A reduction kiln works by intentionally limiting the amount of oxygen available during the firing process, forcing the burning fuel to pull oxygen from the ceramic materials within the kiln.
Understanding Reduction Firing
Kilns are used to fire ceramic pieces, transforming clay and glazes into durable objects through heat. Firing environments typically fall into two main categories: oxidation and reduction. While oxidation firing allows ample oxygen, reduction firing deliberately restricts it to create unique chemical reactions and artistic effects.
The Core Mechanism
Reduction is achieved by burning any of these organic materials with an insufficient quantity of air to achieve complete combustion. This is typically done by restricting the air flow into the kiln at specific high temperatures during the firing cycle.
By starving the kiln of air, the burning fuel (like gas or wood) cannot get enough oxygen from the atmosphere to burn completely. Instead, the burning fuel needs another source of oxygen and finds it in the ceramic oxides, which are part of the work loaded into the kiln. The fuel effectively steals oxygen from the metal oxides present in clay bodies and glazes.
The Process in Action
Here's a simplified look at what happens:
- Heating: The kiln is heated to high temperatures, just like in any firing process.
- Air Restriction: At a certain temperature range (often near peak temperature), the kiln operator reduces the airflow, creating an oxygen-starved environment.
- Incomplete Combustion: The fuel cannot burn completely with the limited air.
- Oxygen Seeking: The oxygen-hungry combustion byproducts (like carbon monoxide) actively seek oxygen from other sources, including the metal oxides in the ceramic pieces.
- Chemical Change: Metal oxides in the glazes and clay bodies lose oxygen atoms. This changes their chemical composition and, consequently, their color and appearance.
For example, copper oxide, which typically turns green in oxidation, loses oxygen in reduction and can transform into metallic copper, resulting in vibrant reds often seen in copper red glazes. Similarly, iron oxide can change from brown or yellow (in oxidation) to shades of green, blue-grey, or black in reduction (like in celadon glazes).
Why Use Reduction Firing?
Artists and potters use reduction firing specifically to achieve these unique and often dramatic visual effects that are impossible to create in an oxygen-rich environment. It adds depth, complexity, and often earthy or metallic tones to glazes and clay bodies.
Common effects include:
- Copper reds
- Celadons (iron greens/blues)
- Lustrous or metallic surfaces
- Darkened clay bodies
- Varied tonal ranges in glazes
In essence, a reduction kiln works by manipulating the atmosphere to trigger specific chemical changes in the ceramic materials, turning a simple firing process into a powerful artistic tool.
