REFINING OF LEAD. 200* 



I have given this fl<etch of the Englidi procefs only to (hew The veflel is 

 thai it is poflible (a refine lead with very little expence of ^^""y ^^^^P* 

 allies tor forming the cupels. Thofe here mentioned do not 

 abforb eighty pounds of oxide out of the large quantity of 

 lead thus refined. 



Hence we fee that metallurgifts have always endeavoured 

 to obtain the grcatefl potlible quantity of litharge and the lead 

 of a (lies containing oxide; but as they did not imagine they 

 could depart from the docimaflic procefs, they have coniiantly 

 made their cupels o^' allies. 



We have feen that in the fmall procefs of cupellation the The litharge 

 lead penetrates the aflies as it becomes oxided, and that when ^"ns oft m the 



II • 1 /• II I .. c ,-, . Imall procefj 5 



no more lead remains the imall button of lilver remains pure 



at the bottom in the fpherical form. This operation is eflfeded 

 with more fpeed becaufe tlie furface of the bath is always con- 

 vex in thefe fmall vetfels, and confequently the litharge runs 

 off on all fides towards the edge of the cupel, whefe it isi im- 

 mediately imbibed. 



This is not the cafe in large cupels of feveral yards diam- — but cannot 

 eter. Bellows muft be ufed, not only to accelerate the oxi- ^" ^^^ ^'^^^* 

 dation by their blaft, but to drive the litharge towards the paf- 

 fage or gutter which is left for its ilTue. 



The inconveniencies and even the impoffibility of caufing It muft be driven 

 all the lead to penetrate the aflies of thefe large cupels have "^^ ^^ ^'^^°''*- 

 been already remarked. This muft be evident on refle6ling 

 that the oxidation can take place only at thofe parts of the 

 bath which are expofed to the contact of the air and the blaft 

 of the bellows. The litharge near the middle of the bafoa 

 not being difpofed to flow towards the edge, would cover and 

 defend the metal from any farther oxidation. Hence it is 

 that the operators have found themfelves obliged to drive out 

 the litharge by the mechanical action of a ftream of air from 

 bellows. 



The oxidation therefore takes place only at the furface of The oxidation is 



the lead, and not lower; if it were otherwife, the allies of the ^5'?'^/"'^' "' 



' the turface, 



cupel would be penetrated with oxide to a depth which wohM 



be more unequal the longer the operation lafted. Now I have 



always remarked that the portion of alhes thus imbibed in the 



large refinery is not thicker towards the center of the balbn 



than towards its edge, though the lead remains thirty or forty 



times longer at the bottom than near the edges, becaufe the 



Vol, XL— July, 1805, P bath 



