12 The Pjdio and Cazo Process. 



The usual charge is one ton ; it is often greater in large and 

 less in small arrastras. When the charge has been introduced, a 

 few buckets of water are thrown in to make a sufficiently consis- 

 tent mud, about half the total quantity used being added at first. 

 If there is too little water, the ore is raised and pushed forward 

 by the mullers without being ground. If there is too much, it 

 packs underneath the mullers. Care is taken to add the water 

 as required, to keep the proper consistence. To do the 

 work most efficiently, the mullers should be made to revolve 

 slowly at first, but when the larger pieces have become 

 reduced, the motion is increased to from six to ten turns a 

 minute. This is sufficiently rapid to prevent the larger and 

 heavier pieces from settling and thus clogging the voladoras, 

 and does not make the charge rise over the sides. When 

 the ore has been ground about eight hours, quicksilver 

 is added in sufficient quantities to amalgamate the free gold and 

 silver. The quicksilver is usually amalgamated with either 

 silver, copper or zinc. The quantity added depends on the 

 quantity of gold and silver in the ore, and on the quantity to 

 be Avorked before a clean-up is made. 



When the arrastra is new, or immediately after a clean-up, 

 from two to five kilograms of mercury are added at once. When 

 the work is going on regularly, it is 0.25 kilog. every second 

 day. If there is no free gold or silver, no mercury is added in 

 the arrastra. When 400 kilograms are treated per day, which 

 makes about 12 tons a month, six kilograms of amalgam, con- 

 taining about 4.5 kilos, of quicksilver, are used. This adts readily 

 as long as there is plenty of free mercury present ; but as this 

 becomes saturated with the precious metals, fresh quantities must 

 be added; and to determine what this quantity should be, assays, 

 tentadura, of the amalgam taken from the bottom, made by wash- 

 ing in a horn spoon, must be made every day or two. Sometimes 

 the assay is made on a red earthen plate, platillo, which is used 

 as a pan. 



It is desirable that the amalgam should not be too liquid, for 

 it is then liable to roll into the crevices and be caught there. If, 

 however, it is too dry, the mercury, being already nearly satura- 

 ted, will not attack the precious metal. A properly constituted 

 amalgam flattens and spreads itself out, and presents large sur- 



