It is prepared by lixiviating calcined albuminous schist, and concentrating the solution to 1.4 or l.5 density, and adding the requisite quantity of chloride of potassium, soap-boilers' ash, or kelp, to supply the alkali. By re-crystallization it is obtained colourless. In some manufactories sulphate of ammonia, from gas-liquor, is added to the lixivium, instead of chloride of potassium. Alum is also manufactured from clay, cryolite or Greenland spa, Bauxite (a mineral containing a large proportion of hydrated alumina), and blast-furnace slag. Roman or cubic alum is crystallized from a solution, the temperature of which is not allowed to exceed 104° F. It differs from common alum in containing a large quantity of base, a portion of which separates, if the solution be heated to 120°. Another kind of alum, sometimes used as a mordant, consists almost entirely of sulphate of alumina, and is probably made by boiling fine clay, free from iron, with sulphuric acid, and cooling the solution so as to obtain a solid mass. See Dyes, etc.