This is a grog clay with 25% Christy Minerals STKO22S grog (20 mesh one size). This piece is about 8 inches tall fired at cone 10R. This body is a Redart, Ball clay base that totally vitrifies to a chocolate brown. But with the added refractory grog it is fairly stable in the kiln and is much more vitreous than other grog bodies. Because it is such a plastic smooth base and because the grog is only one size, this is actually throwable. And it is very resistant to splitting during hand building.
In the ceramic industry, refractory materials are those that can withstand a high temperature without deforming or melting. Refractories are used to build and furnish kilns.