Ceramics artists have a wide array of options for finishing and decorating their pieces. Two of the most common firing techniques are oxidation and reduction, which create very different visual effects through complex chemical reactions. Let's break down the science behind these two methods.
Oxidation firing is the standard and simplest firing process for ceramics. During oxidation firing, oxygen is freely available to interact with the clay body and glazes. Here's a quick definition:
Oxidation firing - A firing technique where oxygen is allowed to interact with clay fully and glazes at high temperatures in a kiln. This causes the materials to reach their most oxidized state, brightening colors.
The abundant oxygen triggers oxidation, which happens when electrons are removed from an element. This almost always causes visual changes like brightening or intensifying colors. Most glazes and clay bodies are designed to be fired in an oxidizing atmosphere.
So, in summary, oxidation firing brings out the fully vibrant colors and effects that ceramics artists intend with their glazes and clay choices.
During oxidation firing, air is allowed to circulate freely through the kiln at high heat. Oxygen molecules interact with the metallic elements in clay and glazes, extracting their electrons.
This oxidation makes the metals more positively charged, changing their energy levels and causing vibrant color changes. Brighter blues, greens, and reds are common oxidation effects.
In an electric kiln, oxidation firing is simple - no steps need to be taken to limit airflow. In a gas kiln, the vents must be adjusted to allow enough oxygen inside for full oxidation.
So what effects can you expect from oxidation firing your ceramics?
Oxidation firing tends to mute earth tones while intensifying primaries and secondaries. Glaze chemistry is built around oxidation, making colors reliable and reproducible.
Reduction firing gives ceramics artists more unusual surface effects by limiting available oxygen. Let's start with a definition:
Reduction firing - A kiln firing technique where oxygen is partially restricted, creating a reducing atmosphere that affects clay and glaze chemistry.
In reduction, oxygen molecules are not free to circulate and oxidize elements. This causes a chemical change called reduction, where elements gain electrons.
Reduction firing typically happens in gas kilns. The artist can manipulate air intake to control the level of reduction and its effects.
Restricting oxygen flow into a kiln creates unique effects:
The reduced atmosphere causes elements in clay and glazes to gain electrons as they give up oxygen. This alters their energy levels, physical structure, and behavior.
Reduction firing requires specific steps with a gas-fired kiln:
The starting oxidation stage prepares the work. The final reduction phase at peak temperature creates the surface effects.
While gas kilns allow the most reduction control, electric kilns can also mimic a reducing effect:
As these carbon sources burn out, they temporarily lower oxygen levels around the pots. This causes a minor reduction causing flashing and speckling.
Reduction firing results in more diverse, earthy surfaces:
The reduction atmosphere gives clay and glazes an unfinished, natural look. Glazes react unpredictably from firing to firing.
Oxidation gives potters vibrant, reproducible results that show off bold glaze chemistry. Reduction offers organic variation from effects like flashing and carbon trapping.
The atmosphere created inside a hot kiln can utterly transform finished ceramics. Oxidation and reduction techniques make science and chemistry work for artists choosing how to finish their clay work beautifully.