Cedar Heights Fireclay is used to formulate stoneware clay bodies. Fireclay is a general name for sedimentary clays usually associated with coal measures they are very ancient deposits. Particle size averages 0.1-5.0 microns. Reasonable plasticity and dry strength. Fired color is buff to light-brown. Used by potters in refractory mixtures and as a body component. If used in large proportions, however, its carbon content can cause trouble such as black core and bloating. - p. 110, Illustrated Dictionary of Practical Pottery, 3rd Ed. by Robert Fournier