Flint Hills Clay - 50 lb - Black - Flint Hills Black Clay is similar to other cone 6 clays. This black clay turns black when fired in a gas kiln or fired in a pit. Black clay is also designed to become a peppery tan reduction when fired in an electric kiln. Flint Hills Black Clay has great working properties and fires at cone 5 to cone 8 oxidation. This clay is a versitile medium firing clay for sculpture and pottery.