Ceramic Technology 2 - Glazes and Glazing

What's in a Glaze?

A glaze is simply a thin coating of glass on the top of a ceramic clay body. The term ‘glaze’ is also used in painting to describe a thin coating of transparent lacquer applied over underlying paint to improve it’s lustre or for it’s protection. In ceramics, a glaze is an integral part of the ceramic work. Heat is required to melt the glaze and during this process begins fuse with the surface of the clay body. This fusion becomes more pronounced with higher firing as the body becomes vitreous, eventually resulting in the translucent glaze and clay body characteristic of porcelain. Further increases in temperature will likely result in the body melting to become a glaze itself.

Imagine crushing a glass bottle, grinding the particles to a fine powder and then adding the powder to a bucket of water. Stir the mixture to suspend the powdered particles, and dip a porous, bisc fired ceramic work into the milky liquid for two to three seconds. Remove the work and watch it quickly dry. A thin layer of the glass powder is now left coated to the surface of the bisc. This is essentially a how a glaze is made, prepared and applied to ceramic.

Bottle Glass left, and Felspar

Although bottle glass and ceramic glass are related, there is a significant difference between them. Ceramic glazes are made from minerals similar to those used in clay bodies, are much higher in Alumina (Al2O3) so they adhere to the clay body better and form a stiffer, more viscous melt than ordinary glasses.

Glaze Chemistry Overview

A deeper understanding of glazes can be found by considering both the physical and chemical reactions that occur during their formation. These conditions, where heat and chemical reactions combine, is called ‘thermochemistry’.

In it’s pure theoretical form, a glaze is usually though of as a colourless, transparent Alumino Silicate Glass. From this basis it is easier to understand how the huge variety of glaze finishes emerge. By altering the ingredients of a glaze, tit’s thickness of application , the temperature to which it is fired, the kiln atmosphere, and the the rate of cooling and heating, a myriad of surfaces, colours and special effects can be achieved. 

A glaze can be thought of as comprising three categories of materials, fluxes, glassformers and stabilizers.

The major stabiliser used in glazes is Alumina, which is also a primary oxide found in Clays. This combines with Silica to form an ‘Alumino Silicate’ glass. Ceramic Glazes differ from bottle or ‘soda’ glasses which have little or no silica present. During kiln firing, Alumina provides a melt that chemically matches the clay and creates a  viscous glass melt which helps prevent the melted glaze from running and adhering objects to the kiln shelves. It follows that by adjusting the amount of alumina in a glaze, it’s maturation or ‘melt’ temperature can be modified. Satin, or Matt glazes are often created this by increasing the amount of Alumina in the glaze. (usually through the addition of  China Clay). The other main contributor to Mattness in glazes comes via crystallization, which is induced by paticular combinations of minerals.

Double dipped glaze with controlled melt

Tang Dynasty Chinese 'Tri-colour' glazed terracotta horse
showing the decorative use of fluid glaze.

Fluxes , in addition to modifying the glaze melt, effect colour reactions and surface quality. Raw materials may behave differently according to their firing. Iron, for example, has more than one form, depending upon it’s degree of oxidation. As Ferric Oxide it is a stabiliser when fired in an oxidation kiln environment. However in a Reduction kiln , Ferric Oxide (Fe2O3) loses oxygen and becomes FeO, which is a flux, and in so doing, alters its colour effect.

See the class notes for more detail on colour an surface in glazes, and check out these videos for a discussion and visual tips on applying glaze.