A-Z of Ceramics - O is for Oxidation

In addition to changing clay into a hard ceramic material, the first firing of a pot transforms its appearance, dramatically altering colours and textures. The challenge to the potter is to control the firing conditions to achieve the desired results. The two most fundamental aspects of firing are the temperature and the amount of oxygen in the kiln atmosphere.

Altering the amount of oxygen in the kiln can result in startling effects. The very same glaze pigments may change entirely, depending on whether the kiln atmosphere is rich in oxygen (favouring 'oxidation' reactions) or starved of oxygen (favouring chemical 'reduction'). In making the pots illustrated below, the same form of copper oxide was added to the glazes. But at a critical moment during the firing of the red vase, the kiln was starved of oxygen, resulting in this rich and luscious effect.

Bowl, Lucy Rie, about 1979. Museum no. C.44-1982. Porcelain with glaze stained with metal oxides and a bronzed rim.

Bottle, made by James Walford, United Kingdom, 1996. Museum no. C.119-1996. Porcelain with copper-red glaze.