214 REFINING OP LEAD. 



the earth repeatedly for feveral days, otherwift it would cracky 

 and thefe cracks, which would become wider by the lhrinkii>g 

 from heat, would afford a lodgment for fome of the lead ; an 

 inconvenience which fand, even if rather loamy, does not 

 prefent. It muftalfobe remarked, that a cupel of clay would 

 become too hard to admit of the excavation for carrying off the 

 litharge ; fo that this part at leaft would require to be made of 

 fand orathes. 



Two kinds of It will be advantageous to ufe two kinds of fand in forming 

 a bafon of the cupel, the one fine fuch as the founders' fand, 

 and the other coarfe. The latter may form the firft ftratum, 

 which, after being well rammed with the implements ufed for 

 this purpofe, muft be left about three inches in thicknefs. 

 Upon this the fine and fomewhat loamy fand is to be fpread 

 and rammed like the firft. A flight degree of rooifture muft 

 be ufed with both thefe, in order that they may more folidly 

 adhere together.. The lower (iratum being more coarfe, will 

 facilitate the efcape of the humidity. 



Repair or re- It will not be neceffary to diflurb the lower ftratum of fand 



newalofthe when a new cupel is to be made; and even of this laft that 

 portion which has not imbibed any oxide may be ufed along 

 with the new fand intended to be applied. The lower firatum 

 mui\ not be touched during this renewal, for fear of mixing 

 coarfe fand with the fine. This inconvenience may be guarded 

 againft by ramming upon the furfacc of the coarfe fand a bed 

 of a thin facing of afhes, at which the operator muft ftop when 

 he takes away the upper liratum. 



We have remarked that the founders' fand muft be rather 

 loamy, and that if it be not fo, it will be neceflary to add a 

 fmall quantity of clay to render it adhefive ; but as it is ne- 

 ceflary that this cfay fliould be equally diffufed through the 

 mafs, it may be diffufed in the water with which the fand is 

 to be fprinkled, and the whole muft be carefully mixed. 



Abforption is of. It might be obje61ed, that fince the cupels of fand do not 



no advantage in ^bforb fo much litharge as thofe of aflies, more time will be 



the old procefs. . , , ^ , ,. . , ^ , • . • n . 



required to complete the rehning, becaule the oxide inltead 



of being in part abforbed, muft by this new procefs be driven 



out of the furnace. This however is a fubjedl which ought 



not to be confidered as of anv importance ; for the biaft of the 



bellows well directed will caufe the oxide of litharge to flow/ 



out more abundantly through the gutter than if the abforption 



took place. 



I have 



