Chrome Oxide

Chrome oxide colours glazes and slips green, and stain makers use it as a starting material for their greens and for the chrome tin pinks. A trace is enough to read as green. More darkens the green and makes it opaque, and a heavy addition also stiffens and mattes the surface, because chrome is refractory and does not melt at any pottery temperature. A slip needs about twice as much as a glaze. The shade of green depends more on the base glaze than on the amount. Alkaline fluxes push it towards yellow green, and a glossy base reads differently from a matte one. Zinc in the base turns chrome brown, so a chrome green needs a base with no zinc. Chrome can also give pink. In an oxidation firing, a glaze that contains tin and a lot of calcium, with little alumina and no zinc, turns chrome pink or maroon instead of green. Those are the chrome tin pinks, and all four conditions have to be met at once. From around cone 6 some of the chrome evaporates out of the glaze and can leave a pink blush on any tin bearing glaze in the same firing, and this happens in an electric kiln as much as anywhere. Chrome does not dissolve readily in a melt, so an overloaded glaze looks muddy, and a glaze with chrome in it should be sieved or it fires speckled.