What is the difference between oxidation and reduction firing

There are many factors to consider when it comes to firing your pottery in a kiln. The terms oxidation and reduction refer to how much oxygen is in the kiln's atmosphere while the kiln is firing.

An oxidation atmosphere has plenty of oxygen for the fuel to burn. A reduction atmosphere occurs when the amount of available oxygen is reduced. This may not sound like things that will affect your pottery, but it can.

The oxidation process, for example, can alter the color of the glazes or paint you've chosen to use. The reduction process, when oxygen is leeched out of your kiln atmosphere and pottery, can change the texture of your clay.

Learn what's going on in your kiln before you fire your next project. When heated sufficiently, many substances oxidize if there is free oxygen available.

Volatile portions of compounds and molecules break free and the free oxygen then attaches to the remaining material, forming oxides. This process is called oxidation. In firing a pottery kiln, the materials will normally convert to their oxide forms.

For example, when copper carbonate is fired, the carbon will detach and burn off. Salt glazed ware typically has distinctive marbled and variegated surface effects. In the broadest terms, there are two types of kilns: intermittent and continuous, both being an insulated box with a controlled inner temperature and atmosphere. A continuous kiln, sometimes called a tunnel kiln, is long with only the central portion directly heated.

We typically fire these saggars in a raku kiln, but can be done in an electric or a pit. We pack the kiln with pieces ensuring there is airflow around each piece. We fire until the foil loses its sheen, its shine. Reduction firing is when the kiln atmosphere, which is full of combustible material, is heated up. As its name indicates, this is a slower version of oxidation where products are slowly ruined over time. Examples include products becoming spoiled and discolored, metal corrosion, rusty car doors, and foods turning moldy.

The reduction process, when oxygen is leeched out of your kiln atmosphere and pottery, can change the texture of your clay. Oxidation occurs when there is an excess of oxygen.

As the kiln heats up compounds in the glaze break off and oxygen attaches itself to the glaze and clay. Reduction Firing Most usually, reduction firing is favored by potters and is done using gas, propane or other fuel burning kilns or methods. Reduction firing is the exact opposite of Oxidation firing. It is the lack of oxygen in the last part of the process that is critical for the desired effects.

Fuel burning kilns give the artist control of how much or how little oxygen enters the firing chamber and when. The percentage of colorant can also be a factor in how similar they will appear. The identity of the colorant is important, some are less prone to differences in kiln atmosphere. Color interactions are also a factor. The rule? There is none, it depends on the chemistry of the host glaze, which color and how much there is.

Notice how the iron is fluxing it more on the left, it is beginning to run. And how the reduction atmosphere amplifies the color of the iron by changing it to the metallic state.

It is not just iron oxide that changes character from oxidation to reduction. Of course, cobalt can fire to a bright blue in oxidation also, but this will only happen if its host glaze is glossy and transparent.

In this case the shift to reduction has altered the character of the glaze enough so that its matte character subdues the blue. By Tony Hansen.