Shale is a black, slaty substance, or a species of clay concreted into a stony consistence, and impregnated with a considerable quantity of bituminous matter. It is of various degrees of hardness, but does not emit sparks when stricken against steel; and, on being heated, it exhales a strong smell.
Large strata of this mineral are dug out in Derbyshire, as well as in those counties which contain fossil coal. On calcination, an acid is evolved, that combines with the argillaceous particles, and forms Alum. The shale is next immersed in water, in consequence of which, the alum thus obtained is dissolved ; and, after undergoing various processes, is formed into the masses usually met with in the shops. - It is computed by Doctor Watson (Chemical Essays, vol. ii.) that 120 tons of the calcined shale will produce one ton of alum.