422

MANUFACTURES.

Manufac-

tures.

Usual pro-cess -withpyritousores.

plying the alum of Europe . Notwithstanding thissentence, however, the work continued, and still con-tinues to be the largest of the kind in England ; thereare, however, larger works, as the Hurlett mine nearGlasgow .

N early all the alum works of Europe , with the excep-tion of the two just mentioned in Italy , obtain the saltfrom pyritous ores. In the process followed in the lattercases the first step is the acidification of the sulphur, andthe formation of the sulphate of alumina; to effect which thecommon practice is to roast the ore as soon as it is obtainedfrom the mine. This is done by laying it in largeheaps and burning it with wood or thorns till it is white, orit is sometimes done by rearing a pile of ore and of coal inalternate layers, and kindling it at thebottom. When it issufficiently calcined.it is put into a pit, where it is mace-rated with water for eight or ten hours, the lie is thendrawn off into large cisterns or reservoirs, and a freshquantity of water is put on the ore in the pit. This,after some time, having acquired a sensible but weakerimpregnation than the former, is likewise run off intothe cistern. From this it is conveyed into pans, whereit is kept boiling for twenty-four hours. There isthen added to it a lie prepared from kelp; and aftersome time, the liquor is drawn off into a vessel, in whichits impurities may subside. When clear, it is run intocoolers, where it is allowed to remain four days and nights,and a quantity of putrid urine is sometimes added, in theproportion of eight gallons to a ton of liquor. When thealum crystallizes, the residual liquor is removed, andmixed with the liquor obtained from a fresh quantity ofmaterials, which is to be evaporated. The crystallizedalum is then washed, and the water allowed to drainfrom it; it is then melted in a pan, and while in itsliquid state is conveyed into tuns, in which it is allowedto remain about ten days, during which it becomes fullyconcreted. The tuns are then unhooped, and the largemasses of alum are broken down and stowed for market.A particular account of this process, observed at Whitby and other works, is given by Mr. Winter, in Nicholson’sJournal, vol. xxv., from which it appears that the stra-tum of aluminous schistus impregnated with pyrites, inthe Yorkshire district, is about twenty-nine miles inwidth, and is covered with strata of alluvial soil, sand-stone, ironstone, shell, and clay. The alum schist isgenerally found disposed in horizontal laminae. Theupper part of the rock is most abundant in sulphur, sothat a cubic yard taken from the top of the stratum isfive times more valuable than the same bulk one hun-

dred feet below.

Yorkshire (599.) According to Mr. Winter’s statement, the ex-process ^ pense of digging and removing to a distance of 200 yardsby S Mr one cu bi c yard of the schistose rock, is about sixpence half-

Winter. penny, and a man can earn from 2s. 6d. to 3s. a day. The

rock broken into small pieces is laid on a horizontalbed of fuel, composed of brushwood, &c. When aboutfour feet in height of the rock is piled on, fire is set tothe bottom and fresh rock is continually poured uponthe pile. This is continued until the calcined heap beraised to the height of 90 or 100 feet. Its horizontalarea has also been progressively extended at the sametime, till it forms a great bed nearly 200 feet square,having about 100,000 yards of solid measurement. Therapidity of the combustion is allayed by plastering upthe crevices with small schist moistened ; but notwith-standing this precaution, a great deal of sulphuric orsulphurous acid is dissipated. 130 tons of calcined

schist produce on an average one ton of alum. This Mechanics result has been deducted from an average of 150,000 Processes^tons.

The calcined mineral is digested in water containedin pits usually measuring about 60 cubic yards. Theliquid is drawn off into cisterns, and afterwards pumpedup again upon fresh calcined mine. This is repeateduntil the specific gravity becomes 1.5. The half-ex-hausted schist is then covered with water to take up thewhole soluble matter. The strong liquor is drawn offinto settling cisterns, in which the sulphate of lime, iron,and earth are deposited, At some works, the liquor isboiled, which aids in purification. It is then run intoleaden pans ten feet long, four feet nine inches wide, twofeet two inches deep at one end, and two feet eight inchesat the other. This slope admits of their being easilyemptied. Here the liquor is concentrated at a boilingheat. Every morning the pans are emptied into a set-tling cistern, and a solution of muriate of potash, eitherpretty pure from the manufacturer’s, or crude and com-pound from the soap boiler’s, is added. The quantity ofmuriate necessary is determined by a previous experi-ment in a basin, and is regulated for the workmen bythe hydrometer. By this addition, the pan liquor, whichhad acquired a specific gravity of 1.4 or 1.5, is reducedto 1.35. After being allowed to settle for two hours, itis run off into the coolers to be crystallized. At a greaterspecific gravity than 1.35, the liquor, instead of crys-tallizing, would, when it cools, present us with a solidmagma resembling grease. Urine is occasionally addedto bring it down to the proper density.

After standing four days, the mother-waters aredrained off, to be pumped into the pans on the succeed-ing day. The crystals of alum are washed into the tuband drained. They are then put into a lead pan withas much water as will make a saturated solution at theboiling point. Whenever this is effected, the solution isrun off into casks. At the end of ten or sixteen days,the casks are unhooped and taken apart. The alum isfound exteriorly in a solid cake, but in the interior cavityin large pyramidal crystals, inserted successively intoone another. This last process is called rocking. Mr-Winter says, that 22 tons of muriate of potash will pro-duce 100 tons of alum, to which 31 tons of the blackashes of the soap boiler, or 73 of kelp, are equivalent.

Where much iron exists in the alum ore, the alkalinemuriate, by its decomposition, gives birth to an uncrys-tallizable muriate of iron.

The alum manufactured according to the precedingmode is a sulphate of alumina and potash. There isanother alum which exactly resembles it; this is a sul-phate of alumina and ammonia. Both crystallize inregular octoedrons, formed by two four-sided pyramidsjoined base to base. Aluin has an astringent, sweetistaste. Its specific gravity is about 1.71. It reddensthe vegetable blues. It is soluble in 16 parts of water a60°, and three-fourths of its weight at 212°. It efflorescessuperficially on exposure to air, but the interior remainslong unchanged. Its water of crystallization is sucient at a gentle heat to fuse it. If the heat be increase ,it froths up and loses fully 45 per cent, of its weigh >nwater. The spongy residue is called burned or calcinealum, and is used by surgeons as a mild escharotic.violent heat separates a great portion of its acid.

Mr. Winter is very averse from the use of urine in

manufactory, observing that at the works of Lord udas, and those of Messrs. Baker and Co., which approa