Black Copper Oxide, also known as Cupric Oxide (CuO), has been used by potters as a coloring oxide for a very long time. It can work as a very strong flux and give glazes a speckled color appearance. This material reacts greatly to atmospheric changes. In reduction, the material can completely change color.

Information gathered from Digitalfire Ceramic Materials Database and The Potter’s Dictionary of Materials and Techniques by Frank Hamer.