COPCA
The main source of green and turquoise in glazes · Green in oxidation; turquoise in a high-alkali, low-alumina base · The route to copper reds in reduction, with tin in the base · Disperses evenly — less specking than black copper oxide · Seven parts stand in for five parts of black copper oxide · Fluxes strongly: heavy additions run, and the green goes black · Volatile above about cone 8, and can blush tin glazes sharing the kiln
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Copper carbonate is the main source of green and turquoise in glazes, and it colours slips as well. It is the more even of the copper additions, dispersing without the specking the oxide can give.
Copper carbonate is where most greens and turquoises in a glaze come from. In oxidation it gives green; in a base high in alkali and low in alumina it turns turquoise. In reduction it is the route to the copper reds — oxblood, peach bloom and the rest — though those need more than a reducing kiln: tin in the base, far less copper than a green takes, and a schedule that reoxidises at the end of the firing. Copper colours slips too, and a slip needs more of it than a glaze does. Copper fluxes strongly, so heavy additions run, and past a point the green goes black or metallic instead. Above about cone 8 it leaves as a vapour and can blush neighbouring pots, which is hardest on the tin whites and celadons sharing the kiln. Against black copper oxide it carries less copper per gram, so seven parts of carbonate stand in for five parts of the oxide.
