# The Air Inside Your Kiln Changes Everything

Unit: The Final Bake: Glaze Firing
Topic: Oxidation vs. Reduction
URL: https://claybook.studio/learn/the-air-inside-your-kiln-changes-everything/

# What Is a Kiln Atmosphere?

The atmosphere inside a kiln during firing is not neutral. It is either rich in oxygen or starved of it, and this difference dramatically changes what your finished work looks like. Understanding atmosphere is one of the most powerful tools a potter has for influencing colour and surface.

## Oxidation: More Oxygen

An **oxidation firing** has plenty of oxygen throughout the kiln. Electric kilns always fire in oxidation because they use heating elements, not combustion. In oxidation:

*   Iron oxides produce warm amber, tan, and orange tones.
*   Copper produces bright, stable greens and turquoises.
*   Cobalt produces clear, reliable blues.

Oxidation firings are predictable and consistent. Most commercial glazes are formulated for oxidation.

## Atmosphere Planning
When choosing glazes:
* Confirm they are formulated for oxidation, reduction, or both
* Note which looks you want that only gas/wood reduction can give
* Decide which projects truly justify the extra complexity of reduction

This keeps expectations realistic between electric and gas results.

## Reduction: Less Oxygen

A **reduction firing** starves the kiln of oxygen, usually by partially closing the damper or burners of a gas kiln. When there is not enough oxygen, the flame draws oxygen from the metal oxides in the clay and glaze, chemically changing them:

*   Iron produces grey-greens (celadon), rust reds, and deep blacks.
*   Copper produces deep reds and purples, impossible to achieve in oxidation.
*   The clay body itself often turns warm grey rather than tan.

Reduction requires a gas or wood kiln. You cannot reduce an electric kiln.

## The Bigger Picture

The colour shifts caused by reduction firing are rooted in [redox chemistry](https://en.wikipedia.org/wiki/Redox), where metal oxides in the glaze gain or lose oxygen atoms depending on the kiln atmosphere. One of the most celebrated results of reduction firing is [celadon](https://en.wikipedia.org/wiki/Celadon), a jade-green glaze produced by firing iron oxide in an oxygen-starved kiln.

## Common Mistakes and Quick Fixes

*   **Expecting gas results from electric oxidation glazes**: Match glaze chemistry to kiln atmosphere.
*   **Treating reduction as on/off only**: Learn light, medium, and heavy reduction impacts.
*   **No atmosphere notes**: Log color outcomes against atmosphere decisions every firing.
*   **Ignoring clay body response**: Atmosphere changes clay tone, not just glaze color.

## Practice Exercise

Compare one glaze on two test tiles: electric oxidation and gas reduction at matched cone maturity. Build a side-by-side board for your studio so atmosphere differences become concrete, not theoretical.

## Check your understanding

### Question 1: Why do electric kilns always fire in oxidation?

- [ ] A. Electric kilns reach lower temperatures that only work with oxidation glazes
- [x] B. They use heating elements rather than combustion, so oxygen is never consumed
- [ ] C. The kiln lid seals tightly, preventing reduction
- [ ] D. Manufacturers chose oxidation because it is safer

Tip: Electric kilns use heating elements rather than combustion, so there is no flame consuming oxygen. The atmosphere inside remains oxygen-rich throughout the firing.

### Question 2: In reduction firing, why does copper produce deep reds instead of green?

- [ ] A. The higher temperature in gas kilns changes the colour
- [x] B. Reduction pulls oxygen from the copper oxide, chemically transforming its colour
- [ ] C. Different copper compounds are used in reduction glazes
- [ ] D. Copper always produces red: green only appears in low-fire

Tip: Reduction starves the kiln of oxygen, causing the flame to pull oxygen from metal oxides in the glaze. This chemically transforms copper, which produces green in oxidation, into deep reds and purples.