LEAD 



G7 



by throwing water upon its surface, and whilst cooling it is kept constantly agitated 

 by means of a long iron stirrer or slice. Crystals soon begin to make their appear- 

 ance, and these as they accumulate and fall to the bottom are removed by means of a 

 large perforated ladle, in which they are well shaken, and afterwards carried over 

 to the next pot to the left of the workman. This operation goes on continually until 

 about 4 tons of crystals have been taken out of the pot r, and have been placed in 

 pot E, at which time the pot F may contain about 40 oz. of silver to the ton, whilst 

 that in E will only yield 10 oz. The rich lead in F is then laded into the next pot o, 

 to the right of the workman, and the operation repeated in F, on a fresh quantity of 

 calcined lead. 



In this way calcined lead is constantly introduced, and the resulting poor lead 

 passes continually to the left of the workman, whilst the rich is passing towards his 

 right. Each pot in succession, when filled with lead of its proper produce for silver, 

 is in its turn crystallised, the poor lead passing to the left of the workman, and the 

 enriched lead to his right. By this method of treatment it is evident that the crystals 

 obtained from the pots to the left of the workman must gradually be deprived 

 of their silver, whilst the rich lead passing to his right becomes continually 

 richer. The final result is, that at one end of the series the poor lead contains 

 very little silver, whilst at the other an exceedingly rich alloy of lead and silver is 

 obtained. 



The poor lead obtained by this process should never contain more than 12 dwts. of 

 silver per ton, whilst the rich lead is frequently concentrated to 500 oz. to the ton. 

 This rich lead is subsequently cupelled in the refining furnace. 



The ladle employed for the removal of the crystals, when manual labour is made 

 use of, is about 16 inches in diameter, and 5 inches in depth, but when cranes are 

 used, much larger ladles are easily managed. A form of crane has been invented 

 which effects considerable economy of labour in this operation. When, during the 

 operation of crystallisation, the ladle becomes chilled, it is dipped into a small vessel 

 containing lead of a higher temperature than that which is being worked, and known 

 by the name of a temper-pot. The pot containing the rich lead is generally called 

 the No. 1 pot ; in some establishments, however, the last pot in which the poor lead 

 is crystallised obtains this appellation. 



Figs. 1347 and 1348 represent a plan and elevation of a set of Pattinson's pots, 

 arranged in the most approved way. A. is the ' market pot,' from which the desil- 

 verised lead is laded out. B, c, D, E, F, G, H, and i, are the working pots, whilst A', B', c', 

 D', E', F 7 , o', H', and i', are their respective fire-places. The ' temper pots,' a a act, are 

 employed for heating the ladles when they have become too much reduced in tem- 

 perature. 



1349 



The figs. 1349 and 1350 are sections showing the manner of setting and the arrange- 

 ment of the pots and flues. A, pot ; B, main flue ; c, ash-pit. 



The erection of nine six-ton pots requires 15,000 common bricks, 10,000 fire-bricks, 

 160 feet of quarles, 80 fire-clay blocks, and 5 tons of fire-clay. 



In some establishments ten-ton pots are employed, and where cranes are made use 

 of they are found to be advantageous. 



Refining. The extraction of the silver contained in the rich lead is conducted in a 

 cupel forming the bottom of a reverberatory furnace called a refinery. 



In this operation the litharge produced, instead of being absorbed by the substance 

 of the cupel, is run off in a fluid state, by means of a depression called a gate. 



