ALUM, is an earthy salt, which occurs in a native state only in small quantities ; but it has been long artificially made and extensively employed in various chemical manufactures. Its basis is sulphate of alumina, combined usually with sulphate of potash, but sometimes with sulphate of soda or sulphate of ammonia : when the first alkaline salt enters into its com position, the product is common or potash alum, the second forms soda alum, and the third am monia alum.
At La Tolfa, in Italy, alum is prepared from an alum-stone, which contains all the ingre dients mixed with silica. At the alum-works near Whitby, in Yorkshire, alum is produced from alum-slate, the stratum of which is nearly thirty miles in length. Near Glasgow alum is manufactured from clay slate ; which is obtained from coal mines, and contains a double sulphate of iron and alumina. Alum is also produced in chemical works, by the direct union of the component ingredients.
The alum mine at Hurlet is well worthy of a visit; it possesses many of the striking cha racteristics of most mines, without being so deep or so dirty as the majority of those un derground workings. The land belongs to the Earl of Glasgow, and contains beneath the surface beds of alum, iron, coal, and lime ; and what is very curious is, that the owner leases these different kinds of mineral riches to dif ferent parties, even though they may occur in immediate juxta-position ; thus, the Hurlet Alum Company leases the alum deposits, but must not touch the other treasures, even though they occur in the same excavations ; and it thus happens that alum miners and other miners may be met with in the same galleries or passages. We descend a square shaft, the mouth of which is visible in the middle of a field ; and after a few lengths of narrow ladder have been descended, the mouth of a dark horizontal passage is reached— rendered by degrees dimly visible by the candles carried by the miners. At one place we encounter a party of coal-miners, with bits of lighted candle stuck in their caps, and occupied in the various operations inci.
dent to coal-mining. At another a party of lime-workers come into view, quarrying the hard white stone which forms the object of their labours, and baring the upper parts of their bodies to render the heat more en durable. At a third spot we fall in with the alum-miners, who do not meddle with the iron, the coal, or the lime, which are around them. The philosophy of this multiform mining is thus explained. There is a stratum of lime over a stratum of coal ; and between the two is a stratum (varying from two to twelve inches in thickness) of ore containing most of the chemical elements of alum. When the coal has been for some years excavated, the alum ore, by being exposed to atmospheric influence, undergoes a slight mechanical and chemical change, which fits it for being used in the manufacture of alum. Sometimes specimens of ore are met with in which a hard slaty sub stance is in terstratified with layers of a greenish white crystalline body ; sometimes the ore presents itself as a brownish-black kind of coaly slate ; but for the most part the ore clings to the roof of the excavated passages as a crumbling powdery substance. To collect, then, these various forms and stages of alum ore, is the work of the miners ; and the pick and the shovel, the basket and the wheelbarrow, are dimly seen by the flickering light of the candles, acting their part in the operations.
Arrived again in open daylight we find a large area of ground occupied by steeping-pits and other arrangements for the manufacture of alum. If the ore is in the efflorescent state, it is steeped in water containing sulphate of iron and alumina, and the water is boiled, evaporated, and crystallized into alum; but if the ore is in the hard stony state, it requires to be roasted or burned in a large heap built up in the open air, before the elements which compose it can be acted on by the liquid solution.