Alum, a triple salt of great importance in the arts, is composed of sulphuric acid, alumina, and potash. It is sometimes found native, but only in small Quantities, and is artificially manufactured, chiefly from the mineral called alumslate. This is found in great abundance in the north-east district of Yorkshire, more particularly between Whitby and Stockton. In the reign of Queen Elizabeth, Sir Thomas Chaloner established a manufactory of alum at Gis-borough, in Yorkshire, and engaged several expert workmen, acquainted with the mode of manufacture, from the dominions of his Holiness the Pope, who fulminated bulls and anathemas against him and them in vain. The alum-slate is also found in abundance at Beckel, in Normandy. There are other aluminous minerals from which alum may be obtained; but none appear so well adapted, nor so plentiful, as the alum-slate, or aluminous schist. The mineral from which alum is manufactured, is broken into small pieces, and placed on a bed of fuel, until it is about 4 feet high.
The fuel is then ignited, and as the calcination proceeds, more of the mineral is added from time to time, until an enormous pile, sometimes equal in area to 200 square feet at the base, is formed The combustion proceeds so rapidly, that it is necessary to prevent access of air, by plastering the crevices with small schist (alum-slate,) made into a lute with water. It requires about 130 tons of the mineral to produce one ton of alum. When the calcination is effected, the residue is digested in pits containing large quantities of water. The liquid is withdrawn by pumps, and added to fresh calcined ore. This is repeated until its spec. grav. becomes about 1.15. This saturated liquor is sometimes placed in pits, to deposit sulphate of lime, and other earthy matter, which may contaminate it, and sometimes boiled to produce the same effect. The purified liquid is then concentrated at a boiling heat, in large sloping leaden pans. This is then removed to a settling cistern, and a solution of muriate of potash, or the impure alkali of the soap-maker, is added to it. The quantity necessary is ascertained by experiment with a small portion in a basin. The alkaline solution reduces the spec. grav. from 1.4 or 1.5 to 1.35.
The quantity of alkali required is, therefore, easily ascertained by an hydrometer. If the spec. grav. of the liquid be more than 1.35, it does not yield crystals, but a solid magma resembling grease, and it becomes necessary to reduce it and re-crystallize. Urine is sometimes added for this purpose; it is crystallized in the usual manner by slow evaporation. These crystals are purified by washing and boiling in a leaden vessel. The saturated solution is then poured into casks, and in about a fortnight the casks are unhooped and taken to pieces, the alum remaining in a solid mass. This last process is called roching. It is calculated that 22 tons of muriate of potash, or 31 tons of the black ashes of the soap boiler, or 73 of kelp, are necessary to make 100 tons of alum. Ammonia may be employed instead of potash, but it is more expensive. The impure sulphate of soda formed in the manufacture of aqua fortis, may be economically employed in the manufacture of alum, as it contains two out of the three ingredients necessary to its formation. The celebrated Chaptal manufactured alum artificially in France to a great extent. A chamber of very large dimensions was constructed of masonry, and floored with brick; the bricks being cemented with a composition of pitch, wax, and turpentine.
The roof was of timber, but the planks were closely grooved into each other, no nails being employed. Lastly, the whole of the interior was covered with a thick coating of the above-mentioned cement, applied as hot as possible. The purest and whitest clay is then, after calcination, strewn on the floor, and sulphur burned in the chamber. By this process sulphuric acid is formed, which in a few days saturates the alumina of the clay, converting is into sulphate of alumina. This is known by the efflorescence that takes place. The salt is then removed and exposed to the air, that the acid may penetrate more effectually the alumina. It is then lixiviated, treated with potash, and crystallized as before described. One of the most ancient manufactories of alum was at Roche, in Lyria, whence is derived the term Roche-alum. According to the analysis of Berzelius alum consists of
The taste of alum is rather sweet, hut astringent. It is a super-sulphate, and reddens the vegetable blues. The spec. grav. is about 1.71. It is soluble in 16 parts of water at 60°, and in 3/4 ths of its weight of boiling water. By long exposure to air, the surface effloresces, hut the interior will remain a long time unchanged. A moderate degree of heat expels its water of crystallization, but an intense heat decomposes it, by separating a great part of its acid. The alum of commerce generally contains much impure matter, particularly if kelp or urine has been used in the manufacture. The sulphate of iron is, perhaps, the most injurious foreign ingredient in its composition; this may be detected by ferro-prussiate of potash. Alum is of great use and importance in many processes of the arts. It is very extensively employed in dyeing, as a mordant. Most of the vegetable colours employed in dyeing have no affinity for the material intended to be dyed. In most of these cases alum is employed as the intermediate medium by which the colouring matter may be fixed, as it has an affinity both for the material and the colouring matter.
An acetate of alumina is frequently employed by the dyer, instead of the ordinary sulphate.
The acetate does not act so corrosively on delicate articles as the common alum, and its affinity for some colours is greater. It is easily prepared by the double decomposition of a solution of the sulphate of alumina and potash, and a solution of the acetate of lead. When these two solutions are mixed together, the sulphuric acid of the alum combines with the lead, forming an insoluble precipitate, which is sulphate of lead, while the liberated acetic acid unites with the alumina and potash, forming the required acetate, which is separated by filtration, from the solid substance. If paper, linen, or wood, be soaked in a solution of alum, it is rendered nearly incombustible, and less liable to be affected by moisture. Paper thus prepared is advantageously employed in wrapping gunpowder and is also useful in whitening silver. Alum has the property of hardening tallow, for which purpose it has been sometimes advantageously employed. It is used with much success in the preparation of skins for tanning, giving them firmness after they have become flaccid by immersion in the lime pits. Turbid water is rendered limpid and transparent by the addition of a small quantity of alum in solution; its purity, however, is by no means promoted by the process.
It is usefully combined with the salt by which cod fish are cured, as it prevents a deliquescence which would otherwise take place. It is used in the manufacture of London bread rather extensively, as it enables the baker to employ flour of inferior quality; the alum promotes a white colour and closeness of texture in the bread, - qualities much admired, although they generally indicate impurity. It improves the colour of the beautiful pigment, Prussian blue, and is extensively employed in the preparation of that article. In medicine it is administered as an astringent and tonic, and also as a collyrium. Alum, deprived by heat of its water of crystallization, which, in medicine, is called alumen ustum (burnt alum), has been greatly extolled as a remedy in cases of colic, and some other disorders. A remarkable substance, called Homberg's Pyrophorus, is prepared from alum. Equal parts of alum and brown sugar are melted over a clear fire, and stirred constantly until they become dry. When cold the mixture is finely powdered, and placed in a phial coated with clay, having an open glass tube luted into its neck. This is to be intensely heated in a crucible containing sand, when gas will issue from the tube, which may be inflamed.
When this ceases to come over, the crucible is removed, and the tube stopped by moistened clay until the bottle is sufficiently cold to be corked. The substance thus formed is a black and light powder, which takes fire when poured from the bottle into the air. If poured into oxygen gas a more vivid combustion takes place.