Reduction

A reducing firing usually takes place in a gas or wood kiln. Reducing means that the oxygen supply is limited. As a result, there is little or no oxygen present in the kiln, the flame seeks oxygen and will extract the available oxygen from the clay and glaze. Due to this extraction, a chemical reaction occurs in the glaze materials, causing them to sometimes take on a different color. Well-known reduction glazes include copper red and celadon. These glazes would turn green and yellow-brown in an electric kiln. Also read this article about this.