Reduction firing is firing a fuel-burning kiln with too little air, so the flames pull oxygen from the clay and glazes. It changes how colouring metals behave, giving effects such as celadon greens, copper reds and toasty flashed stoneware.
What Is Reduction Firing?
In a gas or wood kiln, the potter reduces the air reaching the burners or partly closes the damper during part of the firing. The atmosphere becomes rich in carbon monoxide, which reacts with oxides in the clay and glaze, changing iron and copper into forms that give very different colours from an oxidation firing.
Iron in the clay comes out to the surface as warm orange flashing and dark speckles, iron glazes become celadons and tenmokus, and copper turns from green to red. These effects are the heart of many classic high-fire traditions.
Reduction takes skill, because too much smokes the kiln and wastes fuel, and too little gives weak results. It is mostly done in gas kilns and isn’t recommended in electric kilns, because the reducing atmosphere damages elements.
Raku and saggar firing create local reduction around pieces in other ways, which is how electric kiln users often explore reduction-style surfaces.
This pottery glossary entry from Le Studio Art Space is general educational content. Always follow the clay, glaze and kiln makers’ data for your own materials and equipment.