GLASS: THE BOND IN CERAMICS*

Abstract

A ceramic industry is one which manufactures products by the application of heat to raw materials containing silica or silicates as essential ingredients, and ceramic chemistry is that branch of Chemistry which deals with the chemistry of silica. The property which makes silica the basis of industries as apparently diverse as water glass and refractories and which is conferred on all mixtures in which it is present in adequate amounts is that of forming glasses. While silica itself forms an ideal glass, its high melting point requires the addition of a flux, and the commonest and most powerful flux is soda. The addition of soda to silica produces a remarkable lowering of the melting point, from 1713 to 793° and glass of this composition is remarkably stable against devitrification. It is dissolved by water and forms the basis for the soluble silicate industry. But insolubility in water and acids is essential for most of the glasses of ceramics, and other oxides must be added to obtain this stability. The commonest and cheapest is lime (CaO), and most of the glass of commerce may be regarded as derived from the sodium‐silicate glass by the addition of several per cent of lime, usually also with the addition of small amounts of other oxides, notably magnesia, boric oxide, and alumina. Boric oxide and alumina are especially characteristic of the glassy base of enamels, which chiefly differ from glasses in containing insoluble materials to render them opaque and often coloring agents soluble in the glassy phase. Glazes, too, are of similar composition. Refractories are bonded by glass; either a lime glass in the silica refractories, or an aluminum glass in the clay refractories. But a siliceous glass is the characteristic feature which is found in all products of the ceramic art, and glass is the bond which unites the divers ceramic industries.