Ilmenite supplies titanium and iron oxides to glazes. Increasing amounts cause glaze opacity and, eventually, matte surfaces. Titanate crystallization on cooling can lead to interesting, variegated surfaces. The iron in ilmenite is a colorant, typically muting other coloring oxides. Purer sources of TiO2 for glazes are rutile (96%) and white titanium dioxide (99%).
For more information on ilmenite, see the Digitalfire.com Reference Library of ceramic materials.