Silica, or silicon dioxide, is a widely distributed mineral. It occurs in rocky masses of crystalline formation, clear, or partly opaque, and white in color when pure. It is prepared for use by grinding and removing the moisture by any of the usual processes.

In combination with white lead, this substance has been used for some considerable time as a paint base and some extravagant claims have been made for it. It resembles barytes in its working qualities, except that it is bulkier and requires more oil. Used as a paint, it dries extremely hard, is a neutral pigment, but covers poorly. It is extensively used in paste and liquid wood fillers, in which use its hard drying properties and transparency render it of considerable value.

Silica is of particular interest from the fact that water glass or silicate of soda (soluble glass) is prepared from it. This substance finds extended use in dyeing, color printing, in various cements, and in painting.

Water glass comes into commerce in the form of a thickly fluid transparent mass resembling sugar syrup in appearance. It is made by fuseing, in a crucible, 15 parts of quartz sand (silica), 10 parts of potassium carbonate and 1 part of powdered charcoal. When cold, the fused mass is pulverized and exposed to the air. The powder is washed thoroughly with cold water, and then boiled with 5 parts of water until dissolved. The solution is filtered and evaporated to the proper consistency; when exposed to the air, it dries to a transparent glass. Other methods of manufacture are in vogue, in a general way resembling the one described. It can be purchased very cheaply. Under formulae, in Chapter X (Formulae). various water glass preparations are described.