Copper Oxide Black or Cupric Oxide is one of the oldest colorants used by potters. It is a popular source of copper in glazes and glass. It is a very strong flux, in a mix of 50% Ferro borax frit 3134 it will dissolve a firebrick crucible at cone 6! It is the most stable form of oxidized copper (Cuprous oxide oxidizes to cupric oxide in normal firings).
The oxide form of copper can give a speckled colour in glazes whereas the carbonate form will give a more uniform effect.
Copper normally produces green colours in amounts to 5% where it moves toward black. In reduction firing, it turns to Cu2O and gives vibrant red hues. It the glaze is fluid copper will tend to crystallize heavily. See CuO and Cu2O in the oxides database for more information.
Above 1025C copper becomes increasingly volatile and its crystalline structure breaks down. At 1325C CuO melts. This can affect the colour of other glazes pieces in the kiln. Glazes containing copper can change significantly because of loss of copper. Some potters alternate between reduction and oxidation, and even put a dish filled with copper carbonate in the centre of the kiln to minimize this phenomenon..