THE MANUFACTURE. 1. From Alum Rock.—This rock, which occurs in the volcanic districts around Naples, and at Muszag in Hungary, is composed principally of silica and sulphate of alumina. Analyses of four samples taken from different places have shown it to have the following composition :— The rock ie piled up in heaps in a furnace or kiln, and heated to low redness, the flames being led in an upward direction through the alai's. After a short time the sulphate of alumina ie decom posed into alumina, oxygen, and sulphurous acid, and the calcination is known to be complete when white vapours of this latter gas exhibit themselves. The calcined mass is then placed in cisterns, and constantly moistened with water for three or four months, during which time it crumbles up and is converted into a soft mud. This mud contains a considerable quantity of alum which is dissolved out with water, the liquor being evaporated down until it attains a specific gravity of about 1.114 at 45°. The crystals of alum which separate out on cooling are of a reddish tinge, owing to the presence of iron, and must be subjected to recrystallization, which yields the salt in a very pure state. This product is much valued in commerce. Cubical or Roman alum is prepared in the same manner, except that the pure mineral, alumite, is employed, instead of the alum rock.
2. From Aluminous Shale or Alum Ores.—These are a kind of schismatic clay, containing much iron pyrites and bituminous matter, and very closely resembling the ordinary clay slate. Large beds are found in the Scandinavian peninsula ; in Bohemia, the Hartz, and the mountainous districts of the lower Rhine. In Great Britain, they occur at Hurlet and Carnpsie, near Glasgow, rind near Whitby, in Yorkshire. For many years, these places were the chief seats of the manufacture, alum works at the latter place having been established since the year 1600. The following table represents the composition of different shales from Glasgow and Whitby :— The proem is conducted in the following way :—The mineral is piled up in heaps, which are moistened every now and then with water ; it thee becomes heated, and gradually crumbles up into a pulverulent state. This is usually carried ou, either wholly or in part, on the floor of the
mine. If the ore fails to attain this condition upon mere exposure to air and moisture, it must be broken to pieces and piled up in heaps upon a bed of brushwood and small-coal, in layers of about 4 feet in thickness. Fire is then applied, and when the mass is thoroughly kindled, fresh quantities of the broken shale are thrown upon it until it attains a considerable height and thickness. The bituminous matter contained in the shale is generally sufficient to produce the required heat provided that it he continued long enough ; in some cases, when the shale is not very bituminous, it is necessary to employ slack or sawdust in order to assist the combustion. Calcination is then effected by means of a smothered fire ; care must be taken to prevent the mass from becoming fused and from disengaging sulphurous vapours. To this end, the mass is after a time coven d with a coating of calcined ore, or " mantled." as it is termed, in order to shelter the burning heap from wind and rain, and to moderate the heat and prevent it from progressing too rap•dly, thus causing the sulphur to he lost by volatilization. When the process is complete, a thicker " mantling" is laid on, and the mass is allowed to cool, when it is found to have lost about one-half in bulk and to have become open and porous. It is then laid open to the air and moistened again with a little water. The time occupied by the process of calcination varies, according to the size of the mass and the state of the weather, from three to nine months.
The next part of the process consists in digesting the calcined ore in warm water in a large atone or brickwork cistern, until the soluble portion has been totally extracted ; the lye is then run into another stone or brick cistern, placed in close proximity to a reverberatory furnace, so that the flame and products of combustion are led over the surface of the liquor in the cistern.