Silica. SiO2. Silicon dioxide. Flint. Ground quartz rock. A relatively coarse powdered grade ideally suited for use in clay bodies to thwart crazing and to reduce clay shrinkage. Good for making molds for glass when mixed ~50:50 with Pottery No. 1 Plaster. About 99.7% pure silica.
For more information on silica, see the Digitalfire.com Reference Library of ceramic materials.