20 The Patio and Cazo Process. 



from the evaporation of sea-water ; but this was found too 

 expensive, on account of the long transportation. There are 

 in Mexico a large number of salt lakes, called lagunes, which 

 dry up every year. The residue contains about 20 per cent, 

 of salt,* and fully 50 per cent, of sand. They also contain 

 both sulphate and carbonate of soda. These impure residues, 

 salfierra, are purified to be sent to the works. When purified, 

 they contain from 70 to 90 per cent, of salt — the latter figure 

 being seldom reached — and from 10 to 15 per cent, of carbonate 

 of soda. The impurities make no difference in the reactions, 

 except from there being so much less salt. 



The bed of ore which is prepared with salt should be at least 

 from 25 to 30 centimetres in thickness, depending somewhat on 

 the consistence of the pulp. The thinner the pulp, the thicker 

 the bed may be. In order to make the pile as homogeneous as 

 possible, it is trodden by mules or horses — 8 to 25 being required 

 for treading a pile — the latter number being necessary for a 100 

 ton tor to, ; 16 mules and 8 men are required for a 60 ton tortaA 

 The thickness and consisience of the ore should be such that 

 they can tread it without too much difficulty, as the work is 

 extremely laborious. In order to have a perfectly uniform 

 action, the slimes should not be too thick — the thickness being- 

 settled by the hoof of a mule being able to penetrate to the bottom? 

 and to be withdrawn without difficulty — leaving a hole which 

 does not close up for several seconds. J Whenever the mules 

 stop for rest, the spading is continued. In this way the salt is 

 thoroughly incorporated through the whole mass. This opera- 

 tion of treading is called repaso. During this time no chemical 

 action takes place, but only a mixture of the ore and salt has 

 been accomplished. Every possible effort has been made to do 

 away with this treading, as it is so fatiguing to the animals, 

 and if not well done does not allow of a full treatment of the 

 ore. Mechanical devices of many kinds have been invented 

 with more or less success. Weighted wheels, § moved in various 

 ways by mechanical devices, more or less complicated, have been 



* Laur, Metallurgie de 1' Argent auMexique; Annates des Mines, Series 6, 

 Vol. 20, p. 65. f Ibid. p. 144. % Ibid. p. 141. 



^ Percy's Silver and Gold. Part I, pp. 611 and 613, 



