ALU

ALU

by alluvial soil, by sandstone lying upon orsometimes inclosing a thin bed of coal, by iron-stone, shale and clay.

The alum slate itself, although tender nearthe surface, is at the depth of about 100 feetequal in hardness and compactness to commonroof-slate. The upper part, whether from beingactually richer in pyrites, or in a state of partialdecomposition, is found to yield four or fivetimes as much alum as the lower part, thislatter being at the same time considerably merebituminous. Ammonites, nautilites, and tro-chites, together with belemnites, abound inthe schisms; vertebrae and other bones ofamphibious animals are also met with. Woodconverted by bituroinization into a more or lessperfect jet is of not unfrequent occurrence, andseveral subordinate beds of red iron ore areworked at the depth of about 200 feet.

The alum slate being procured by means ofthe pickaxe, the first process to which it is sub-jected is roasting. For this purpose a bed offaggots and brushwood four or five yards broadand two yards high is covered to the heighth ofabout four feet with the ore broken very smal 1;it is then kindled, and fresh ore is continuallyadded ; the breadth of the pile is also increasedby successive additions of faggots and ore tillat length the mass reaches the heighth of SO or100 feet,covering an area 150 or 200 feet square,and containing about 100,000 cubic yards ofore. The combustion is moderated by coveringthe surface with the more earthy part of theore, and after a time it is extinguished spon-taneously.

Of this calcined mine it takes on an average130 tons to produce one ton of alum. For thispurpose the ore is steeped in pits containingabout 00 cubic yards at a time, and the solution(technically called alum liquor) is drawn offinto a cistern, from which it is again pumpedupon fresh calcined mine. This process is re-peated till the liquor acquires a specific gravityof 1.15. The ore after having been once steepedis lixiviated again a second and third time, andthese weaker solutions are used instead of waterin subsequent steepings.

The strong liquor is drawn off into cisternswhere it deposits much selenite and oxyd ofiron ; it is then boiled for a short time and al-lowed to cool, by which nearly the whole ofthe grosser impurities are got rid of. The liquorthus clarified is transferred to a leaden boiler,where it is mixed with a quantity of motherliquor and kept briskly boiling during the whole

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day, the loss from evaporation being from timeto time supplied by fresh liquor. The con-centrated solution from the whole of the pansis run every morning into a vessel called asettler, where its specific gravity is taken, andis generally found to vary between 1.45 and1.5: a solution of crystallized muriate of pot-ash is then added, sufficient to lower the specificgravity to 1.35, and the whole is carefully mixedtogether. The liquor then stands in the settlerabout two hours in order to deposit the sedi-ment which it contains, and when clear is trans-ferred into the coolers to crystallize. Afterstanding four days the mother liquor is drainedoff, and the crystals of alum washed with waterin a tub, whence they are conveyed to a binwith holes in the bottom that the moisture mayrun off. The washed alum is then put into alarge leaden pan with as much water as is re-quisite to hold the salt in solution at a boilingheat -, the solution being compleat the hot liquoris immediately run into casks. Here it remainsabout sixteen days; at the end of which timethe casks are taken to pieces in order to getout the hollow mass of alum entire ; this latteris then pierced, the mother liquor is let out andthe alum itself being cleared from any adheringdirt is broken into pieces of about 1 cwt. each,and is ready for sale.

] SO tons of calcined mine afford 1 ton of alum.

The daily produce of each pan is about 4 cwt.of alum, for which about 18 bushels of coal arerequired.

22 tons of muriate of potash are necessaryfor every 100 tons of alum.

§ 2. Analysis of Alum.

The most accurate analysis of alum is byVauquelin ’ ; in his comparative examination ofseveral varieties of this salt, for the purpose ofascertaining whether there is any such real su-periority of the Roman alum, and of certainother foreign varieties of the same substance,over the French manufactures of this article asit seems, is generally supposed by the French dvers and calico-printers. The four most in-teresting varieties out of the six which wereanalysed by this able chemist, were the follow-ing: i. Genuine Roman alum. 2 Ditto froma manufacture at Liege. 3. Ditto English .4. Ditto French from the manufacture of Ivl.Ribaucourt. _ -

The method pursued was to dissolve thealum in water, and by the addition of ammo-nia in excess to separate the alumine and oxideof iron, the precipitate being perfectly well

c Ann. Chim. J.p. 154.