Tea Cup Tianmu (Heaven's eye)
Tianmu in Chinese or Tenmoku in Japanese. 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. Please look closely. They are a little bit different.