A I- U ALU 



t!)« ore lias now acquired t!ie fwoeriili aariiigent tallc of .'cc. 179.;,) 13:) pnrt^ fulplat of poiafli arc compofcd of 



^"'■r^il r J r 1 • 54-'^ I'^^al^ a"J 45-J fiilpliiiric aciJ ; and 100 paitti alum of 



the l-condproeefs hegir.sby pilinfrtl.ccalciiud (loncs ^,3.75 alii.ninc and 36.25 fulpluiric acid. TWcfurc, the 



Ml long beds, oiia noping floor, the lower fide of «liiel. is 25 parts fulplmric acid in the ore re.p.irc 37 1 aluminc and 



terminated by a ditch of water, cxtetidinjj along its whole 

 lencjlh ; from this ditch the beds are fr^q-iently f;.rinklcd, 

 and the water draining frofn them reti-rns again into the 

 rer.Tvoir. Tn about a fortiii^rht the ftones begin to crack 

 and break down, and are at IcTigth, in f.nty day:, more or 

 lei's, ovcrCprcad with a reddifli efflorefcence, and reduced into 

 a kind of pallc. A leaden boiler is now half filled with 

 water, and when hot, frefh portions of the prepared ore arc 

 contimially ftirred in till a foltition of fnlTici-nt Urcngth is 

 procured ; the liquor as yet turbid is di-.uvn off into another 

 boiler, where it is fuhjeacd to a verj- gentle evaporation, 

 at the fame time that it becomes clear by the depofition of 

 its earth. H iving arrived at the point of crydalli/.ation, it 

 is transferred by means of a pipe into a fquare woo«*-n velTel, 

 eight feet high by live wide, fo conftrueted as to be readily 

 taken to pieces ; after remaining here for a few days, the 

 mother water is poured out, to be boiled again with frelh 

 alum ore in the firll cauldron, and the cryllals, when dried, 

 are ready for falc. 



From this account of the procefs, by an eye-witnefs (the 

 Abbe Mazeas), it would ap]iear that no potafh or ammonia 

 is added to the lixivium ; it follows, therefore, that one or 

 both thefe alkalies muft be found in the ore, and this is 

 actually the cafe, according to the analyfis, by Vauquelin, 

 already quoted iil the preceding article. 



The nature of this ore has been long mifunderftood, as 

 well as the rationale of its manufafture, and the analyfes of 

 it undertaken by Bergman and Monnet have only ferved to 

 perpetuate the error. Both thefe chemifts found a lai-'.-^e pro- 

 portion of fulphur in it, while Vauquelin finds only fuliihuric 

 acid ; this apparent contradiftion, however, may ealily be 

 reconciled, by confideringlhat the ore contains carbonaceous 

 matter enough to blacken it, and to give out a light flame 

 wlie-n powdered and fpread on a hot iron ; hence, if the 

 analyfis of Bergman and Monnet was begun by diftiUation 

 in a clofe retort, as it probably was, the decompofition of 

 tlie acid and production of fulphur is readily accounted for. 

 Admitting then the proportions of this ore, as afccrtained 

 by Vauquelin, to be lufficie.itly corf?ft, viz. alumine 43.92 ; 

 fulphuric acid 2J.; potafh 3.00 ; watery.; lilex 24.; it 

 oug'nt to be conlidered as a native fa'urated alum, with 

 excels of earth and deficiency of alkali, intimately mixed 

 with filex and inllHmninble matter. The a.^ion of the fire 

 in the roalling is to drive off the i;',fl^;nmab!e matter, and 

 from the fweet aluminous tafle which is thus communicated 

 to the ore, notwitiiilanding the lofs by volatiiizat'on of part 

 of its i'u'phuric acid, it Teems alfo to ell'efl a fep:i'-ation be- 

 tween the alum and the cxccfs of earth. The fubfcqucnt 

 cracking and breaking down upon expofure to the air and 

 moifhtre, is probably caufed by the ablorption of water of 

 cryllallizatlon. 



But though a confiderable proportion of alum is thus ob- 

 tained, without the addition of potalli, it may be worth 

 while to enquire whether a larger quantity might not be 

 procured by a trifling additional expcnce. The alum of 

 La Tolfa contains by Vauquelin's analyfis 

 49. fulphat of alumine 



7. fulphat of potafh 

 44. water 



100 

 And according to Kirwa:i, (on the pr.iportionof real acid, 



I 



i.j potalli. But the ore only conlaiiii at mcft 3.CH potaDi, 

 lo that no more than 16 parts of fulphuric acid will be con- 

 verted into.ahiin; the nniainiiig ij. will be left incoinbiiiaiioii 

 with altiniine in the mother water; and thi» agrees with the 

 obfervation of Marcaa, who fpcaks of an uiK^iiioun acid, 

 cfflorefccnt fall being left in tlit rcliduc of the lixiviated ore. 

 The 9 parts of acid that arc thus loft, may, however, btf 

 converted into alum, by the addition of I.42 potalli, or 

 about 3. fulphat of potaih. 



From thefe data the ore of La Tolfa ought to yield L» 

 the prefent method of working it 78.5 per cent, of cryllal- 

 lizcd alum ; or by the addition of 3 per cent, fulph.il of 

 potaih, 12, per cent, of cry(la!li/ed alum. In thiscalcul.ition, 

 however, no allowance has been made for thi f.ilphuric acid 

 volatilized in the reading, and that portion of the fall winch 

 cannot be cxtracled by lixiviation in the large way from the 

 prepared ore ; both thefe circumftances will, no doubt, di- 

 minilli confiderably the produce of alum, but the proportions 

 muft vary- much according to the <kill and attention of the 

 manufadurcr. 



§ 4. MtinufaSure of Alum from the Pyrilout oret. 



All the European alum, except what is manufadurcd at 

 Solfatara and La Tolfa, as defcribed in the preceding fec- 

 tions, is prepared from the alum flate or alum earth, and 

 thefe containing only the remote principles of this fait, a 

 much more complicated procefs is required than where tha 

 alum exifts ready formed in the ore. 



The only necilfary ingredients in the pyrito-aluminous 

 ores are clay, and pyrites, or fulphuret of iron. Belidcs 

 thefe, however, there is generally a variable proportion of 

 bitumen, lime, and magnefia. The befl alum is procured 

 from the black micaceous fpecics in which the pyrites is 

 thoroughly diffeminated through the mafs in fucli Imall par- 

 ticles as to be indillinguifhable from the reft. Such, how- 

 ever, as contains even large nodules of pyrites, is very ca- 

 pable of being manufattured, much of the Swedilh ore 

 being of this kind. 



The firft thing to be done is to difpofe the p)-ritc8 to 

 deeompofe into fTilphat of iron, (green vitriol), and this at 

 the manufacture of Flone, in the department of Ourte, in 

 Fr;ince, is brought about by fimple cxpofiirc of the ore to 

 the adlion of air and moiilure ; this ore, however, is of the 

 very bell kind, mod.eratcly foft, free from bitumen, and with 

 the ingredients well mixed, and even with thefe advantageii, 

 the procefs requires three years. The more ftony and bitu- 

 minous kinds, fiich as thofe of England and Sweden, are 

 fubjerted to a previous roafliiig. For this purpofe a layer 

 of billet wood or coals is placed on a floor of rammed clav, 

 and fet fire to ; upon this are thrown by degrees modcratCiy 

 fmall pieces of unburnt ore, till a ftratum is fonncd, about 

 half a foot in t'licknefs ; thefe piefently take fire, by their 

 own bitumen, and arc then covered with a ftratum of^ nearly 

 the fame thicknefs of ore that has been already roafted and 

 lixiviated ; to this fucceedsa layer of unburnt ore, and thus 

 alternate layers, eight or nine in number, arc gradually 

 added, till the pile is completed. Can: is taken by protcA- 

 ing it from heavy rains, and covering thofe parts expofed to 

 the wind, to keep up the heat of a moderate equable degree 

 till the bitumen being coiifumed, the fire goes out of itlclf. 

 If the ore is now examined it will be found to be of a reddilh 

 colour, containing a fmall quantity of fulpliatcd iron and 



alumiue, 



