Alum—Alum Mines of Tolfa—Borax—The Suffioni in the Florentine Lagoons—China Clay: how formed?—Its Manufacture in Cornwall—Plumbago—Emery—Tripolite.

Alum, a double salt, consisting of sulphate of alumina (the peculiar earth of clay) and sulphate of potash, or sulphate of alumina and sulphate of ammonia, was known to the ancients, who used it in medicine, as it is now used, and also as a mordant in dyeing, as at the present day. Their alum was chiefly a natural production, which was best and most abundantly obtained in Egypt. In later times Phocis, Lesbos, and other places supplied the Turks with alum for their magnificent Turkey red; and the Genoese merchants imported large quantities from the Levant into Western Europe for the dyers of red cloth. In 1459 Bartholomew Perdix, a Genoese merchant who had been in Syria, observed a stone suitable for alum in the Island of Ischia, and burning it with a good result, was the first who introduced the manufacture into Europe. About the same time John di Castro learnt the method at Constantinople, and manufactured alum at Tolfa. This discovery of the mineral near Civita Vecchia was considered so important by John di Castro that he announced it to the Pope as a great victory over the Turks, who annually took from the Christians 300,000 pieces of gold for their dyed wool. The manufacture of alum was then made a monopoly of the Papacy; and instead of buying it as before from the East, it was considered a Christian duty to obtain it only from the States of the Church. With the progress of Protestantism, however, the manufacture began to spread to other countries. Hesse-Cassel began to make alum in 1554, and in 1600 alum slate was found near Whitby in Yorkshire.

Alum-stone is a rare mineral, which contains all the elements of the salt, but mixed with other matters from which it must be freed. For this purpose it is first calcined, then exposed to the weather, in heaps from two to three feet high, which are continually kept moist by sprinkling them with water. As the water combines with the alum, the stones crumble down and fall eventually into a pasty mass, which must be lixiviated with warm water, and allowed to settle in a large cistern. The clear liquor on the surface, being drawn off, is to be evaporated and then crystallised. A second crystallisation finishes the process, and furnishes a marketable article. Thus the Roman alum is made; but its production being far from enough for the supply of the world, the greater portion of the alum found in British commerce is made from alum-slate and analogous minerals, which contain only the elements of two of the constituents, namely, clay and sulphur, and to which, therefore, the alkaline ingredient must be added.

Borax, or Borate of Soda, is a substance extensively used in the glazes of porcelain, and recently in the making of the most brilliant crystal when combined with oxide of zinc. Formerly its chief supply was obtained under the name of Tincal, from Thibet, where the crude product was dug in masses from the edges and shallow parts of a salt lake; and, in the course of a short time, the holes thus made were again filled. Crude borax is also found in China, Persia, the Island of Ceylon, and in South America; but at present by far the largest quantity used in commerce is derived from the lagoons of Tuscany, where vapours charged with a minute quantity of boracic acid rise from volcanic vents.

Before the discovery of this acid, in the time of the Grand Duke Leopold I., by the chemist Hoefer, the fetid odour developed by the accompanying sulphuretted hydrogen gas, and the disruptions of the ground occasioned by the appearance of new suffioni or vents of vapour, had made the superstitious natives regard them as a diabolical scourge, which they vainly sought to remove by priestly exorcisms; but since 1818 the skill, or rather the industrial genius, of Count Larderel, originally a simple wandering merchant, has rendered these once abhorred fugitive vapours a source of prosperity to the country, and were they to cease, all the saints of the calendar would be invoked for their return.

By a most ingenious contrivance the waters which have been impregnated by the volcanic steams are concentrated in leaden reservoirs by the heat of the vapours themselves. The liquid, after having filled the first compartment, is diffused very gradually into the second, then into the third, and successively to the last, where it reaches such a state of concentration that it deposits the crystallised acid, which the workmen immediately remove by means of wooden scrapers. As this mode of gradual concentration requires very few hands and no artificial heat, a circumstance of great importance in a country without fuel, it may almost be said that the acid is obtained without expense. The produce of the lagoons amounts to more than 2,000 tons annually.

China-clay, or kaolin, a substance largely used for the fabrication of porcelain and in paper-making, is quarried from amidst the granitic masses of Dartmoor and Cornwall. It results from the decomposition of felspar, the chief constituent of granite, and is thus, to a certain point, provided by Nature. As it is, however, mixed with many grosser particles, it requires repeated washings in a series of small pits or tanks, until finally the water, still holding in suspension the finer and purer china-clay, is admitted into larger ponds. Here the clay is gradually deposited, and the clear water on the surface is from time to time discharged by plug-holes on one side of the pond. This process is continued until by repeated accumulations the ponds are filled. The clay is then removed to large pans about a foot and a half in depth, where it remains exposed to the air until it is nearly dry--a tedious operation in our damp climate, as during the winter at least eight months are necessary, whilst during the summer less than half the time suffices.

When the clay is in a fit state it is cut into oblong masses and carried to the drying-house, a shed the sides of which are open wooden frames constructed in the usual way for keeping off the rain, but admitting the free passage of the air. The clay thus prepared is next scraped perfectly clean, and is then packed up into casks and carried to one of the adjacent ports, to be shipped for the potteries or the paper manufactories. It is of a beautiful and uniform whiteness, and is perfectly smooth and soft to the touch.

China-clay is an important article of commerce. The average annual production of Cornwall and Devonshire amounts to 105,000 tons, and it is reckoned that more than 200,000l. is annually spent in Cornwall in obtaining and preparing it.

As in our times no branch of industry remains unimproved, a machine has recently been invented which greatly accelerates the drying. The lumps of china-clay are placed in the compartments of the drying machine, and the whole is then rotated with great velocity. Thus the water is thrown off by the operation of the centrifugal force, and two tons of clay can be dried in five minutes.

The remarkable contrast between the extreme hardness and softness of the same primitive rock is a curiosity of geology well worth notice. ‘A stone,’ says the author of ‘Cornwall and its Mines,’ ‘is primevally fused and cooled into as hard a substance as nature affords; so hard that it paves our streets in long enduring slabs and blocks. Tens of thousands of feet passing daily over it make no impression upon it. Years of travel only see it smooth and shining. The men who pass over it pass away, but it is still durable and unsoftened. Halls and mansions, clubs and palaces, are built of it, and it endures unmarked by Time’s devouring tooth. Is it not adamantine? But lo! what is that elegant cup from which you are sipping your tea? It is of Worcestershire china, fine and almost pellucid. Well, it is made out of the soft ruins of that very granite whose endurance is proverbial. How remarkable that the best types of firmness and fragility should be found in the same stone! What the immense billows of the Atlantic are now beating upon, at the Land’s End, without effect, that very substance has been found in such a friable natural condition as to be finally moulded into that vase which stands, elegant and admired, upon your mantelpiece, and which one puff of wind, or one whirl of a lady’s silk dress, would dash down into innumerable and unmendable fragments.’

Valuable kaolins are likewise found in China and Japan, in Saxony and France; but the production is nowhere so extensive as in Cornwall.

The best quality of Plumbago, graphite, or black-lead, from which the finest lead-pencils were made, was formerly abundantly supplied by the mine of Borrowdale in Cumberland. At one time as much as 100,000l. was realised in a year, the plumbago selling at forty-five shillings per pound. Its high value, however, proved a source of loss to the proprietors, for robbery found here a most profitable field. Even the guard stationed over it by the owners was of little avail against determined thieves, for about a century ago a body of miners broke into the mines by main force, and held possession of it for a considerable time. The treasure was then protected by a building consisting of four rooms upon the ground floor, and immediately under one of them is the opening, secured by a trapdoor, through which alone workmen could enter the interior of the mountain. The mine has now, however, not been worked for many years. Its ‘nests’ of plumbago seem to be exhausted, and it is no longer an object capable of exciting greed, or worthy of anxious protection. At present the mines of Batougal, near Irkutsk in East Siberia, furnish large quantities of graphite of the very best quality; and the plumbago from Ceylon is likewise highly esteemed. Inferior qualities--employed for counteracting friction between rubbing surfaces of wood or metal, for making crucibles and portable furnaces, for giving a gloss to the surface of cast iron, &c. &c.--are supplied by Lower Austria, Bohemia, Bavaria, Ticonderago in the State of New York, New Jersey, Arendal in Norway, Finland, &c. &c.

The name black-lead applied to plumbago, from its resembling lead in its external appearance, is very inappropriate, as not a particle of that metal enters into its composition. Plumbago might indeed more justly be called black-diamond, for, consisting, in its pure state, of carbon, it is consequently but another form of the chief of precious stones.

The beautiful oriental ruby has likewise a humble cousin among the useful minerals, as it is identical in composition with Emery, which, since the remotest antiquity, has been employed for the grinding of metals and glass. Emery was formerly almost exclusively furnished by the island of Naxos; but since 1847 new mines have been discovered in some other isles of the Grecian Archipelago, and in the neighbourhood of Smyrna, from which place it is now imported in large quantities. In 1850 the total production of the East amounted to 1,500 tons. In 1864 a considerable mine of emery was discovered at Chester (Hampden County, Massachusetts), and as might be expected in a country where none of the gifts of nature are allowed to be wasted, already provides for the wants of all the manufactories of the United States.

Tripolite, a mineral related to the precious opal, as it consists almost entirely of silica, is likewise used for polishing stones, metals, and glasses. Its composition is truly remarkable, as it is actually formed of the exuviÆ, or rather the flinty envelopes of diatoms, which belong to the minutest forms of vegetable life. They are recognised with such distinctness in the microscope that the analogies between them and living species may be readily traced, and in many cases there are no appreciable differences between the living and the petrified. As every cubic inch of tripolite contains millions of these exuviÆ, and the stone not seldom occurs in deposits often many miles in area, imagination is at a loss to conceive the innumerable multitudes of organised atoms whose flinty remains have been piled up in these masses of hard rock.