Zinc J)e^lIrcrizatio)}. 89 



lioiirs. Before clrtiwiiig them, they are mixed with coal on top 

 of the melted charge, to reduce any oxide of lead, and are then 

 drawn ; and if they form again they are removed. When 

 they no longer form, the fnrnace is cooled gradually, but is 

 kept above the melting-point of lead. The crasses are 

 drawn from the working-door and are collected in a bin, 

 where they are allowed to accumulate until there is enough to 

 work. 



When litharge commences to form, the crasses are no longer 

 drawn, but are left in the furnace after the lead has been tapped. 

 In refining the next charge, they give up their oxygen to more 

 easily oxidized metals, and thus help to separate them from the 

 lead. Quick-lime is usually added as soon as they commence to 

 form, to keep the litharges from cutting. 



Sometimes all the impurities have been removed at the end of 

 12 hours or less, but the charge in the furnace must stand until 

 the desilverizing kettles are ready. This is done by simjoly shut- 

 ting the dampers, and adding only just enough fuel to the fireplace 

 to keep the charge melted; but as all the compounds of arsenic and 

 antimony are very fusible, the softening must be kept up as long 

 as these form W^ith a charge of 26 tons, at the Pennsylvania 

 Lead Works, from 24^ to 25s tons of softened lead remain in the 

 furnace. 



It often happens that the charge is ready for tapping, but the 

 desilverizing pots are in use ; so that the lead is kept in the fur- 

 nace at the melting point until the pots are free. It is cheaper, 

 even if the lead is extremely pure, to keep it melted in the 

 furnace during the time necessary, rather than to cast it and 

 re-melt it. 



At Cheltenham, the tap-hole opens into a deep but narrow 

 trough lined with brasque, from which the lead is syphoned off 

 with a Steitz syphon, Fig. 6. The brasque is made of * clay 

 and 5 coke-dust. It is made as dry as it can be stamped, and is 

 then carefully shaped and cut down to make the arch leading 

 into the furnace. When the kettles are ready, the furnace is 

 tapped. The tapping-spout is very large, and during the time 

 of casting exposes a large surface to oxidation, thus increasing 

 the losses in lead. If the furnace was sufficiently high above 

 the pot, the lead could be tapped by a gutter directly into the 



