52 



EXTRACTION OF SILVER. 



quantity of silver in the ore, is judged of beforehand by experiments on 

 a small scale.) The mass is well mixed by treading with the feet and 

 working with hoes. A little calcined iron pyrites, called magistral, is 

 also added — about four pounds to the caxon. The pile is often exam- 

 ined to see that the amalgamation is going on well. In some conditions 

 the mass is called hot ; in others, cold. The state of heat is cured by 

 adding a little lime and rotten dung ; that of cold, by a little magistral 

 or oxide of iron. Practice and experience alone will enable one to 

 judge of these states. It is then left to stand for eight or nine days, 

 (occasionally re-trodden and re-worked,) until the amalgamation is com- 

 plete, which is also judged of by experiment. It is then carried to an 

 elevated platform of stone, and thrown, in small quantities at a time, 

 into a well sunk in the middle of the platform ; a stream of water is 

 turned on, and four or five men trample and wash it with their feet. 

 The amalgam sinks to the bottom, and the mud and water are let off, 

 by an aperture in the lower part of the well, into a smaller well below, 

 lined with a raw-hide, where one man carries on the washing with his 

 feet. More amalgam sinks to the bottom of this well, and the mud 

 and water again flow off though a long wooden trough, lined, with 

 green baize, into a pit prepared for it, where the water percolates 

 through the soil, leaving the mud to be again re-washed. When the 

 washing is finished for the day, the green baize lining of the trough, 

 with many particles of the amalgam clinging to it, is washed in the 

 larger well. The water, which by this time is clear, is let off, and all 

 the amalgam, called "pella" is collected, put in hide-bags, and weighed. 

 Two caxons are washed in a day. The pella is then put into conical 

 bags of coarse linen, which are hung up, and the weight of the mass 

 presses out a quantity of the quicksilver, which oozes through the 

 interstices of the linen, and is caught in vessels below. The mass, now 

 dry, and somewhat harder than putty, is carried to the ovens, where the 

 remainder of the quicksilver is driven off by heat, and the residue is 

 the plata pina, or pure silver. This is melted, run into bars, stamped 

 according to the ley or quality of the silver, and sent to Lima, either for 

 the mint or for exportation. 



In the refining process the fumes of the mercury are condensed, and 

 it is used again. Two pounds, however, are lost to every pound of 

 silver. The proportion of pure silver in the pella seca, or amalgam, 

 after the draining off of the mercury through the bag, is about twenty- 

 two per cent. A careful experiment made by Mr. Gait, a jeweller of 

 this city, on a bit of the pella which I brought home from Cerro Pasco, 

 gave but eighteen and thirty-three per cent, of pure silver. 



