LEAD 



61 



.iron, from the sole-plate to the horizontal tuyere in its middle ; but above this point 

 it is made of sandstone. The tuyere is from 1 to 2 inches in diameter. In front of 

 the fore-hearth b, a cistern, e, is placed, through which water continually flows, so 



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1336 



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that the slags which spontaneously overflow the fore-hearth may become inflated and 

 divided, whereby the lead disseminated through them may be readily separated by 

 washing. The lead itself flows from the fore-hearth b, through an orifice into an iron 

 pot, /, which is kept over a fire. The metal obtained from this slag-hearth is much 

 less pure than that extracted directly from the ore. 



The whole bottom of the furnace is filled to a height of 17 inches, that is, to within 

 2 or 3 inches of the tuyere, with the rubbish of coke reduced to coarse powder and 

 beat strongly down. At each smelting shift, this bed must be made anew, and the 

 interior of the furnace above the tuyere repaired, with the exception of the front, con- 

 sisting of cast iron. In advance of the furnace there is a basin of reception, which is 

 also filled with coke rubbish. Farther off is the pit, full of water, replenished by a 

 cold stream, which incessantly runs in through a pipe. The scoriae, in flowing out of 

 the furnace, pass over the coke bed in the basin of reception, and then fall into the 

 water, whose coolness makes them fly into small pieces, after which they are easily 

 washed, so as to separate the lead that may be entangled among them. 



These furnaces are urged sometimes by fans or by wooden bellows,/^. 1337. But 

 at the smelting works of Lea, near Matlock, the blowing-machine consists of two 

 casks, which move upon hori- 

 zontal axes. Each of these 

 casks is divided into two 

 equal parts by a fixed plane 

 that passes through it's axis, 

 and is filled with water to a 

 certain height. The water of 

 one side communicates with 

 that of the other by an open- 

 ing in the lower part of the 

 division. Each cask possesses 

 si movement of oscillation, 

 produced by a rod attached 

 to a crank of a bucket-wheel. 

 At each demi-oscillation one of the compartments, being in communication with the 

 external air, is filled ; whilst the other, on the contrary, communicates with the nozzle, 

 and supplies wind to the furnace. 



Instead of being blown by a cold blast, these furnaces are sometimes supplied with 

 heated air. When smelting with cold air, it is often found difficult to proportion the 

 quantity of slag or other substance operated on, so as to preserve the nose or cone of 

 slag which forms at the end of the tuyere from growing too long, to the prejudice of 

 the operation. When the substance operated on is poor for metal, and very refractory, 

 it frequently happens that the smelter is obliged to break the nose, or introduce some 

 very fusible substance in order to melt it off. By the introduction of hot air this in- 

 convenience is removed, since by increasing or lowering the temperature of the blast, 

 the nose may be allowed to lengthen or shorten, according as the nature of the slags 

 may require. The temperature found to answer best is from 250 to 300 Fahr. ; 

 since when it is heated to from 500 to 600, it is found impossible to form a nose of 

 sufficient length to convey the blast to the front of the hearth, and therefore the back, 

 which is expensive to rebuild, is quickly destroyed. 



The advantage to be derived from the use of the hot blast will be evident, from the 

 result of two experiments which were tried some years since : 



Twenty-eight tons of slag smelted with cold blast consumed 392 cubic feet of air 

 per minute. 



Labour cost 378 



Coke, 7 tons, at 24s. 6d 8 11 6 



Total . 11 19 2 



