Glazing is a process of providing a glassy layer on the surface of clay product or ceramics. The glazing layer is fused to a ceramic body by burning at high temperature. The thickness of glazing is generally about 0.1 to 0.2 mm. Different methods of glazing and uses of glazing are explained below.
Uses of Glazing
Following are the uses of glazing.
- It acts as an impervious layer and makes the product durable.
- It provides an aesthetic appearance to the product.
- It provides a smooth surface to the product.
- It resists the action of chemicals, various atmospheric agencies, etc.
Methods of Glazing of Clay Products
There are two methods of glazing of clay products namely
- Transparent glazing
- Opaque glazing
1. Transparent Glazing
Transparent glazing of clay products is achieved by two methods
In this method, a mixture of lead oxide and tin oxide particles is prepared. The clay product to be glazed is burnt in the kiln and taken out. This clay product is immersed in the prepared mixture of lead and tin oxides. After that, the clay product is taken out and placed in the kiln and heated at high temperature.
The particle of oxides of lead and tin are melted and a glass-like layer is formed on the surface of the clay product. The lead glaze formed is smooth and of better quality. Unlike the other glazing methods, the layer formed in this method does not penetrate into the body of clay product. It can be separated easily from the object surface if required. Fire-clay products, terra-cotta products are glazed by lead glazing.
Slat glazing on a clay product is achieved by using salt or wet sodium chloride. In this method, the clay object is placed in the kiln and heated at high temperature. During the burning of a clay product, a small quantity of wet sodium chloride (wet salt) is thrown into the kiln.
The wet salt is vaporized and the vapor formed is of sticky nature. This salt vapor forms a glass-like layer on the surface of a clay product. The salt glazing generally produces brownish color on the product. Stoneware products, sanitary pipes, etc are glazed by this method.
2. Opaque Glazing
Opaque glazing gives a better appearance than transparent glazing. In this method, a finely powdered and dried clay is taken and an adequate amount of water is added to it. Both are mixed until required consistency (generally plastic consistency) is achieved. The mixture formed is called slip.
The object to be glazed is dipped into the slip and afterward heated in the kiln. The slip surface melts and forms a layer on the surface of the object. The layer formed is penetrated into the pores of clay object and hence it is difficult to detach the layer from the surface of the object. Sanitary pipes made of clay are glazed by this method.