Tenmoku (also spelled "temmoku" and "temoku") is a dark glaze with a surface that resembles oil spotting. It is comprised of feldspar, limestone, and iron oxide. The more quickly a piece is cooled, the blacker the glaze will be. Tenmoku's are known for their variability. During their heating and cooling, several factors influence the formation of iron crystals within the glaze. A long firing process and a clay body, which is also heavily colored with iron, increase the opportunity for iron from the clay to be drawn into the glaze. While the glaze is molten, iron can migrate within the glaze to form surface crystals, as in the "oil spot" glaze, or remain in solution deeper within the glaze for a rich glossy color. Tenmoku glazes can range in color from dark plum (persimmon), to yellow, to brown, to black – as seen in this superb example.