ZIOX

Zinc oxide is a white powder that works as a flux from the middle of the firing range upward. It brightens a copper green, turns chrome to a dull grey-brown, and in quantity it grows crystals.

Zinc oxide melts far above any pottery kiln on its own, so it does its fluxing in combination with what is around it, which is why it belongs in mid- and high-fire glazes rather than low-fire ones. It lowers a glaze's thermal expansion, well below any of the alkalis, and it brightens a copper green while turning chrome to a dull grey-brown. A small addition mattes a surface, a middling one comes out clear and glossy, and a quarter of the recipe grows the zinc silicate crystals a crystalline glaze is made of, with the cooling curve deciding as much as the recipe does. It does not survive reduction, where it reduces to the metal and leaves as vapour.