September 29, 2023

Exploring Oxidation and Reduction Firing in Ceramics

Explore the science behind oxidation and reduction firing techniques for ceramics. Learn how manipulating oxygen levels in a kiln creates different effects.

Oxidation in clay

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.

What is Oxidation Firing?

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.

How Does Oxidation Work in a Kiln?

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.

Oxidation Firing Results

So what effects can you expect from oxidation firing your ceramics?

  • Bright, saturated glaze colors
  • Deep brick red clay bodies
  • Carbon burn-out from raw clay is complete
  • White glazes stay pure white

Oxidation firing tends to mute earth tones while intensifying primaries and secondaries. Glaze chemistry is built around oxidation, making colors reliable and reproducible.

Understanding Reduction Firing

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.

What Does Reduction Do?

Restricting oxygen flow into a kiln creates unique effects:

  • Muted and grayed glaze colors
  • Darkened clay bodies
  • Increased metallic luster in glazes
  • Reduction speckling and flashing in the glazes

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.

Creating Reduction Conditions

Reduction firing requires specific steps with a gas-fired kiln:

  1. Heat the kiln to around 1650°F with open vents for full oxidation
  2. Partially close the kiln vents to restrict air flow between 1650-2200°F. This removes oxygen
  3. Fire from 2200°F down to 1900°F or lower in full reduction (vents closed)
  4. Open vents fully until kiln cools to avoid re-oxidizing the work

The starting oxidation stage prepares the work. The final reduction phase at peak temperature creates the surface effects.

Using Reduction in an Electric Kiln

While gas kilns allow the most reduction control, electric kilns can also mimic a reducing effect:

  • Pack pots with carbon materials like sawdust or paper
  • Spray a carbonate solution on pots
  • Add a reducing agent like magnesium to the kiln wash

As these carbon sources burn out, they temporarily lower oxygen levels around the pots. This causes a minor reduction causing flashing and speckling.

The Effects of Reduction Firing

Reduction firing results in more diverse, earthy surfaces:

  • Gray, muted greens and blues
  • Rich dark browns and blacks
  • Metallic luster in copper and iron glazes
  • Carbon trapping in white glazes

The reduction atmosphere gives clay and glazes an unfinished, natural look. Glazes react unpredictably from firing to firing.

Choosing Oxidation vs Reduction

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.

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