Chemically, there are two types of copper oxide: cuprous oxide (Cu2O) and cuprous oxide (CuO), the former called red copper oxide or copper suboxide, and the latter black copper oxide or simply copper oxide.
Native to Japan, copper green glazes had a style of using copper oxide called hege, which was used by forgers to make copper sheets. The composition of ancient Egyptian glass paints shows that those with low copper content are blue and those with high copper content are green. The leaden green glazes of the Han and Tang dynasties in China are also thought to be colored by copper oxide.
When fired by reduction, however, the colloidal nature of copper produces a beautiful crimson color, or cinnabar glaze.
