COOXB

Black copper oxide is the more concentrated of the two commonest copper additions, carrying more copper per gram than the carbonate. It colours glazes and slips.

Black copper oxide is cupric oxide, and it gives a glaze the same greens and turquoises the carbonate does, and the same reds in reduction. What differs is concentration and grind. It carries more copper per gram, so five parts of it stand in for seven parts of copper carbonate. It is also the coarser of the two and does not dissolve as readily, so a glaze carrying it wants sieving; without that the copper arrives as specks, which some recipes are after and most are not. Against red copper oxide the difference is not the fired colour: in an ordinary oxidation firing the red converts to this same black oxide on the way up, and both finish green. The red is slightly the stronger by weight, and it floats on the glaze water rather than mixing in, where the black gives no such trouble. Which green arrives depends on the glaze around it, as it does for the carbonate, and like all copper it fluxes strongly and turns volatile above about cone 8.