On Smelting of Lead. 375 



. *' It is not always an easy matter to meet with a current 

 of water sufficient to move the bellows icquired in smeltinf 

 on an hearth furnace ; and lo carry the ore from the mine 

 where it is dug to a considerable distance (o be smelted is 

 attended with great cxpeiise : this expense is saved by smelt- 

 ing in the cupola furnace, which, not requiring the use of 

 bellows, may be constructed any where ; wood is very scarce 

 in every mining country in England, and the pit coals cost 

 ten or twelve shiihngs a ton in Derbyshire, yet thcv can 

 smelt a definite quantity of ore in the cupola at a far less 

 expense, by means of pit coal than of wood. The flame 

 which piays upon the surface of the ore, and smdlls it in a 

 cupola iurnacej is not driven against it with much violence; 

 by this means small particles of ore called helland may be 

 smelted in a cupola furnace with great convenience, which 

 would be driven away if exposed to the fierce blast of a pair 

 ot bellows in a hearth furnace, lliese are son)e of the ad- 

 vantages attending the use of a cupola in preference to a 

 hearth furnace, and to these may be added, one superior to 

 all the rest, the preservation of the workmen's lives : the 

 noxious particles of the lead are carried np the chimney in 

 a cupola, whilst they are driven in the face of the hearth 

 smelter at every blast of the bellows. 



"They generally put into the cupola furnace a ton of ore, 

 previously broken small, and properly dressed, at one time ; 

 this they call a. charge: if the ore is very poor in lead, they 

 put in sotTiewhat more, and thev uork oft' three charges of 

 ore in every twenty- four hours. In about six hours from 

 the time of charging, the ore becomes as fluid as milk. 



" Before the ore becomes fluid, and even whilst it con- 

 tinues in a state of fusion, a considerable portion of its 

 weight is carried oft' through the chimney ; what remains 

 in the furnace consists of two different substances, of the 

 lead, for the obtaining of which the process was com- 

 menced, and of the 5/00 or scoria. The proportion between 

 these parts is not always the same, even in the same kind 

 of ore : it depending much upon the nianagement of the 

 fire. The lead, being heavier than the slag, sinks through 

 it as it is formed, and settles into the concavity of the bot- 

 tom of the furnace. The pure slag, according to the idea 

 here given, is that part of the ore of lead wiiich is neither 

 driven ofl" by the heat of the furjiace, nor changed nito lead. 

 ]n order to obtain i/ie lead free from the slag wliick swims 

 over it, the smelters muatly throw in about a bushel of lime ; 

 not, as is usually supposed, ii\ order to contribute towards 

 the more perfect fusion of the ore, but to dry up the slag 

 A a -i which 



