The Patio mid. Oazo Process. ->;> 



in teams of not more than eight or nine. They are usually tied 

 together four abreast, and are driven, by a man -who stands in 

 the centre of the torta holding the halter, and who, by the aid 

 of a long whip, makes them walk in such a way, commencing 

 at the outer edge, as to cover every part of the torta. Some- 

 times two teams are at work on the same torta when it is very 

 large. A day's work is from 6 A. M. to 3 P. M. It is very fatiguc- 

 ing. When the work of treading stops, the feet of the mules are 

 carefully washed in a tank provided especially for that purpose, 

 not only to recover the rich material, but also to keep the mules 

 healthy ; otherwise, being in constant contact with so much 

 mercury, they would soon become diseased. They cannot be 

 prevented from licking themselves, however, to get the salt the 

 mud contains. Balls of amalgam, which often weigh* from 50 

 to 100 grams, are sometimes found in their stomachs ; which, 

 however, contain but little mercury. 



The reactions in the torta commence at once after the mas - - 

 istral is added. It is said to work cold or hot. There are two 

 kinds of heat : the first is due to an excess of the reagents ; the 

 second results from cold, and is called calor de frio. They dif- 

 fer as to their cause, but the result is the same, and increases 

 the loss in mercury while it diminishes the extraction of the 

 silver. On cold mornings, the heat of the pile being greater 

 than that of the air, the pile steams ; but as the sun rises higher 

 this vapor ceases. This is called the calor de frio. When there 

 is an excess of magistral, the chloride of mercury acts on the sul- 

 phide of silver and makes chloride of silver and sulphide of 

 mercury — which latter is entirely lost. A large amount of heat 

 is produced in this way. When the heat is thus caused by the 

 excess of the reagent, wood-ashes or lime is added to decompose 

 the chloride of copper which is formed. Lime or ashes are, 

 however, never added when it can be avoided ; they do not re- 

 vivify the mercury, and they retard the operation and diminish 

 the yield of both gold and silver. When lime is used, it should 

 be in fine powder, and only just enough should be added to 

 produce the effect. If large pieces of it were used, they w r ould 

 not be likely to be wholly acted on by the time the torta was right 

 again, and their effect would have to be counteracted, as the 



* Phillip's Gold and Silver, p. 341. 



