire or a violent blow, while the glass would still -crack, the fragments would be held together by the wire network, -and the plates of glass as a whole would remain in place, neither -causing destruction through flying fragments nor allowing fire or, -for the matter of that, burglar a free passage. The utility of such -a material has been readily recognised, but the difficulty lies in -its production. These difficulties arise from two causes. The most -serious of these is the considerable difference between the thermal -expansion of the glass and of the wire to be embedded in it. The wire -is necessarily introduced into red-hot glass while the latter is being -rolled or cast, and therefore glass and wire have to cool down from a -red heat together. During this cooling process the wire contracts much -more than the glass, and breakage either results immediately, or the -glass is left in a condition of severe strain and is liable to crack -spontaneously afterwards. An attempt has been made to overcome this -difficulty by using wire made of a nickel steel alloy, whose thermal -expansion is very similar to that of glass; but, as a matter of fact, -this similarity of thermal expansion is only known to hold for a short -range of moderate temperatures, and probably does not hold when the -steel alloy is heated to redness. In another direction, greater success -is to be attained by the use of wire of a very ductile metal which -should yield to the stress that comes upon it during cooling; probably -copper wire would answer the purpose, but the great cost of copper is a -deterrent from its use. A second difficulty is met with in introducing -wire netting into glass during the rolling operation, and this lies in -effecting a clean join between glass and wire. Most metals when heated -give off a considerable quantity of gas, and when this gas is evolved -after the wire has been embedded in glass, numerous bubbles are formed, -and these not only render the glass very unsightly but also lessen the -adhesion between the wire and the glass. This difficulty, however, can -be overcome more readily than the first, since the surface of the metal -can be kept clean and the gas expelled from the interior of the wire -by preliminary heating. On the whole, however, wired glass is perhaps -still to be regarded as a product whose evolution is not yet complete, -and there can be no doubt that there are great possibilities open to -the material when its manufacture has been more fully developed. - - - - -CHAPTER X. - -SHEET AND CROWN GLASS. - - -In the preceding chapter we have dealt with the processes of -manufacture employed in the production of both the crudest and the -most perfect forms of flat glass as used for such purposes as the -glazing of window openings. The products now to be dealt with are -of an intermediate character, sheet-glass possessing many of the -properties of polished plate, but lacking some very important ones; -thus sheet-glass is sufficiently transparent to allow an observer to -see through it with little or no disturbance--in the best varieties -of sheet-glass the optical distortion caused by its irregularities -is so small that the glass appears nearly as perfect as polished -plate--but in the cheap glass that is used for the glazing of ordinary -windows, sheets are often employed which produce the most disturbing, -and sometimes the most ludicrous, distortions of objects seen through -them. It is a curious fact that even in good houses the use of such -inferior glass is tolerated without comment, the general public being, -apparently, remarkably nonobservant in this respect. In another -direction sheet-glass has the great advantage over plate-glass that -it is very much lighter, or can at least be produced of much smaller -weight and thickness, although this advantage entails the consequent -disadvantage that sheet-glass is usually much weaker than plate, and -can only be used in much smaller sizes. In recent times the production -of relatively thin plate-glass has, however, made such strides that -it is now possible to obtain polished plate-glass thin enough and -light enough for almost every architectural purpose. Finally, the most -important advantage of sheet-glass, and the one which alone secures its -use in a great number of cases in preference to plate-glass, is its -cheapness, the price of ordinary sheet-glass being about one-fourth -that of plate-glass of the same size. - -The raw materials for the manufacture of sheet-glass are sand, -limestone, salt-cake, and a few accessory substances, such as arsenic, -oxide of manganese, anthracite coal or coke, which differ considerably -according to the practice of each particular works. In a general way -these materials have already been dealt with in Chapter III., and we -need only add here that the sheet-glass manufacturer must keep in -view two decidedly conflicting considerations. On the one hand the -requirements made in the case of sheet-glass as regards colour and -purity render a rigorous choice of raw material and the exclusion of -anything at all doubtful very desirable; but on the other hand the -chief commercial consideration in connection with this product is its -cheapness, and in order to maintain a low selling price at a profit -to himself the manufacturer must rigorously exclude all expensive raw -materials. For this reason sheet-glass, works such as those of Belgium -and some parts of Germany, which have large deposits of pure sand -close at hand, possess a very considerable advantage over those in -less favoured situations, since sand in particular forms so large a -proportion of the glass, and the cost of carriage frequently exceeds, -and in many cases very greatly exceeds, the actual price of the sand -itself. The same considerations will apply, although in somewhat lesser -degree, to the other bulky materials, such as limestone and salt-cake; -but both these are more generally obtainable at moderate prices than -are glass-making sands of adequate quality for sheet manufacture. - -Ordinary “white” sheet-glass is now almost universally produced in -tank furnaces, and a very great variety of these furnaces are used or -advocated for the purpose. It would be beyond the scope of the present -book to enter in detail into the construction of these various types -of furnace or to discuss their relative merits at length. Only a brief -outline of the chief characteristics of the most important forms of -sheet-tank furnaces will therefore be given here. - -Sheet tanks differ from each other in several important respects; these -relate to the sub-division of the tank into one, two, or even three -more or less separate chambers, to the depth of the bath of molten -glass and the height of the “crown” or vault of the furnace chamber, -to the shape and position of the apertures by which the gas and air -are admitted into the furnace, and the resultant shape and disposition -of the flame, and finally to the position and arrangement of the -regenerative appliances by which some of the heat of the waste gases is -returned into the furnace. - -Taking these principal points in order, we find that in some sheet tank -furnaces the whole furnace constitutes a single large chamber. In this -type of furnace the whole process of fusion and fining of the glass -goes on in this single chamber, and an endeavour is made to graduate -the temperature of the furnace in a suitable manner from the hot end -where the raw materials have to be melted down to the colder end where -the glass must be sufficiently viscous to be gathered on the pipes. It -is obvious that this control of the temperature cannot be so perfect -in a furnace of the single chamber type as in one that is sub-divided. -Such sub-divided furnaces are, as a matter of fact, much more frequent -in sheet-glass practice; but this practice differs widely as to the -manner and degree of the sub-division introduced. In the extreme form -the glass practically passes through three independent furnaces merely -connected with one another by suitable openings of relatively small -area through which the glass flows from one to the other. If it were -possible to build furnaces of materials that could resist the action of -heat and of molten glass to an indefinite extent, it is probable that -this extreme type would prove the best, since it gives the operator -of the furnace the means of controlling the flow of glass in such a -way that no unmelted material can leave the melting chamber and enter -the fining chamber, and that no insufficiently fined glass can leave -the fining chamber and find its way into the working chamber. But in -practice the fact that this extreme sub-division introduces a great -deal of extra furnace wall, exposed both to heat and to contact with -the glass, involves very serious compensating disadvantages--the cost -of construction, maintenance and renewal of the furnace is greatly -increased, while there is also an increased source of contamination of -the glass from the erosion of the furnace walls. It is, therefore, in -accordance with expectations to find that the most successful furnaces -for the production of sheet-glass are intermediate in this respect -between the simple open furnace and the completely sub-divided one. -In some cases the working chamber is separated from the melting and -fining chamber by a transverse wall above the level of the glass, while -fire-clay blocks floating in the glass just below this cross wall serve -to complete the separation and to retain any surface impurities that -may float down the furnace. - -As regards the depth of glass in the tank, practice also varies very -much. The advantages claimed for a deep bath are that the fire-clay -bottom of the furnace is thereby kept colder and is consequently less -attacked, so that this portion of the furnace will last for many -years. On the other hand the existence of a great mass of glass at a -moderate heat may easily prove the source of contamination arising from -crystallisation or “devitrification” occurring there and spreading -into the hotter glass above. Also, if for any reason it should become -necessary to remove part or all of the contents of the tank, the -greater mass of glass in those with deep baths becomes a formidable -obstacle. On the whole, however, modern practice appears to favour the -use of deeper baths, depths of 2 ft. 6 in. or even 3 ft. being very -usual, while depths up to 4 ft. have been used. - -The question of the proper height of the “crown” or vault of the -furnace is of considerable importance to the proper working of the -tank. For the purpose of producing the most perfect combustion, it is -now contended that a large free flame-space is required. The earlier -glass-melting tanks, like the earlier steel furnaces, were built with -very low crowns, forcing the flame into contact with the surface -of the molten glass, the object being to promote direct heating by -immediate contact of flame and glass; the modern tendency, however, is -strongly in the direction of higher crowns, leaving the heating of the -glass to be accomplished by radiation rather than direct conduction -of heat. There can be little doubt that up to a certain point the -enlargement of the flame-space tends towards greater cleanliness of -working and a certain economy of fuel, but if the height of a furnace -crown be excessive there is a decided loss of economy. Flame-spaces as -high as 6 ft. from the level of the glass to the highest part of the -crown have been used, but the more usual heights range from 2 ft. to 5 -ft. - -The “ports” or apertures by which pre-heated gas and air enter the -furnace chamber differ very widely in various furnaces. In some cases -the gas and air are allowed to meet in a small combustion chamber just -before entering the furnace itself, while in other cases the gas and -air enter the furnace by entirely separate openings, only meeting in -the furnace chamber. The latter arrangement tends to the formation of -a highly reducing flame, which is advantageous for the reduction of -salt-cake, but is by no means economical as regards fuel consumption. -On the other hand, by producing a perfect mixing of the entering gas -and air in suitable proportions, the other type of ports can be made to -give almost any kind of flame desired, although their tendency is to -form a more oxidising atmosphere within the furnace. The latter type -of ports, although widely varied in detail, are now almost universally -adopted in sheet tank furnaces. - -All modern tank furnaces work on the principle of the recovery of heat -from the heated products of combustion as they leave the furnace, and -the return of this heat to the furnace by utilising it to pre-heat the -incoming gas and air; but the means employed to effect the application -of this “regenerative” principle differ considerably in various types -of plant. Perhaps the most widely-used form of furnace is the direct -descendant of the original Siemens regenerative furnace, in which four -regenerator chambers are provided with means for reversing the flow of -gas and air in such a way that each pair of chambers serves alternately -to absorb the heat of the outgoing gases and subsequently to return -this heat to the incoming air that passes through one, and the incoming -gas that passes through the other of these chambers. In these furnaces, -the regenerator chambers themselves are generally placed underneath -the melting furnace, and they are built of fire-brick and filled with -loosely-stacked fire-bricks, whose function it is to absorb or deliver -the heat. In the most modern type of furnaces of this class, the -gas-regenerators are omitted entirely, the air only being pre-heated -by means of regenerators, while the gas enters the furnace direct -from the producer, thus carrying with it the heat generated in the -producer during the gasification of the fuel. While this arrangement -is undoubtedly economical, it has the serious disadvantage, especially -in the manufacture of sheet-glass, that the gas, rushing direct from -the producer into the furnace, carries with it a great deal of dust and -ash, which it has no opportunity of depositing, as in the older types -of furnace, in long flues. - -The most serious disadvantages of the ordinary types of regenerative -furnaces are due to the considerable dimensions of the regenerative -apparatus, necessitating a costly form of construction and occupying -a large space, while the necessity of periodically reversing the -valves so as to secure the alternation in the flow of outgoing and -incoming gases requires special attention on the part of the men -engaged in operating the furnace, as well as the construction and -maintenance of valves under conditions of heat and dirt that are not -favourable to the life of mechanical appliances. It is claimed that -all these disadvantages are overcome to a considerable extent in one -or other of the various forms of furnace known as “recuperative.” In -these furnaces there is no alternation of flow, and the regenerator -chambers are replaced by the “recuperators.” These consist of a large -number of small flues or pipes passing through a built-up mass of -fire-brick in two directions at right-angles to one another; through -the pipes running in one direction the waste gases pass out to the -chimney, while the incoming gas and air pass through the other set of -pipes. A transference of heat between the two currents of gas takes -place by the conductivity of the fire-brick, and thus the outgoing -gases are continuously cooled while the ingoing gases are heated--the -transference of heat being somewhat similar to that which takes place -in the surface condenser of a steam engine. Theoretically this is a -much simpler arrangement than that of separate regenerator chambers, -and to some extent it is found preferable in practice, but there -are certain disadvantages associated with the system which arise -principally from the peculiar nature of the material--fire-brick--of -which the recuperators must be constructed. In the first place, the -heat-conductivity of fire-brick is not very high, so that, in order -to secure efficiency, the recuperators must be large, and while the -individual pipes must be of small diameter, their area as a whole must -be large enough to allow the gases to pass through somewhat slowly. -Next, owing to the tendency of fire-brick to warp, shrink and crack -under the prolonged effects of high temperatures, it becomes difficult -to prevent leakage of gases from one set of pipes into the other. If -this occurs to a moderate extent its only effect will be to allow some -of the combustible gas to pass direct to the chimney, and at the same -time a dilution of the gases entering the furnace by an addition of -products of combustion from the waste-gas flues. This, of course, will -materially reduce the efficiency of the furnace and require a higher -fuel consumption if the temperature of the furnace is to be maintained -at its proper level. If, however, the leakage should become more -serious, a disastrous explosion might easily result, particularly if -the nature of the leakage were such as to allow the incoming gas and -air to mix in the flues. It follows from these considerations that, -although the recuperative furnace is somewhat simpler and cheaper to -construct, it requires, if anything, more careful maintenance than the -older forms of regenerative furnace. - -Tank furnaces for the production of sheet-glass in this country are -generally worked from early on Monday morning until late on Saturday -night, glass-blowing operations being suspended during Sunday, although -the heat of the furnace must be maintained. On the Continent, and -especially in Belgium, the work in connection with these furnaces goes -on without any intermission on Sunday--a difference which, however -desirable the English practice may be, has the effect of handicapping -the output of a British furnace of equal capacity by about 10 per -cent. without materially lessening the working cost. - -The process of blowing sheet-glass in an English glassworks -is generally carried out by groups of three workmen, viz., a -“pipe-warmer,” a “gatherer” and a “blower,” although the precise -division of the work varies according to circumstances. The -pipe-warmer’s work consists in the first place in fetching the -blowing-pipe from a small subsidiary furnace in which he has previously -placed it for the purpose of warming up the thick “nose” end upon -which the glass is subsequently gathered. The sheet-blower’s pipe -itself is an iron tube about 4 ft. 6 in. long, provided at the one -end with a wooden sleeve or handle, and a mouthpiece, while the other -end is thickened up into a substantial cone, having a round end. -Before introducing the pipe into the opening of the tank furnace, the -pipe-warmer must see that the hot end of the pipe is free from scale -or dirt and must test, by blowing through it, whether the pipe is free -from internal obstructions. He then places the butt of the pipe in the -opening of the furnace and allows it to acquire as nearly as possible -the temperature of the molten glass. When this is the case the pipe is -either handed on to the gatherer, or the pipe-warmer, who is usually -only a youth, may take the process one step further before handing -it on to the more highly skilled workman. This next step consists in -taking up the first gathering of glass on the pipe. For this purpose -the hot nose of the pipe is dipped into the molten glass, turned slowly -round once or twice and then removed, the thread of viscous glass that -comes up with the pipe being cut off against the fire-clay ring that -floats in the glass in front of the working opening. A small quantity -of glass is thus left adhering to the nose of the pipe, and this is now -allowed to cool down until it is fairly stiff, the whole pipe being -meanwhile rotated so as to keep this first gathering nicely rounded, -while a slight application of air-pressure, by blowing down the pipe, -forms a very small hollow space in the mass of glass and secures the -freedom of the opening of the pipe. When the glass forming the first -gathering has cooled sufficiently, the gatherer proceeds to take up the -second gathering upon it. The pipe is again introduced into the furnace -and gradually dipped into the molten glass, but this must be done with -great care so as to avoid the inclusion of air-bells between the glass -already on the pipe and the new layer of hotter glass that is now taken -up. This freedom from air-bells is secured by a skilful gatherer by -a gradual rotation of the pipe as it is lowered into the glass, thus -allowing the two layers of glass to come into contact with a sort of -rolling motion that allows the air time to escape. When completely -immersed, the pipe is rotated a few times and is then withdrawn and the -“thread” again cut off. The mass of glass on the end of the pipe is now -considerably larger than before and requires more careful manipulation -to cause it to retain the proper, nearly spherical, shape. During the -cooling process which now follows the pipe is laid across an iron -trough, kept brimful of water; this serves to cool the pipe itself, and -also allows the pipe to be readily rotated backwards and forwards by -rolling it a little way along the trough. When the whole mass of glass -has again cooled sufficiently to be manipulated without risk of rapid -deformation, a third gathering of glass is taken up, in precisely the -same manner as that already described for the second gathering, and -if the quantity of glass required is large, or the glass itself is so -hot and fluid that only a comparatively small weight adheres at each -time of gathering, the process may be repeated a fourth or even a fifth -time, but as the weight of pipe and adhering glass increases with each -gathering, each step becomes more laborious, while the hot glass, being -now held on a much larger sphere, tends to flow off more readily, so -that greater skill is required to avoid “losing” the gathering. - -The care and skill with which these operations of gathering are carried -out determine, to a large extent, the quality of the resulting sheet -of glass; any want of regularity in the shape of the gathering leads -inevitably to variations of thickness in different parts of the sheet, -while careless gathering will introduce bubbles or “blisters” and other -markings. During the intermediate cooling stages the glass must be -protected from dust and dirt of all kinds, since minute specks falling -upon the hot glass give rise to an evolution of minute gas bubbles -which become painfully evident in the sorting room. - -When the last gathering has been taken up and the mass cooled so far -as to allow of its being carried about without fear of loss, the glass -forms an approximately spherical mass, with the nose-end of the pipe -at or near the centre of the sphere. The next stages of the process -consist in the preliminary shaping of this mass in such a way as to -bring the bulk of the glass beyond the end of the pipe, and then in -forming just beyond the end of the pipe a widened shoulder of thinner -and therefore colder glass, of the diameter required for the cylinder -into which the glass is to be blown. This is done by bringing the -glass into the successive shapes shown in Fig. 12, the forming of the -glass being effected by the aid of specially shaped blocks and other -shaping instruments in which the glass is turned and blown. The final -shape attained at this stage is a squat cylinder containing the bulk of -the glass at its lower end, and connected to the pipe by the thinner -and colder neck and shoulder already mentioned. - -[Illustration: FIG. 12.--Early stages in the formation of cylinders for -sheet glass.] - -At this point of the process the pipe with its adherent glass is handed -over to the blower proper. This operator works on a special stage -erected in front of small furnaces, called “blowing holes,” although -in some works these are dispensed with, and the stages are erected in -front of the melting furnace itself. The sheet-blower’s stage is simply -a platform placed over or at the side of a suitable excavation which -gives the blower the necessary space to swing the pipe and cylinder -freely at arm’s length. The blowing process itself involves very little -actual blowing, but depends rather upon the action of gravitation -and on centrifugal effects for the formation of the large, elongated -cylinder from the squat cylinder with which the blower commences. The -process consists in holding the thick, lower end of the cylinder in -the heating-furnace, and when sufficiently hot, withdrawing it and -swinging the pipe with a pendulum movement in the blower’s pit. The -cylinder thus elongates itself under its own weight, and any tendency -to collapse is counteracted by the application of air-pressure by the -mouth, the pipe being also, at times, rotated rapidly about its own -axis. The re-heating of the lower end of the cylinder is repeated -several times, until finally the glass has assumed the form of a -cylinder of equal thickness all over, but closed with a rounded dome at -the lower end (Fig. 13). This rounded end is now opened. In the case -of fairly thin and light cylinders this is done by holding the thumb -over the mouthpiece of the pipe in such a way as to make an air-tight -seal, and then heating the end of the cylinder in the blowing-hole. The -heat both softens the glass at the end and at the same time causes -considerable expansion of the air enclosed in the cylinder, with the -result that the end of the cylinder is burst open. After a little -further heating, during which the glass at the end of the cylinder -becomes very soft, and takes a wavy, curly shape, the blower withdraws -the cylinder from the furnace, and holding it vertically downwards in -his pit, spins it rapidly about its longitudinal axis. The soft glass -at the lower end immediately opens out under the centrifugal action, -and the blower increases the speed of rotation until the soft glass has -opened out far enough to form a true continuation of the rest of the -cylinder, and in this position it is allowed to solidify. With thick, -heavy cylinders, the first opening of the end is done in a different -way. A small quantity of hot glass is taken up by an assistant on -an iron rod, and is laid upon the centre of the closed end of the -cylinder. The heat of this mass of hot glass softens the glass of the -cylinder, and the operator, with the aid of a special pair of shears, -cuts out a small circle of this softened glass, thus opening the end of -the cylinder. The final operation of straightening out the opened end -is carried out in the same way as described above for lighter cylinders. - -[Illustration: FIG. 13.--Later stage in sheet glass blowing.] - -The completed cylinder, still attached to the pipe, is now carried away -from the blowing-stage and laid upon a wooden rack; then the blower -takes up a piece of cold iron, and placing it against the neck of glass -attaching the cylinder to the pipe, produces a crack; a short jerk then -serves to completely sever the pipe from the cylinder. A boy now takes -the pipe to a stand where it is allowed to cool and where the adhering -glass cracks off from it prior to passing it back to the pipe-warmer -for fresh use. - -On the wooden rack the cylinder of glass is allowed to cool to a -certain extent, and then the remaining portion of the neck and shoulder -(see Fig. 13) are removed. This is done by a boy who passes a thread -of soft, hot glass around the cylinder at the point where it is to -be cut off; the thread of hot glass merely serves to produce intense -local heating, for as soon as it has become stiff, the thread of glass -is pushed off and a cold or moist iron is applied to the cylinder at -the point where it had been heated by the thread. As a rule a crack -immediately runs completely round the cylinder along the line of the -thread, and the “cap” is thus removed. The glass is now in the form of -a uniform cylinder open at both ends, but it must be opened out into a -flat sheet before it can assume the familiar form of sheet-glass. - -The first stage in the opening-out process is that of splitting. For -this purpose the cylinders are carried to a special stand, upon which -they are laid in a horizontal position, and here a crack or cut is -made along one of the generating-lines of the cylinder. This may be -done either by the application of a hot iron, followed, if necessary, -by slight moistening, or by the aid of a cut from a heavy diamond -drawn skilfully down the inside of the cylinder. It will be seen -from the account of the process so far given, that the glass has as -yet undergone no real annealing, although the blower is expected to -“anneal” his cylinder during the blowing process, as far as possible, -by never allowing it to cool too suddenly, and this degree of annealing -is usually sufficient to save the cylinder from breaking under its -internal stresses when left to cool on the racks. The surface of the -glass, however, is left in a decidedly hardened condition, especially -on the outside, which has necessarily been most rapidly cooled. For -this reason--among others--the splitting cut is always made on the -inside of the cylinder. The difference between the rates of cooling -of the outside and inside of the cylinder has a further effect, which -becomes evident as soon as the cylinder is split. The outside having -become hard while the inside was still relatively soft, the outer -layers of glass are in a state of compression and the inner layers in -a state of tension in the cold cylinder. As soon as the cylinder is -split, however, these stresses are to some extent relieved, the inner -layers being then free to contract and the outer layers to expand; the -result is an increase in the curvature of the cylinder, which slightly -decreases in diameter, the cut edges overlapping. If the cylinder has -been cooled rather too quickly, or if the glass itself has a high -coefficient of expansion, this release of internal stresses at the -moment of splitting becomes very marked, and each cylinder splits with -the sound of a small explosion, while if the internal stresses are -still more severe, the cylinders may even fly to pieces as soon as they -are cut. - -The next stage in the manufacture of a sheet of glass is the flattening -and annealing process. For this purpose the split cylinders are taken -to a special kiln, generally known as a “lear,” or “lehr,” where they -are first of all raised to a dull red-heat; they are then lifted, one -at a time, on to a smooth stone or slab placed in a chamber of the -kiln where the heat is great enough to soften the glass. Here the -cylinder is laid down with the split edges upwards, and by means of a -wooden tool the glass is slowly spread out, being finally rubbed down -into perfect contact with the slab or “lagre.” From the flattening -slab, the sheet as it now is passes into the annealing kiln, which -communicates with the flattening chamber. This consists, similarly to -other continuous annealing kilns already described in connection with -other varieties of glass, of a long tunnel, heated to the temperature -of the flattening kiln at one end and nearly cold at the other. The -sheets are moved down this tunnel at a uniform slow rate by the action -of a system of grids which, at intervals, lift the sheets from the -bottom of the kiln, move them forward by a short distance, and again -deposit them on the bottom, the grids themselves returning to their -former position by a retrograde movement made below the level of the -kiln-bottom, and therefore not affecting the glass. - -On leaving the annealing kiln the sheets of glass are sometimes covered -with a white deposit arising from the products of combustion in the -kiln and their interaction with the glass itself. This deposit can be -removed by simple mechanical rubbing, but it is usual to dip the glass -into a weak acid bath, which dissolves the white film and leaves the -glass clear and bright, ready for use. - -From the annealing kiln the finished sheets of glass are taken to the -sorting room, where they are examined in a good light against a black -background, and are sorted according to their quality for different -purposes. - -The defects which are found in sheet-glass are of a very varied -nature, as would be anticipated from the long and complicated process -of manufacture which the material undergoes in the course of its -transformation from the raw materials into the finished sheet of glass. -A full enumeration of all possible defects, with their technical names, -need not be given here, but a description of the more important and -frequent ones will be useful. The defects may be conveniently grouped -according to the stage of the process from which they originate. - -The first class of defects accordingly embraces those that arise from -the condition of the glass as it exists in the working-end of the -furnace. Chief of these are white opaque enclosures, known as “stones.” -These may arise from a variety of causes within the furnace, such as an -admixture of infusible impurities with the raw materials, insufficient -heat or duration of melting, leading to a residue of unmelted raw -material in the finished glass, or from defective condition of the -interior of the furnace, leading to contamination of the glass with -small particles of fire-brick. Further, if any part of the furnace has -been allowed to remain at too low a temperature, or if the composition -of the glass is unsuitable, crystallisation may occur in the glass, -and white patches of crystalline material may find their way into the -finished sheets. Another defect that may arise from the condition of -the glass in the furnace is the presence of numerous small bubbles, -known as “seed” in the glass. By the blowing process these are -drawn out into pointed ovals, and they are rarely quite absent from -sheet-glass. They arise from either incomplete fining of the glass in -the furnace or from allowing the glass to come into contact with minute -particles of dust during the gathering process. Another possible defect -to the glass itself may be found at times in too deep a colour. This -is only seen readily when a sheet of some size is examined edgewise, -as most varieties of ordinary sheet-glass are too free from colour to -allow this to be judged by looking through the sheet in the ordinary -way. It follows from this fact that for practical purposes, where -the light always traverses one thickness of the glass only, a slight -difference of colour should be regarded as a very minor consideration, -at all events as compared with freedom from other defects. - -The gathering process in its turn is responsible for further defects of -sheet-glass. Some of these, such as defects arising from the use of a -dirty pipe, are never allowed to pass beyond the sorting-room, and are -therefore of no interest to the user of glass. Of those whose traces -are seen in the glass that passes into use, “blisters” and “string” are -the most important. “Blisters” are somewhat larger, flat air-bells, -arising from the inclusion of air between successive layers of the -gathering. “String” is a very common defect in all sheet-glass. To some -extent it may arise from want of homogeneity in the glass itself. If -this consists of layers of different densities and viscosities, the -gatherer will take these up on his gathering, and ultimately they will -form thickened ridges of glass running around the cylinders and across -the sheets. Such striæ, due to want of homogeneity in the glass, are -much more common in flint glass than in the soda-lime glasses used for -sheet manufacture, but are not unknown in the latter. On the other -hand, even if the glass be as homogeneous as possible, the gatherer -can produce these striæ if he takes up his glass from a place close -to the side of the fire-clay ring that floats in the furnace in front -of his working opening. Glass always acts chemically upon fire-clay, -gradually forming a layer of glass next to the fire-clay that contains -much more alumina than the rest of the contents of the furnace. Such -a layer is formed on the surface of each ring in a sheet tank, but if -the gathering is taken from the centre of the ring, this thick viscous -layer of aluminiferous glass remains undisturbed. If, however, the -gatherer brings his pipe too near the side of the ring, the glass -will draw some of this different layer on to the gathering, and this -glass will form thick ridges and striæ running across the sheet in -all directions. Another defect for which the gatherer is generally -responsible is that of variation of thickness within the same sheet. -The blower, however, can also produce this defect. - -During the blowing proper, a further series of defects may be -introduced, principally by allowing particles of glass derived from -certain stages of the process to fall upon the hot glass of the -cylinder and there become attached permanently. More serious, and also -more frequent, is the greater or less malformation of the cylinder. If -the glass as it leaves the blower is of any shape other than that of -a true cylinder, it becomes impossible to spread it into a truly flat -sheet in the flattening kiln. Sometimes, in practice, the “cylinder” is -wider at one end than at the other, or, worse still, it is of uneven -diameter, showing expanded and contracted areas alternately. When such -a cylinder comes to be spread out on the slab it cannot be flattened -completely, and various hollows and hillocks are left, which mar the -flatness of the sheet and interfere with the regular passage of light -through it when in use. - -Finally, the process of flattening is apt to introduce defects of its -own. The most common of these are scratches arising from marks left by -the flattening tool; indeed, in all sheet glass it is quite possible -to see, by careful examination of the surfaces, upon which side the -flattening tool was used. Sheet-glass thus has one side decidedly -brighter and better in surface than the other, the better side being -that which rested upon the “lagre” during the flattening process. On -the other hand, if the slab itself be not quite perfect, or if any -foreign body be allowed to rest upon it, that side of the glass will be -marked in a corresponding manner. - -In the account of the manufacture of sheet-glass given above, we have -outlined one typical form of the process, but nearly every stage is -subject to modifications according to the practice and particular -circumstances of each works. We will now describe one or two special -modifications that are of more general importance. - -First, as regards the melting process, although the tank-furnace -has almost entirely superseded the pot furnace for the production -of ordinary sheet-glass, there are still some special circumstances -under which the pot furnace is capable of holding its own. Thus, -where for special purposes it is desired to produce a variety of -sheet-glass which, as regards all defects arising out of the glass -itself, and especially as regards colour, is required to be as perfect -as possible, melting in pots is found advantageous, and for some very -special purposes even covered (hooded) pots are used. For such special -purposes, too, sulphate of soda is eliminated from the raw materials -and carbonate of soda (soda ash) substituted. For the production of -tinted glasses also, whether they are tinted throughout their mass, -or merely covered with a thin layer of tinted glass (“flashed”), -manufacture in pot rather than tank furnaces is generally adopted, -the exact nature and composition of the glass being far better under -control in the case of pots. - -The blowing process is also subject to wide variations of practice. The -most important of these variations concerns the shape and dimensions -of the cylinders. In English and Belgian works the dimensions of the -cylinders are so chosen that the length of the cylinder constitutes the -longest dimension of the finished sheet, the diameter of the cylinder -forming the shorter dimension. In some parts of Germany, however, the -practice is the reverse of this, the cylinders being blown shorter -and much wider, so that the circumference of the cylinder constitutes -the longest dimension of the finished sheet. It is, however, pretty -generally recognised that the latter method has very serious -disadvantages, although it is claimed that somewhat more perfect -glass can be obtained by its means. For the production of a special -variety of glass, known as “blown plate glass,” this method of blowing -short wide cylinders is still adhered to. This is a very pure form of -sheet-glass, blown into thick, small sheets which are subsequently -ground and polished in the same manner as plate-glass. Here the great -thickness of glass required seems to render the blowing of long -cylinders very difficult, and the other form is therefore adopted. On -the other hand, English patent plate-glass, which is made by grinding -and polishing the best quality of ordinary sheet-glass, is made from -glass blown into long narrow cylinders in the manner described in -detail above. - -The process of blowing described above is capable, with slight -modifications, of yielding glass with surfaces other than the plain -smooth face of ordinary sheet-glass. Thus fluted and “muffled” glass -are produced in a very similar manner to that described above for -ordinary sheet, except that the fluting or the irregular surface -markings which constitute the peculiarities of these two varieties of -glass, are impressed upon the surface of the cylinder at an early stage -in the process. - -From the outline description given above of the usual method of -manufacture of sheet-glass, it will readily be seen that this is a -long, complicated, and laborious process, involving the employment -of much skilled labour, and involving the production of a relatively -complicated form, viz., the closed cylinder, as a preliminary to the -production of a very simple form, viz., the flat sheet. It is therefore -by no means surprising to find that a great many inventors have worked -and are still working at the problem of a direct mechanical method of -producing flat glass possessing a natural “fire polish” at least equal -to that of ordinary sheet-glass. The earlier inventors have almost -uniformly endeavoured to attain this object by attempting to improve -the process of rolling glass, with a view to obtaining rolled sheets -having a satisfactory surface. We have already indicated why these -efforts have never met with success and what reasons there are for -believing that they are never likely to attain their object. A totally -different line is that taken by Sievert, to whose inventions we have -already referred in connection with the mechanical production of blown -articles. This inventor has endeavoured to utilise his process for -blowing large articles of glass for the direct production of sheets -of flat glass. His method is to blow, by the steam process described -in another chapter, a large cubical vessel, having flat sides, the -flatness of these sides being ensured by blowing the vessel into or -against a mould having flat sides. This flat-sided vessel is ultimately -to be cut up into five large sheets. This process also appears to -involve some of the main difficulties of rolling as regards the -means of transferring the glass from the furnace to the plate of the -blowing machine, and in practice the inventor has not yet succeeded in -producing glass of sufficiently good surface for the purposes of sheet -glass. - -Another class of processes entirely avoid all means of transferring -molten glass from the furnace to any machine, by working on glass -direct from the molten bath itself. Some of these processes are in -actual use in America, and others are being experimented with in -Europe, but their complete technical and commercial success has yet to -be proved; there can, however, be little doubt that they have overcome -the greatest of the many difficulties that stood in the way of the -mechanical production of sheet-glass, and that they are therefore -destined very shortly to solve the problem completely, in which case -they would, of course, rapidly supersede the hand process. - -One of the earliest of these direct processes proposed to allow the -molten glass to flow out from the furnace, downward, through a narrow -slit formed in the side or bottom of the tank. The impossibility of -keeping such a narrow orifice open and at the same time regulating the -flow of glass made this proposal impracticable, although the use of -drawing orifices has been revived in one of the latest processes. - -The American process, which is said to be at work under commercial -conditions, is not entirely satisfactory in this respect--that it is -a mechanical process for the production of cylinders and not of flat -sheets, so that the subsidiary processes of splitting and flattening -still remain to be carried out as before. In this process an iron -ring is lowered into the bath of molten glass through an aperture -from above; the glass is allowed to adhere to the ring which is then -slowly raised by mechanical means, drawing a cylinder of glass with -it. If left to itself, such a cylinder, owing to the effects of -surface tension in the glass, would soon contract and break off, but -the American invention avoids this action by chilling each bit of the -cylinder as soon as it is formed. This is done by the aid of air blasts -delivered upon both sides of the glass as it emerges from the bath, -and it is claimed that by this means cylinders of any desired length -and diameter may be drawn direct from the bath. The obviously great -mechanical difficulties connected with these operations have probably -been overcome, but not without sacrificing much of the simplicity of -the arrangement, and the relative economy of this process as a whole, -compared with the hand process, has yet to assert itself. - -The inventions of Fourcault, which are at present being developed on -the Continent by a syndicate of glass manufacturers, aim at a much more -direct process. Here also the glass is drawn direct from the molten -bath by the aid of a drawing-iron that is immersed in the glass and -then slowly raised, but in this case the piece immersed is simply a -straight bar, and the aim is to draw out a flat sheet. In this case -the tendency, under surface tension, is to contract the sheet into -a thread, and apparently the simple device of chilling the emerging -glass is not adequate to prevent this in a satisfactory manner, and -subsidiary devices have been added. Those that have been patented -include a mechanism of linked metal rods so arranged as to be immersed -and drawn out of the glass continuously with the emerging sheet, in -such a manner as to support the vertical edges of the glass and so -aid in resisting the tendency of the glass to contract laterally. -Another device consists in the use of a slit or orifice formed in a -large fire-brick that floats on the surface of the glass. Through this -orifice the glass is drawn, of the desired thickness and width. The -use of this orifice, however, interferes markedly with the perfection -of the product, and in fact all the glass produced in this way shows -quite plainly a set of longitudinal striations due to the inevitable -irregularities in the lips of the drawing slot. Further, it appears -to be impracticable to draw _thin_ glass in this way, a thickness of -from 2½ to 3 millimetres (about 1/8 inch) being the least that is -practicable, on account of the large amount of breakage that occurs -with weaker sheets. This process, in its present stage of development, -however promising, does not appear to have solved the problem of -mechanical manufacture of sheet-glass, since it is just in the -thinner, lighter kinds of glass that the advantages of sheet are most -pronounced. On the other hand, it is quite possible that this drawing -process, or some development arising from it, may shortly supplant the -casting process in the production of polished plate-glass, although for -the largest sizes of this product also, the difficulty and danger of -handling the weights involved may prove a serious obstacle. - -_Crown Glass._--Although this is a branch of manufacture that is -nearly obsolete, it deserves brief notice here, partly because it is -still used for the production of special articles, and also because it -illustrates some interesting possibilities in the use and manipulation -of glass. - -The process of blowing crown glass may be briefly described as that of -first blowing an approximately spherical hollow ball, then opening this -at one side and expanding the glass into a flat disc by the action of -centrifugal forces produced by a rapid rotation of the glass in front -of a large opening in a special heating furnace. The actual process -involves, of course, the preliminary of gathering the proper quantity -of glass, much in the manner already described in connection with -sheet-glass manufacture. This gathering is then blown out into a hollow -spherical vessel. This vessel is now attached to a subsidiary iron -rod by means of a small gathering of hot glass, applied at the point -opposite the pipe itself, the glass being thus, for a moment, attached -to both the pipe and the “pontil” or “punty” (as the rod is called). -The pipe is, however, detached by cracking off the neck of the original -glass, which now remains attached to the pontil in the shape of an open -bowl. This bowl is now re-heated very strongly in front of a special -furnace, the open side of the bowl being presented to the fire. The -pontil is meanwhile held in a horizontal position and rotated. As the -glass softens the rotation spreads it out, until finally the entire -mass of glass is formed into a simple flat disc spinning rapidly before -the mouth of the furnace. This flat disc or “table” of crown glass is -allowed to cool somewhat, is detached from the pontil by a sharp jerk, -and is then annealed in a simple kiln in which the glass is stacked, -sealed up, and allowed to cool naturally. - -It is obvious that by this process no very large sheets of glass can -be produced; tables 4 ft. in diameter are already on the large side, -and these can only be cut up into much smaller sheets on account of the -lump of glass by which the table was originally attached to the pontil, -and which remains fixed in the centre of the finished disc. For certain -ornamental purposes, where an “antique” appearance is desired, these -bullions are valued, but for practical purposes they interfere very -seriously with the use of the glass. As a matter of fact, even several -inches away from the central bullion itself, crown glass is generally -marked with circular wavings, which render it readily recognisable in -the windows of older buildings, but which decidedly detract from the -perfection of the glass. On the other hand, crown glass is still valued -for certain purposes, such as microscope slides and cover glasses, -where entire freedom from surface markings, such as those found in -sheet glass as a result of the flattening operations, is desirable. -While, therefore, the process has merely an historical interest so far -as ordinary sheet-glass purposes are concerned, it is still used in -special cases. - - - - -CHAPTER XI. - -COLOURED GLASSES. - - -In various chapters throughout the foregoing portions of this book -we have had occasion to refer to the colour of glass and the causes -affecting it, but these references have chiefly been made from the -point of view of the production of glasses as nearly colourless as -possible under the circumstances. While it is obvious that for the -great majority of the purposes for which it is used the absence of -all visible coloration is desirable or even essential in the glass -employed, there are numerous other uses where a definite coloration is -required. Thus we have, as industrial and technical uses of coloured -glass, the employment of ruby, green and purple glasses for signalling -purposes, as in the signal lamps of our railways, the red tail-lights -of motor-cars, or even the red or green sectors of certain harbour -lights and lighthouses; again, coloured glasses, ruby, green, and -yellow, are extensively employed in connection with photography. Rather -less exacting in their demands upon the correctness of the colour -employed are the architectural and ornamental uses to which coloured -glass is so extensively put in both public and domestic buildings, -while, finally, coloured glass is largely the foundation upon which -the stained-glass worker builds up his artistic achievements; in -another direction, coloured glass is also utilised in the production of -ornamental articles and of some table-ware. While it must be admitted -that in a great many cases the colour-resources of the glass maker are -hopelessly misapplied, yet in really artistic hands few other materials -are capable of yielding results of equal beauty. - -By the “colour” of a glass is generally understood the tint or colour -which is observed when it is viewed, in comparatively thin slices, by -transmitted light; the actual colour is thus a property, not so much of -the kind or variety of glass as of each individual piece, since thick -pieces out of the same melting will show a different tint from that -seen in thinner pieces. As we have already pointed out, such glasses as -sheet or plate, which appear practically colourless when viewed in the -ordinary way, show a very decided green colour when viewed through a -considerable thickness. In the same way, a very thin layer of the glass -known as “flashing ruby” shows a brilliant red tint, but a thickness of -one-sixteenth of an inch is sufficient to render the glass practically -opaque, giving it a black appearance by both transmitted and reflected -light. Again, cobalt blue glass, when examined with a spectroscope in -thin layers, is found to transmit a notable proportion of red rays, -but thicker pieces entirely suppress these rays. These phenomena will -be readily understood when we recollect that colour in a transparent -medium arises from the fact that the medium has different absorbing -powers for light of different colours. All transparent substances, -and certainly glass, are only _partially_ transparent: all light -waves passing through such a substance are gradually absorbed, and -the extent to which they are absorbed differs according to the length -of these waves. It always happens that for some special wave-lengths -the substance has the power of absorbing the energy of the entering -waves and converting it into heat-vibrations of its own molecules or -atoms. In the most transparent and colourless glasses this process, so -far as the waves of ordinary light are concerned, only goes on to a -negligibly slight extent; if, however, we extend our view beyond the -range of ordinary visible light, and consider the region of shorter -waves that lies in the spectrum beyond the violet, we find that -ordinary colourless glass becomes strongly absorbent; thus to waves -of about half the length of those which produce upon our eyes the -impression of yellow light, ordinary glass is as opaque as is a piece -of metal to white light. In this wider sense, then, we may fairly say -that all glasses are coloured--_i.e._, all have a power of selective -absorption; but in the case of those which are nearly colourless in -the ordinary sense, this absorption takes place only for waves which -are either decidedly shorter or decidedly longer than those to which -our eyes are sensitive. Those glasses which appear coloured in the -ordinary sense, on the other hand, owe this property to the fact that -the power of absorption for light-waves extends into the region of the -visible spectrum; thus a blue or violet glass is practically opaque to -red rays, while a red glass is opaque to blue, green or violet rays. -This statement may be verified in a striking manner by holding over one -another a piece of deep blue or green glass and a similar piece of ruby -glass--the combination will be found to be very nearly opaque even when -each glass by itself is practically transparent. - -The question which now naturally presents itself to us is, what is -the essential difference between, for instance, a piece of red glass -and a piece of “white” glass that confers upon the former the power -of absorbing blue light? A perfectly complete and satisfactory answer -to this question is not, in the writer’s opinion, available in the -present state of our knowledge, but to a certain extent the difference -between the two kinds of glass can be explained. The difference is -_produced_, in the first instance by introducing into the colourless -glass some additional chemical element or elements, the substances -in question being generally known as “colouring oxides,” although -they are by no means always introduced in the form of oxides, and are -frequently present in the glass in entirely different forms. To a -certain extent the colour of the glass may be ascribed to a definite -“colouring” property of the chemical elements concerned; thus most -of the chemical compounds of such elements as nickel, cobalt, iron, -manganese and copper are more or less deeply coloured substances, -and it would seem as if the atoms or “ions” of these elements had -the specific power of absorbing certain varieties of light-waves -while not materially affecting others. But this specific “colouring” -property is not so easily explained when we recollect that the colours -of iron compounds, for example, may be green or red according to the -state of combination in which that element is present, and that iron -has also the power of imparting either a green or a yellow colour to -glass according to circumstances. The detailed discussion of these -questions, however, lies outside our present scope, and we must -confine ourselves to the broad statement that colouring substance in -glass may be roughly divided into two kinds or groups; the first -and probably the largest group are those bodies which occur in glass -in true solution, the element itself being present in the combined -state as a silicate or other such compound (borate, phosphate, etc.) -which is soluble in the glass. In this class, the colouring effect -upon the glass is specifically that of the element introduced, and is -brought about in the same way as the colouring of water when a coloured -salt--such as copper sulphate--is dissolved in it. The second class of -colouring substances, however, behave in a different manner; they are -probably present in the glass in a state of extremely fine division, -and held not in true solution, but really in a sort of mechanical -suspension that approximates to the condition of what is known as a -“colloidal solution.” The point which is known beyond doubt, thanks -to the researches of Siedentopf and Szigmondi on ultra-microscopical -particles, is that in certain coloured glasses, of which ruby glass is -the best example, the colouring substance, be it gold or cuprous oxide, -is present in the form of minute but by no means atomic or molecular -particles suspended in the glass. The presence of these particles has -been made optically evident, although it can hardly be said that they -have been rendered visible, and it is at all events probable that these -suspended particles act each as a whole in absorbing the light-waves -characteristic of the colour which they produce in glass. This being -the case, it is easy to understand how readily the colour of such -glasses is altered or spoilt by manipulations which involve heating -and cooling at different rates--too rapid a rate of cooling producing -a different grouping of the minute particles, altering their size or -shape, or even obliterating them entirely by allowing the element in -question to go into or to remain in solution in the glass. - -While it would be entirely foreign to the purpose of this volume to -give in this place a series of recipes for the production of various -kinds of coloured glass, it will be desirable to state in general terms -the colours or range of colours which can be produced in various kinds -of glass by the introduction of those chemical elements which are -ordinarily used in this way. In general terms it may be said that the -lighter elements do not as a rule tend to the production of coloured -glasses, while the heavier elements, so far as they can be retained in -the glass in either solution or suspension, tend to produce an intense -colouring effect. The element lead appears to form a striking exception -to this rule, but this is due to the fact that while the silicates -of most of the other heavy elements are more or less unstable, the -silicate of lead is very stable, and can only be decomposed by the -action of reducing agents. When lead silicates are decomposed in this -way, however, the resulting glass immediately receives an exceedingly -deep colour, being turned a deep opaque black, although in very thin -layers the colour is decidedly brown. On the other hand, glasses -very rich in lead are always decidedly yellow in colour, and it has -been shown that this coloration is due to the natural colour of lead -silicates and not to the presence of impurities. What has just been -said of lead applies, with only very slight modification, also to the -rare metal thallium and its compounds, which have been introduced into -glass for special purposes. Leaving these two exceptional bodies on -one side, we now pass to a consideration of the elements in the order -of their chemical grouping. The rare elements will not be considered -except in certain cases where their presence in traces is liable to -affect results attained in practice. - -The _Alkali Metals_, sodium, potassium, lithium, etc., and their -compounds, have no specific colouring effect, although the presence -of soda or of potash in a glass affects the colours produced by such -substances as manganese, nickel, selenium, etc. - -_Copper_, as would be anticipated from the deep colour of most of -its compounds, produces powerful colouring effects on glass. Cupric -silicates produce intense green, to greenish-blue tints. Copper, -either as metal or oxide, added to glass in the ordinary way, always -produces the green colour; but when the full oxidation of the copper is -prevented by the presence of a reducing body, and the glass is cooled -slowly, or is exposed to repeated heating followed by slow cooling, -an intense ruby coloration is produced. In practice this colour is -produced by introducing tin as well as copper into the mixture, and -so regulating the conditions of melting as to favour reduction rather -than oxidation of the copper. Under these circumstances the copper is -left in the glass in a finely divided and evenly suspended state; if -exactly the right state of division and suspension is arrived at, a -beautiful red tint is the result, although the coloration of the glass -is so intense that it can only be employed in very thin sheets, being -“flashed” upon the surface of colourless glass to give it the necessary -strength and thickness for practical use. It is further very easy to -slightly alter the arrangement of the copper in the glass, with the -result of producing an opaque, streaky substance resembling sealing-wax -in colour and appearance, this product being, of course, useless from -the glass-maker’s point of view. Finally, by exceedingly slow cooling, -and under other favouring conditions which are not really understood, -the particles of suspended colouring-material--be it metallic copper or -cuprous oxide--grow in size and attain visible dimensions, appearing as -minute shimmering flakes, thus producing the beautiful substance known -as “aventurine.” - -_Silver_ is never introduced into glass mixtures, the reason being that -it is so readily reduced to the metallic state from all its compounds -that it cannot be retained in the glass except in a finely-divided -form, causing the glass to assume a black, metallic appearance -resembling the stains produced by the reduction of lead in flint -glasses. On the other hand, silver yields a beautiful yellow colour -when applied to glass as a surface stain, and it is widely used for -that purpose. - -_Gold_ is introduced into glass for the production of brilliant ruby -tints; its behaviour is very similar to that of copper, except that -the noble metal has a great tendency to return to the metallic state -without the aid of reducing agents. No addition of tin is therefore -required, but the rate of cooling, etc., must be properly regulated, -since rapidly cooled glass containing gold shows no special colour, -the rich ruby tint being only developed when the glass is re-heated -and cooled slowly. The colouring effect of gold is undoubtedly more -regular and uniform than that of copper, and it is accordingly possible -to obtain much lighter shades of red with the aid of the noble metal. -“Gold ruby” can therefore be obtained of a tint light enough to be used -in sheets of ordinary thickness, and the process of “flashing” is not -essential. - -The elements of the second group, such as magnesium, calcium, -strontium, barium, zinc and cadmium, exert no strong specific colouring -action on glass, with perhaps the exception of cadmium, and that -element only does so to any considerable extent in combination with -sulphur, sulphide of cadmium having the power of producing rich yellow -colours in glass. The sulphur compounds of barium also readily produce -deep green and yellow colours, and the formation of these tints is, -indeed, very difficult to avoid in the case of glasses containing much -barium. A colouring effect has sometimes been ascribed to zinc, but -this is not in accordance with facts. - -Of the elements of the third group, only boron and aluminium are ever -found in glass in any notable quantity. Boron is present in the form of -boric acid or borates, and as such produces no colouring effect, nor -does there seem to be any tendency for the separation of free boron. -The compounds of aluminium also possess no colouring effect, although -certain compounds of this element are utilised for imparting a white -opacity to glass for certain purposes--such glass being known as “opal.” - -The elements of the fourth group are of greater importance in -connection with glass. Carbon is capable of exerting powerful colouring -effects when introduced into glass. These effects are of two kinds, -viz., indirect in consequence of the reducing action of carbon on other -substances present, and direct from the presence of finely-divided -carbon or carbides in the glass. The latter are similar in kind to -those produced by the presence of other finely-divided elementary -bodies (copper, gold, lead, etc.) except that the lightness of the -carbon particles tends to the production of yellow and brown colours -rather than of red and black, while the chemical nature of carbon -renders the glass in which it is suspended indifferent to rapid -cooling, so far as the carbon tint is concerned. The indirect effects -of carbon, in reducing other substances that may be present in the -glass, become evident with much smaller proportions of carbon than are -required to produce visible direct effects. As we have seen above, -carbon, in the form of coke, charcoal or anthracite coal, is regularly -introduced, as a reducing medium, into glass mixtures containing -sulphate of soda. If even a slight excess of carbon be used for this -purpose, the formation of sulphides and poly-sulphides of sodium and -of calcium results, and these bodies, like all sulphides, impart a -greenish-yellow tint to the glass, at the same time bringing other -undesirable results in their train. - -_Silicon_, in the form of silicic acid and its compounds, is a -fundamental constituent of all varieties of glass, and in this form -is in no sense a colouring substance; on the other hand, there is no -doubt that under some conditions silicon may be reduced to the metallic -state at temperatures which normally occur in glass-furnaces, and it -is practically certain, that if present in glass in this condition, -silicon would colour the glass. It is just possible that some of the -colouring effects produced in ordinary glass by powerful reducing -agents, such as carbon, either in the solid form or as a constituent of -furnace gases, may be due to the reduction of silicon in the glass. - -_Tin_ by itself does not appear to have any colouring effect upon -glass, except that its oxide, in a finely suspended state, produces -opalescence and, in large quantities, white opacity. Tin, however, is -used in conjunction with copper in the production of copper-ruby, to -which reference has already been made. - -_Lead and Thallium_ have already been dealt with, and it only remains -to add that their presence in the glass, although not in itself -producing any intense colouring action, increases the colouring effects -of other substances. This is probably merely a particular case of the -fact that dense glasses, of high refractive index, are more sensitive -to colouring agencies than the lighter glasses of low refractive index; -this applies to barium as well as to lead and thallium glasses. - -_Phosphorus_ occurs in some few glasses in the form of phosphoric -acid, and this substance, as such, has no colouring effect. Calcium -phosphate, however, is sometimes added to glasses for the purpose of -producing opalescence. Its action in this respect is probably similar -to that of tin oxide and aluminium fluoride, these substances all -remaining undissolved in the glass in the form of minute particles in a -finely divided and suspended state. - -_Arsenic_ does not exert a colouring effect on glass, and owing to its -volatile nature it can only be retained in glass in small quantities -and under special conditions. A “decolourising” action is sometimes -ascribed to arsenic, but if this action really exists it can only be -ascribed to the fact that arsenic compounds are capable of acting as -carriers of oxygen, and their presence thus tends to facilitate the -oxidation of impurities contained in the glass. A further reference -to this subject will be found below in reference to the compounds of -manganese. - -_Antimony_, although frequently added to special glass mixtures, does -not appear to produce any very powerful effects, except possibly -in the direction of producing white opacity if present in large -proportions. The sulphide of antimony, however, exerts a colouring -influence, although its volatile and unstable character renders the -effects uncertain. - -_Vanadium_, owing to its rarity, is probably never added to glass -mixtures for colouring purposes, although it is capable of producing -vivid yellow and greenish tints when present even in minute -proportions. On the other hand, vanadium occurs in small proportions -in a number of fire-clays, including some of those of the Stourbridge -district, and glass melted in pots containing this element is liable to -have its colour spoilt by taking up the vanadium from the clay. - -_Sulphur_ is an element whose presence in various forms is liable -to affect the colour of glass in a variety of ways. The colouring -effects of sodium-, calcium-, cadmium-, and antimony-sulphides have -already been referred to. Sulphur probably never exists in glass in -the uncombined state at all, but sulphur and its oxides, which are -often contained in furnace gases, sometimes exert a very marked action -upon hot glass. The presence of sulphur gases in the atmospheres of -blowing-holes and annealing kilns is liable to produce in the glass -a peculiar yellowish milkiness which penetrates for a considerable -depth into the mass of the glass and cannot be removed by subsequent -treatment. Glass vessels, particularly if made of glass produced from -raw materials among which salt-cake has figured, are also affected by -contact with fused sulphur or its vapour, the effect being a gradual -disintegration of the glass. The precise mechanism of these actions -is not known at present, but they probably consist in the formation -of sulphur compounds within the glass, possibly giving rise to an -evolution of minute bubbles of gas. - -_Selenium_, which is chemically so closely related to sulphur, is -a relatively rare element, which is, however, finding some use -in glass-manufacture as a colouring and a decolouring agent. The -introduction of selenium or of its compounds under suitable conditions -into a glass mixture produces or tends to produce a peculiar -yellowish-pink coloration, the intensity of the colour produced being -dependent upon the chemical nature of the glass as a whole and, of -course, upon the amount of selenium left in the glass at the end of the -melting process, this latter in turn depending upon the duration and -temperature of the process in question. The pink colour of selenium -glass is best developed in those containing barium as a base, but it is -also developed in lead glasses, while soda-lime glasses do not show the -colour so well. As a “decolouriser” the action of selenium is entirely -that of producing a complementary colour which is intended to “cover” -the green or blue tint of the glass; where the depth of the tint to -be “covered” is small, selenium can be used very successfully in this -way, although it is a relatively costly substance for such a purpose. -No oxidising or “cleansing” action can be ascribed to selenium or its -compounds. - -_Chromium_ is one of the most intensely active colouring substances -that are available for the glass-maker, and it is accordingly used -very extensively. It has the advantage of relative cheapness, and can -be conveniently obtained and introduced into glass in the form of pure -compounds whose colouring effect can be accurately anticipated; the -colours produced by the aid of chromium have the further advantage of -being very constant in character, being little affected by oxidising -or reducing conditions, and only very slightly by the length or -temperature of the melting process. The rate of cooling, in fact, -appears to be the only factor that materially affects the colours -produced by compounds of chromium. The colours produced by chromium -alone are various depths of a bright green, the depth varying, of -course, with the proportion of chromium that is present in the glass -and with the purity of the glass itself. Very frequently, chromium -is used in conjunction with either iron or copper to produce various -tints of “cold blue” and “celadon green” respectively. This element -is most usually introduced into the glass mixture in the form of -potassium bichromate; although other compounds might be employed, -this substance presents several advantages to the glass maker. In -the first place, since the colouring effect of chromium is very -intense, it must be used in very small quantities, and if chromic -oxide itself were used, the weighing would have to be carried out with -extreme care; potassium bichromate, however, contains a much smaller -proportion of the effective colouring substance, so that much larger -weights can be employed, and the accuracy of weighing required is -proportionately reduced. A further consideration arises from the fact -that chromic oxide is itself an extremely refractory body, and is -therefore comparatively difficult to incorporate with glass, while its -presence tends to make the glass itself more viscid and refractory; the -simultaneous introduction of the alkali, as provided by the use of the -bichromate, is thus an advantage in restoring the fluidity and softness -of the glass when finished, while also facilitating the solution of -the chromium in the glass during the fusion process; this process of -solution, however, takes some time, chromium glasses being liable to -appear patchy if insufficient time is given to the “founding.” - -_Uranium_ is one of the rarer and more costly elements, but is -nevertheless used in glass-making for special purposes on account of -the very beautiful fluorescent yellow colour which it imparts when -added in small proportions. This yellow is quite characteristic and -unmistakable, so that none of the other varieties of yellow glass -can ever be used as a substitute for uranium glass, but the great -cost of the latter prevents its extended use. Uranium is usually -introduced into glass mixtures in the form of a chemical compound, -such as uranyl-acetate or uranyl-nitrate, both these substances being -obtainable in the form of small, intensely bright yellow crystals. - -_Fluorine_ occurs in a number of glasses in the form of dissolved or -suspended fluorides, principally fluoride of aluminium. The element -is not essentially a colouring substance, and is only mentioned here -because the fluoride named is the most frequently used means of -producing “opal” glass. The fluoride is most frequently introduced -into the glass mixtures as calcium fluoride, used in conjunction with -felspar, or as cryolite, a natural mineral which consists of a double -fluoride of sodium and aluminium. - -_Manganese_ is one of the most important colouring elements used by -the glass-maker. When introduced into glass in the absence of other -colouring ingredients, compounds of manganese produce a range of -colours lying in the region of pinkish-purple to violet, according -to the chemical nature of the glass. The exact colour produced varies -according as the glass has lead, lime or barium as its base, and it -also depends upon the presence of soda or potash as the alkaline -constituent. The nature and intensity of the colour, however, which -the addition of a given percentage of manganese will produce depends -upon other factors besides the chemical composition of the bases used -in the mixture. The heat and duration of the “found” and the reducing -or oxidising conditions of the furnace in which it has been carried -on very materially affect the result. Thus, a glass having a slight -tinge of pink or purple derived from manganese can be rendered entirely -colourless by the action of reducing gases or by introducing into the -glass a reducing substance, such as a piece of wood. It will thus be -seen that while manganese is a most useful element for the glass-maker, -its employment requires much skill and care, and generally involves -some troublesome manipulations before the desired result is attained. - -In practice, manganese is most frequently used with other colouring -ingredients for the production of what may be called “compound” -colours, the function of the manganese being to provide the “warm” -element, _i.e._, the pink or purple component, required. One of the -most important uses of manganese coming under this head is its use as -a “decolouriser.” By a “decolouriser” the glass-maker understands a -substance which can be used to improve the colour of a glass which, -from the nature of its raw materials and conditions of melting, would -have a greener colour than is thought desirable for the product -in question. It may be said at once that the most perfect and -satisfactory method of obtaining the better colour required is to -adopt the use of purer raw materials and methods of melting less liable -to lead to contamination of the glass. On the other hand, this radical -course is often impossible on the ground of expense, and the less -satisfactory course must be adopted of covering one undesirable colour -by another complementary colour which would, in itself, be equally -undesirable. The rationale of this procedure depends upon the fact -that a slight amount of absorption of light is not readily detected by -the human eye if it be uniformly or nearly uniformly distributed over -the whole range of the visible spectrum, _i.e._, if the colour of the -resulting light is nearly neutral, while an equally slight absorption -in one region of the spectrum, while actually allowing more light -to pass through the glass, is at once detected by the eye owing to -the colour of the transmitted light. Now it has been found that the -colour produced in glass by the addition of very small proportions of -manganese is approximately complementary to the greenish-blue tinge of -the less pure varieties of ordinary glass; the addition of manganese in -suitable proportions to such glass therefore results in the production -of a glass which transmits light of approximately neutral, usually -slightly yellow, colour, the increased total absorption only becoming -noticeable in large pieces. This “covering” of the greenish tinge is -generally most completely successful in the case of soda-flint glasses, -but the method is also used to a certain extent in the case of the -soda-lime glasses used for sheet and plate-glass manufacture. Manganese -added to glass for this purpose is generally introduced into the -mixture in the form of the powdered black oxide (manganese dioxide), -which is available as a natural ore in a condition of sufficient -purity. Added in this form, the manganese compound exerts a double -action, the decomposition of the dioxide resulting in the liberation of -oxygen within the mass of melting glass, and this oxygen itself exerts -a favourable influence on the resulting colour of the glass, since it -removes organic materials whose subsequent reducing action would be -deleterious, and it also converts all iron compounds present into the -more highly-oxidised (ferric) state in which their colouring effects -are less intense. The actual colouring effect of the manganese itself -is, of course, afterwards developed, and produces the effects discussed -above. - -The “covering” of the greenish tints due to iron and other compounds -is only possible when these are present in very small proportions. -When larger quantities of these substances have been introduced into -the glass the addition of manganese modifies the resulting colour, but -is no longer able to neutralise it. A very large range of colours can -be obtained by using various proportions of iron and manganese, the -best-known of these being the warm brown tint known as “hock-bottle,” -while all shades between this and the bright green of iron and the -purple of manganese can be obtained by suitable mixtures. What has been -said above as to the sensitiveness of manganese colours applies with -even greater force to these mixed tints, since here both the iron and -the manganese compounds are liable to undergo changes of oxidation. -Copper-manganese and chromium-manganese colours are also used, as -indeed almost any number of colouring ingredients may be simultaneously -introduced into a glass mixture, the resulting colour being, as a rule, -purely additive. - -_Iron_ is so widely distributed among the materials of the earth’s -crust that it is exceedingly difficult to exclude it entirely from -any kind of glass, although the purest varieties of glass contain the -merest traces of this element. Cheaper varieties of glass, however, -always contain iron in measurable quantity, while the cheapest kinds of -glass contain considerable proportions of this element. The colouring -effects of iron have already been alluded to at various points in the -earlier chapters as well as in the section on manganese just preceding. -Little further remains to be said here. Just as the less highly -oxidised compounds of iron--_i.e._, the “ferrous” compounds--always -show a decided green tint, so glasses containing iron when melted under -the usually prevalent reducing conditions of a glass-making furnace, -show a decided green tint whose depth depends upon the amount of -iron present, provided no manganese or other “decolouriser” has been -introduced. “Ferrous” compounds are, however, readily converted into -the more highly oxidised or “ferric” state by the action of oxidising -agents, and this change can also be brought about in molten glass by -the action of such substances as nitrates or other sources of oxygen. -The ferric compounds, however, show characteristic yellow tints which -are much less intense and vivid than the corresponding green colours -of the “ferrous” series, and a similar result is brought about by the -oxidation of iron compounds contained in glass; hence the “washing” or -cleansing effects ascribed to oxidising agents introduced in the fusion -of glass. It should, however, be borne in mind that the oxidation of -other substances besides iron compounds, viz., organic matter, carbon -and sulphur compounds, may, and probably does, play a most important -part in this process in the case of most varieties of glass. - -_Nickel_ exerts a powerful colouring influence on glass, in accordance -with the fact that most of the other compounds of this element are -also deeply coloured. The exact colour produced in glass depends -upon the nature of the glass and on the condition of oxidation in -which the nickel is present. The colours, however, are usually of -a greenish-brown tint, although brighter colours can be produced -by nickel under special conditions. This element is not, however, -much used as a colouring agent in practice, although it has been -advocated as a “decolouriser.” The writer is not, however, aware -that it has ever been successfully used for this purpose, and, in -fact, the colours to which it gives rise do not appear to be even -approximately complementary to the ordinary green and blue tints which -“decolourisers” are intended to cover. - -_Cobalt_ is one of the most powerful colouring agents in glass, and is -very largely used in the production of all varieties of blue glass. The -blue colour produced by cobalt is, in fact, probably the most “certain” -of the colours available to the glass-maker, this tint being least -affected by all those circumstances that lead to variations in other -tints. Almost the only difficulty involved in the use of cobalt is the -great colouring power of this element, which requires that for most -purposes only very small quantities may be added to the glass mixture. -Formerly cobalt was added to glass mixtures in the form of “zaffre,” -which was a very impure form of cobalt oxide. At the present time, -however, the more expensive but much more satisfactory pure oxide of -cobalt is in almost universal use. This substance shows a perfectly -constant composition and, by means of accurate weighing, enables the -glass-maker to introduce precisely the right amount of cobalt into his -batch. - -The range of colours which are available to the modern glass -manufacturer are, as will be seen from a consideration of the list of -colouring elements given above, practically unlimited, particularly as -these substances can be used in almost any combination to produce mixed -or intermediate tints. This practically infinite variety of possible -tints, indeed, involves the principal difficulty encountered by the -manufacturer of coloured glass, _i.e._, that of matching his tints, or -of keeping the colour of any particular variety of glass so constant -that pieces produced at various times can be used indiscriminately -together. This ideal is, perhaps, never entirely realised, but in the -case of glasses intended for special technical uses the ideal degree of -constancy is very closely approached. - -In addition to being called upon to produce a large variety of -different tints, the glass-maker is also called upon to produce -various depths of the same tint. In many cases this can be readily -done by the simple means of varying the amount of colouring material -added to the glass. Where the colouring effect of small quantities of -these substances is not excessively powerful there is no very great -difficulty in doing this, but in certain cases this mode of regulating -the intensity of the colour is not available. Thus copper-ruby glass -cannot readily be made of so light a tint as to appear of reasonable -depth when used in sheets of the thickness of ordinary sheet-glass. -As has already been indicated, the desired tint is obtained by the -process of “flashing,” _i.e._, of placing a very thin layer of deep -ruby-coloured glass upon the surface of a sheet of ordinary more -or less colourless glass of the usual thickness. This is generally -accomplished by having a pot of molten ruby glass available close to a -pot from which colourless glass is being gathered. A small gathering -of ruby glass is first taken up on the pipe, and the remaining -gatherings required for the production of the sheet are taken from -the pot of colourless glass. When such a composite gathering is blown -into a cylinder in the manner described in the previous chapter, the -ruby glass lies as a thin layer over the inner face of the cylinder, -but special care and skill on the part of the gatherer and blower is -required to ensure that this layer shall be evenly distributed and -of the right thickness to produce just the tint of ruby required. -Since the whole layer of red glass is so thin, a very slight want of -uniformity in its distribution leads to wide variations of tint, and in -practice these are often seen in the less successful cylinders of such -glass. - -The chemical composition of the ruby and the colourless glass which -are to be employed for this purpose must also be properly adapted -to one another in order to produce two glasses which shall have as -nearly the same coefficient of thermal expansion as possible. If this -requirement is not met, the resulting glass is subjected to internal -strains which may lead to fracture, while, if the ruby glass has the -higher coefficient of expansion, the sheet after flattening tends to -draw itself up on the “flashed” side and cannot be passed out of the -annealing kiln in a properly flat condition. - -Although most usually applied to copper-ruby glass, the flashing -process is often used with other colours also. Coloured glass of -this kind is at once recognised when looked at through the edges. -Thus examined the glass simply shows the greenish tint of ordinary -sheet-glass which constitutes practically the entire thickness of the -sheet. In the same way, if such “flashed” glass be cut or etched in -such a way that the layer of coloured glass is removed in places, the -resulting pattern appears in white on the coloured ground--a feature -which is utilised for certain decorative purposes. The flashing -process just described, it should be noted, is applicable to any form -of glass-ware which is blown from a gathering, and the coloured layer -can be applied either upon the inside or outside of any object thus -produced. - -In addition to the palette of colours which the glass-maker is able to -supply, the artist in stained glass has a further range of colours at -his disposal in the form of stains and transparent colours which can be -applied to the surface of glass and developed and rendered more or less -permanent by being properly “fired.” The colours produced in this way -are also, in one sense, coloured glasses, or rather glazes, whose raw -materials are put upon the glass by the brush of the painter, and only -subsequently caused to combine and melt by suitable heating. The degree -of heat applicable under these circumstances is, however, very limited -by the necessity of avoiding any great softening of the substratum of -glass, while many of the colours themselves are composed of materials -which could not resist very high temperatures. The fluxes used in -the composition of these colours must for this reason be of a very -fusible kind, with the inevitable result of a greatly reduced chemical -stability as compared with the glass itself. - -The whole subject of painting on glass, even from the purely technical -as apart from the æsthetic point of view, is a very wide one, and lies -outside the scope of the present volume. Only one further technical -point in connection with glass-painting and stained glass work will -therefore be touched upon here. This is an example of the fact that -the more technically “perfect” modern product is not always preferable -for special purposes which have been well served by older and far -less “perfect” products. The production of technically excellent -coloured glass in modern times was, somewhat surprisingly at first, -accompanied by a very marked decline in the artistic beauty of stained -glass windows produced with this modern material; the ancient art of -stained glass was, therefore, for a time regarded as a “lost art,” and -glass-makers were blamed for being unable to produce the brilliant and -beautiful tints which had been formerly available. More careful study, -however, revealed the fact that while the actual colour of modern -glass was at least as brilliant and varied as that of ancient glass, -the difference lay in the fact that the modern glass was practically -entirely free from such imperfections as air-bubbles, striæ, and -other defects which improved appliances and methods had enabled the -glass-maker to eliminate from his products. Finding the beauty of his -wares greatly improved by this increased purity of the glass in the -case of window glass and table ware, it was natural for the glass-maker -to endeavour to produce the same “improvement” in the coloured glasses -intended for artistic purposes and, indeed, it is more than likely that -the stained-glass workers themselves pressed this line of improvement -upon him by a demand for “better” glass. It turned out, however, on -close examination, that this very perfection of modern glass rendered -it less adapted for these artistic purposes. A perfect piece of glass, -having smooth surfaces and no internal regularities, allows the rays -of light falling upon it to pass through undeflected in direction, -and merely changed in colour, according to the tint of the glass in -question. On looking at the glass, external objects can be quite -clearly seen, and much of the interest and mystery of the glass itself -is lost. On the other hand, when falling upon a piece of glass having -an irregular surface, and containing all manner of irregularities -such as striæ, air-bells, and even pieces of enclosed solid matter, -the light is scattered, refracted, and deflected into all manner of -directions until it almost appears to emanate from the body of the -glass itself, which thus appears almost to shine with an internal -light of its own; the eye can hardly perceive the presence of external -objects, and the whole window appears as a brilliant self-luminous -object. - -Once their attention had been drawn to these facts, modern glass-makers -endeavoured, and with much success, to reproduce the desirable -qualities of the ancient glass, while still availing themselves of -modern methods to produce more stable glasses and a wider range of -colours. The irregular surface of the old glass is imitated by using -rolled or “muffed” instead of ordinary blown glass, while the internal -texture is rendered non-homogeneous by the deliberate introduction of -solid and gaseous impurities and by manipulations so arranged as to -leave the glass in layers of different density, which appear in the -finished glass as “striæ.” As a consequence, it is probably not too -much to claim that the modern workers in coloured glass have materials -at their disposal which are at least as suitable for the purpose as -those that were available in the best days of the ancient art. - -Some reference has already been made to the technical uses of coloured -glass, but one or two further points in that connection remain to be -discussed. For such technical purposes as railway and marine signals, -the consensus of practical experience has decided in favour of certain -colours of glass, such as red and green of particular tints. On the -other hand, for various purposes in connection with photography, -the glass-maker does not appear to have been able to meet the new -requirements, with the result that flimsy and otherwise unsatisfactory -screens made of gelatine or celluloid stained with organic dyes are -employed in place of coloured glass in such cases, for example, as the -covering of lamps for use in photographers’ “dark” rooms, and for the -light-filters used for orthochromatic and tri-chromatic photography. -In all these cases it is necessary to use a transparent coloured -medium which transmits only light of a certain very definite range of -wave-lengths, and there is no doubt that for the glass-maker, who is -confined to the use of a number of elementary bodies for his colouring -media, it is by no means easy to comply with these requirements of -exact transmission and absorption. On the other hand, the field of -available coloured glasses has not been fully explored from this point -of view, the only extensive work on the subject having been done in -connection with the Jena firm of Schott, who have put upon the market -a series of coloured glasses of accurately-known absorbing power. -There is, however, little doubt that a much greater extension of this -field is possible, and that it will be opened up by a glass-maker who -undertakes the exhaustive study of coloured glasses from this point of -view, although it must be admitted that there is considerable doubt -whether the results obtainable by the aid of aniline and other dyes as -applied to gelatine can ever be equalled by coloured glasses. - - - - -CHAPTER XII. - -OPTICAL GLASS. - - -Optical glass differs so widely from all other varieties of glass that -its manufacture may almost be regarded as a separate industry, to -which, indeed, a separate volume could well be devoted. In the present -chapter we propose to give an outline of the most important properties -of optical glass, and in the next chapter to describe the more -important features of the processes used in its production. - -The properties which affect the value of optical glass may roughly be -divided into two groups. The first group comprises the specifically -“optical” properties--_i.e._, those directly influencing the behaviour -of light in its passage through the glass, while the second group -covers those properties of a more general nature, which are of special -importance in glass that is to be used for optical purposes. - -_Optical Properties of Glass._--The most essential property of glass -in this respect is homogeneity. We have already indicated that glass -can never be regarded as a definite chemical substance or compound, -but that it usually consists of mutual solutions of various complex -silicates, borates, etc. Solutions being of the very nature of mixtures -of two or more different substances, it follows that they can only -become homogeneous when _complete_ mixing has taken place. We have -a familiar example of the formation of such a solution when sugar is -dissolved in water. The water near the sugar becomes saturated with -sugar and of different density from the remaining water; if the liquid -is _slightly_ stirred a very characteristic phenomenon makes its -appearance--the pure water and the dense sugar solution do not at once -mix completely, the denser liquid remaining for a time disseminated -throughout the whole fluid mass in the form of more or less fine lines, -sheets, or eddies, and these are visible because the imperfectly mixed -liquids have different effects on the light passing through them. In -the case of sugar-water we are, however, dealing with a very mobile -liquid, and a few turns of a tea-spoon suffice to render the mixture -complete, and the liquid, which for a few moments had appeared turbid, -becomes homogeneous and transparent. In the case of glass, when the -raw materials are melted together, a mixture is formed of liquids of -differing densities similar to that which was temporarily formed in -the sugar-water solution. Molten glass, however, is never so mobile -a liquid as ordinary water, nor is it in the ordinary course of -manufacture subjected to any such thorough mixing action as that which -is produced by a spoon in a glass of water. In glass as ordinarily -manufactured, therefore, it is not surprising to find that the lack of -homogeneity which originates during the melting persists to the end. -Its effects can be traced whenever a thick piece of ordinary glass is -carefully examined, when the threads or layers of differing densities -can be recognised in the form of minute internal irregularities in the -glass. These defects are known as striæ or veins, and their presence -in glass intended for the better kind of optical work renders the -glass useless. As will be seen below in the production of optical -glass, special means are adopted for the purpose of rendering it -as homogeneous as possible; in fact, the early history of optical -glass manufacture is simply the history of attempts to overcome this -very defect. The problem is, however, beset by chemical and physical -difficulties of no mean order, and even in the best modern practice -only a small proportion of each melting or crucible full of glass -is entirely free from veins or striæ. In many cases these defects -are very minute, and sometimes escape observation until the stage of -the finished lens is reached. At that stage, however, their presence -becomes painfully evident from the fact that they interfere seriously -with the sharp definition of the images formed by the lens in question. -It will be seen that in such a case time and money has been wasted -by grinding and polishing what turns out to be a useless piece of -glass. Methods are, therefore, used for examining the glass before it -is worked, whereby the existence of the smallest striæ can scarcely -escape detection. These methods depend upon the principle that a beam -of parallel light passing through a plate of glass will meet with no -disturbance so long as the glass is homogeneous, but if striæ are -present, they will cause the light to deviate from parallelism wherever -it falls upon them. Under such illumination, therefore, the striæ -will appear as either dark or bright lines, when they can be readily -detected. One form of apparatus used for this purpose is illustrated in -Fig. 14. - -[Illustration: FIG. 14.--Diagram of striæ-testing apparatus. - -_L_, source of light; _S_, slit; _A_ and _B_, simple convex lenses; -_G_, glass under test; _E_, eye of observer. The arrows indicate the -paths of light-rays.] - -_Transparency_ and _colour_ are obviously fundamentally important -properties of glass. In one sense homogeneity is essential to -transparency, but the aspect of the subject which we are now -considering is that of the absorption of light in the course of regular -transmission through glass. It may be said at once that no glass is -either perfectly transparent or, what comes to nearly the same thing, -perfectly free from colour. In the case of the best optical glasses it -is true that the absorption of light is very slight, but even these, -when considerable thicknesses are viewed, show a greenish-yellow or -bluish colouring. On the other hand, certain optical glasses which are -used at the present time for many of our best lenses absorb light so -strongly or are so deeply coloured that a thickness of a few inches -is sufficient to reveal this defect. To some extent public taste or -opinion which objects to the use of even a slightly _greenish_ glass -in optical instruments of good quality is to blame for the tint of -these glasses. In many cases glass-makers could produce a very slightly -greenish glass, but in order to overcome this colour they deliberately -add to the glass a colouring oxide imparting to the glass a colour more -or less complementary to the natural green tint. The result is a more -or less neutral-tinted glass which, however, absorbs much more light -than the naturally green glass would have done. Since such glass is -frequently used for photographic lenses, it is interesting to note that -the light rays whose transmission is sacrificed in order to avoid the -green tint are those lying at or near the blue end of the spectrum, so -that the photographic rapidity of the resulting lenses is decidedly -reduced by the use of such glass. - -_Refraction and Dispersion._--The quantitative properties of glass, -governing its effect upon incident and transmitted light, are, of -course, of fundamental importance in all its optical uses. The -fundamental optical constant of each variety of optical glass is known -as its refractive index; this number really represents the ratio of -the velocity with which light waves are propagated through the glass -to the velocity with which they travel through free space. Not only -does this ratio vary with every change in the chemical composition and -physical condition of the glass, but it also varies according to the -length of the light waves themselves. In other words, the short waves -of blue light are transmitted through glass with a different velocity -from that with which the longer waves of red light are transmitted. -The consequence is that when a beam of white light is passed through a -prism it is split up and spread out into a number of beams representing -all the colours of the spectrum in their proper order, the blue light -suffering the greatest deflection from its original path, while the -red light suffers least deflection. Both the actual and relative -amount by which light rays of various colours are deflected under -such circumstances depends upon the nature of the glass in question; -therefore, to fully characterise the optical properties of a given -kind of glass it is necessary to state not only its refractive index -but to specify the refractive indices for a sufficient number of -different wave-lengths of light, suitably distributed through the -spectrum. For this purpose a number of well-marked spectrum lines have -been chosen, the systematic use of the particular set of lines which is -now usually employed being due to the initiative of Abbé and Schott at -Jena, who initiated the system of specifying the optical properties of -glass in this way. The actual lines chosen are the line known as A′, -corresponding to a wave-length of 0·7677 micro-millimetres, and the -lines known as C, D, F, and G′, whose wave-lengths, in the same units, -are 0·6563, 0·5893, 0·4862, and 0·4341 respectively. The A′ line, -however, lies so near the extreme red end of the spectrum that the data -concerning it are seldom required. - -As a matter of fact, the actual refractive index is only stated in most -tables of optical glasses for sodium light (D line), the dispersive -properties of the glass being indicated by tabulating the differences -between the refractive indices for the various lines, the table thus -containing columns marked C-D, D-F, F-G′. These figures are usually -described as the “dispersion” of the glass from C to D, D to F, etc. -In addition to these figures it is usual to tabulate what is called -the “mean dispersion” of the glass, which is simply the difference -between the refractive indices for C and F lines; this interval is -usually taken as representing that part of the spectrum which is of -the greatest importance for visual purposes. A further constant which -is of great importance in the calculations for achromatic lenses is -obtained by dividing the mean dispersion into the refractive index for -the D line minus one (usually written (C-F)/(n_{D}-1)=ν). This term, -for which no satisfactory name has yet been suggested, characterises -the ratio of the dispersive power of the glass to its total refracting -power. It is usually denoted by the Greek letter ν. The following table -(taken from the Catalogue of the Optical Convention, 1905) gives a list -of optical glasses produced by Messrs. Chance, of Birmingham. This -list, although it is not nearly so long as that issued by the French -and German firms who manufacture optical glass, contains examples of -the most important types of optical glass which are available at the -present time. Those, however, who wish to use the data for the purpose -of lens calculation are advised to consult the latest issues of the -optical glass-makers’ catalogues, since the range of types available, -and even the actual figures for some of the glasses, are liable to -variation from time to time. - - -TABLE OF OPTICAL PROPERTIES. - - --------------+------+----+------+----------------------------------- - | | | | Partial and - | | |Medium| Relative Partial Dispersions. - | | | Dis- +------+----+------+----+------+---- - Name. |n_{D}.| ν. | per- | |C-D | |D-F | |F-G′ - | | | sion.| C-D. |--- | D-F. |--- |F-G′. |--- - | | | C-F. | |C-F.| |C-F.| |C-F. - --------------+------+----+------+------+----+------+----+------+---- - Extra Hard | | | | | | | | | - Crown |1·4959|64·4|·00770|·00228|·296|·00542|·704|·00431|·560 - Boro-silicate | | | | | | | | | - Crown |1·5096|63·3|·00803|·00236|·294|·00562|·700|·00446|·555 - Hard Crown |1·5175|60·5|·00856|·00252|·294|·00604|·706|·00484|·554 - *Medium Barium | | | | | | | | | - Crown |1·5738|57·9|·00990|·00293|·296|·00697|·704|·00552|·557 - *Densest | | | | | | | | | - Barium | | | | | | | | | - Crown |1·6065|57·9|·01046|·00308|·294|·00738|·705|·00589|·563 - Soft Crown |1·5152|56·9|·00906|·00264|·291|·00642|·708|·00517|·570 - *Medium Barium | | | | | | | | | - Crown |1·5660|56·3|·01006|·00297|·295|·00709|·704|·00576|·572 - Barium Light | | | | | | | | | - Flint |1·5452|53·5|·01020|·00298|·292|·00722|·701|·00582|·570 - Extra Light | | | | | | | | | - Flint |1·5316|49·0|·01085|·00313|·288|·00772|·711|·00630|·580 - Extra Light | | | | | | | | | - Flint |1·5333|48·5|·01099|·00322|·293|·00777|·707|·00640|·582 - Boro-silicate | | | | | | | | | - Flint |1·5623|47·4|·01187|·00343|·289|·00844|·711|·00693|·584 - *Barium Light | | | | | | | | | - Flint |1·5833|46·6|·01251|·00362|·288|·00889|·711|·00721|·576 - Soda Flint |1·5482|45·8|·01195|·00343|·287|·00852|·713|·00690|·577 - Light Flint |1·5472|45·8|·01196|·00348|·291|·00848|·709|·00707|·591 - Light Flint |1·5610|43·2|·01299|·00372|·287|·00927|·713|·00770|·593 - Light Flint |1·5760|41·0|·01404|·00402|·286|·01002|·713|·00840|·598 - Light Flint |1·5787|40·7|·01420|·00404|·284|·01016|·715|·00840|·591 - Dense Flint |1·6118|36·9|·01657|·00470|·284|·01187|·716|·01004|·606 - Dense Flint |1·6214|36·1|·01722|·00491|·285|·01231|·715|·01046|·608 - Dense Flint |1·6225|36·0|·01729|·00493|·286|·01236|·715|·01054|·609 - Extra Dense | | | | | | | | | - Flint |1·6469|33·7|·01917|·00541|·285|·01376|·720|·01170|·655 - Densest Flint |1·7129|29·9|·02384|·00670|·281|·01714|·789|·01661|·678 - --------------+------+----+------+------+----+------+----+------+---- - -In the table on p. 212 the first column contains the ordinary trade -names by which the various types of glass are known. These names, -while somewhat arbitrary, indicate in a rough way the chemical nature -of the glass concerned. Thus the word “flint” always implies a glass -containing lead and therefore having a comparatively high refractive -index and low value of ν, while the word “crown,” originally applied -only to lime-silicate glasses, is now used for all glass having a high -value of ν. In the next column of the table are given the refractive -indices of the glasses, while the third column contains the values of -ν. It will be seen that the glasses are arranged in descending order -of magnitude in respect of this constant. An inspection of the figures -in these two columns will reveal the fact that for the majority of -the glasses contained in this table the value of ν decreases as the -refractive index increases. The glasses which are an exception to this -rule are indicated by an *. As a matter of fact this rule applied -to all glasses that were known or were at all events commercially -available prior to the modern advances in optical glass manufacture -which were initiated by Abbé and Schott of Jena. It was Abbé’s insight -into the requirements of optical instrument design that led him to -realise the importance of overcoming this limitation in the ratio -between the dispersive and refractive powers of glass. With the -collaboration of Schott he succeeded in producing a whole series of -previously unknown varieties of optical glass in which the relation -between n_{D} and ν is not that of approximately simple inverse -proportionality which holds for the older crown and flint-glasses. -Most valuable and in many ways most typical of these new glasses are -those known as the “barium crown” glasses, which combine the high -refractive index of a light flint or even a dense flint-glass with the -high ν value of an ordinary crown glass. It would lead too far into -the subject of lens construction to explain in detail the possibility -opened up to the optician by the use of these newer varieties of glass. -We must content ourselves with pointing out that the great forward -strides marked by the production of apochromatic microscope objectives, -of anastigmatic photographic lenses, and the modern telescope -objectives are all based upon the employment of these new optical -media; and although optical glasses of these newer types are at the -present time produced in the optical glass manufactories of France and -England, in quality and quantity at least equal to the output of the -Jena works themselves, these great optical achievements stand as a -lasting monument to the pioneer work of Abbé and Schott in this field. - -The last six columns of the table of optical glasses given above -contain figures which define the manner in which each of the glasses -named distributes the various sections of the spectrum. The columns -C-D, D-F, and F to G′ give as already indicated the differences between -the refractive indices for the C, D, F and G′ lines respectively; the -smaller figures in the intermediate columns indicate the ratio of -each of these differences to the mean dispersion of the glass. If all -kinds of glass distributed the various portions of the spectrum in the -same proportionate manner, merely differing in the total amount of -dispersion produced, these figures would be identically the same for -all glasses. In actual fact it will be seen that the figures differ -very widely from one type of glass to another. A moment’s consideration -will show that when two glasses are used in a lens for the purpose -of achromatising one another, _i.e._, when one is used to neutralise -the dispersion of the other, such achromatisation can only be perfect -if these ratios (the relative partial dispersions) are the same for -both glasses. To put the same statement in more concrete terms, if -the spectrum produced by one glass is comparatively long-drawn out -at the red end, relatively compressed at the blue end, while in the -other glass the opposite relation holds between the two ends of the -dispersion spectrum, it is evident that the two spectra can never be -superposed in such a way as to entirely neutralise one another--the -spectrum produced by the one glass will predominate and leave a -residual colour at the blue end, while the other will predominate -at the other end. In the case of lenses achromatised by the use of -such glasses, there will always be a slight fringe of colour around -the borders of the images which they produce. One of the aims which -Abbé and Schott set themselves in the production of new varieties of -optical glass was to obtain one or more pairs of glasses in which the -relative partial dispersions should be as nearly alike as possible -while the actual values of ν should differ as widely as possible. Some -success in this direction was at first claimed by the Jena workers, -but unfortunately some of the most promising glasses in this respect -were found to be too unstable for practical use and had ultimately to -be abandoned. At the present time the only pair of really perfectly -achromatic glasses offered by the Jena firm is that tabulated below, -and it will be seen that although the relative partial dispersions -are very closely alike, the ν values of the two glasses only differ -by 10, and at least one of these glasses is not readily obtainable in -really satisfactory optical quality. On the other hand, practically -perfectly achromatised lenses (generally known as “apochromatic”) have -been produced, especially by Zeiss of Jena, for microscopic purposes, -by the careful selection of glasses suited to each other in this -respect. Such a solution of the problem is further facilitated by the -fact that in these lenses more than two varieties of glass can be -used to neutralise one another, while a natural mineral (fluorite) -is also employed. From the glass-maker’s point of view, however, the -problem of producing a satisfactory pair of glasses capable of entirely -achromatising one another has yet to be solved. - - ---------+------+----+-------+------+------+------+------+------+----- - | | | | | c-d | | d-f | | f-g′ - Name. | n_{D}| ν | C-F. | C-D. | --- | D-F. | --- | F-G′.| --- - | | | | | c-f. | | c-f. | | c-f. - ---------+------+----+-------+------+------+------+------+------+----- - Telescope| | | | | | | | | - Crown |1·5254|61·7|·00852 |·00250| ·292 |·00602| ·707 |·00484| ·568 - Telescope| | | | | | | | | - Flint |1·5211|51·8|·001007|·00297| ·294 |·00710| ·705 |·00577| ·573 - ---------+------+----+-------+------+------+------+------+------+----- - -The table of optical glasses given above, although brief as compared -with the lists issued by French and German optical glass-makers, -fairly covers the range of practically available glasses, and a rapid -inspection will at once show how extremely limited this range really -is. Thus the refractive index varies only between the limits 1·49 -and 1·71, and even if we admit as practical glasses such extreme -types--offered by some makers--as would extend this range to 1·40 -in one direction and to 1·80 in the other, this does not affect the -present argument. Of course, a glass of a refractive index as low -as 1·0, or even 1·10, is not theoretically possible, since the mere -density of any substance enters into the factors that affect its -refractive index, and a glass having a density lower than that of -water (whose refractive index is about 1·3) is scarcely conceivable. -In the other direction, however, the limits met with in the case of -glass are considerably exceeded by certain natural mineral substances. -Thus the diamond has a refractive index of 2·42, while the garnets -show refractive indices from 1·75 to 1·81. The values of ν found in -the table of optical glasses are still more narrowly restricted, -lying between 67 and 29, while such a mineral as fluorite shows a -value of 95·4. These facts show that it is physically possible to -obtain transparent substances having optical properties lying far -beyond the limited range covered by our present optical glasses, and -it scarcely needs showing that if such an extended range of materials -were available greatly increased possibilities would be opened up to -the designer of optical instruments. It is consequently interesting -to inquire as to the actual causes which limit the range of optical -glasses at present available. It will be found that these limits are -set by the properties of glass itself. While the more ordinary kinds -of glass, having average optical properties and showing dispersive -powers roughly conforming to the law of inverse proportionality with -refractive index which governs the older varieties of optical glass, -are chemically stable substances, showing little tendency to undergo -either chemical changes or to crystallise during cooling, the more -extreme glasses exhibit these undesirable features to an increasing -extent the more nearly the limit of our present range is approached. -As the chemical composition of a glass is “forced” by the addition -of special substances intended to affect its optical properties in -an abnormal direction, so the chemical and physical stability of the -glass is rapidly lessened. The more extreme glasses, in fact, behave as -active chemical agents readily entering into reaction or combination -even with relatively inert substances in their environment--they act -vigorously upon the fire-clay vessels in which they are melted, and -they are readily attacked by acids, moisture or even warm air, when in -the finished condition, while many of them can only be prevented from -assuming the condition of a crystalline (and opaque) agglomerate by -being rapidly cooled through certain critical ranges of temperature. -A limit to the possibility of production is set by these tendencies -when they exceed a certain amount--a point being reached where it -ceases to be practicable to overcome the tendency of the glass to -self-destruction. On the lines of our present glasses, therefore, it -does not appear hopeful to look for any considerable extension of the -range of our optical media. On the other hand, as the known optical -properties of transparent crystalline minerals show, a much greater -range of optical constants would become available if it were possible -to manufacture artificial mineral _crystals_ of sufficient size and -purity for optical purposes, and the author believes that in this -direction progress in optical materials is ultimately bound to lie[1]. - - [1] See a Paper by the present author on “Possible Directions - of Progress in Optical Glass”--Proceedings of the Optical - Convention, London, 1905.] - -In addition to possessing the requisite optical constants, a good -colour and perfect homogeneity, certain other properties are essential -in good optical glass. These are the general physical and chemical -qualities which are essential in all good glass, but especially -emphasised by the fact that the requirements for optical glass are more -stringent than for any other variety of the material. Thus chemical -stability is of the greatest importance, for the best lenses would -soon become useless if the action of atmospheric moisture were to -affect them appreciably--the polished surfaces would rapidly become -dull and the whole lens would soon be rendered useless. The conditions -governing the chemical stability of glass and the methods of testing -this quality have already been indicated (Chapters I. and II.). The -harder varieties of optical glass, such as the glasses quoted in the -above table under the names of “Hard Crown” and Boro-Silicate Crown, -are probably among the most durable and chemically resistant of all -varieties of glass, but as we have already indicated, when extreme -optical properties are required, the necessary chemical composition of -the glass always entails a sacrifice of this great chemical stability, -until a limit is reached where valuable optical properties no longer -counterbalance the serious disadvantage of a chemical composition -which renders the glass liable to rapid disintegration. In certain -special cases it is, perhaps, possible to protect lenses made of such -unstable glass by covering them with cemented-on lenses of stable -glass, but this device entails concomitant limitations in the design -of the optical system and is, therefore, rarely used. In any case, -however, it is well for the lens-designer to consider the relative -stability of the glasses employed when arranging the order in which -they are to be used, since it is obviously preferable to put a hard, -durable glass on the outside of his system, where it is most directly -exposed to atmospheric moisture, and is also subject to handling and -“cleaning” by inexpert hands. This latter factor is a very important -one for the life of any lens. In the first place, a glass surface is -very seriously affected by the minute film of organic matter which is -left upon it when it has been touched with even a clean finger; unless -the glass is of the best quality in this respect, such fingermarks -readily develop into iridescent spots and may even turn into black -stains. Particles of dust allowed to settle on the surface of the glass -will affect it in the same way, so that the protection afforded by mere -mechanical enclosure in the tube of an instrument is of decided value -in preserving a glass surface. It should, however, be noted that in -some instances the interior metal surfaces of optical instruments are -varnished with substances that give off vapours for a long time after -the instrument is completed, and in that case the inside lenses are apt -to be tarnished in consequence. On the other hand, outside lenses are -also exposed to direct mechanical injury from handling and “cleaning.” -As far as the latter operation is concerned, it frequently happens, -particularly in glasses containing soda, that a slight surface dimming -is formed on the glass when it has been left in a more or less damp -place for a long time. This dimming is chiefly due to the formation on -the surface of a great number of very minute crystals of carbonate of -soda, which are hard and sharp enough to scratch the glass itself if -rubbed about over it. If such a lens be wiped with a dry cloth, however -clean and soft, the effect is a permanent injury to the polished -surface, which could readily be avoided by first washing the lens with -clean water, or even by using a wet cloth instead of a dry one for the -first wiping. - -The mechanical hardness of the glass is an important factor in -determining its resistance to such injurious treatment or to the -effects of accidental contact with hard, sharp bodies. The subject of -the hardness of glass has already been discussed in a general way in -Chapter II., and little remains to be added here. Broadly speaking, a -high degree of hardness and a low refractive index are found together. -This statement is certainly true where any considerable difference of -hardness is considered, as, for example, in comparing a hard crown -glass with a dense flint; but where the difference of refractive index -or of density is small, it is not at all certain that the lighter -glass will also be the harder. - -The properties involved in the quality known as “hardness” also affect -in a very marked manner the behaviour of glass when subjected to the -grinding and polishing processes. The ease with which a good polish -can be obtained varies very much in different kinds of glass, both the -hardest and the softest glasses showing themselves difficult in this -respect. The harder glasses are certainly less liable to accidental -scratching during the polishing operations, and generally work in a -cleaner manner; but the time required to produce a satisfactory polish -is much greater owing to the resistance to displacement offered by the -molecules. Both the speed of working and the pressure exerted during -the polishing operation have, in fact, to be carefully adapted to the -quality of the glass in this respect if the best possible results are -to be obtained. - -Another property which is essential in optical glass of the highest -quality is that of freedom from internal strains. This subject will be -again referred to later in connection with the annealing processes used -in the manufacture of optical glass, and it need only be mentioned here -that the presence of internal strain is readily recognised in glass, -by the aid of the polariscope. Perfectly annealed glass, entirely free -from internal strains, produces no effect upon a beam of polarised -light passing through it, while even slightly strained glass becomes -markedly doubly-refracting. For many purposes of optics this double -refraction becomes undesirable or even inadmissible, especially as it -is accompanied by small variations in the effective index of refraction -of various portions of the mass of glass. Further, if the amount of -double refraction observed is at all serious it indicates a state -of strain which may easily lead to the fracture of the whole piece, -particularly when undergoing the earlier stages of the grinding process -or if exposed to shocks of any sort. As will be seen below, perfectly -annealed glass is obtainable, but very special means are required for -its production, and the optician should for that reason avoid making -unnecessarily extreme demands in this direction. The very _small_ -amount of double refraction frequently found in the better class of -optical glass is entirely harmless for most purposes. - - - - -CHAPTER XIII. - -OPTICAL GLASS. - - -The process of manufacturing the best qualities of optical glass may -be briefly described as consisting in obtaining a crucible full of -the purest and most homogeneous glass, and then allowing it to cool -slowly and to solidify _in situ_. From the resulting mass of glass -the best pieces are picked and moulded into the desired shape for -optical use. It will be seen at once that in this process there is -an essential difference from all others that have been described in -this book--viz., that the glass is never removed from the melting-pot -while molten, and that none of the operations of gathering, pouring, -rolling, pressing, or blowing are applied to it. The reason for this -apparently irrational mode of procedure lies in the fact that the -perfect homogeneity essential for optical purposes can only be attained -by laborious means, and can then only be retained if the glass is left -to solidify undisturbed; any movement by the introduction of pipes or -ladles would result in the contamination of the glass by striæ and -other objectionable defects. - -The choice and proportion of raw materials used in the production -of any given quality of optical glass is governed by the chemical -composition which experiment has shown to be necessary to yield the -desired optical properties. The composition of optical glass mixtures -cannot therefore be varied to suit the conditions of the furnace or -to facilitate ready melting and fining, so that many of the usual -resources of the glass-maker cease to be available in the very case -where their aid would be most welcome to facilitate the production of -technically perfect glass. On the other hand, the manufacturer has a -certain amount of choice as to the precise form in which the various -chemical ingredients are to be introduced into the mixture, and he -makes his choice among oxides, carbonates, nitrates, and hydrates, -according to the behaviour that it is desired to impart to the mass -during the earlier stages of fusion. The state of purity in which the -various substances are commercially obtainable also enters largely into -the question, since the greatest possible degree of purity in the raw -materials is essential to the production of glass of good colour, or -rather freedom from colour. - -Since homogeneity is so essential in the finished product, very -thorough mixing of the raw materials is necessary in the case of -optical glass, and the ingredients are for this purpose generally used -in a state of finer division than is necessary with other varieties -of glass. As a rule the quantities of mixture of any one kind that -are required are not large enough to justify the use of mechanical -appliances, and very careful hand-mixing is carried out. - -Although it is quite possible to obtain successful meltings from -raw materials alone, it is preferable to mix with these a certain -proportion of “cullet” or broken glass derived from a previous melting -of the same sort. The broken glass used for this purpose is first -carefully picked over for the purpose of rejecting pieces that contain -visible impurities, although pieces showing striæ are not usually -rejected. The greater part of this cullet is generally mixed as evenly -as possible with the raw materials, but a certain proportion is -reserved for another purpose, as explained below. - -The furnaces used for the production of optical glass vary very -much in type in different works. In some the old-fashioned conical -coal furnaces are still used, the disadvantages attached to their -employment being outweighed--in the opinion of the manufacturers--by -their simplicity and ease of regulation. In other works gas-fired -regenerative furnaces of the most recent type are installed, and in -these also optical glass of the highest quality can be produced. As a -rule, however, optical glass furnaces differ from other pot-furnaces -found in glass-works in this respect--that the former are usually -constructed to receive one pot or crucible only, while in other glass -furnaces from four to twelve or even twenty pots are heated at the same -time. The reason for this restriction in the capacity of the furnaces -lies in the fact that since the mixtures used for optical glass cannot -be adjusted to suit the furnace, the latter must be worked as far as -possible in such a way as to suit the mixture to be melted in it, and -this implies that every pot will require its own adjustment of times -and temperatures, and this it would be difficult, if not impossible, -to secure if more than one pot were heated in the same furnace. It is -further to be remembered that the amount of care and attention required -during the melting of a pot of optical glass is out of all proportion -to that needed with other varieties, so that little would be gained -by having a number of pots in one furnace, since several sets of men -would be required to tend them. - -In addition to the single-pot melting furnace, a very important part of -the equipment of the optical glass works is formed by a number of kilns -or ovens which are used for the preliminary heating, and sometimes for -the final cooling of the various crucibles or pots. Similar kilns are -used in other branches of the industry, but in those cases the pots, -once introduced into the furnace, are expected to last for a number of -weeks, or even months. In optical glass manufacture, on the other hand, -a pot is used once only, so that fresh pots are required for every -new melting. The kilns in which these pots are heated up before being -placed in the melting furnace are thus in very frequent use. As a rule -they are simply fire-brick chambers provided with sufficient grate-room -and flue-space to be gradually raised to a red heat in the course of -four or five days, while for the purpose of gradual cooling they can be -sealed up like the annealing kilns used for polished plate-glass. - -The pots or crucibles in which optical glass is melted are usually of -the same shape as the covered pots used for flint-glass as illustrated -in Fig. 2. The optical glass pots, however, are made considerably -thinner in the wall, since they are not required to withstand the -prolonged action of molten glass in the same way as pots used for -flint-glass manufacture. On the other hand, the fire-clays used for -this purpose must be chosen with special care so as to avoid any -contamination of the glass by iron or other impurities which might -reach the glass from the pot. For the production of certain special -glasses, in fact, pots made of special materials are required, since -these glasses, when molten, produce a rapid chemical attack upon -ordinary fire-clays. A certain amount of the aluminiferous material -of the pot is, in fact, always introduced into the glass by the -gradual dissolving action of glass on fire-clay which we have already -described. The glass contaminated with these aluminiferous substances -is generally more viscous than the rest of the contents of the pot, and -therefore ordinarily remains more or less adherent to the walls of the -crucible, but the inevitable disturbances which accompany the processes -of melting and fining lead to the dissemination of some of this viscous -glass through the entire pot in the form of veins or striæ, which are -only removed during the stirring process. On the other hand, more of -this viscous glass is constantly being formed so long as the glass -remains molten, and if disturbances are not sufficiently avoided during -the later stages of the process fresh veins may easily be formed. - -The actual operations of producing a melting of optical glass begin by -the gradual heating-up of the pot in the kiln just described. When the -pot has reached a full red heat the doors of the kiln are opened and -the pot drawn out by means of a long heavy iron fork running on wheels; -this implement is run into the mouth of the kiln and the tines of the -fork are pushed under the pot, and the latter is then readily lifted up -and withdrawn from the kiln. Meanwhile the temperature of the furnace -has been regulated in such a manner as to be approximately equal to -that attained by the heating kiln, so that the pot, when transferred -as rapidly as possible from the kiln to the furnace, is not subjected -to any very sudden heating; were it attempted to place the new pot in -a furnace at full melting heat the fire-clay would shrink rapidly and -the entire vessel would fall to pieces. Even under the best conditions -it is not possible to avoid the occasional failure of a pot by cracking -either at this or a slightly later stage of the process. The latter -occurrence is apt to be particularly disastrous, as the pot may then be -full of molten glass, which runs out and is lost. - -As soon as the empty pot has been put into place, the melting furnace -is carefully sealed up by means of temporary work built of large -fire-bricks, the whole being so arranged that the mouth of the hood of -the pot is left accessible by means of an aperture in the temporary -furnace wall. This aperture can be closed by one or more slabs of -fire-clay, and when these are removed an opening is left by which the -raw materials are introduced, and through which the other manipulations -are carried out. - -When this stage of the process is reached, the wagons containing the -mixed raw materials are usually wheeled into place in front of the -furnace, but the introduction of the materials themselves into the -pot is not begun until several hours later, when the furnace has been -vigorously heated and an approach to the melting heat has been attained. - -When the furnace and pot have attained the necessary temperature, -but before the raw materials are introduced, a small quantity of the -cullet, which has been reserved for this purpose, is thrown into the -pot and allowed time to melt, and then only is the first charge of -mixture put into the pot. The object of this proceeding is to coat the -bottom and part of the walls of the pot with a layer of molten glass -which serves to protect it from the chemical and physical attack of -the raw materials during the violent action which takes place when they -are first exposed to the furnace heat. - -The gradual filling of the pot with molten glass is now carried out by -the introduction of successive charges of raw material; as the mixture -not only occupies more space than the glass it forms, but also froths -up a good deal during melting, the quantities introduced each time -must be carefully adjusted so as to avoid an overflow of half-melted -glass through the mouth of the pot. As the pot is more and more nearly -filled, the space left for the raw materials is proportionately -diminished, and the later charges are therefore much smaller than the -first few. - -When, finally, sufficient material has been introduced to fill the pot -completely, the next stage of the process commences. When the last -charge of raw materials has melted, the glass in the pot is left in the -state of a more or less viscous liquid full of bubbles of all sizes; -it is essential that these bubbles should escape and leave the glass -pure and “fine,” and this result can only be achieved by raising the -temperature of the furnace and allowing the glass to become more fluid, -while the rise of temperature also causes the bubbles to expand owing -to the expansion of the gas contained in them. In both ways, rise of -temperature facilitates the escape of the bubbles, and the furnace -is therefore heated to the full, and this extreme heat is maintained -until the glass is free from bubbles. In the case of the more fusible -glasses the temperature required for this purpose is not excessively -high, and, indeed, in the case of these glasses care is taken to avoid -too high a temperature, as it entails other disadvantages. In the case -of the harder crown glasses, however, the difficulty lies in producing -an adequately high temperature without at the same time endangering -the life of furnace and crucible. The difficulty of freeing the molten -glass from bubbles constitutes one of the causes that limit the range -of our optical glasses in one direction--still harder glasses could be -melted, but it would not be feasible to maintain a temperature high -enough to render them fluid enough to “fine.” - -In the case of other kinds of glass, again, it becomes impossible to -entirely remove the bubbles from the molten mass even when very hot and -very fluid. The exact cause is not known, but in some kinds of glass -the bubbles formed are so minute that even when the glass is perfectly -mobile the bubbles show no tendency to escape, while in other kinds of -glass there appears to be a steady evolution of minute bubbles as soon -as the temperature is raised with a view to removing those already in -the glass. As this property attaches to some of the most valuable of -the newer varieties of optical glass, opticians and the public have -learnt to put up with the presence of minute bubbles in the lenses -and prisms made of these glasses. These bubbles are, however, very -minute and do not interfere with the optical performance of the lenses, -&c., except to the extent of arresting and scattering the very small -proportion of light that falls upon them; their presence is therefore -to be regarded as a small but unavoidable drawback to the use of -glasses which offer advantages that completely outweigh this defect. - -Returning to the melting process, we find that the extreme heating -required for the purpose of “fining” the glass is continued for a -considerable period of time, as long as thirty hours in some cases, -the glass being examined from time to time to test its condition as -regards freedom from bubbles. This is done by taking a small sample -of glass out of the pot and examining it to see if it still contains -bubbles. In some works this test is made by taking up a very small -gathering of glass on the end of a small pipe and blowing it into a -spherical flask; on looking at such a flask in a suitable light the -presence of even minute bubbles is readily detected. In other works a -simpler process is adopted, a small quantity of glass being ladled out -of the pot on the surface of a flat iron rod. It is allowed to cool -on the rod, and when pushed off forms a small bar of glass some eight -or ten inches long and about an inch wide; in this also the presence -of bubbles is easily detected. These test pieces are known among -glass-makers as “proofs.” - -When proofs, taken as just described, have shown that the glass is free -from bubbles, the extreme heat of the furnace is allowed to abate, -and the fire-clay slabs in front of the mouth of the pot are removed. -The next step is that of skimming the surface of the glass. Since -most of the materials liable to contaminate the contents of a pot are -specifically lighter than the molten glass, they will be found floating -on the surface, and the surface glass is therefore removed with a -view to ridding the glass of anything that may have been accidentally -introduced and that has not melted and become incorporated with the -molten mass. - -The next steps in the process are those of stirring the molten glass -with a view to rendering it homogeneous and free from striæ. The -stirrer used for this purpose is usually a cylinder of fire-clay, -previously burnt and heated. This is provided with a deep square hole -in one end, and it is held at first by means of a small iron bar -passed into this hole. By this means the red-hot cylinder of fire-clay -is introduced into the open mouth of the pot, and when it has attained -approximately the temperature of the molten glass it is dipped into -the glass itself, in which it ultimately floats. When stirring is to -begin, the square, down-turned end of a long iron bar is introduced -into the corresponding square hole in the upper end of the stirrer, and -by this means the fire-clay cylinder is held in a vertical position in -the glass and given the steady rotatory movement which constitutes the -stirring process. For this purpose the long iron bar just mentioned is -made to pass over a swivel-wheel, while a workman moves it steadily by -the aid of a large wooden handle. This operation is always laborious -and trying; the workman is necessarily exposed to the intense heat -radiated from the open mouth of the crucible, so that men have to -relieve each other at frequent intervals. - -During the earlier stages of the stirring process the glass is very -hot and mobile, but the stirring is continued, with short intervals, -until the glass is so cold and stiff that the stirrer can scarcely be -moved in it at all, so that the work of moving the stirrer becomes -heavy towards the end of the operation. The actual amount of stirring -required varies according to the nature of the glass, and the size -of the pot or crucible in question. Some meltings are found to be -satisfactory after as little as four hours’ stirring, while for others -as much as 20 hours are required. - -When the glass has stiffened to such an extent that it is no longer -possible to continue the stirring, preparations are made for the final -cooling-down of the pot of glass. The fire-clay stirrer is sometimes -withdrawn from the glass, but this is laborious, and entails dragging -a considerable quantity of glass out of the pot with the clay cylinder; -more usually, therefore, the stirrer is simply left embedded in the -glass. - -The next object to be accomplished is that of cooling the glass as -rapidly as safety will permit until it has become definitely “set”--the -purpose being to prevent the recrudescence of striæ as a result of -convection currents or other causes which might disturb the homogeneity -of the glass. This rapid cooling is obtained in various ways; in one -mode of procedure the furnace is so arranged that by opening a number -of apertures provided for the purpose cold air is drawn in and the pot -and its contents chilled thereby without being moved. This method has -the advantage that the pot containing the viscous glass is never moved -or disturbed in any way, but on the other hand the cooling which can be -effected within the furnace itself is never very rapid, and the furnace -as well as the pot is chilled. Further when the glass has been chilled -down to a certain point this rapid rate of cooling must be arrested, as -otherwise the whole contents of the pot would crack and splinter into -minute fragments. Where the pot has been left in the furnace this can -only be done by sealing up the whole furnace with temporary brickwork -and lutings of fire-clay, leaving it to act as an annealing kiln until -the glass has cooled down approximately to the ordinary temperature, a -process that occupies a period of from one to two weeks according to -the size of the melting. Such enforced idleness of a melting furnace -is of course very undesirable from an economical point of view, and it -is generally avoided by adopting the alternative method of drawing -the pot bodily out of the furnace as soon as the stirring operation is -ended. For this purpose the temporary brickwork forming the front of -the furnace is broken down, and with the aid of a long crow-bar the -bottom of the pot is levered up from the bed or siege of the furnace -to which it adheres strongly, being bound down by the sticky viscous -mass of molten glass and half-molten fire-clay which always accumulates -on the bed of the furnace. The pot being temporarily held up by the -insertion of a piece of fire-brick, the tines of a long and heavy -iron fork running on a massive iron truck are introduced beneath the -pot; an iron band provided with long handles is then passed around -the pot, and the latter is then drawn forward by the aid of suitable -pulley blocks. The tines of the fork are then raised, and the pot is -wheeled out of the furnace and deposited upon a suitable support. Here -it is allowed to cool to the requisite extent, when it is again picked -up on the tines of the fork and deposited in an annealing kiln which -has been previously warmed to a suitable temperature. It will be seen -that this handling of a heavy mass of intensely hot material involves -much labour, while there is also a risk of losing the glass if the pot -should break before the glass has set sufficiently. Every care is taken -to prevent such an accident, the pot being wrapped round with chains or -otherwise supported in such a way that a small crack could not readily -develop into a large gap. - -When such a melting of glass has cooled sufficiently, either in the -furnace or in the annealing kiln, to be safely handled, the whole -pot is drawn out, and the fire-clay shell, which is generally found -cracked into many pieces, is broken away by the aid of a hammer. Under -favourable circumstances the whole of the glass may have cooled intact -as one solid lump sometimes weighing over half a ton. Unless special -care is taken, however, it is more usual to find the glass more or less -fissured, a number of large lumps being accompanied by a great mass of -small fragments. These are now picked over, and all those which are -free from visible imperfections or which can be readily detached from -such imperfections by the aid of a chipping hammer are put upon one -side for further treatment. - -The next step of this treatment consists in moulding the rough broken -lump into the shape of plates, blocks, or discs according to the -purpose for which the glass may be required by the optician. The -plant used for the moulding process varies widely, but in all cases -the operation consists in gradually heating the glass in a suitable -kiln until it is soft enough to adapt itself to the shape of the -mould provided for the purpose. In some cases these moulds are made -of fire-clay, and the glass is simply allowed to settle into them by -its own weight; in other cases iron moulds are used, and the glass -is worked into them by the aid of gentle pressure from wood or metal -moulding tools. In yet other cases, particularly where the glass is -required in the form of small thin discs or where it is to be formed -into the approximate shape of concave or convex lenses, the aid of a -press is sometimes invoked. - -In all cases the moulding process is followed by the final annealing, -which consists in cooling the glass very gradually from the red heat at -which it has been moulded, down to the ordinary temperature. The length -of time occupied by such cooling depends very much upon the size of the -object and also upon the degree of refinement to which it is necessary -to carry the removal of small internal strains in the glass. For many -purposes it is sufficient to allow it to cool down naturally in a -large kiln in the course of six or eight days. For special purposes, -however, where perfect freedom from double refraction is demanded, much -greater refinements are required, and special annealing kilns, whose -temperature can be accurately regulated and maintained, are employed. -In these the annealing operation can be carried out so gradually that a -rate of cooling in which a fall of 1° C. occupies several hours can be -maintained, so that very perfectly annealed glass can be produced even -in discs or blocks of large size. - -When removed from the annealing kiln the plates or discs of optical -glass are taken to a grinding or polishing workshop, where certain of -their faces or edges are ground and polished in such a way as to permit -of the examination of the glass for bubbles, striæ and other defects -in the manner indicated in the previous chapter. As the amount of -sorting that can be done while the glass is still in rough fragments -is necessarily very limited, it follows that a considerable proportion -of the glass which has been moulded and annealed must be rejected as -useless when thus finally examined. A yield of perfect optical glass, -amounting to 10 or at most 20 per cent. of the total contents of each -pot, is therefore all that can be expected, and smaller yields are by -no means infrequent--a consideration that will serve to explain the -relatively high price of optical as compared with other varieties of -glass. - -A consideration of the various factors that are involved in the -production of a piece of perfect optical glass will make it apparent -that the cost and difficulty of its production increases rapidly with -the weight of the piece to be produced, so that it is not surprising to -find that the price of very large discs of perfect optical glass such -as those required for large astronomical telescopes, reaches figures -which become prohibitive when very large sizes are considered. Thus, -while it is quite possible to obtain say 100 pounds of good glass from -a single melting if the glass is to be used in the form of pieces not -weighing more than five or six pounds each, it is only rarely that a -single block of perfect glass can be found weighing 100 pounds. In the -former case the best pieces can be picked, the worst defects can be -eliminated by chipping the rough fragments, and at a later stage other -defective pieces can be cut off or ground away; not so where a large -single block is required. A single fine vein, perhaps too small to be -visible to the unaided eye, may be found to run through a whole block -in such a way that it cannot be removed without breaking or cutting up -the whole piece, and it will be seen that the frequency with which this -is liable to occur increases with the volume of the piece required. -The difficulties of re-heating and moulding are also increased -enormously with the size of the individual pieces of glass that have -to be dealt with, and where very large pieces have to be heated and -cooled accidental breakage becomes a serious risk. In view of these -difficulties it is not surprising to find that the dimensions of our -astronomical refractors appear to have approached their limit, but -rather are we led to admiration of the skill and enterprise that has -pushed this limit so far as to produce discs of optical glass measuring -as much as one metre in diameter. - - - - -CHAPTER XIV. - -MISCELLANEOUS PRODUCTS. - - -The field of glass-manufacture is so wide and the number and variety of -its products so great, that in the limited compass of this volume it is -impossible to fully enumerate them all; there are, however, a certain -number of these products which, while of considerable importance in -themselves, yet do not fall readily under any of the headings of the -preceding chapters. A short space will therefore be devoted to some of -these in this place. - -_Glass Tubing._--A widely-useful form of glass is that of tubes of -all sizes and shapes, ranging from the fine capillary tubes used in -the construction of thermometers to the heavy drawn or pressed pipes -that have been employed for drainage and other purposes. The process -of manufacture employed varies according to the size and nature of -the tube that is required. Thus lamp-chimneys are really a variety of -tube, used in short lengths and made of relatively wide diameter and -thin walls. These are not, however, ordinarily made in the form of long -tubes cut into short sections, but--as has already been mentioned--they -are blown into moulds in the form of a thin-walled cylindrical bottle, -whose neck and bottom are subsequently removed. By this process the -various forms of chimneys for oil-lamps, having contractions at -certain parts of their length, can be readily produced. - -The articles more strictly described as glass tubes are, however, -produced by a process in which actual blowing plays only a very minor -part. A gathering of suitable size is taken up on a pipe, a very small -interior hollow space is produced by blowing into the pipe, and then -the gathering is elongated by swinging the pipe in a suitable manner. -The end of the elongated gathering furthest from the pipe is then -attached to a rod or “pontil” held by a second workman, and the two men -then proceed to move apart, drawing out the gathering of glass between -them. According to the bore and thickness of wall required in the tube, -the men regulate the speed at which they move apart; the thinner the -tube is to be the more rapidly they move, in order to draw the glass -out to a sufficient extent before it hardens too much. The rate of -drawing must, of course, also be adapted to the nature of the glass in -question, and this will vary very widely. For the production of the -smaller bored tubes the men find it necessary to separate at a smart -trot, while heavy tubes such as are used for gauge-glasses, are drawn -of hard glass by a very gradual movement. In some cases, the setting -of the glass, when the tube has attained the desired thickness, is -hastened by the aid of an air-blast, or--in more primitive fashion--by -boys waving fans over the hot glass. In any case, suitable troughs are -provided for receiving the tube when drawn, and from these the tube is -taken to an annealing kiln to undergo this necessary operation. - -The glass used for the production of tubing varies very widely -according to the purpose for which the product is intended. Almost -any of the more usual varieties of glass can be readily drawn out -into tubes, and the choice of the kind of glass to be employed is -therefore left to other considerations. Tubing required for the use of -the lamp-worker, _i.e._, for the production of instruments or other -articles by the aid of the glass-blower’s blow-pipe, must have the -capacity of undergoing repeated cooling and heating without showing -signs of crystallisation (devitrification), while reasonable softness -in the flame is also required. For this purpose, also, glass containing -lead is not admissible, since this would blacken under the influence -of the blow-pipe flame. Soda-lime glasses rather rich in alkali are -most frequently used for these purposes; one consequence of their -chemical composition, however, is that such glass tends to undergo -decomposition when stored for any length of time, more especially in -damp places. Frequently this decomposition only manifests itself on -heating the glass in a flame, when it either flies to pieces or turns -dull and rough on the surface. Such glass is sometimes said to have -“devitrified,” but this is not really the case; what has actually -happened is that the atmospheric moisture has penetrated for some -little distance into the thickness of the glass, probably hydrating -some of the silica; on heating, this moisture is driven off, with the -result that either a few large cracks, or innumerable fine ones, are -formed. In the latter case these do not readily disappear when the -glass is softened and the dull, rough surface is left at the end of the -operation. - -For purposes where the glass is to be exposed to high temperatures, -tubing made of so-called “hard glass” is employed. This is practically -a form of Bohemian crystal glass, the chemical composition being -that of a potash-lime glass rather rich in lime. To some extent this -Bohemian hard glass has been superseded by the special “combustion -tube” glass manufactured by Schott, of Jena. This is a very refractory -borosilicate glass containing some magnesia; it certainly withstands -higher temperatures than hard Bohemian glass, and is rather less -sensitive to changes of temperature; on the other hand, it has the -inconvenient property of showing a white opalescence when it has once -been heated, and this, after a time, renders the glass completely -opaque. - -For many purposes, where heat-resisting qualities are chiefly required, -ordinary glass has now a formidable rival in the shape of vitrified -silica, which is now available as a satisfactory commercial product. -This substance offers the great advantage that for most ordinary -purposes it may be regarded as entirely infusible, since the intense -heat of an oxygen-fed flame is required to soften or melt the silica. -Further, vitreous silica has an extremely low coefficient of expansion, -and appears also to have a rather high coefficient of thermal -conductivity. The result is that tubes and other articles made of -this material possess an astonishing amount of thermal endurance (see -Chapter II.). - -A white-hot tube or rod of this material can be plunged into cold water -with impunity, and no special care need be exercised in heating or -cooling articles made of this substance, unless articles of great size -and thickness are involved, and even with these only little caution -is needed. The only disadvantages which must be balanced against the -great advantages just named lie in the relatively high cost of the -articles and in their somewhat sensitive behaviour to certain chemical -influences. As regards cost, vitreous silica is at present available in -two different forms; in the first form it resembles ordinary glass very -closely in appearance, the shape and finish of the tubes and vessels -of this kind having undergone very great improvements quite recently. -This silica glass has, in fact, been worked from molten silica in a way -more or less analogous to that in which ordinary glass is worked, the -great extra cost of the silica ware being due, in part, at all events, -to the extremely high temperature required for melting and working this -material; ordinarily, in the production of the class of silica ware -now referred to, this heat is generated by the liberal--and therefore -expensive--use of oxygen gas. In great contrast to this glass-like, -transparent silica ware is the other form in which this material is -available. This is a series of products obtained from the fusion of -silica in special forms of electric furnace; in this ware the minute -bubbles so readily formed in the fusion of all forms of quartz are not -even partially eliminated, and by their presence--often in the form of -long-drawn-out, capillary hollows--they impart to this ware its very -characteristic milky appearance. The price of this product, which is -mostly used in the form of tubes, although such articles as basins, -crucibles, and even muffles of considerable size are available, is much -lower than that of the transparent variety, being in fact decidedly -lower than that of the best porcelain; on the other hand, even this -price is considerably above that of the best glass tubing. - -Apart from the question of cost, the use of silica ware is further -limited by its sensitiveness to all forms of basic materials. Thus -alkaline solutions cannot be allowed to come into contact with this -substance, since they attack it vigorously, especially when warm. At -high temperatures all basic materials produce a rapid attack on silica -ware, the silica, in fact, behaving as a strongly acid body at and -above a red heat. The attack which occurs when such a substance as iron -or copper oxide is allowed to come into contact with heated vitrified -silica is, in fact, so rapid that a tube is completely destroyed in -a few minutes, the formation of silicates resulting in the cracking -and disintegration of the whole piece. While, therefore, silica ware, -especially in its cheaper forms, undoubtedly possesses great advantages -and possibilities, its use must be carried on with careful reference to -its chemical nature. - -Vitreous silica, in addition to the uses and advantages just named, -has also an interest from the optical point of view; this arises from -the fact that it is transparent to short (ultra-violet) light waves -to which all ordinary varieties of glass are completely opaque. Quite -recently, the Jena works have produced special glasses which are more -transparent to these ultra-violet rays than ordinary glass, but even -these fall far short of silica in this respect. This property of -transparence to ultra-violet light is utilised in two widely different -directions. One of these is in the production of ultra-violet light -when required for medical or other special purposes; a most energetic -source of such rays is available by the use of tubes of vitrified -silica within which the mercury-vapour arc is produced. In another -direction the employment of quartz lenses makes it possible to take -advantage of the optical properties of ultra-violet light in connection -with microscopy; for the purpose of constructing a perfect optical -system, crystalline quartz would be useless, since its property of -double refraction would interfere hopelessly with the performance of -the lenses. This is now overcome by the use of vitreous silica lenses, -in the case of the “ultra-violet microscope,” as made by Carl Zeiss, -of Jena. So far, however, it has only been possible to produce quite -small pieces of vitreous silica sufficiently free from bubbles to be -used for optical purposes. The great difficulty lies not so much in -merely melting the quartz down as in freeing it from the air-bubbles -enclosed within it; the course usually adopted with glass, of raising -the temperature and allowing the bubbles to rise to the surface, -becomes impossible in this case, because the silica itself begins to -vapourise and even to boil vigorously at temperatures not very far -above its melting point. Quite recently, however, two American workers -have claimed to be able to overcome this difficulty by the use of both -vacuum and high pressure applied at the earlier and later stages of -the fusion process respectively, so that it may shortly be possible to -produce vitreous silica in large and perfectly clear blocks. - -We have already indicated that glass tubing and rod form the basis -upon which the glass-worker, with the aid of the blow-pipe or “lamp,” -fashions his productions, which, of course, include a great number of -scientific instruments and appliances used more especially in the field -of chemistry. In another direction also glass tubing serves as a basis -for a branch of the glass industry; this is the manufacture of certain -classes of glass beads, which are formed by cutting up a heated glass -tube of suitable diameter and colour into short, more or less spherical -sections. In some cases the colour of the beads is secured by using -glass of the desired tint, but in other cases the beads are made of -colourless glass, and a colouring substance is placed in the interior -of the bead. - -Solid glass rods are also employed for a variety of purposes; their -mode of manufacture is exactly analogous to that of tubing, except -that the gathering is drawn out without having first had a hollow -space produced at its centre by the blower. In its most attenuated -form glass rod becomes glass thread or fibre; this is produced by -drawing hot glass very rapidly, the resulting thread being wound on a -large wheel. At one time this material found considerable use, since -it was found possible to spin and weave the thinnest glass fibres into -fabrics which could be used for dress purposes. It is not, however, -to be regretted that this fashion has neither extended nor survived, -since it was certainly liable to produce serious injury to health. -It is a well-known fact that there are few more injurious or even -dangerous substances to be inhaled into the human throat and lungs than -finely-divided glass; glass fibre, moreover, when subjected to constant -bending and wear, is bound to undergo frequent fracture, and the -atmosphere of a ball-room, for example, in which several such dresses -were worn would soon be contaminated with innumerable fine, sharp -particles of glass which would produce an injurious effect on those -inhaling them. At the present time glass fibre is used for little else -than the “glass wool” required for certain special purposes in chemical -laboratories. - -Fused quartz or silica fibres, of extreme tenuity, but of relatively -very great strength, are employed in many scientific instruments, where -their extreme lightness and perfect elasticity and freedom from what -is known as “elastic fatigue” renders them of very great value. These -fibres are not drawn from a mass of molten silica, as is done with -glass, but are produced by attaching a nail or bolt to a small bead of -fused silica produced by the aid of an oxygen-fed blowpipe; the nail or -bolt is then suddenly shot away down a long passage or similar space by -means of a cross-bow, drawing a very fine fibre of silica with it; the -most difficult part of this operation, however, consists in finding and -handling the fibres thus produced. - -_Artificial Gems._--The fact that pieces of suitably-coloured glass can -be made to show a superficial, but sometimes more or less deceptive, -resemblance to precious stones, has led to the manufacture of imitation -jewels of all descriptions. The glass used for this purpose is usually -a very dense flint-glass whose high refractive index facilitates the -imitation which is aimed at. The external shapes of gems are, of -course, readily imitated by cutting and grinding the glass, while the -requisite colours are attainable by means of the colouring materials -described in Chapter XI. To a casual observer the difference in sparkle -and brilliance which arises from the difference between the refractive -index of the heavy flint-glass (about 1·8) and that of minerals -(which ranges from 1·7 to 2·2) is not readily apparent, but closer -examination will at once reveal the difference. The determination of -the optical constants by means of a refractometer would at once reveal -the true character of the imitation, but an even readier test is that -of hardness. The dense flint-glass is naturally soft, and is readily -scratched by most of the harder minerals, while the precious stones, -more particularly garnets, rubies and diamonds, are very hard. If an -attempt is made to scratch an ordinary sheet of window-glass, it will -be found that most real precious stones will do so readily, while -flint-glass imitations will fail to make more than a slight mark, -which is more smear than scratch. The test by determining the specific -gravity is also obviously applicable, since the flint-glass will -readily betray its presence by its high density (over 4). - -In quite a different class from the imitation gems made of cut -flint-glass are the artificial gems, which in nature and composition -are exact reproductions of natural gems, but which have been produced -by artificial processes. As far as the writer is aware these are -only found in any large numbers in the case of the ruby, but in that -case, at all events, it is said that the production of the artificial -crystals is at least as costly as the purchase of the natural stones. -There can, however, be very little doubt that as the processes of -fusion and crystallisation become better known and understood, and the -chemistry of silicate minerals is developed, the artificial production -of mineral crystals in, at all events, moderate sizes will become -increasingly possible; it is even to be hoped that their production -will be so far perfected as to place their really valuable properties -at the service of man. - -_Chilled Glass._--In all the processes of glass manufacture described -in the present book, annealing has always played an important part. The -glass, after it has undergone its last treatment under the influence -of heat, is subjected to a gradual cooling process with the object of -freeing it from the internal strains which it would otherwise retain, -and which would, ordinarily, endanger its existence and interfere with -its use. It is, however, well known that surfaces of glass subjected -to such internal strains as result in a compressive stress on the -glass near the surface, are less liable to injury, and are apparently -stronger than when the glass is annealed and the stresses are removed. -On the other hand, glass surfaces under tension are extremely delicate -and fragile. In some respects, therefore, glass which has not been -annealed may appear to be stronger than the annealed product. The -well-known case of the Rupert’s drop is an example of this kind. -Rupert’s drops are produced by dropping molten glass into water; they -generally take the form of a more or less spherical body having a long -tail, tapering off into a thread, attached to it. Such a Rupert’s drop -may be struck with a heavy hammer, and will safely resist a blow that -would splinter a similar body made of annealed glass. If, however, the -surface be scratched, or the tip of the tail be broken off, the entire -“drop” breaks up, sometimes with a violent explosion, into minute -fragments. Numerous inventors, among whom De la Bastie and Siemens -figure most conspicuously, have endeavoured to utilise these properties -of chilled glass, not exactly by endeavouring to produce that extreme -degree of internal strain which is characteristic of the Rupert’s drop, -but by producing what they describe as “tempered” glass, in which the -internal strains have been reduced by less violent cooling to such an -extent as to retain some of the advantages of the hardened, internally -strained condition while approximating more or less to the safer state -of annealed glass. At one time articles of this kind were frequently -seen as curiosities, such as tumblers that could be dropped on the -floor without breaking, etc., but these articles generally ended by -receiving a slight scratch or chip and promptly falling into fragments. -As a matter of fact, however, some tempered glass is actually -manufactured by the firm of Siemens at the present time for special -purposes. De la Bastie’s process was tried in England, and some success -was claimed for it; but it is not in commercial operation at the -present time, and never appears to have attained any great importance. - -_Massive Glass._--Enthusiasts for the extension of the use of glass -have endeavoured to apply it to a great variety of purposes, including -the construction of buildings and the paving of streets. In the -former case, which was exemplified at the Paris Exhibition of 1900, -advantage was taken of the light-transmitting power of the material, -but although the buildings erected with large blocks of cast glass were -not displeasing in effect, this use has not found any considerable -extension. For paving purposes, the hardness and durability of glass -are the only useful qualities, and here also--although several trials -have been made in France--no signs of any considerable application of -the new products are as yet visible. What has been said above with -reference to the injurious character of glass dust applies, further, to -glass pavements, since their natural wear would result in the formation -of considerable quantities of this dust. The advocates of glass paving, -however, suggest that the hardness of glass would greatly reduce the -actual amount of wear, and that consequently the dust would be reduced -considerably. This is a matter which prolonged experience alone can -decide, but it does not seem obvious that glass blocks should wear more -slowly than stone setts made of good granite, for example. On the other -hand, the glass blocks could probably be produced more cheaply, since -the labour of cutting to size would be obviated by casting the blocks -to the desired dimensions. - -Water-glass, or silicate of soda or potash is perhaps scarcely to be -classed under the heading of “Glass Manufacture” at all, but it bears a -certain relationship to glass in several ways. Thus one of the modes of -manufacturing water-glass is by the fusion of sand and alkali in tank -furnaces somewhat resembling those used for glass production; the fused -silicate, moreover, solidifies as a vitreous mass, in which respect -it also resembles such substances as borax, etc. The uses of silicate -of soda and potash are, however, so far removed from the field of -glass-manufacture that we cannot enter into them here. - -In concluding this chapter, we wish to describe one more product of the -glassworks, and this includes some of the most impressive and splendid -examples of the glass-maker’s art. These are the great mirrors and -lenses by whose aid our lighthouses and searchlights send forth their -powerful beams of light. Although these objects are called “mirrors” -and “lenses,” since they fulfil the functions of such optical organs, -yet in their nature and mode of manufacture they are so far removed -from the glass used for the production of other kinds of lenses that -they could not be included under the heading of “optical glass.” - -The characteristic feature in the manufacture of optical glass is the -manner in which each separate pot or melting is allowed to cool down -and to break up into irregular fragments which are subsequently moulded -to the desired shape. Were it attempted to manufacture the large glass -bodies required for lighthouse purposes in this manner, the cost would -approximate to that of the large discs used for telescope objectives, -and this would of course be entirely prohibitive. The requirements as -regards colour, homogeneity and freedom from other defects, which must -be met in lighthouse lenses, are further not nearly so stringent as -those which are essential in ordinary optical work of good quality. -The reason for this difference arises from the fact that lighthouse -lenses and searchlight mirrors are used merely to impart a desired -direction to a beam of light, and not for the purpose of producing -sharply-defined images; slight irregularities in the glass are -therefore not of such serious importance. - -Lighthouse glass can therefore be produced by rather less elaborate -means; although every care is taken to make the glass as perfect as -possible, it is brought into approximately the desired form by casting -the molten glass in iron moulds of the proper shape. When removed -from these moulds and annealed, the glass is fixed on large revolving -tables and ground and polished to the final shape of lenses and annular -lens-segments as required for the various types of Fresnel lighthouse -lenses. In this way complete rings, forming annular lenses, are -produced up to 48 inches diameter. Rings of larger size are usually -built up of a number of segments, and these built-up rings sometimes -have a radius as large as 7 feet. For the majority of lighthouse -lenses, it should be added, a hard soda-lime glass having a refractive -index of 1·50 to 1·52 is used, but for special purposes a dense -flint-glass having a refractive index of 1·63 is employed. - -Mirrors for searchlight purposes are of very varied forms and sizes, -the shape depending largely upon the particular form of beam which -they are designed to project. For many purposes a parabolic form -is required, while in others, where a flat, fan-shaped beam is to -be produced, a form having an elliptical section in a horizontal -plane and a parabolic section in the vertical plane is required. In -most cases these mirrors are produced by bending plates of glass, -previously raised to the necessary degree of heat, over suitably -shaped moulds, the surface being subsequently re-polished to remove -any roughness resulting from the bending process. Another type of -mirrors is that known as “Mangin,” which has two spherical surfaces -placed eccentrically in such a way that the centre of the mirror is -considerably thinner than the periphery; in this type of mirror the -reflecting action of the back surface is modified by the refracting -action of the front surface, but both are spherical, and can therefore -be accurately ground and polished by the usual mechanical means. Such -mirrors are manufactured of single pieces of glass up to 6 feet in -diameter. - - - - -APPENDIX - -BIBLIOGRAPHY. - - -The existing literature of glass manufacture is so limited that a -complete bibliography could almost be given on a single page; in the -English language, in particular, there are exceedingly few books -and papers on the subject. The French and German literature of the -subject is a little more extensive. In giving a list of the works, -and more particularly in referring to those which he has consulted in -the preparation of the present volume, the author thinks it will be -an advantage to indicate their scope, and, to some extent, what he -believes to be their value, in order to save the student the trouble -of seeking out comparatively inaccessible works only to find that they -contain little that is of value for his purpose. - - -_English Books and Papers on Glass Manufacture._ - -The Principles of Glass Making (George Bell & Sons). By Powell & -Chance. An elementary book giving a clear and concise account of -the older processes, more especially in connection with flint and -plate-glass. - -Glass. Articles in 9th Edition of Encyclopædia Britannica. A detailed -account of processes, more or less covering the entire subject, but the -processes described are mostly obsolete at the present time. - -Glass. Article in Supplement to 9th Edition of Encyclopædia Britannica. -By Harry J. Powell. A brief summary of more recent developments. -Particularly valuable in reference to artistic English flint-glass. - -Jena Glass. By Hovestadt, translated by J. D. and A. Everett. -Contains a full account of the scientific work on glass and its -practical application, done in connection with the Jena Works of -Schott. Particularly interesting in connection with the subjects of -Chapters I., II., XII., and XIII. As the title indicates, the book is -written from the Jena point of view, and scarcely does justice to work -done elsewhere. The book has gained considerably at the hands of the -translators. - -Some Properties of Glass. By W. Rosenhain. (Transactions of the Optical -Society of London, 1903.) Gives a brief account of the properties of -glass as affecting its optical uses. - -Possible Directions of Progress in Optical Glass. By W. Rosenhain. -(Proceedings of the Optical Convention, London, 1905.) Has been -referred to in the text of this book (Chapter XII.). - -Catalogue of the Optical Convention Exhibition, London, 1905. Contains -historical and general notices of optical and lighthouse glass, -glass-working machinery, etc. - -Glass for Optical Instruments. By R. T. Glazebrook. (Cantor Lectures -to the Society of Arts.) Gives an account of modern optical glass -manufacture. - -Old English Glasses. By Albert Hartshorne. Gives an account of the -history of glass-making in England. - -The Methods of Glass Blowing. By W. Shenstone. Describes the -manipulation of glass-blowing for experimental purposes, _i.e._, lamp -work. - - -_French Books on Glass Manufacture._ - -Guide du Verrier. By G. Bontemps. A classical work by one of the -greatest experts of his day. Much of the contents of the book is, -however, entirely out of date at the present time. The book is -interesting as being the work of the man who introduced optical glass -manufacture into England. - -Verres et Emaux. By L. Coffignal. Chiefly of interest in connection -with the subjects of Chapter VIII. - -Le Verre et le Crystal. By J. Henrivaux. (P. Vicq Dunod et Cie., -Paris.) A lengthy book profusely illustrated and giving a great wealth -of detailed information. The writer was for some time the general -manager of one of the largest plate-glass manufactories in Europe; his -account of plate-glass manufacture is, therefore, especially valuable. -Much space in this book is devoted to historical and æsthetical matter. - -La Verrerie au XX^{ieme} Siècle. By J. Henrivaux. (Paris, R. Bernard -et Cie., 1903.) Practically a supplement to the preceding; some of -the processes and products described are, however, not of a practical -nature. Chiefly valuable for recent developments in plate-glass and -bottle-glass manufacture. - - -_German Books on Glass Manufacture._ - -Die Glasfabrikation. By R. Gerner. (A. Hartleben’s Verlag, Vienna -and Leipzig, 1897.) A concise and clear account of most of the more -important processes of glass manufacture. Very practical in character. -The information given appears to be reliable, although far from -complete. - -Die Herstellung Grosser Glaskoerper and Die Bearbeitung Grosser -Glaskoerper. By C. Wetzel. (Hartleben’s Verlag, Vienna and Leipzig, -1900 and 1901 respectively.) Describes numerous special processes and -appliances devised for use in connection with large glass objects. -Some of these descriptions, however, appear to be little more than -transcripts from patent specifications. - -Glasfabriken und Hohlglasfabrikation. By R. Dralle. (Leipzig, -Baumgaertner, 1886.) Looked upon as a classic in Germany. Gives -detailed plans and drawings of entire bottle-works, including furnaces -and all accessories. Deals principally with bottle manufacture. - -Die Glasfabrikation. By Dr. E. Tscheuschner. (Weimar, B. H. Voigt, -1888.) A full detailed account of all processes known at the time. The -rapid progress of modern practice has, however, already rendered this -book to some extent obsolete. - -Jenaer Glas. By Hovestadt. Already referred to in respect of the -English translation. - -Der Sprechsaal. (Schmidt, Weimar.) A trade journal devoted to the -discussion of technical matters relating to the glass and ceramic -industries. Occasionally contains articles and abstracts of technical -or scientific interest in connection with glass manufacture. - - * * * * * - -In addition to the books and papers named in the above list, a great -number of scientific papers, notes, etc., are to be found scattered -throughout the technical and scientific publications of the world; -those that have proved of real interest and importance have, however, -left their mark on the industry, and will be found described or -referred to in connection with the various branches of manufacture -described in the present volume or in the books named above. - - - - -INDEX - - - A. - - Abbé, 8, 10, 210, 218 - - Absorption of light in glass, 32, 179 - - Acid, action of, on glass, 11 - boric, action of, on glass, 11, 186 - carbonic, action of, on glass, 12 - hydrofluoric, action of, on glass, 12 - phosphoric, action of, on glass, 11 - - Air, compressed, 91, 105, 117 - - Alkali chlorides, use of, in glass manufacture, 41 - content of hygroscopic glass, 6 - metals, 184 - nitrates, 44, 78 - sources of, 40 - - Alkaline liquids, action of, on glass, 11 - - Aluminium, 51, 186 - - Ammonia soda, 41 - - Anastigmatic photographic lenses, 213 - - Ancient windows, colours of, 16, 202 - - Annealing bottles, 103 - kiln, 103 - for optical glass, 235 - for plate glass, 135 - for rolled plate glass, 127 - - Anthracite coal, 42, 53, 79 - - Antimony, 188 - - Apochromatic objectives, 213 - - Arsenic, 52, 78, 105, 117, 188 - - Artificial gems, 246 - - Auerbach, 22 - - Aventurine, 185 - - - B. - - Bacteria, action of, on glass, 13 - - Barium compounds, 47, 186 - crown glass, 212 - glass, 7 - - Barytes, 48 - - Bases other than alkalies, sources of, 45 - - Beads, 244 - - Behaviour, chemical, of glass, 6 - - Bending plate glass, 144 - - Bevelling, 145 - - Black ash, 41 - - Blisters in sheet glass, 160, 168 - - Blocks, fire-clay, 58 - tank, 59 - - Blower, sheet glass, 158 - - Blower’s chair, 111 - - Blowing glass, 89 - holes, 91, 161, 189 - sheet glass, 161 - - Blown glass, decoration of, 114 - plate glass, 171 - - Bohemian glass, 109, 240 - - Boiling up, 81 - - Bottles, annealing of, 103 - blowing, improvements in, 99 - machines, 100 - colour of, 96 - manufacture, furnace for, 97 - moulds for blowing, 98 - production of, by hand, 98 - raw materials for, 95 - strength of, 18 - - Boric acid, 11 - - Boron, 186 - - Boro-silicate crown, 212 - - Boucher’s bottle-blowing machine, 101 - - Bricks, fire-clay, 58 - silica, 60 - - Bubbles in optical glass, 230 - removal of, 81 - - Burning, pots, 58 - - - C. - - Cadmium, 186 - - Calcium carbonate, 46 - oxide, 45, 186 - sulphate, 47 - - Carbon, 53, 79, 186 - - Carbonate of soda, 41 - - Carbonic acid, action of, on glass, 12 - - Carboys, blowing of, 104 - - Casting plate glass, 132 - - Chair, glass-blower’s, 111 - - Chalk, 46 - - Chamotte, 57 - - Chance, 211 - - Charcoal, 42, 58, 79 - - Charging furnaces, 75 - - Chemical behaviour of glass, 6 - composition of glass, 5 - of optical glass, 217 - reactions during fusion, 76 - - Chilled glass, 247 - - Chimneys, gaslight, 23 - lamp, 238 - - Chromium, colouring effect of, 190 - - Cleaning of lenses, 220 - - Coal, anthracite, 42, 53, 79 - - Cobalt, colouring effect of, 197 - - Coke, 42, 53, 79 - - Colour of ancient windows, 16 - glass, 32 - theory of, 181 - optical glass, 208 - sheet glass, 167 - - Coloured blown glass, 113 - glass, 178 - technical uses of, 203 - - Combustion tubing, 7, 241 - - Compressed air for glass blowing, 91, 105, 117 - - Conductivity, electrical, of glass, 30 - thermal, of glass, 24, 29 - - Copper, colouring effect of, 184 - ruby, 184, 188, 198 - - Corrosion of glass, 11 - - Covered pots, 56, 109 - - Crown, boro-silicate, 219 - glass, 175, 211 - hard, 212, 219 - soft, 212 - telescope, 215 - - Crowns, furnace, 60 - - Crucibles, manufacture of, 56 - for glass melting, 54 - - Crushing strength of glass, 19 - - Cryolite, 52 - - Crystallisation of glass, 3 - - Crystals, mineral, 218 - - Cullet, 74 - for optical glass, 224 - - Cutting rolled plate glass, 128 - - Cylinders, sheet glass, 161, 171 - - - D. - - Decolourisation of glass, 52, 188, 190, 193, 197 - - Decoration of blown glass, 114 - - Defects in rolled plate glass, 129 - sheet glass, 166 - - Definition of glass, 1 - - De la Bastie, 248 - - Devitrification, 3, 11 - - Diamond, refractive index of, 216 - - Dimming of glass surfaces, 12 - - Dinas bricks, 61 - - Dipping of sheet glass, 166 - - Dispersion of optical glass, 209 - partial, 214 - - Double refraction in optical glass, 221 - rolling machine, 130 - - Drawing tubes, 239 - - Ductility of glass, 20 - - Durability of glass, tests for, 14 - - Dust, action of, on lenses, 220 - glass, 245 - - - E. - - Elasticity of glass, 20, 24 - - Electrical properties of glass, 29 - - Epinal, 39 - - Etching of glass, 12 - - Expansion, coefficient of thermal, 24, 25 - - - F. - - Felspar, 40, 44 - - Fibres, glass, 245 - silica, 245 - - Figured rolled plate glass, 87, 130 - cutting of, 131 - - Finger-marks on lenses, 219 - - Fining of glass, 81 - optical glass, 229 - - Fire-clay, action of, on glass, 6 - for pots, 55 - wetting up, 57 - - Fire-polish, 117 - - Flashed glass, 25, 199 - - Flint, 40 - boro-silicate, 212 - dense, 212, 246 - densest, 212 - extra dense, 212 - glass, 7, 49, 78, 108, 211 - light, 212 - soda, 212 - telescope, 215 - - Fluorite, refractive index of, 216 - - Fontainebleau, 38 - - Founding of optical glass, 227 - - Fourcault process, 174 - - Fresnel, 251 - - Furnace crowns, 60 - gas, 63 - - Furnaces for bottle manufacture, 97 - glass melting, 54, 62 - optical glass, 225 - plate glass, 133 - rolled plate glass, 122 - sheet glass, 151, 170 - ports, 67 - recuperative, 66, 156 - regenerative, 66, 155 - tank, 59, 69 - economy of, 72 - - Fusion, process of, 73 - temperature of glass, 5 - - Freezing of glass, 2 - - - G. - - Gaslight, chimneys for, 23 - - Gas producers, 62, 64 - - Gatherer, 158 - - Gathering of glass, 85, 88, 158 - - Gauge tubes, 10, 18, 23, 26 - - Gems, artificial, 246 - - Ghosts, photographic, 16 - - Glauber’s salt, 43 - - Gold, colouring effect of, 185 - - Grinding plate glass, 137 - - Gypsum, 47 - - - H. - - Hardened glass, 20 - - Hardness of glass, 21 - tests for, 22 - - Heavy spar, 48 - - Henrivaux, 19 - - Hertz, 22 - - Hock-bottle colour, 195 - - Hohenbocka, 38 - - Hollow glass-ware, 108 - - Horse-shoe flame, 69 - - Hydrofluoric acid, action of, on glass, 12 - - Hygroscopic glass, alkali content of, 6 - - - I. - - Indentation modulus, 22 - - Index, refractive, 216 - - Insulating properties of glass, 29 - - Iron, 96 - colouring effect of, 196 - oxidation of, in glass, 195 - - Irregularities caused by rolling, 86 - - - J. - - Jena, 7, 10, 14, 26, 29, 203, 210, 213, 241 - - - K. - - Kelp, 40 - - Kowalski, 19 - - - L. - - Laboratory ware, 10, 23 - - Ladling glass, 85 - rolled plate glass, 124 - - Lagre, 166 - - Lamp-chimneys, 110, 238 - - Lamp-work, 240, 244 - - Large vessels, production of, 105 - - Lead, 49, 183, 188 - - Lear for rolled plate glass, 127 - sheet glass, 165 - - Leighton, 39 - - Lenses, cleaning of, 220 - finger-marks on, 220 - pressing small, 94 - - Light, action of, on glass, 15 - - Lighthouse glass, 178, 250 - - Lime, slaked, 45 - - Lime-stone, 46 - - Limited range of vitreous bodies, 4 - - Lippe, 38 - - Lynn, 39 - - - M. - - Machines, bevelling, 145 - double rolling, 130 - grinding, 139 - polishing, 141 - - Magnesia, 48, 186 - - Manganese, 15, 52, 80 - - Mangin mirrors, 252 - - Marver, 111 - - Massive glass, 249 - - Mechanical properties of glass, 18 - - Metal, attachment of, to glass, 26 - - Minerals, crystalline, 217 - - Mirrors, 145 - searchlight, 251 - - Mixing of materials, 73 - - Moulds for glass-blowing, 90, 110, 116 - pressed glass, 119 - - Muffled glass, 172 - - Muranese glass, 123 - - - N. - - Nickel, 96 - colouring effect of, 197 - steel, 27, 148 - - Nitrates, alkali, 44, 78 - - - O. - - Objectives, apochromatic, 213 - telescope, 218 - - Opal glass, 45, 52, 186 - - Opaque plate glass, 146 - - Open pots, 56 - - Optical glass, annealing, 235 - chemical composition of, 217 - cooling of, 233 - cost of, 237 - fining, 229 - founding, 227 - furnaces for, 225 - hardness of, 220 - moulding, 235 - pressing, 93 - range of, 216 - raw materials for, 223 - sorting, 235 - stability of, 219 - strain in, 221 - stirring, 231 - yield of, 236 - properties of glass, 205 - - - P. - - Painting on glass, 201 - - Parason, 102 - - Patent plate glass, 171 - - Paving stones, glass, 249 - - Pearl ash, 43 - - Phosphoric acid, 11 - - Phosphorus, 188 - - Photographic ghosts, 16 - lenses, anastigmatic, 213 - colour of, 209 - - Pipe, glass-maker’s, 89 - sheet-blower’s, 158 - warmer, 158 - - Plate glass, annealing kiln for, 135 - bending of, 144 - blown, 171 - casting, 132 - colour of, 33 - figured rolled, 87 - flatness of, 134 - furnaces for, 133 - grinding machines, 139 - of, 137 - mirrors, 145 - opaque, 146 - polishing machines, 141 - polishing of, 137 - raw materials for, 132 - rolled, 86, 123 - silvering, 146 - sizes of, 143 - strength of, 15 - striæ in, 143 - wired, 27, 147 - - Platinum, 27 - - Polishing, theory of, 141 - - Pontil, 98, 176, 239 - - Potash, 43 - - Potato, use of, in glass melting, 81 - - Ports, furnace, 67 - - Pots, burning of, 58 - covered, 56 - drying of, 58 - for flint glass, 109 - optical glass, 226 - manufacture of, 56 - open, 56 - - Pouring of glass, 85, 87 - - Pressed glass, 92, 118 - composition of, 120 - - Presses for glass, 119 - - Proofs, 82, 231 - - Purity of materials, 36 - - - Q. - - Quartz, 40 - - - R. - - Range, limited, of vitreous bodies, 4 - - Recuperative furnaces, 66, 156 - - Red lead, 49 - - Refraction, double, in optical glass, 221 - of light in optical glass, 209 - - Refractive index, 216 - - Regenerative furnace, 66, 155 - - Reichsanstalt, 10 - - Resistance to crystallisation of glass, 4 - - Rings for lighthouse lenses, 251 - - Rod, glass, 245 - - Rolled plate glass, 86, 123 - annealing, 127 - cutting, 128 - defects of, 129 - figured, 130 - furnaces, 123 - ladling, 124 - raw materials for, 124 - rolling, 126 - sorting, 129 - surface of, 122 - - Rolling of glass, 86 - - Rubies, artificial, 247 - - Ruby, copper, 184, 188, 198 - flashed, 184 - gold, 185 - - Rupert’s drops, 248 - - - S. - - Salt-cake, 37, 42, 79, 189 - - Sand, 38 - - Sandstone, 39 - - Schott, 8, 19, 203, 213, 241 - - Scratches on sheet glass, 169 - - Searchlights, 250 - - Seed in sheet glass, 167 - - Selenium, colouring effect of, 190 - - Sheet glass, 70 - blisters in, 160, 168 - blowing, 161 - colour of, 33, 167 - compared with plate, 149 - cylinders, 161, 171 - defects of, 166 - dipping, 166 - flattening, 165 - furnaces, 151, 170 - lear, 165 - mechanical production of, 173 - raw materials for, 150 - sorting, 166 - splitting, 164 - strength of, 18 - - Siedentopf, 182 - - Siege blocks, 59 - - Siemens, 248 - - Sievert, 92, 105, 117, 172 - processes, 105, 117 - - Signal glasses, 203 - - Silica bricks, 61 - glass, 5, 26, 241 - sources of, 37 - - Silicon, colouring effect of, 187 - - Silver, colouring effect of, 185 - - Silvering plate glass, 146 - - Sizes of plate glass, 142 - - Soda ash, 41 - carbonate, 41 - sulphate, 37, 42, 79 - sulphide, 80 - sulphite, 79 - - Solidification of glass, 1 - - Solutions, analogy of, with glass, 206 - - Sorting rolled plate glass, 129 - - Specific heat of glass, 25, 29 - inductive capacity of glass, 29 - - Stains, coloured, 200 - - Stassfurth, 44 - - Stones in rolled plate glass, 129 - sheet glass, 167 - - Storage of materials, 37 - - Strain in optical glass, 221 - - Strength of glass, 19 - - Striæ in coloured glass, 203 - optical glass, 206, 227 - plate glass, 143 - testing apparatus, 207 - - String in sheet glass, 168 - - Strontium, 86 - - Structure of glass, 1 - - Sulphur, colouring effect of, 189 - - Surfaces, chemical behaviour of glass, 8, 10 - - Szigmondi, 182 - - - T. - - Table, rolling, 126 - - Tank blocks, 59 - furnaces, 59, 69 - economy of, 72 - for sheet glass, 152 - - Telescope objectives, 213 - - Temperature of fusion of glass, 5 - - Tempered glass, 20, 248 - - Tensile strength of glass, 19 - - Thallium, 183, 188 - - Theory of colours in glass, 181 - polishing, 141 - - Thermal endurance of glass, 23 - properties of glass, 23 - - Thermometer glass, 7, 8, 28 - - Tin, colouring effect of, 187 - - Tonnelot, 7 - - Transparency of glass, 31 - optical glass, 208 - - Trautwine, 19 - - Tubing, 238 - combustion, 7 - drawing of, 239 - - Tumblers, 111 - - - U. - - Ultra-violet microscope, 243 - - - V. - - Vanadium, colouring effect of, 189 - - Veins in optical glass, 206, 227 - - - W. - - Water, action of, on glass, 10 - glass, 250 - - Wetting up clay, 57 - - Winkelmann, 19 - - Wired plate glass, 27, 147 - - Witherite, 48 - - Wool, glass, 245 - - - Y. - - Young’s modulus, 20 - - - Z. - - Zaffre, 197 - - Zeiss, 213, 244 - - Zinc, colouring effect of, 49, 186 - - Zschimmer, 14 - - -BRADBURY, AGNEW, & CO. LD., PRINTERS, LONDON AND TONBRIDGE. - - - - -D. VAN NOSTRAND COMPANY - -Publish the following Catalogues which will be sent gratis on -Application.... - - -CHEMICAL. - -_144 Page_ Alphabetical and Classified List of Books on Chemistry, -Chemical Technology and Physics. Arranged by subjects and authors. - - -ELECTRICAL. - -_112 Page_ Classified Catalogue of Electrical Books. Indexed. _8 page_ -list of Electrical Books issued in 1907. - - -TECHNICAL. - -_122 Page_ List arranged by Authors. - -_48 Page_ List of Scientific Books recently issued and in preparation. - -_8 Page_ Supplemental List of Books on Manufactures and Industries. - -_8 Page_ List of Books just issued. - - -MONTHLY RECORD. - -_16 Page_ Monthly Record of Scientific Literature Sent regularly to any -address, on request. - - - 23 Murray and 27 Warren Streets, New York. - - - - -D. Van Nostrand Company - -Announce the early issue of the - -WESTMINSTER SERIES. - - 8vo, cloth, fully illustrated, Each, $2.00 net. - - - Much of the information forthcoming in these volumes is either - not available to the general public, or if available, is widely - scattered. These volumes will be issued at short intervals. - - -_NOW READY._ - - =INDIA-RUBBER AND ITS MANUFACTURE=, with Chapters on Gutta-Percha - and Balata. By H. L. Terry, F.I.C., Assoc. Inst. M.M. - - =ELECTRIC POWER AND TRACTION.= By F. H. Davies, A.M.I.E.E. - - =LIQUID AND GASEOUS FUELS, AND THE PART THEY play in Modern Power - Production.= By Professor Vivian B. Lewes, F.I.C., F.C.S. - - =COAL.= By James Tonge, M.I.M.E., F.G.S., etc. (Lecturer on Mining - at Victoria University, Manchester). - - =THE BOOK: Its History and Development.= By Cyril Davenport, V.D., - F.S.A. - - =IRON AND STEEL.= By J. H. Stansbie, B.Sc., F.I.C. - - =TOWN GAS FOR LIGHTING AND HEATING.= By W. H. Y. Webber, C. E. - - =GLASS.= By Walter Rosenhain, Superintendent of the Department of - Metallurgy in the National Physical Laboratory, late Scientific - Adviser in the Glass Works of Messrs. Chance Bros. & Co. - - -_IN PREPARATION._ - - =PATENTS. Trade Marks and Designs.= By Kenneth R. Swan (Oxon.), of - the Inner Temple, Barrister-at-Law. - - =THE MANUFACTURE OF PAPER.= By R. W. Sindall, F.C.S. - - =WOOD PULP AND ITS APPLICATIONS.= By C. F. Cross, E. J. Bevan, and - R. W. Sindall. - - =STEAM ENGINES.= By J. T. Rossiter, M.I.E.E., A.M.I.M.E. - - =PRECIOUS STONES. With a Chapter on Artificial Stones.= By W. - Goodchild, M.B., B.Ch. - - =ELECTRIC LAMPS.= By Maurice Solomon, A.C.G.I., A.M.I.E.E. - - =STEAM LOCOMOTIVES.= By Vaughan Pendred, M.I.Mech.E. - - =GOLD AND PRECIOUS METALS.= By Thomas K. Rose, D.Sc., of the Royal - Mint. - - =ELECTRO-METALLURGY.= By J. B. C. Kershaw, F.I.C. - - =PHOTOGRAPHY.= By Alfred Watkins, President-Elect of the - Photographic Convention, 1907. - - =COMMERCIAL PAINTS AND PAINTING.= By A S. Jennings, Hon. Consulting - Examiner, City and Guilds of London Institute. - - =BREWING AND DISTILLING.= By James Grant, F.S.C. - - - Publishers and Booksellers, - - 23 Murray and 27 Warren Streets, NEW YORK. - - - - -Transcriber’s Notes - - -Punctuation, hyphenation, and spelling were made consistent when a -predominant preference was found in this book; otherwise they were not -changed. - -Simple typographical errors were corrected; occasional unbalanced -quotation marks retained. - -Ambiguous hyphens at the ends of lines were retained. - -The appearance of the Table of Contents has been altered slightly. - -Index not checked for proper alphabetization or correct page references. - -In the Index, it was unclear to the Transcriber whether some of the -indentations indicated sub-entries or implied ditto marks, so the -indentations have been treated here as multiple levels of sub-entries. - - - - - -End of the Project Gutenberg EBook of Glass Manufacture, by Walter Rosenhain - -*** END OF THIS PROJECT GUTENBERG EBOOK GLASS MANUFACTURE *** - -***** This file should be named 52724-0.txt or 52724-0.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/5/2/7/2/52724/ - -Produced by Chris Curnow, Charlie Howard, and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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