Was used up until the 1970s but has not been available since. This material was popular since it fired to a very warm brown in oxidation and deep brown in reduction.
Cone 10 reduction (top), cone 10 down to 6 oxidation below that (top to bottom). A refractory material.
Fireclays are non-kaolin non-ball clay materials similar to stoneware clays but lacking fluxing oxides. Many fireclays have a PCE of 28 or more.