The process of raku firing involves removing the pieces from the kiln at the moment the glaze has melted and then putting it into another chamber full of leaves, paper, wood shavings, or anything else that easily burns. Since the usual melting point for these glazes is between 1700-1800 degrees Fahrenheit, they burn easily indeed!
The chamber is then closed to restrict the amount of oxygen available to these materials. This is called a reduced oxygen, or reduction atmosphere. By creating such an atmosphere, the oxygen within the glaze is drawn out of the glaze. This varies with factors such as airspace around the piece; mass of combustibles; and the tightness of the lid. These differing amounts of the reduction bring out the varied copper, violet and green palette of raku.
Each piece is unique because their very shapes contribute to the flow of air (or the lack thereof) around them. When the pieces are removed from the reduction chamber, they are sprayed with water to freeze the colors since they are often hot enough to reoxidize the glaze.