Other things you need to know about glazing of ceramic articles

Glazes are useful both in case of porous as well as dense bodies. In porous bodies the glazes acts as seals and prevent penetration of water, while in dense bodies they make the surface smoother and brilliant. A glaze is formed on the surface of a shaped ceramic article during the firing. Before firing, the glaze cover the face of the ceramic article in form of dry powder. A glaze on the surface of ceramic articles must not get too fluid. If it were fired to the degree of fluidity required for flowing a glass bottle. It would run off the ceramic article. The glazing mixture free from iron and other colouring pigment forms colourless glaze. Coloured metal oxides or pigments are mixed in proper proportions for colour glazing. For example for red and brown colours, iron oxide is used. For Cream and yellowish tint, iron oxide and lime are used. For green colours copper oxide is used and for blue colours Cobalt blues are used. Glazes can be divided into the following three classes, according to the ways in which they are prepared

(a) Raw glazes : prepared from materials which are insoluble in water

(b) Frit glazing : in these materials, which are soluble in water are first melted with silica and then mixed with other materials soluble in water

(c) salt glazing .

Salt glazing is a special type of glazing in which common salts are used for getting glossy film over the earthenware. In this process common salts in thrown into the furnace in which the articles to be salt glazed are in red hot condition. The common salts undergo volatalisation under intensive heat and react with silica of the article to form a glossy film of sodium silicate.