Types of Firing: Oxidation, Reduction, Salt, Wood, Raku
Oxidation firing is typically done in an electric kiln, but can also be done in a gas kiln. Oxygen is free to interact with the glazes when firing. Oxidation firing allow very bright, rich colors. High temperatures may be used, or low temperatures.
Reduction firing is typically done in a gas or other fuel burning kiln, since the corrosive effects wear out the elements on an electric kiln. In reduction firing, oxygen is prevented from interacting with the glazes during glaze maturation. This is done by adjusting the fuel in a gas kiln, or by adding organic material such as in Raku. As the organic material burns it uses up oxygen, leaving an oxygen-less environment. Reduction firing typically is used to obtain mottled, rich, earthy colors, often the iron from the clay shows through and gives a speckled look. Typically high temperatures are used in reduction firings.
In Salt firing, and Sodium firing, salt or sodium are introduced during the final stages of firing. The atmosphere allows pieces to get a glazed like finish without glazes. Sometimes it takes on the appearance of orange peel. Interiors of pots should be glazed since the salt atmosphere often won't reach deep into them.
Wood firing uses wood as fuel instead of gas or propane. In pit firing, a large hole is dug in the ground, wood and pots placed in, wood started on fire, and the whole thing covered with dirt to smolder. Pots are then dug out.
Raku is a technique where a pot is heated until glowing, then pulled with tongs from the kiln and placed into a reduced atmosphere, such as a metal can full of pine needles, newspaper or leaves. After cooling somewhat, the piece is often plunged into cold water, creating crackle effects. In Raku, unglazed areas of clay become black due to the carbon from the burning fuel. When the carbon is scrubbed off the glazed areas they are often bright metallics such as copper and bronze.