# Oxidation vs Reduction Firing: What Changes in Your Glaze

Cluster: Firing & Kilns
URL: https://claybook.studio/blog/oxidation-vs-reduction-firing/

# Oxidation vs Reduction Firing: What Changes in Your Glaze

The same glaze can come out of two kilns looking like two different glazes. The reason is not the recipe. It is the air.

A kiln fires in one of two atmospheres: **oxidation**, with plenty of oxygen, or **reduction**, with limited oxygen. The atmosphere changes how metals in clay and glaze behave, and that changes color.

## Oxidation: plenty of oxygen

In oxidation, there is enough air for the fuel (or electric heating) to burn cleanly, and for oxygen to remain available to the glaze and clay.

* **Typical kilns:** electric kilns, which are by far the most common in community studios and schools.
* **Look:** clear, bright and consistent colors. Many commercial glazes are formulated for oxidation.
* **Strengths:** predictable, repeatable and easy to learn.

Oxidation is where most beginners start, because electric kilns are simpler and cleaner to run. A cone 6 oxidation firing is a studio standard. See the [cone chart](/blog/pottery-cone-chart/).

## Reduction: limited oxygen

In reduction, the kiln is fed more fuel than the available air can burn. The flames seek oxygen wherever they can find it, including from the metal oxides in the clay and glaze.

* **Typical kilns:** gas, wood and some oil kilns.
* **Look:** deeper, more variable colors, with subtle breaks, flashing and variation across a pot.
* **Strengths:** surfaces that are hard to imitate in electric kilns.

Reduction is used at many temperatures, and often at high fire, such as cone 10.

## How atmosphere changes color

Metal oxides respond to the amount of oxygen around them. A few classic examples:

* **Iron** can turn from tan or brown in oxidation to green-gray in reduction, the basis of celadon glazes.
* **Copper** often gives green in oxidation and deep red in reduction.
* **Clay bodies** with iron can darken and speckle in reduction.

Because reduction is hard to control, results can vary within the same kiln. That is part of the appeal, and part of the frustration. Our lesson on [reduction timing](/learn/reduction-timing-and-the-beauty-of-unpredictability/) explains why.

## Can you reduce in an electric kiln?

Technically yes, by adding combustible materials, but it is generally discouraged. The atmosphere can damage the heating elements. Many studios simply do not allow it. If you want reduction effects, use a studio with a gas or wood kiln, or look at techniques like [raku firing](/blog/raku-firing-explained/) that create reduction after the main firing.

## Safety matters in reduction

Reduction produces carbon monoxide and other combustion gases.

* **Ventilation** must be good and the kiln should be outdoors or in a properly vented space.
* **Carbon monoxide** is odorless. Never rely on smell. Use a detector.
* **Follow the kiln builder's instructions**, and only operate a gas or wood kiln after training.

Read [the reduction chamber and safety](/learn/the-reduction-chamber-and-safety/) before your first firing.

## Choosing glazes for your atmosphere

Glaze labels list cone and atmosphere. A glaze designed for reduction may look dull or different in oxidation, and the reverse is also true. Test any glaze on a tile in your own kiln before you glaze a pot. See [compare oxidation and reduction results](/practice/compare-oxidation-and-reduction-results/).

## Keep your results

Write down the atmosphere, the cone and the glaze for every test. In Claybook you can attach firing notes and photos to every piece so you can compare how a glaze behaves across firings.