TiO2. High purity, for use in glazes. Increasing amounts of titania cause glaze opacity and, eventually, matte surfaces. Titanate crystallization on cooling can lead to interesting, variegated surfaces. Other, less expensive sources of TiO2 for glazes include rutile (95% TiO2) and ilmenite, a ferrous titanate.
For more information on titanium dioxide, see the Digitalfire.com Reference Library of ceramic materials.