Glass and Glass Manufacture — Themes and Context

n for a long time before this as a centre for the mines producing , and eventually this clay was adopted for making glass-house pots; now many other sources are available for these fire-clays. Much of the antiquity of the glassmaking of England is hidden in the neighbourhood of Stourbridge, and the writer has himself found a few antique specimens of old green devitrified window glass embedded in the subsoil of some fields near Oldswinford, probably relics of the Huguenots, who practised and extended the art of glassmaking in that district. Other important centres for glassmaking now are York, London, Manchester, Edinburgh, Newcastle, and Birmingham; but, although glassmaking has reached a high degree of excellence in this country, there is nothing yet comparable with the extensive factories which exist abroad. The conservatism of many English manufacturers, and the adverse influence of the Glass Makers’ Society, considerably restrict the progress of this trade compared with the broad and progressive manner in which it is carried on abroad.[1]

_See_ article “Trade Unionism,” in last chapter.

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THE CHEMISTRY OF GLASS-MAKING AND THE MATERIALS USED

The term “glass,” in a general sense, is applied to the hard, brittle, non-crystalline, transparent, opaque or translucent vitreous substance which results from fusing silica with active mineral solvents or fluxes, such as the alkalies, earthy bases, or metallic oxides. Silica exists in great abundance, in a free natural state, in the form of flints, quartz, and sand; and in the latter form it is now most generally used for glassmaking. When sand alkali and lead oxide are heated together to a high temperature, the sand is dissolved by the solvent action of the fused alkali and lead oxide until the whole becomes a molten mass of glass. The solvent action of the alkalies, soda potash or lead oxide, is very energetic whilst being heated, and the mass boils with evolution of gases until, at last, the solution, becoming complete, settles down to a clear quiescent molten liquid metal, which is quite soft and malleable, after the nature of treacle. In this condition it is ready for working. The time and temperature necessary for melting such mixtures vary according to the proportions and composition of the ingredients.

=Silica=, combined with alumina and other oxides, is freely distributed in nature in the form of clays, granites, and feldspars, which are also available for use in glassmaking. Originally glass was made by using crushed and ground flint stones as the source for the silica: hence is derived the old name of “flint” glass; but now the large extensive deposits of white sand present a much more convenient and less expensive source, and sand has become universally used. Fine white sand is obtained from Fontainebleau, near Paris; other sources are Lippe, Lynn, Aylesbury, Isle of Wight, Holland, and Belgium.[2] These are the sources preferred by crystal glass manufacturers and makers of fine quality glass, such as chemical ware pressed glass, tube, cane, and medical bottles, on account of their greater purity. The commoner varieties of sand from Reigate and Bagshot and even red sand are being used in the manufacture of the lower grades of glass such as beer bottles and jam jars, where a greater latitude in the chemical impurities present is permissible. Only the best and purest silica sands are used for making cut crystal and optical glasses. In these trades the sand is always cleaned by washing it in water to clear it from any salt, chalk, or other impurities which may possibly be present. The sand, after washing, is heated to redness, or “burnt,” in order to burn off any organic or vegetable matter, and when cold it is sifted through a fine screen to take out any coarse grains or lumps. In this prepared state, the sand is ready for weighing out into the proportions desired for mixing with the other materials, and is stored for use in covered wooden compartments situated in or near the mixing rooms, along with the other materials which may be used in the glass mixtures.

_See_ “British Glass Sands” (Boswell), “British Glassmaking Sands” (Peddle); papers read at the third meeting, Society of Glass Technology, Sheffield, for further information.

The alkalies, potash or soda, or a mixture of both, are commonly used in making glass in the form either of carbonates, sulphates, or nitrates. The soda and potash silicates form very fusible glasses, but they are not permanent, being soluble in water; therefore they cannot be used alone. In making glassware for domestic use, other bases, such as lead oxide, barium, or lime, have to be added to form more insoluble combinations with the silica or sand.