Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/684

Rh out, and little or nothing left for the manufacturer to extract. When the stone has by this process been reduced to a sufficiently fine powder, it is thrown into a leaden boiler filled two-thirds with water. During the boiling the powder is frequently stirred up, and the water that eva porates is replaced. When the boiling has been continued for a sufficient time, the fire is withdrawn, and time allowed for the earthy matter to subside to the bottom. A cock is then opened, which allows the clear liquor to flow out into deep wooden square vessels, so made that they can be easily taken to pieces. Here the alum gradu ally crystallises, and attaches itself to the sides and bottom of the vessel. The mother liquid is then drawn off into shallower wooden troughs, where more alum crystals are deposited. The liquid has now a red colour, and is muddy ; and the last alum crystals are mixed with this red matter. They are washed clean in the mother liquor, which is finally pumped into a trough, and used in subse quent processes. The alum obtained at Tolfa is known by the name of Roman alum, and is in very high estimation. It is always mixed with a little reddish powdery matter, which is easily separated from it. Alum-slate, being very different in its composition, re quires a different treatment to fit it for yielding alum. If the alum-slate contain a notable quantity of lime or mag nesia, it does not answer the purposes of the manufacturer so well. The essential ingredients in alum-slate, for the alum-makers, are alumina and iron pyrites. The first process is to roast the ore. In Sweden, where the fuel is wood, and consequently expensive, it is cus tomary to use the alum-slate itself as fuel for roasting the ore. For this purpose a small layer of brushwood is covered with pieces of alum-slate, and set on fire ; and, as the combustion proceeds, new layers of alum-slate are added. It is usual to place alternate layers of roasted and unroasted alum-slate. The combustion continues for a month or six weeks. At Whitby, coal is employed for roasting the alum-slate. Indeed the alum-slate of Whitby is lighter coloured than that of Sweden, and probably would not burn of itself. So great is the quantity of com bustible matter in the Swedish alum-slate that it is em ployed as fuel for burning limestone. Great quantities of limestone are burnt in this manner at Hunneberg, near the south side of the lake Wener. The roasted ore has usually a brown colour. When it is red the quantity of alum which it yields is considerably diminished. By this roasting the pyrites is oxidised into sulphate of iron and sulphuric acid, thus : FeS 2 + 7 + H 2 = FeS0 4 + 11,80,. The sulphuric acid as it is produced is, however, at once neutralised by the large excess of alumina producing sul phate, so that the result of the action is to produce a mixture of the sulphates of iron and alumina. The roasted ore has an astringent taste, owing to the sulphate of iron and sulphate of alumina which it con tains. The next process is to lixiviate it with water, in order to dissolve these salts. For this purpose it is put into reservoirs made of wood or masonry, with a stop cock at the bottom to draw off the water. The usual method is to keep the water for twelve hours in contact with ore that has been twice lixiviated ; then to draw it off, and allow it to remain for an equal period on ore that has been once lixiviated. Lastly, it is run upon fresh ore, and allowed to remain on it for twelve hours longer. If the specific gravity of the liquid thus treated be T25 at the temperature of 55, it may be considered as saturated with sulphate of alumina and sulphate of iron ; but probably this specific gravity is not often obtained. The liquid, thus impregnated with salt, is now boiled down in leaden vessels to the proper consistency for crys tallisation. In Sweden the fuel employed for this pur pose is alum-slate. By this means a double effect is produced the liquid is evaporated, and the alum-slate is roasted. During the boiling abundance of oxide of iron falls, mixed with selenite, if lime be one of the con stituents of the alum-slate. When the liquid is suffi ciently concentrated it is let into a square reservoir, in order to crystallise. Great quantities of sulphate of iron crystals are usually deposited in this vessel. These are collected by drawing the liquid off into another reservoir. When all the sulphate of iron that can be obtained has been separated, a quantity of sulphate of potash or am monia, muriate of potash, or putrid urine, is mixed with the liquid. The sulphate of potash is procured from the sulphuric acid makers, and the muriate of potash from the soap-makers. By this addition alum is formed in the liquid, and it gradually deposits itself in crystals on the sides of the vessel. These crystals are collected, and dis solved in the smallest quantity of boiling water that will take them up. This solution is poured into large wooden casks. In a fortnight or three weeks the alum crystal lises, and covers the sides and bottom of the cask. The hoops are now taken off, and the staves of the cask removed. A mass of alum crystals, having the shape of the cask, remains. This mass is pierced, the mother liquor allowed to run out, and preserved for a subsequent process. The alum, being now broken in pieces, is fit for sale. The manufacture of alum from bituminous shale and slate-clay bears a considerable resemblance to the manu facture from alum-slate, but differs in several particu lars. We shall give a sketch of the processes followed in two works of this kind that are in operation in the neigh bourhood of Glasgow. The bituminous shale and slate- clay employed are obtained from old coal-pits, which are very extensive near Glasgow. The air in these coal-pits is moist, and its average temperature about 62. The shale having been exposed for many years, has gradually opened in the direction of its slaty fracture, so as to re semble in some respects a half-shut fan ; and all the chinks in it are filled with a saline efflorescence in threads. This salt is white, with a shade of green, has a sweetish as tringent taste, and consists of a mixture of sulphate of iron and sulphate of alumina. In order to obtain these salts in a state of solution, nothing more is requisite than to lixiviate this shale with water. The lixiviated ore being left exposed to the weather, forms more salt, which is gradually washed out of it by the rain-water, and this water is collected and preserved for use. The next step in the process is to boil down the liquid to a sufficient state of concentration. At Campsie all the boilers are composed of stone, and the heat is applied to the surface. This is a great saving, as leaden vessels are not only much more expensive, but require more fre quent renewal. When the liquid is raised to a sufficiently high temperature in the stone reservoir, pounded sulphate of potash, or muriate of potash, as they can be procured, is mixed with it ; and there is an agitator in the vessel, by which it is continually stirred about. This addition con verts the sulphate of alumina into alum. The liquid is now let into another trough, and allowed to remain till it crystallises. In this liquid there are two salts contained in solution viz., sulphate of iron and alum ; and it is an object of great consequence to separate them completely from each other. The principal secret consists in drawing off the mother liquor at the proper time ; for the alum is much less soluble in water than the sulphate of iron, and therefore crystallises first. The first crystals of alum formed are very impure. They have a yellow colour, and