Glass is a poor conductor of heat. When a piece of glass has been expanded under the influence of heat, and is rapidly cooled, the superficial outer portions become intensely strained and contracted upon the interior portions, which retain the heat longer. Under these conditions of cooling, glass is apt to “fly,” or collapse and fall to pieces, owing to the outer portions giving way under the great strain. These stresses or strains are relieved in the process of annealing, under which they are gradually eased by a slow and regular cooling from the heated condition. Certain glasses, the composition of which shows considerable differences in the density of the respective bases present, are more subject to this defect than those in which the bases are of more even density and homogeneous in character. Such glasses should be “de-graded” and re-melted in order more thoroughly to diffuse and distribute the denser portions throughout the mass. In de-grading glass, the hot glass is ladled out and quenched in cold water, dried, and re-used as “cullet.”
CHAPTER IV
THE COMPOSITION OF THE DIFFERENT KINDS OF GLASS
The composition of glasses may be simple, compound, or complex, according to the number of bases or acids which may be present in the mixture.
The Simple types of glass are exhibited in the soda silicate, potash silicate, and lead silicate. The two former silicates are of most industrial value.
Soda Silicate is made from a fusion of 100 parts of sand with 50 parts of soda carbonate and 5 parts of charcoal. The charcoal is added to facilitate the decomposition. The fused mass when cool is transparent and of a pale, bluish, sea-green colour. Upon boiling it in water it dissolves and gives a thick viscid solution called “Water Glass.” This is extensively used in the various arts and manufactures. Textile fabric and woodwork saturated with this solution and dried are rendered fireproof. In the manufacture of artificial stone it forms, with lime and other basic oxides, very stable cements. Mixed with silicious or ganister it forms the well-known fire cements for repairing the cracks in retorts, muffles, etc. Water glass is also used in soap, and colour making, and for preserving eggs.
Potash Silicate is less used, being more expensive. It is produced from a fusion of 100 parts sand, 60 parts potash carbonate, and 6 parts charcoal.
Lead Silicate is composed of 100 parts sand and 66 parts of red lead fused together. This silicate is mostly used in the manufacture of soft enamels and artificial gems, and goes under the names of “Rocaili flux,” “strass metal,” and “diamond paste.”
There is another form of soluble glass which is a combination of the soda and potash silicates. This is really a double silicate and may be produced by fusing sand 100 parts, soda carbonate 25 parts, potash carbonate 30 parts, and 6 parts of charcoal. This silicate is used in soap making. Soluble glass can also be formed by using sulphate of soda as the alkali. In this case, a larger proportion of the alkaline salt has to be used, also a larger amount of carbon, in order to complete the decomposition of the sulphate. A mixture of sand 100 parts, saltcake 70 parts, and carbon 16 parts would produce sodium silicate. The boron silicate and borate of alumina are two other forms of soluble glass used in their simple states.
The Compound Glasses may be flint or crystal glass, soda-lime glass, Bohemian glass, pressed glass, and sheet glass. These are the general type of glasses used in the manufacture of domestic glasswares.