High Firing in Ceramics: Oxidation and Reduction Explained
Let’s recall what we mentioned in previous posts: the high firing (glaze firing) is a second firing, after the bisque firing. This is when the glaze is fully matured and bonded to the piece.
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The most commonly used technique is oxidation firing, which is what you usually get in electric kilns. It’s the simplest method and the one adopted by most potters. If you’re just starting out, we don’t recommend any other firing technique than this one.
The oxides develop the colours typical of an oxidation atmosphere (for example, copper oxide will give a green colour).
Image of a typical electric kiln in oxidation firing:
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Reduction firing, on the other hand, is done in gas kilns (or others such as wood-fired, diesel, etc.). It consists of cutting off the oxygen supply to the kiln when the reduction phase begins (around 1000–1050 °C). In this way, the carbonaceous materials undergo incomplete combustion.
When there is more carbon present than oxygen available in the kiln, combustion cannot be completed (since full combustion requires enough oxygen), and carbon and carbon monoxide are formed. These “steal” oxygen from the clay body and the glazes, and this reducing effect changes the colour of the oxides in the clay and glaze. For example, copper oxide will no longer give a green colour as in oxidation, but instead will produce a red colour.
We’ll close this post by stressing that reduction firing is generally harder to learn and master than oxidation firing.