| 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.