Kiln Atmosphere

June 06, 2018 3 Comments

Before I started pottery, I imagined that firing pottery was like baking. Just put your pot in the oven and set the temperature. These 3 bowls are made of exactly the same clay blend and shino glaze, yet look totally different because of how they were fired. This has to do with the kiln atmosphere.

The first one was (accidentally) fired in an oxidizing atmosphere. This means oxygen (from the air) was present inside the kiln during firing. This firing method did little to bring out the colors of the clay and glaze. The resulting color of the clay is a pale-buff color. The glaze is translucent with a very small amount of orange flashing where thin. The reason this bowl was fired in oxidation is that I forgot to open the gas valve during the firing. Oops...

The second one was fired in a reduction atmosphere. Gas was added as a kiln fuel during the firing. It actually takes an excess of gas that's beyond what the kiln can consume that pushes the atmosphere into reduction. It’s called reduction firing because the unstable carbon monoxide molecules created by the burning gas steal oxygen atoms from the iron oxide in the clay and glaze during firing, reducing them. This amazing reaction makes the color of the clay darker, and also creates more of an interaction between the clay and the glaze. As a result of the reduction, the high-iron clay turned a nice reddish brown and the glaze developed red-orange highlights where the application was thinner. 

The third pot was also fired in reduction. The difference is that it was fired longer than the second one and there was reduction during cooling. This made the clays color become very dark as it was not followed by an oxidation cycle at the end of the firing. A reduction firing cycle is often followed by a period of oxidation. Without being able to re-oxidize at the end of the firing the clay continued to lose oxygen molecules to the reduction atmosphere till near the end of the firing. This caused the clay to turn almost black. Although it's not visible in the photo, there were some places where the glaze developed an iridescent, metallic sheen. 

It takes a lot of trial and error to find the results that are most pleasing from each clay and glaze combo. There's no guarantee how the results will turn out. Each clay and glaze fire differently in different kilns with differing firing atmospheres.

Even though these bowls were all fired to the same temperature and for a similar amount of time, you can clearly see the difference in the outcomes due to differing kiln atmospheres. 

The kiln I use in my studio in Taiwan is a hybrid kiln. It uses both electricity and gas. That means I have a lot of control over my firing atmosphere. Hybrid kilns are very uncommon in the west, almost unheard of. It’s a small efficient kiln and I'm very happy with what I'm able to produce with it. 

Beautiful ceramics can be produced with any kind of kiln whether it fires in oxidation or reduction.  It’s up to the artist to be creative with their glazing techniques to produce beautiful pots.

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