ZrSiO4. Zirconium silicate. Zircon. A purified, micronized glaze opacifier. 10%-15% Ultrox Extra added to most clear glazes produces a good white. Add to coloured glazes to make opaque pastels. It can be used in recipes calling for Zircopax, Superpax, etc. (alternate brands and grades differ somewhat in effective opacifying power depending on how finely ground they are; Ultrox Extra is a slightly more effective opacifier than Ultrox 500W).
For more information on Ultrox, see the Digitalfire.com Reference Library of ceramic materials.