LEAD 



65 



and when this layer has in its turn become sufficiently heated, more ore is thrown 

 on. In this way ore is from time to time added, until the kiln has become full, when 

 the orifice of the doorway is closed by an iron plate, and the operation proceeds regu- 

 larly and without further trouble until the greater portion has become eliminated. 



This usually happens at the expiration of about four weeks from the time of first 

 ignition, and the brick-work front is then removed, and the ores broken out, and after 

 being mixed with proper fluxes, passed through the blast-furnace. 



The proportion of wood necessary for the roasting of a ton of ore by this means 

 must necessarily depend on the composition of the minerals operated on ; but with ores 

 of the description above-mentioned, and in a neighbourhood where wood is moderately 

 cheap, the desulphuration may be effected at a cost of about 5s. per ton. 



Calcining. ^-The lead obtained by the various processes above described generally 

 contains a sufficient amount of silver to render its extraction of much importance ; 

 but, in addition to this, it is not unfrequently associated with antimony, tin, copper, and 

 various other impurities, which require to be removed before the separation of the 

 silver can be effected. 



This operation consists in fusing the hard lead in a reverberatory furnace of peculiar 

 construction, and allowing it to remain, when in a melted state, exposed to the oxi- 

 dising influences of the gases passing through the apparatus. By this treatment the 

 antimony, copper, and other impurities become oxidised, and on rising to the surface 

 of the metallic bath are skimmed off, and removed with an iron rake. The hearth of 

 the furnace in which this operation is conducted consists of a large cast-iron pan, 

 which may be 10 feet in length, 5 feet 6 inches in width, and 10 inches in depth. The 

 fire-place, which is 1 foot 8 inches in width, has a length equal to the width of the 

 pan, and is separated from it by a fire-bridge 2 feet in width. The height of the arch 

 at the bridge-end is 1 foot 4 inches above the edge of the pan, whilst at the outer 

 extremity it is only about 8 inches. 



The lead to be introduced into the pan is first fused in a large iron pot fixed in 

 brick -work at the side of the furnace, and subsequently laded into it through an iron 

 gutter adapted for that purpose. The length of time necessary for the purification of 

 hard lead obviously depends on the nature and amount of the impurities which it 

 contains ; and, consequently, some varieties will be sufficiently improved at the ex- 

 piration of twelve hours, whilst in other instances it is necessary to continue the 

 operation during three or four weeks. The charge of hard lead varies from eight to 

 eleven tons. 



When the metal is thought to be in a fit state for tapping, a small portion taken out 

 with a ladle, and poured into a mould used for this purpose, is found on cooling to 



1344 



assume at the surface a peculiar crystalline appearance, which when once seen is 

 readily again recognised. As soon as this appearance presents itself, an iron plug is 



1345 



withdrawn from the bottom of the pan, and the lead run off into an iron pan, from 

 which it is subsequently laded into moulds. 

 VOL. in. F 



