Red copper oxide (Cu2O) is a reddish powder that is a reduced form of black copper oxide that produced by heat-treating the black form. It is used as a colorant in many ceramics applications, including the preparation of slips and glazes. Red copper oxide will maintain its structure and produce reds in reduction firings. However, red copper oxide has a low solubility in water and beads up at the surface of a solution, which makes it difficult to work with at times.
Suggested concentrations of copper in the preparation of slips and glazes are as follows:
In slips: 2 - 8%
In glazes: < 5%
If concentrations above 5% are used, glazes often change to a metallic pewter/black.
Red copper oxide is harmful if swallowed and a potential skin and respiratory tract irritant. Please use protective coverings when handling this substance.