jSo A L 
taining pure argillaceous earth confifts in precipitating it 
in this manner from alum. In this way Bergman found 
one hundred parts of cryftallized alum to contain forty- 
four of water, eighteen of clay, and confequently thirty- 
eight of vitriolic acid, and probably water, which could 
not be made to quit the acid. 
Alum is produced, but in very fmall quantity, in the 
native (fate ; and this is mixed with heterogeneous.matters. 
It efflorefees in various forms, upon ores during calcina¬ 
tion, but it feldom occurs cryftallized. The greater part 
of this fait is factitious, being extracted from various mi¬ 
nerals called alum ores, fuch as, i. Sulphurated clay, which 
Contiitutes the pureft of all aluminous ores, namely, that 
of la Tolfa, near Civita Vecchia, in Italy. It is white, 
compact, and as hard as indurated clay; from whence it 
is called petra aluminaris. It is taftelefs and mealy: one 
hundred parts of this ore contain above forty of fulphur 
and fifty of clay, a fmall quantity of fixed vegetable alkali, 
and a little iron. Bergman fays it contains forty-three of 
fulphur in one hundred, thirty-five of clay, and twenty- 
two of (iliceous earth. This ore is firft torrefied to de- 
compofe the fulphur, whofe vitriolic acid re-aids on the 
clay, and forms the alum. 
2. The pyritaceous clay, which is found at Schwemfal, in 
Saxony, at the depth of ten or twelve feet, is a black and 
hard, but brittle, tub fiance, confiding of clay, pyrites, and 
bitumen. It is expofed to the air for two years, by which 
means the pyrites are decompofed, and the alum is formed. 
The alum ores of Hefle and Liege are of this kind; but 
they arefirft torrefied, which isfaidto be adifadvantageous 
method. 
3. The fhifius aluminaris contains a variable proportion 
of petroleum and pyrites intimately mixed with it. When 
thefe lafi are in a very large quantity, this ore is rejected 
as containing too much iron. Profellbr Bergman very 
properly fuggefts, that by adding a proportion of clay this 
ore may turn out advantageoufly for producing alum ; but 
if the petrol is confiderable it muft be torrefied. The 
mine of Becket in Normandy, and thofe of Whitby in 
Yorklhire, are of this fpecies. 
4. Volcanic aluminous ore: fuch is that of Solfaterra, 
jsear Naples. It is in the form of a white faline earth, 
after it has etfiorefeed in the air ; or elfe it is in a ltony form. 
5. Bituminous alum ore is called fliale, and is in the form 
of a Ihiftus, impregnated with fo much oily matter of bi¬ 
tumen as to be inflammable. It is found in Sweden, and 
alfo in the coal-mines at Whitehaven, and elfewliere. 
6. Alum might alfo be extracted from many fpecies of 
pyrites; but is in general fo contaminated with iron, as 
icarcely to pay the expences of the operation. 
The preparation of alum confifis in rendering the ores 
aluminous in the firft place, and next in dilfolving and pu¬ 
rifying the fait. Moft of the alum ores contain clay and 
fulphur, which lafi requires to be converted into vitriolic 
acid before it can form the aluminous combination. For 
this purpofe expofure to the air and occafional watering is 
effectual, and is in many places tiled: but it is lefs expe¬ 
ditious than the conversion of the fulphur into vitriolic 
acid by actual combuftion. The chief difficulty of the 
procefs is, that the fulphur muft be gradually and diffi¬ 
dently burned without expelling it by too great a heat, 
which would be likewife attended with a fufion of the 
earthy parts that would render them capable of refilling 
the fubfequent aCtion of the water. After the ore has 
been, partly by calcination, and partly by fpontaneous ef¬ 
florescence, reduced to fuch a ftate as that it can be made 
into a pafte by the hand, it is fit for boiling, which is ufu- 
ally performed in large leaden caldrons. In many manu- 
f t dories a cold elixation is performed, until the faline 
fo'ution is faturated; after which it .is boiled until it be 
iufliciently ftrong for cryftallization. This fhould not be 
too much loaded, becaufe otherwife it would begin to de- 
polit cryltajs by cooling before its earthy impurities had 
riibtided. Some take the floating of a newly-laid egg as 
a. token of the boiling being fini/hed; a rough method in- 
IT M. 
deed, as well becaufe the part above the liquor may be 
very different, and likewife becaufe the fpecific gravity of 
eggs foon changes. The fpecific gravity of a fluid which 
juft prevents a new-laid egg from finking is about 1.081. 
The lixivium, fufficiently concentrated by evaporation, 
is conveyed through channels into coolers, where, in about 
an hour, it is freed, by depofition, from the groffer hete¬ 
rogeneousparticles; it is then put into either ftone or 
wooden receptacles. In eight or ten days the lixivium, 
commonly called magiftral water, flow's into another vef- 
fel, leaving behind a number of cryftals, generally fmall 
and impure, which incruft the bottom and fides of the vef- 
fel. 'I liefe are collected, and waffled from the impurities 
which adhere externally with cold w'ater: the impurities 
remaining in the refervoir after walking are kept by them- 
felves. The waffled cryftals are then put into the boiler 
ufed for purifying them, and are diffblved in a quantity 
of water fo fmall as barely to fufpend the fait when boil¬ 
ing hot. The alum liquor is then poured into large ftrong 
wooden calks, whole (laves and hoops are all numbered 
that they may be more readily put together. In thefe the 
alum gradually (hoots into large cryftals about the (ides ; 
the liquor in the middle is then let off by a cock in the 
bottom, and the veffel turned upfide down for the more 
effectual draining of the remaining fluid. Laftly, the 
cryftals are dried in a (love, and packed up in calks for 
(ale. 
In order to obtain the alum more pure at the fecond 
cryftallization, Come additions are employed in many of the 
manufactories, fuch as alkalies, lime, or urine. It is even 
aflerted that good cryftals of this fait cannot be obtained 
without fuch additions. Wc have not any clear account 
w hat it is that thefe additions perform. Urine is thought 
to be ufeful only by virtue of the volatile alkali it con¬ 
tains. It is thought that thefe additions engage with a 
fuperabundant acid, which from the decompofition of alum 
by alkaline fubftances does not feem to be the only ufe. 
As this proceeding is attended with advantage in the treat¬ 
ment of many kinds of aluminous ley, it feems not un¬ 
likely but that it may decompofe vitriols and other metal¬ 
lic (alts which might interrupt the cryftallization of the 
alum, and at the fame time render it impure. Thus mar¬ 
tial vitriol, a fait frequently abounding in the alum ores 
after etflorefcence, w ould be converted into calx of iron 
and felenite by the addition of lime ; both which fubftan¬ 
ces would fall down, on account of their very (paring 
folubility in that quantity of water which is fufficient to 
fufpend the alum. So likewife the addition of p>ot-a(h, or 
the vegetable alkali, would produce calx of iron and vi- 
triolated tartar, the latter of which requires a very conft • 
derable quantity of water to dilfolve it. It is fcarcely ne- 
ceflary to purfue thefe obfervations, as they w ould require 
to be varied according to the nature of the impurities in 
the ley, and other circumftances, among which the difpo- 
fition to form triple combinations, which exhibits itfelf 
where a variety of principles are fufpended in the fame 
folvent, is not one of the lead conliderable. 
In fuch cafes (as it is afeertained that the alum is ren¬ 
dered more foluble, and confequently lefs cryftallizable, 
by a fuperabundance of vitriolic acid) the moft obvious 
and profitable remedy feems to be that of Bergman, who 
advifes the addition of clay, which, at the fame time that 
it prevents the noxious fuperabundance, increafes the 
quantity of alum. This ftiould be added at the very firft, 
when the lixivium is put into the boiler. All the lixivium 
may then be reduced to a tenacious mafs, and formed into 
cakes, which may be expofed to the open air under a fhed. 
In this (filiation the iron of the vitriol becomes more 
perfectly calcined by the action of the air, fo as to retain 
its acid much lefs forcibly ; w hich likewife, by its (tronger 
attraction, is difpofed to unite with the clay. Calcination 
accelerates this effcCt; but it muft be cautioufly employed 
for fear of expelling the acid. In all thefe manipulations, 
it is evident that much may be gained or loft in the treat¬ 
ment of alum ores, according as the (kill and intelli¬ 
gence 
