GLASS CRAB, an immature condition of certain crabs (Palinurus and its allies) which for a time are flattened and perfectly trans parent, as if formed of a sheet of glass, and have no resemblance to the parent form.
As found ordinarily in commerce, glass consists of an alkaline sili cate with one or more basic silicates melted to gether. From its molten state glass gradually stiffens as it cools, becoming solid eventually without having gone through a molecular change; that is to say, it has not crystallized. In this glass mixture silica takes the role of the principal acid, uniting with at least two basic oxides, one being of the alkaline metals. In many of the common forms of glass boron • accompanies silica as the acid component. A general classification of the ordinary commer cial varieties of glass would group together (1) those in which silica is the only acid com ponent; (2) those in which only an alkaline base is combined with the silica; (3) those in which other acid components are present with the silica; (4) those in which no silica is present, The first group would contain the commonest of all glass— the soda-lime glass as used for window glass, white druggists' bottles, table glass and hollow ware, etc. Its composition is silica, 72 per cent ; soda, 13 per cent; lime 13 per cent; impurities 2 per cent. French window glass has slightly less silica, a trifle more soda, and a little larger percentage of lime. The second group would comprise what is known as soluble glass, or water glass. The third group includes most of the optical glasses, and the enamels and imitation gems used by jewelers. The fourth group is made up of the borate and phosphate glasses used in certain combinations in correcting the aberra tions of lenses.
Raw It is imperative that the materials put into the crucible for fusing into glass shall be scrupulously pure, as there is no way of separating impurities after the mass is once melted. The silica which constitutes from 52 to 72 per cent of the mass of the raw material before melting is obtained chiefly from a high grade of quartz sand. It is required to test
at least 98 per cent pure silica. Of the impuri ties not more than one-half of 1 per cent of oxide of iron is permissable if the glass is to be used for fine table ware, cut glass, etc. If for optical glass, the proportion of iron oxide must not be more than 0.015 of 1 per cent. If for common window glass of a slightly greenish hue, the oxide of iron may run up to 2 per cent. Some of the "sand" used in making the finest glass is obtained by crushing quartz ; and, as it is important that the sand be uniform in size of gra'n, it becomes neces sary to screen the crushed quartz twice to get rid of the dust and fine stuff, and also of the particles which are too coarse. There are some soft sandstones which provide very good material but the extra cost of crushing and screening restricts their use to high-priced articles.
In the United States glass sands are found in commercial quantities in 18 States. In 1916 the amount produced was over 2,000,000 tons, chiefly from Pennsylvania and Illinois, some what less from West Virginia, and smaller, though substantial, quantities from New Jersey and Ohio. The finest quality comes from Berk shire County, Mass., and is practically pure silica; the Pennsylvania and West Virginia sands contain less than one-tenth of 1 per cent of iron. The New Jersey sand is higher in iron, but of good window-glass quality.
The principal source of soda is sodium car bonate, to be had in the market in a sufficiently high degree of purity. The cheaper sodium sulphate is used in making plate and window glass, but it is more trouble to use, as carbon must be added, usually as anthracite coal, and this is likely to carry other impurities. Sodium nitrate is employed to oxidize any organic mat ter present and to change the iron from the ferrous to the ferric state. In potash glasses the alkali is obtained from commercial pearl ashes or from salts of tartar.