GLAZING CLAY PRODUCTS

Bricks, tiles, earthenwares and stonewares are glazed by an impervious film to protect the surface from chemical attack and other weathering agencies. The different types of glazing in use are as to follow.

Transparent Glazing :

There are many methods for imparting transparent glazing, but salt glazing is most commonly used, since this makes the items impermeable. It consists of throwing sodium chloride in thekiln when burning is at peak (1200°–1300°C). The heat of the kiln volatises the salt, which enters into the pores of the burning item and combines with the silica in clay to make soda silicate. The soda silicate so formed combines with alumina, lime and iron in the clay to form a permanent thin, transparent surface coating.

Lead Glazing :

Clay items are burned thoroughly and then dipped in a solution of lead oxide and tin oxide. The particles of lead and tin adhere to the surface of clay items. After this, the articles are returned in potter’s kiln where these adhered particles melt and form a thin transparent layer on the outer surface. This method of glazing is used for items of inferior clay which cannot withstand high temperature required for salt glazing.

Opaque Glazing :

This is also known as enamelling. Borax, kaolin, chalk and colouring matter is fired with total or a part of felspar, flint, and lead oxide. The resulting molten glass is poured into water to give shattered frit. The frit is then ground with remaining materials and water and is made of the consistency of cream known as slip. Fully burnt earthenwares known as biscuits are dipped in the slip. The biscuits absorb water and form thin layer of glaze on the surfaces. After drying the products, these are once again fired to a lower temperature so as to fuse the glaze.