Memc'ir on the Refning of Lead, d^c. 211 



ftruded of the moft porous bricks, in order that the water, 

 with which it is necellary to moillen the adies, may pene- 

 trate them in evaporating, may proceed to the bed of fcoriae, 

 and efcape by the fpiracles which are at the bafe of the fur- 

 nace. 



To afcertain the proportion of lead in filver, it is fufficicnt 

 to put fome pennyweights into a fmall cupcll of bone-aflies 

 placed under the' muffie of an alTaying furnace. In propor- 

 tion as the lead becomes oxidated, it iufinuates itfelf into the 

 cupel!, and the filver at lad aflumes that vivid appearance 

 which announces that the whole lead is diffipatt-d, that the 

 filver it contained is refined, and has attained to its maximum 

 of purity. 



In refining on a large fcale, the obje6l alfo is to feparate 

 the filver from the lead, but not to make the whole of the 

 latter penetrate into the cupell, which is even impoffible ; 

 for in that cafe it would be neceffiiry to have a much larger 

 quantity of aflies for the total abforption of the metal : be- 

 fides, the operation would require a period ten times as long 

 as that ufcd in general for refining, and would occafion ten 

 times the expenle in fuel, and a much greater lofs of the 

 metals than by the ufual procefs, where the greater part 

 of the lead is obtained in litharge, while a portion pene- 

 trates into the cupell for about two inches of its thicknefs, 

 which muft be fufed to revive the lead. This redu£lion is 

 alfo more expenfivc, and experiences a greater lofs than the 

 litharge, which is eafily fufed, and which, without pafTing 

 through the furnace, may be employed as an article in com- 

 merce. 



Lead ore and litharge may be fufed as in England, and 

 the department of the ci-devant Brittany, in a reverberating 

 furnace the folcs or bafons of which are formed of pounded 

 and moificned clay. Thefe foles can ftand the a6lion of 

 heat and of the oxide of lead for fix or eight months of un- 

 interrupted labour. 



The durability of thefc earthen foles gave me the firft idea 

 of the method, which I fliall hereafter propole, for refining- 

 furnaces, where the only thing required is to oxidate the 

 lead to obtain it in litharge, and not to caufe the cu])ells to 

 imbibe the whole of it, as is done when the objeft is to ailay 

 the metal in order to know whether it contains filver. In 

 operating on a large fcale, the cupell, though of afhes, ab- 

 forbs only a part of the lead, as I have already faid, obferv- 

 ing at the fame lime that it would be much more advan- 

 tageous to obtain the whole transformed into litharge, the 

 rcdu6tibn of which into lead is much eaficr than that of the 

 O 2 oxide 



