Fire clay

"A siliceous refractory clay rich in hydrous aluminium silicates." (Wikipedia Seatearthe page) "Fire clay is a range of refractory clays used in the manufacture of ceramics, especially fire brick. The United States Environmental Protection Agency defines fire clay very generally as a "mineral aggregate composed of hydrous silicates of aluminium (Al2O3·2SiO2·2H2O) with or without free silica." " (Wikipedia Fire clay page)