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 silicic acid with active mineral solvents or fluxes, such as the alkalies, earthy bases, or metallic oxides. Silicic acid, or 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 became 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 felspars, which are also available for use in glass-making. 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.1 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 have been 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.
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 silicic acid or sand.
1 See " British Glass Sands " (Boswell), " British Glassmaking Sands " (Peddle); papers read at the third meeting, Society of Glass Technology, Sheffield, for further information.
Carbonate of Potash or Pearlash, which before the war was imported into this country by glass makers from Strassfurt, is much prized by crystal glass makers on account of the colourless silicate it forms when fused with the best white sand. It is now very expensive and difficult to get, and is less used on this account. Potash carbonate is very hygroscopic and absorbs much moisture from the air; therefore it is necessary to keep it within sealed chests while in store.
Potash and soda each have an influence upon the colour of the resulting glasses in which they are respectively used. The potash silicate gives better and clearer glasses than the soda silicate.
Carbonate of Soda, or Soda Ash, is now more generally used. Being a less expensive form of alkali, it constitutes a base in most of the commoner varieties of glassware. Carbonate of soda is manufactured in England from common salt, of which there are large deposits in the Midlands. This common salt, or chloride of sodium, is treated chemically and converted into the carbonate, in which form it is supplied to the glass manufacturers as soda ash.
Sulphate of Soda (Salt Cake) is the form of alkali used in window and bottle glassmaking. In mixtures containing sulphate of soda it is necessary to use a small proportion of carbon in some form, such as charcoal or coal, in order to assist the decomposition of the salt and the formation of the sodium silicate. Sulphate of soda is used in this class of glassware on account of its cheapness. Glasses made from sulphate of soda mixtures are not so clear and colourless as those in which the source of alkali is potash or soda carbonate. On this account, the best crystal glasses cannot be made from sulphate of soda.
Potash Nitrate (Saltpetre) is used in glass mixtures to oxidise the molten metal and improve the colour of the glass. In fusing it disengages oxygen gas, which purifies the glass while melting, and assists the decolor-izers in their action by keeping up an oxidising condition within the molten mass.
Soda Nitre, or Chili Nitre, is the corresponding soda salt to potash nitre. It is much cheaper, but less pure; it has a similar but not nearly so powerful an oxidising action in the glass as potash nitre. It is exported from Chili, where it exists naturally in a crude state as " Calliches," from which the nitrate is refined by recrystallisation.