This is a short overview. For the background — history, value over time and notable mines — see Fireclay →
A refractory clay found beneath coal seams as the fossil soil the coal forest grew in, and usually won as a by-product of coal mining. Its resistance to heat made it the raw material for firebricks, furnace linings and sanitary ware, and many collieries ran a brickworks alongside the pit to use it.