ALU 



alumine, and in fomc of the Swcdidi manufaftoiics is ac- 

 cordingly lixiviated without any further preparation. In llie 

 Engliih and German alum-works, liowcver, the roafted ore 

 is watei-cd lijjhtly, and expofed for a gi-eater or Icfa time to 

 the aftion of the air, by which thefulphurof thcp)Titcs is 

 more completely oxygenated, and in confequeuce a larger 

 proportion of alum is obtained. In the manufnftory of 

 Flone, already mentioned, the fingularly judieiou, practice 

 is obfervcd, of lightly roafting the ore aj'icr Ipontaaeous 

 efflorefecnce. 



The acid being thus developed, and in pnvt united to the 

 aluminc, the proccfs of lixiviation takes place. For this 

 purpof* the ore is thrown into large refervoirs of ftone or 

 wood, furniflied with a falfe bottom, to fervc the purpofe of 

 a iiltcr ; water is then poured on, and remains for twenty- 

 four hours or more, in which time it difTolves the greater 

 part of the falts ; this being let out by means of a cock 

 fixed nearly level with the bottom of the refervoir, a frelli 

 -quantity of water is added, in order to cxhauil the ore of all 

 foluble matter. The fecond lixivium is weaker than the 

 Hrft, but is afterwards concentrated by being ufed inftead of 

 water for the firll lixiviation of the next parcel of ore. The 

 water with which the lixiviation is performed is cold, and it 

 might feem at firft to be an obvious improvement to make 

 ufe of boiling water ; the experiment has, however, been 

 tried without the defired refuk, the increafed ftrength of the 

 lixivium not being adequate to the time and cxpence of 

 fuel. Where the hxivium is kept in large refervoirs, ex- 

 pofed to the weather, much depends on the drynefs of the 

 feafon, a few heavy rains weakening the liquor to fuch a de- 

 gree, as to add eonfiderably to the coft of boiling down. In 

 Sweden and the northern countries, various attempts have 

 been made to concentrate the liquor by freezing, but the 

 fuccefs has not anfwered cxpeftation ; for a faturated folu- 

 tion of alum congeals at nearly the fame temperature with 

 common water. 



The procefs of boiling down fuccecds to that of lixivia- 

 tion, and is always perfomied in leaden boilers, copper being 

 for the mofl part too dear a material, and iron being at- 

 tended with the inconvenience of decompofing alum. The 

 lixivium is mixed in the boiler with the mother-water of a 

 preceding boiling, and this is done either by filling the 

 boiler with a mixture of mother-water and liquor, and fup- 

 plying the lofs by evaporation with frefh liquor, or by filling 

 the boiler at firfl with liquor, and fupplying the w'afte by the 

 above mixture. The evaporation lafts from twenty-four to 

 forty-eight hours, according to the proportion of mother- 

 water. In Saxony, where the proportion of mother-water 

 is large, and the lixivium is brought to a high degree of con- 

 centration, the boiling continues without interruption for 

 eight days. At the end of thefe refpeilive periods the fpe- 

 cific gravity of the liquor is affayed by a leaden hydrometer, 

 or, with greater exaclnefs, by filling a bottle of known fize 

 with the liquor, and then, by weighing it, to afcertain the 

 comparative fpecitic gravity between it and water. This 

 being done, an alkaline folution is added, and the firfl cryf- 

 tallizatlon is brought about. In the Saxon manufaftories, 

 where the liquor is uncommonly concentrated, as foonasthe 

 evaporation is finifhed the contents of the boiler are let out 

 into a refervoir, where they are flrongly agitated for half an 

 hour, during which time a certain proportion of foap- 

 makers lees and putrefied urine is added ; and the liquor 

 being then let into another vat, the cryftals of alum begin 

 immediately to be depofited ; at the end of a few days the 

 mother-water is laded out, and the cryftals are coUefted and 

 walhed. The method followed in the Englifh works differs 

 fomewhat from the Saxon pradice ; in thefe, when the li- 



ALU 



<juor appears by the hydrometer to be fufficicntly evaporated, 

 the lire is withdrawn from the boiler, and a ftream of impure 

 alkaline lixivium, from kelp and foap-makcr's allies, is let into 

 the liquor already in the boiler ; at the fame time the cock 

 at the bottom of the boiler is turned, lo as to allow the con- 

 tents of it to flovv' into a refei-voir, by which management the 

 two liquors are fpecdily and effectually mixed. It remains 

 in this refervoir for three hours, during which it depofits an 

 earthy and ferruginous fediment by the aftion of the alkali, 

 and becomes of a clearer colour ; it is now transferred into 

 another large vat, and has its fpecific gravity again taken, 

 according to which a greater or lefs quantity of putrid urine is 

 added to lower it to the proper ftandard ; being then agitated 

 briikly for a quarter of an hour it is left at reft, and in the 

 courfc of five days the cryftals are depofited. In fome French 

 and Swedilh manufaftories the hquor,after being boiled down, 

 is merely agitated for fome time without adding any alkali, 

 and tiien pafied into the cr\-ftallizing tub. The rough alum 

 being waflied in order to fcparate it from the green vitriol 

 which is depofited along with it, is put into a fmall pan with 

 a little water, and when diffolved and boihng hot, fome bul- 

 locks blood, or other fimilar fubftance, is ufiially added for 

 the purpofe of clarification : when this is etfefted, the hquor 

 is run into caflcs, where the cryftals are depofited in large 

 maffes ; after ten or twelve days the mother-water is poured 

 out, and the fait, being then dried, is ready for fale. By 

 keeping in mind the analyfis and experiments in § i. of this 

 article, it is eafy to underftand the rationale of the manufac- 

 ture, as well as the advantages and faults of each procefs. As 

 foon as the pyrites is converted into fulphat of iron, whether 

 by roafling or by fpontaneous efflorefcence, it begins to be gra- 

 dually decompofed by the lime and magnelia that may hap- 

 pen to be in the ore, therefore the lefs there is of thefe two 

 earths, the greater ceteris paribus will be the produce of alum. 

 Clay is incapable of decompofing fulphat of iron ; but by ex- 

 pofure to the air, efpecially when affifted by the aftion of 

 heat, the metal becomes highly oxygenated, and is no longer 

 combinable with the acid which th«n unites with the .clay, 

 as being the fubftance in the ore of next affinity. Hence 

 arifes the advantage of the practice at Flone of roafting the 

 ore after the formation of the fulphat of iron. We have al- 

 ready feen in § I. that fulphat of alumine, even with excefs 

 of earth, is foluble in water, but that it becomes in foluble on 

 the addition of potafh ; on this account, therefore, coal, 

 which contains little or no potafli, is a far preferable fuel for 

 roafting the ore than v.-ood which yields a great deal, as all 

 the alum, thus rendered incapable of extraction by lixivia- 

 tion, is loft. The bitumen in the ores, however, diminifhes 

 the confumption of wood, and the lixivium confifts of the 

 fulphats of iron, of alumine, of lime, and magnefia. By long 

 boihng and evaporation the iron becomes fo far oxygenated, 

 that the addition of an alkali will decompofe the fulphat of 

 iron, rather than the fulphat of alumine. If the alkali is ever 

 fo little in excefs, the aluminous fulphat will be the next de- 

 compofed ; this is therefore to be carefully avoided. Nor is 

 the kind of alkali a matter of indifference, for fince only am- 

 monia and potafh are capable of forming cryftallizable alura> 

 it would appear that the ufe of foda in the EngUfh manufac- 

 tories might be advantageoufly fuperfeded by potafh ; in- 

 deed the chief ufe of the kelp feems to confift in the potafh 

 which this impure foda contains. The principal thing to be 

 attended to in the boiling down is to bring the liquor to fuch 

 a degree of concentration, that the alum fliall be depofited 

 with as little as poffible of the other falts. 



The mother-water, when thrown away, holds in folution 

 fulphats of potafh or foda, and fulphat of magnefia, the CX- 

 traftion of which was made the fubjett of one of Lord Dun- 



donald's 



