IROXB

Black iron oxide colours a glaze anywhere from tan, through red-brown, to the darkest browns and blacks, depending on how much goes in. It is magnetite, so the black of the powder is not the colour that comes out of the kiln.

Black iron oxide is magnetite, and it goes into glazes, slips and dark clay bodies as a colourant. In an oxidising firing it begins turning over to hematite within a few hundred degrees, long before a glaze melts, so the colour of the powder says nothing about the fired one. Glazes commonly take anything from a part in a hundred for a tan up to a tenth of the recipe for the darkest browns and blacks. At that tenth the atmosphere sets the surface as much as the colour: in oxidation the iron acts refractory and the glaze fires stiff and dry, and in reduction it turns fluxing, so the same recipe goes glossy and can run.