Zinc Desilvcrization. 97 



here made of one casting ; at the Germania Works it is cast in 

 three pieces, as shown in the section A-B, Fig. 5. The 

 arch is 2 feet 9 inches above the floor of the laboratory. It has 

 three openings, 4- inches square, in the fire-bridge, and two on 

 its side, for the introduction of air. The charge remains in this 

 furnace from 18 to 24 hours. The surface is constantly exposed 

 to the air entering the furnace by the air-holes at the bridge. 

 At the end of the first half of the time that the charge is to re- 

 main in the furnace, the bath is skimmed. The skimmings 

 amount to from one to one and a half tons. They contain from 

 45 to 50 per cent, of lead,, and most of the zinc and other re- 

 maining impurities. The charge is rabbled, after the oxides 

 have been removed, but any others which form are allowed to 

 remain until the furnace is tapped into the polling-kettle, which 

 is usually about twenty hours after the charge is made, and are 

 then polled. At Mansfield Valley, the refining is done in twelve 

 hours. The lead is not polled, but is cast into pigs directly from 

 the furnace. At Cheltenham, the jjolling-kettle is placed at the 

 flue end of the furnace. The lead flows into a deep cast-iron 

 channel lined with brasque, from which it is syphoned off. The 

 top of the kettle is about six feet from the floor. Directly in 

 front of the kettle, and about two feet below the floor-level, 

 there is a sunken track upon which a car is run, the top of 

 which comes up to the level of the floor. The car is about 

 six feet wude, and receives the pigs and carries them to the store- 

 house. There is a space of four feet between the car and the 

 furnace. 



The polling is done in eight hours. The wood is held at 

 the bottom of the kettle by a crutch, Fig. 7. Tlie same 

 apparatus is used at the Germania works, except that instead 

 of the crutch, the bars are straight and i3ointed, and holes 

 are bored in the wood" to receive them. Short sticks of green 

 wood are used, but to insure a plentiful escape of steam, all the 

 wood for this purpose is kept soaking in a pool of water. 

 Three or exceptionally four pollings are made, the number de- 

 pending on the quality of the lead ; each polling lasts about an 

 hour, so that the furnace is ready to receive a new charge as soon 

 as the one refined in the softening-furnace is desilverized. The 



