CuO.  2% added to most glazes produces a clear green. In alkaline glazes the colour shifts to turquoise blue.  At 5% or more, metallic surfaces develop. Fired in a reduction atmosphere, red colours are achieved with ½% or less. To substitute for copper carbonate, use 71% of the recipe requirement.

Note: Glazes become increasingly susceptible to food acid attack as copper oxide levels increase.  

For more information on copper oxide, see the Digitalfire.com Reference Library of ceramic materials.