Tenmoku glazes can range in color from dark plum (persimmon), to yellow, to brown, to black.
It is made of feldspar, limestone, and iron oxide. The more quickly a piece is cooled, the blacker the glaze will be.
Tenmokus 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. Oil spots are more common in an oxidation firing.
A longer cooling time allows for maximum surface crystals. Potters can "fire down" a kiln to help achieve this effect. During a normal firing, the kiln is slowly brought to a maximum temperature by adding fuel, then fueling is stopped and the kiln is allowed to cool slowly by losing heat to the air around it. To fire down a kiln, the potter continues to add a limited amount of fuel after the maximum temperature is reached to slow the cooling process and keep the glazes molten for as long as possible.