WATER GLASS, a substance which, when solid, resembles glass, but is slowly soluble in boiling water, although it remains unaffected by ordinary atmospheric changes. It consists of soluble silicates of potash or soda, or a mixture of both. The soda glass has the formula, Nat0.3SiOs A substance of this kind was first discovered, so far as is known, by Van Helmont in 1640. In the 19th century Dr. Johann Fuchs of Munich did much to improve the mode of preparing and applying it. Water glass may be prepared in two ways, called wet and dry. The former method is used where the proportion of alkali may be so large as 50 per cent, in which case the sand and alkali may be simply boiled together. Where a water glass richer in silica in required the mode of prepara tion consists n breaking down and calcining flint nodules, the fragments or particles of which are then added to a solution of caustic potash or soda, whereupon the whole is ex posed for a time to intense heat at 60 pounds pressure. Sand may be used instead of the flint. According to the product when prepared in this way is viscid, and may be used in this state or further diluted. In this method of
manufacture the caustic soda or potash may be replaced by the carbonate or the sulphate. Potash water glass is more soluble than soda water glass, and if both metals be present a still more soluble glass, called double soluble glass, is obtained. Among the purposes to which water glass is applied are painting on glass, coating stone, wood and other materials to render them waterproof or fireproof, glazing scenery and paintings, etc. It is also used mixed with sand to make an artificial stonc, and it forms an ingredient indispensable in the manufacture of fireproof cements, such as are used in joining firebricks in furnace construc tion. For this purpose it is commonly mixed with lime. One of the most valuable of its applications is in the fixing of wall-paintings, spraying with the water glass. They arc thus rendered more durable than in the or dinary process of fresco-painting.