ALUM.— Boiling tiie Lie. 1C3

time to time upon the heap collects, and is thenceconducted to the crude liquor cisterns. The solublesalts are thus removed at certain periods, as they areproduced during decomposition. The strength ofthe crude liquor in the cistern must be regulated bythe respective prices of labour and fuel, weak liquorrequiring more fuel, and strong liquor more labour;it is never advisable, however, to concentrate theliquor to the degree at which it would be saturatedwith crystallizable salts, such -as green vitriol andsulphate of magnesia, as these would crystallize withthe slightest amount of evaporation or rise of tem-perature before it was desirable they should do so.In general, the density indicated by 20° B., = sp. gr.T157, is not exceeded. Simon found the crude liquorfrom Gleissen, in Neumark, to contain the followingingredients:—

Aluminium sulphate,. 11*085 ~|

Ferrous sulphate,. 9*773

Sodium sulphate,. 2*035

Magnesium sulphate,. 1*754

Manganese sulphate,. 0*174

Potassium sulphate,. 0*095 00 r7ft

Sulphate of lime, . 0*120 '

Sesquichloride of iron, . 1*872

Magnesium chloride,., . 0*334

Aluminum sesquichloride, . 0*419

Sulphuric acid,. 0*563

Hydrochloric acid,.. 1*454 J

Water,. 70*322

100*000

These constituents are always accompanied by acertain quantity of ready-fonned alum, which maybo potash or ammonia alum, or both, if a red heatlias not been employed in the manufacture; but if ahigh temperature has been applied, both are decom-posed, leaving only alumina. To the production ofthe potash alum, the alkali naturally contained in theores contributes, and still more that which exists inthe ash of the wood used as fuel; the production ofammonia is attributable to the nitrogen contained inthe coal. During the time that the crude lie isclarifying in the large vats, a chemical decompositionensues by the action of the air upon the ferroussulphate, which is not prevented by the state ofsolution of the salt

In the foregoing analysis nearly the whole of theoxide of iron is precipitated, and only a very smallportion remains in solution. This behaviour is turnedto account by mixing the crude liquor with the mudobtained in a subsequent operation. This mud is akind of basic alum, which is soluble and deficient ina certain quantity of, but does not contain enoughsulphuric acid to make alum. Tlie basic sesquisaltof iron is called in German vilriolschmand, and iscollected from time to time from the bottom of thecisterns, and heated to redness; it then parts withits acid, and the peroxide remains, which is used asa red pigment.

Boiling the Crude Lie— The further treatment ofthe lie depends upon the quantity of green vitriolwhich it contains, and in most cases there is quite asmuch of this salt as of alum. When this happensthe lie is used for the production of both salts, andalum and copperas works are generally carried on con-

jointly. The process of separating the two salts bycrystallization varies much in different manufactories.

When, however, the quantity of ferrous sulphateis too small to admit of being profitably extracted,the liquor is at once evaporated until it has attainedthe specific gravity 1*40. Basic sulphate of iron isdeposited, and the liquor assumes a brick red colour.

In order to clear it, it is run off into tanks, and afterthe suspended matter lias deposited is syphoned offand transferred to the precipitation tanks.

"When the crude lie is very much charged with theiron salt it is evaporated in pans, into which old ironis thrown. The ferric oxide formed by the action ofthe air is thereby partly precipitated as a basic salt,and the sulphuric acid set free converts a part of theiron into sulphate, hydrogen gas being at the sametime given off. The latter reduces the precipitatedferric sulphate to ferrous sulphate, and prevents itsfurther oxidation during the evaporation.

In order to afford more points of attachment forthe crystals and facilitate their removal, the work-men hang peeled sticks and branches in the solution.

The mother lie contains the whole of the sulphate ofalumina, which is separated in a manner to be subse-quently described.

In other places—as at Keschwitz, near Saalfeld , atSchwemmsal—the greater portion of the water is re-moved by causing the liquor to trickle through skele-ton towers filled with brushwood placed in a strongdraught, as in the salt-works, when, in consequenceof the increased action of the air, a considerable por-tion of the green vitriol is decomposed, and muchbasic sulphate of iron, mixed with gypsum, is left asan incrustation upon the thorns. The concentratedlie then yields green vitriol on evaporation, and themother liquor contains the alum.

Another process consists in separating the copperas,not by cooling the saturated solution, but by simpleevaporation or abstraction of water. When evapo-ration has been carried on for some time, and theloss of water and the strength of the lie have reacheda certain degree, a point of saturation is attained—when the specific gravity is from 1*35 to 1*37, or 38°to 40° B.—at which the remaining water is just suf-ficient to retain the copperas in solution. The eva-poration of every additional quantity of water thencauses the precipitation of a portion of copperas, anda sediment, consisting of very small anhydrous orslightly hydrated crystals, is rapidly deposited. Inthe meantime, the evaporated water is constantlyreplaced by fresh liquor. It is obvious that, uponthis plan, in which the liquor is always saturated, thequantity of dissolved green vitriol cannot accumulatein the pan, whilst the amount of the other solublesalt, aluminium sulphate, must increase synchron-ously with the deposition of the former until theliquid is saturated with it. At this stage the liquoris carefully watched by means of the hydrometer, toavoid any supersaturation with aluminic salt, as intime it would precipitate with the ferrous sulphate.

The precipitated sulphate of iron is then removedand purified by crystallization, when it acquires sevenequivalents of water, and a mother liquid remains,