CHOX

Chrome oxide is a green colouring oxide for glazes and slips, and a strong one — a trace of it already reads as green. It is also the raw material behind many commercial green and pink ceramic stains.

Chrome oxide is a green colouring oxide, used in glazes and slips where a recipe colours with a raw oxide rather than a prepared stain, and it is one of the materials the stain makers themselves start from, in the greens and in the chrome-tin pinks. A trace of it already reads as green, more darkens the green and turns it opaque, and a heavy addition stiffens and mattes the surface as well, because chrome is refractory and does not melt at any pottery temperature. A slip needs about twice what a glaze does. Which green arrives is mostly the base recipe's doing rather than the amount: alkaline fluxes pull it toward yellow-green, and the hue shifts again with whether the glaze is glossy or matte. Zinc in the base turns chrome brown, so a chrome green wants a base with no zinc in it. Tin changes the outcome altogether — with tin present, in oxidation, in a base high in calcium, low in alumina and free of zinc, chrome gives pink and maroon instead. Those are the chrome-tin pinks, and they need all of those conditions at once. From around cone 6 chrome leaves the glaze as a vapour that will blush pink on any tin-bearing glaze sharing the firing, and because the reaction carrying it off consumes oxygen, an electric kiln in oxidation is where this happens most rather than least. It does not dissolve readily in a melt either, so an overloaded glaze muddies, and a glaze carrying chrome wants sieving or it fires speckled.