ALUM, or ALUMEN, in natural history, a peculiar kind of salt, sometimes found pure, but oftener separated from several substances, as a soft reddish stone in Italy, several kinds of earth, and, in England, from a whitish or bluish stone, called Irish slate. Alum, in medicine, is a powerful astringent. In dying, it fixes the colours upon the stuff. See CHEMISTRY.
Process of making ALUM. At Whitby, in Yorkshire, alum is made thus: Having burnt a quantity of the ore with whins, or wood, till it becomes white, then they barrow it in a pit, where it is steeped in water for eight or ten hours. This liquor, or lixivium, is conveyed by troughs to the alum-house, into cisterns, and from them into the pans, where it is boiled about 24 hours. They add a certain quantity of the lee of kelp; the whole is drawn off into a settler; where having remained about an hour, that the sulphur and other dregs may have time to settle to the bottom, it is conveyed into coolers. This done, to every tun of the liquor they add about eight gallons of urine; and having stood four days and nights, till quite cool, the alum begins to crystallize on the sides of the vessel, from which being scraped off, it is washed with fair water, and then thrown in a bing, to let the water drain off. After this it is thrown into a pan, called the roching pan, and there melted; in which state it is conveyed by troughs into tuns, where it stands about 10 days, till perfectly condensed. Then staving the tuns, the alum is taken out, chipped, and carried to the store-houses.
This is what we commonly call roche or rock alum, as being prepared from stones cut from the rocks of the quarry; and stands contradistinguished from the common alum, or that prepared from earths.