The rock, broken into small pieces, is laid on a horizontal bed of fuel, composed of brushwood, etc. When about four feet in height of the rock is piled on, fire is set to the bottom, and fresh rock continually poured upon the pile, until the calcined heap is raised to the height of 90 or 100 feet. Its horizontal area is at the same time progressively extended, till it forms a great bed nearly 200 feet square, having about 100,000 yards of solid measurement. The rapidity of the combustion is allayed by plastering up the crevices with small schist moistened; but notwithstanding this precaution, a great deal of sulphuric or sulphurous acid is dissipated. One ton of alum is produced from 130 tons of calcined schist; this result has been deduced from an average of 150,000 tons. The calcined mineral is digested with water in pits usually containing about 00 cubic yards. The liquid is drawn off into cisterns, and afterward pumped up again upon fresh calcined "mine." This is repeated until the specific gravity becomes 1.15. The half exhausted schist is then covered with water to take up the whole soluble matter. The strong liquor is drawn off into settling cisterns, where the sulphate of lime, iron, and earth are deposited. At some works the liquid is boiled, which aids its purification.
It is then run into leaden pans 10 feet long, 4 feet 9 inches wide, 2 feet 2 inches deep at one end, and 2 feet 8 inches at the other. This slope facilitates the emptying of the pans. Here the liquor is concentrated at the boiling heat. Every morning the pans are emptied into a settling cistern, and a solution of chloride of potassium (either pretty pure from the manufacturer, or the crude compound from the soap boiler) is added. The quantity of chloride necessary is determined by a previous experiment in a basin, and is regulated for the workmen by the hydrometer. By this addition, the pan liquor, which had acquired a specific gravity of 1.4 or 1.5, is reduced to 1.35. After being allowed to settle for two hours, it is run off into the coolers to be crystallized. At a greater specific gravity than 1.35, the liquor, instead of crystallizing, would on cooling solidify in a magma resembling grease. After standing four days, the mother waters are drained off. to be pumped into the pans on the succeeding day. The crystals of alum are washed in a tub and drained. They are then put into a lead pan, with as much water as will make a saturated solution at the boiling point. Whenever this is effected, the solution is run off into casks.
At the end of 10 or 16 days the casks are un-hooped and taken asunder, when the alum is found exteriorly in a solid cake, but in the interior cavity in large pyramidal crystals, consisting of octahedrons, inserted successively into one another. This last process is called "rocking." Mr. Winter says that 22 tons of chloride of potassium, or an equivalent of 31 tons of the black ashes of the soap boiler or 73 of kelp, will produce 100 tons of alum. Where much iron exists in the alum ore. the alkaline chloride, by its decomposition, gives rise to an uncrystallizable chloride of iron. For this reason it is preferable to the sulphate of potassium. - Alum may also be obtained from cryolite by heating the mineral with three times its weight of strong sulphuric acid, whereby anhydrous neutral sulphate of aluminum and acid sulphate of sodium are obtained; treating the resulting mass with a small quantity of cold water to remove the acid sodium salt; then digesting the anhydrous sulphate of aluminum with warm water, to convert it into the hydrated salt, and adding the proper quantity of sulphate of potassium.
As the alum from cryolite is remarkably free of iron, it is highly prized by many manufacturers. - The discovery of an aluminous earth in the neighborhood of Baux, France, and hence called bauxite, has added to the class of materials for the manufacture of alum. To this the addition both of sulphuric acid and of a salt of potash is necessary. Bauxite is extensively employed in the manufacture of the sulphate of alumina (alum cake) and of the metal aluminum. - At the chemical works of Harrison Brothers, Philadelphia, ammonia alum is manufactured from a pure clay mostly obtained from New Jersey. The clay is dried and then ground and calcined in a reverberatory furnace. When thoroughly calcined and purified, it is while hot digested for some hours in sulphuric acid contained in large vats. The product is washed with water and concentrated, sulphate of ammonia having been previously introduced, and it is further purified by re-dissolving, then boiled by steam, and finally transferred to the crystallizing tubs. These are about eight feet high, and made of strong staves. At the end of eight or ten days, the staves of the tub being removed, a cylindrical mass of apparently solid alum is revealed.
This being pierced near the bottom, the mother water at the centre flows off along the sloping floor into leaden subterranean cisterns, whence it is subsequently pumped and variously utilized. Each crystallizing vat yields about 21 barrels of alum ready for market. -The composition of pure potash alum is:
Sulphate of potash............................................
Sulphate of alumina.................
Its specific gravity is 1.724. It is soluble in 18 parts cold water and in equal weight of boiling water. It consequently rapidly crystallizes out of a hot saturated solution. Alum has a sweet astringent taste, an acid reaction, and, like sulphuric acid, dissolves many metals, for example iron and zinc, with evolution of hydrogen gas. - Burnt alum, or dried alum, is made by gently heating alum till the water is driven off. Ammonia alum readily loses all its ammonia when heated, and the sulphuric acid, may be driven off the remaining sulphate, so that the pure earth alumina will remain. - The employment of alum in medicine and the arts is very extensive. It precipitates albuminous liquids and combines with gelatine. It causes dryness of the mouth and throat, and. checks the secretions of the alimentary canal, producing constipation, and in large quantities nausea, vomiting, and purging. Its principal use is in dyeing. The goods are mordanted with it and put in the dye, when the colors are precipitated and fixed in the texture of the cloth by the alumina. Alum is added to the size in the manufacture of paper to prevent decomposition, and also to bookbinders1 paste for a similar purpose. Baths of alum are used in the tanning of leather, and it is applied in the printing baths of photographers.
It has also been employed in refining sugar and in the manufacture of pigments called lakes. The leather of Hungary is made by impregnating strong hides with alum, common salt, and suet; and in the coloring of morocco the puce tint is communicated- by logwood with a little alum. When alum is added to tallow, it makes it harder. Printers' cushions and the blocks used in the calico manufactory are rubbed with burnt alum to remove any greasiness which might prevent the ink or color from adhering. Water can be purified by means of alum; the mud that water holds in suspension collects on the addition of 0.001 part of alum (this is equal to seven grains per gallon) in long thick streaks, coagulates as it were, and is immediately precipitated. This process, the principle of which is inexplicable, was first introduced by the Chinese, and has been imitated in various parts of the world. The operation was well known from a very early period in the highlands of Scotland, according to Dr. Clark, where it is practised with peat water.
The Parisian laundresses use it, but it has not been introduced into any of the establishments for the purification of drinking water, partly because alum is a substance never naturally combined in water, and may be received as a real impurity, and partly on account of public prejudice. In bottling fruits for preservation, alum water is used. A novel application of alum is seen in the lining of some iron safes with a mixture of alum, and sulphate of lime; as the alum contains 24 equivalents of water, when the safe is heated it keeps the sides cool from the evaporation of the water, the contents of the safe remaining uninjured. It is used in the manufacture of bread to increase the whiteness of the flour. According to Liebig, this is very injurious, as he supposes the soluble phosphates to combine with the alumina, forming insoluble salts, and the beneficial action of the phosphorus is lost to the system. In the manufacture of lard alum is used as an adulterant. Dr. Hassall says that alum is generally put into the vat in breweries to give the beer a smack of age; it also imparts a heading to porter, which landlords are so anxious to raise to gratify their customers.
Alum dissolved in water is used in the adulteration of gin; and it is added to artificial port wine, to increase the brilliancy of the color.