ALLUM

ALLUM, or Alum, Aluimen, a kind of Mineral Salt, of an acid Taste, leaving in the Mouth a Sense of Sweetness accompanied with a considerable Degree of Astringency. See SALT.The ancient Naturalists allow of two sorts of Allum; the one Native, the other Factitious. The Natural is found in the Island of Milo, being a kind of whitish Stone, very light, soft, and porous; and streaked with Filaments resembling silver. The Factitious Alum, is prepared in different manners, according to the different Materials whereof it is made. Allum is of divers Kinds, Red, Roman, Citron, Plumose, Saccharine, and Burat, The three last of which, are not proper native Alums. England, Italy, and Flanders, are the Countries where the Allum is principally produced. The English Allum, called also Rock-Allum, Allumen Rupeum, is made from a bluish mineral Stone, frequent in the Hills in Yorkshire and Lancashire. "This Stone they calcine on a Hearth or Kilns then steep it successively in several Pits of Water: then boil it for about 24 Hours: Lastly, letting it stand for about two Hours; the Impurities subside, and leave a pure Liquor; which, removed into a Cooler, and some Urine added to it, begins, in three or four Days, to gather into a Mass; which being taken out, washed, and melted over again, is fit for use. The Mineral Stone, before it is calcined, being exposed to the Air, will moulder in pieces, and yield a Liquor whereof Copperas may be made; but being calcined it is fit for Allum.—As long as it continues in the Earth, or in Water, it remains a hard Stone.—Sometimes a Liquor will issue out of the Side of the Mine, which by the Heat of the Sun is turned into a Natural Alla. In the Alum-Works at Civita Vecchia, the Process, as described by M. Geoffroy, is somewhat different.—The Stone, which is of a ruddy hue, being calcined, they boil and dissolve the Calx in Water; which imbibing the Salt, i.e., the Alum, separates itself from the useless Earth. Lastly, leaving the Water thus impregnated with Salt to stand for some Days, it crystallizes of itself, like Tartar about a Butt, and makes what they call Roche or Roman-Allum. At Solfatara, near Pozzuoli, is a considerable oval Plain, the Soil whereof is wholly saline; and so hot, that the Hand cannot long bear it.—From the Surface hereof, in Summertime, there arises a sort of Flour, or saltish Dust; which being swept up, and cast into Pits of Water at the bottom of the Plain; the Heat of the Ground, without any other Fire, evaporates the Water, and leaves an Alla behind. Allum dissolves in Water, and what remains undissolved at bottom, is a sort of Calx, which dissolves readily enough in Oil, or Spirit of Vitriol—And hence there arises some doubt, whether Allum, as it does not leave an Earth behind, does properly belong to the Class of Salts—Mr. Boyle assures us, that Allum Ore robbed of its Salt, does in tract of Time recover it again in the Air, See AIR.The Swedish Allum is made of a Mineral which contains a great deal of Sulphur and Vitriol, not to be taken away but by Calcination or Distillation. The Matter remaining in the Iron Vessels used in separating the Sulphur from the Mineral, being exposed to the Air for some time, becomes a kind of bluish Ashes, which they lixiviate, crystallize, and convert into Allum.The Word Allum comes from the Greek ἅλς (hals), Salt; or perhaps from the Latin Lumen, Light; because it adds a Lustre to Colours. Allum is of some use in Medicine, in quality of an Absorbent; but being apt to excite Vomiting, is not much used inwardly, and rarely without some smooth Aromatic, as a Corrector.—It is used outwardly in astringent Lotions, and is an Ingredient in several Dentifrices. It is a principal Ingredient in Dying and Colouring; neither of which can be well performed without it.— It serves to bind the Colour upon the Stuffs, and has the same Uses there, that Gum-water and glutinous Oils have in Painting. It likewise disposes Stuffs to take the Colour, and adds a degree of Briskness and Delicacy to it; as we see visibly in Cochineal, and the Grain of Scarlet. The Effects of Alla seem owing to its styptic, or astringent Quality, by which it binds the finer Parts of Colours together, and prevents their exhaling. Hence also it preserves Barge that has been dipped in its Water, from sinking when wrote upon. See COLOUR, DYEING, etc.Saccharine Alum, bears a near resemblance to Sugar.— It is a Composition of common Alum with Rose-water, and Whites of Eggs, boiled together to the Consistence of a Paste, and thus moulded at pleasure. As it cools, it grows hard as Stone. Burnt Alum, Alumen Ustum, is Alum calcined over the Fire, and thus rendered whiter, more light, and easily pulverized.Plumose Alum, Alumen Plumosum, is a sort of saline, mineral Stone, of various Colours, most commonly white bordering on green; resembling Venetian Talc, except that instead of Scales, it rises in Threads or Fibres, resembling those of a Feather; whence its Name, from Pluma, Feather. Some will have this to be the Lapis Amianthus of the Ancients. See AMIANTUS.