ALUM. One of the most useful salts manufactured ; it is extensively employed in dyeing and calico-printing, to which it supplies the mordant. In candle-mak ing, it is used to harden and whiten tal low; in bread-making, it is often used for a similar effect ; it is added to paste to prevent its decomposition ; it is em ployed in preparing and preserving skins, and also in pharmacy. Our alum was unknown to the ancients, who under that name used a different substance, as sul phate iron, or the latter mixed with alum earth. Alum is composed of sulphuric acid, alumina, or the earth of clay and pot ash. It is a double sulphate of alumina and potash. Crystals of alum are sometimes found ready formed in the earth, as along the chain of the Andes—as aluminitc in Germany, New-York, and other parts of the United States. The materials tbr alum are however, found in a state of combi nation almost fit for manufacture into aluM in various parts of the world. In Italy, Hungary, Sweden, Scotland, and North of England, there are rocks and earths termed alum stone—alum slate, A. clay, slate clay, and bituminous shale. These furnish the main material, and when treated in various ways yield the greater part of the aluin of commerce. These clays and rocks are abundant in this continent. The Genesee black slate of the New-York system of rocks is highly impregnated with bitumen and with iron pyrites (sulphuret of iron); these by re acting on each other produce a decompo sition of the rock, and an approach to the formation of the salt ; and from these is prepared in various ways the alum of commerce. In the slaty stratum there are, among other elements, sulphur, alumina, and iron ; and these, by the long-continued action of air and mois ture, lead to the formation of sulphate of alumina and sulphate of iron. Heat aids this transformation, and hence there are two modes of treatment—one for the efflorescent, or powdery ore, and one for the stony ore.
Alum slate, or alum shale; is a very abundant source of alum. It occurs in Tennessee, and in New-York in the small lake district. It occurs in the lower se condary rocks, and in the latter district is exposed by the ravines, which have cut their over and through the strata in the passage to tbe Lakes -, the , Falls of Lodi, in Seneca Co., N. Y., is over alum slate, and the rough mineral crystallizes out on the surface of the cas cade. Alum shale is a grayish or blu ish colored rock, splitting readily, and friable on exposure to the air : it dries or effloresces on the surface, and acquires an astringent taste. The first step in the process is the roasting of the ore. Some times the shale contains so much bitu minous matter that after being fired it keeps its own combustion : in most eases it is necessary to add additiona: fuel, either brushwood or coal : a thin layer of wood is generally spread on th( and then above it a layer o: slate. This is set fire to, and while burn. ing an additional layer of brushwood anC of slate are added, and alternate layer: are supplied as those below become roast ed : by this process the iron pyrites ir the ore is decomposed, the sulphur anC the iron are both oxydized,-the sulphur being converted into sulphuric acid, anC the iron into oxide of iron—then unite t( form sulphate of iron; or copperas, and any additional sulphuric acid unites with the alinnina to form sulphate of alumina.
These salts are then removed from the ore by washing it. The ore is putt into large flat pans of wood, or masonry, called "steeps ," and the water is left upon it for twelve or sixteen hours when it is fresh ore : this process is repeated three times on each batch of ore, dimin ishing the time, of maceration as the pro cess is repeated. The water, which has lain upon a weak slate, is transferred to one containing more saline matters. From these steeps the liquor is pumped into a series of long-arched boilers, so formed as to apply heat. By this means the water is evaporated, and the highly con centrated liquor is then transferred to large coolers, where it remains for a fort night undisturbed. During this interval crystallization goes on : the liquor con tains sulphate of iron and sulphate of alumina, and the foriher of these separ ates from the latter by gradually crystal lizing out. Sticks are immersed in the liquid in the coolers, and around these stickw large bundlei of beautiful green crystals collect, forming the well-known green vitriol or copperas of the shops. When the crystals of copperas have been removed, the remaining liquor is drawn off into an evaporating boiler in order that the sulphate of alhmina may go through the same process as the cop peras; and after being boiled down to a certain strength, the liquor is drawn off into a cooler. Sulphate of alumina will not crystallize without Wash or some other alkali be added, and this substance is therefore added to the cooler, in which, after some days standing, crystals of alum are produced : it is thus a double salt, a sulphate of alumina and a sulphate of potash. This is crude alum, and it is further purified by other processes of boiling, evaporating, and crystallizing. When the bard or stony OTC is used, a preparatory process is necessary. This ore is in appearance midway 'between slate and stone coal, contains sulphur, iron, and alumina, like the decomposed ore ; but these three elements have not yet been combined into the sulphates of iron and aluMina: the aid of heat is ne cessary for this transformation. The ore after being broken into small pieces is built up into long ridges with fuel be neath, and air-holes in different parts, and it is then fired ; after which the ore undergoes the same treatment as before described. The copperas is thus always an extra product in the manufacture of alum. It is impure, and is usually roasted at a strong heat ; and when washed yields more alum. The red residue after roasting is ground to a fine powder, and when dried is used as a Venetian red pig ment : by altering the temperature at which it is dried, a yellow ochre is ob tained instead of a red. In France alum is made from clay, which is first finely ground, and mixed with half its weight of crude sulphate of potash : these are formed into balls five inches in diameter, and calcined in the furnace ; they are 'then transferred to the bottom of the chamber in which sulphuric acid is made, they swell up, and open on all sides, owing to the acid vapor entering them. They are then lixiviated with water, and crystals of alum are obtained by evaporating the liquid.