ALUM-SLATE, a rock from which, as its name implies, alum is prepared. It is found in Germany, Sweden, &c.; and in Yorkshire a stratum occurs, which, according to Mr. Winter (Nicholson's 'Journal,' No. 25, p. 241), is 23 miles in length, extending from 10 miles to the southward of Whitby to 18 miles to the northward ; the cliffs are in general precipitous, and their height is from 100 to 750 feet. The colour of this slate is bluish-gray : its hardness varies ; at the top part of the stratum it may be crumbled between the fingers, whereas at a considerable depth it is as hard as roofing-slate. The specific gravity is about 2'43. By exposure to the air it effloresces, and acquires tho taste of alum. Alum-slate has not been accurately analysed ; it contains silica, alumina, and, before effloresceuce, probably pyrites or bisulphuret of iron.
At Hindett, near Paisley, and Campsie, near Glasgow, alum is manufactured from what appears to he slate-clay impregnated With bisulphuret of iron ; it is obtained from old coal-pits, and having been long exposed to air and moisture, sulphate of iron and sulphate of alumina are formed, and crystallise so as completely to destroy the texture of the slate.
This double sulphate of iron and alumina occurs in the form of soft delicate fibres, easily separable from each other ; it is nearly colourless, of a silky lustre, and reseinbles in appearance. It is readily soluble in water ; the solution yields crystals of sulphate of iron ; and when potash-salts are added to the remaining solution of sulphate of alumina, crystals of alum are immediately formed; and this is the process of alum-making already noticed.