The Patio and Oazo Process. 33 



must be added in the morning, so as not to cool down the torta 

 after it has once become heated, and thus disturb the reactions 

 which are taking place. The pile must be trodden several times, the 

 object being to keep renewing the surface of the silver which, 

 without this, would become rapidly covered with a bed of solid 

 amalgam which would prevent further action. The operation 

 lasts from three to six weeks, according to the way in which it 

 is conducted, the temperature of the air and the size of the heap. 

 A succession of cloudy days or cold weather in the summer time 

 will retard the operation. Continued or heavy rains may so thin 

 the pulp as to prevent the reactions taking place, and stop all the 

 working until the pulp thickens up again from evaporation. 

 When all the conditions are the most favorable, the inwrporo can 

 be completed in 15 to 18 days. When they are unfavorable, it 

 may take from 40 to 50 days. Taking several months together, 

 20 to 25 days will be the average time. In winter, when the 

 torta always works slow, it may last as long as two or three 

 months. 



The day after the mercury is added, assays, ientaduras, are 

 made, to see how the torta is working, to learn if any one 

 of the reagents used is required, or if any of them is in 

 excess. To do this, a probe-sample, which will weigh about 

 250 grams, is taken from as many different parts of the pile, 

 as possible. The assay is washed in a norn spoon or in an 

 earthen plate, 2^tatillo i 0.18 m. in diameter, and 0.02 m. deep, 

 a rotating motion being given to it. The lighter particles 

 are carried off, and the heavier ones deposited on the bottom 

 in the order of their gravity — the heaviest being in the centre. 

 The mercury which has not yet acted, is generally in the 

 centre, the silver- white amalgam, ceja, which, when moved, 

 shows a distinct tail, lista, next to this, then the undecomposed 

 black sulphurets, then pyrites, and generally a fifth ring of mer- 

 cury in flour. Three assays are generally made on the torta each 

 day, one in the morning before the work commences, one after 

 the treading is about half done, and a third after it has been 

 completed. During the first few days, the appearance of the mer- 

 cury remaining unacted upon shows the workman what is taking 

 place. The mercury is always more or less attacked. If during 

 the first day it looks dull, is of a deep gray or lead color, there 



