LEAD 



1347 



The construction of a furnace of this description requires 5,000 common bricks, 

 3,500 fire-bricks, and 2 tons of fire-clay. 



Figs. 1344 and 1345 represent an 

 1348 elevation and vertical section of the 



calcining furnace. A is the fire-place ; 

 B, ash-pit; c, fire-bridge; D, cast-iron 

 pan ; E, flue ; F F F, channels for allow- 

 ing the escape of moisture ; o, one of 

 the working doors ; H, spout for running 

 off calcined metal. Fig. 1346 repre- 

 sents the pan removed from the 

 masonry, and shows a groove in the 



1346 



lip for the introduction of a sheet-iron 

 dam, tightened with moistened bone-ash 

 for keeping in the fused metal. 



In the more modern furnaces of this 

 description, the corners are usually 

 rounded to prevent breakage from ex- 

 pansion, whilst the tapping is effected 

 by means of a hole through the bottom 

 near one of the sides. This, when 

 closed, is stopped by means of an iron 

 plug kept in its place by a weighted 

 lever. 



Concentration of the silver. This 

 process is founded on the circumstance, 

 first noticed in the year 1829, by the 

 late H. L. Pattinson, of Newcastle-on- 

 Tyne, that when lead containing silver 

 is melted, in a suitable vessel, after- 

 wards slowly allowed to cool, and at 

 the same time kept constantly stirred, 

 at a certain temperature near the 

 melting point of lead, metallic crystals 

 begin to form. These, as rapidly as 

 they are produced, sink to the bottom, 

 and on being removed are found to 

 contain much less silver than the lead 

 originally operated on. The still fluid 

 portion, from which the crystals have 

 been removed, will at the same time 

 be proportionally enriched. 



This operation is conducted in a 

 series of 8 or 10 cast-iron pots, set in 

 a row, with fire-places beneath. These 

 are each capable of containing about 

 6 tons of calcined lead, and, on com- 

 mencing an operation, that quantity of 

 metal, containing, we will suppose, 

 20 oz. of silver per ton, is introduced into a pot (say F, jig. 1347) about the centre 

 of the series. This, when melted, is carefully skimmed with a perforated ladle, and 

 the fire immediately withdrawn. The cooling of the metal is also frequently hastened 





