THE PATIO PROCESS. 47 



and water. At the centre a large and thicker stone is placed, called cepo (stock), 

 intended to receive a vertical post, called peon, which is turned by means of a pole, 

 to which two mules are set to move the apparatus, dragging a large stone, hung bv 

 means of chains to each one of the cross-arms. These stones are called voladoras 

 (runners). Their length is a little shorter than the radius of the arrastre. They are 

 cut in four-face prisms, and weigh about nine hundred weight, and are, as well as 

 those forming the bottom of the arrastre, a hard quartzose porphyry. 



In constant operation, the runners last about two months, and the bottom stones, 

 about sixteen months. The quantity of ore ground in one arrastre of the given 

 dimensions, de viarca (standard), varies from eight to twelve hundred weight. Dur- 

 ing the twenty-four hours required to grind said quantity of ore, twenty or twentv- 

 six hundred weight of water is required. 



The quantity of water put into an arrastre, and the manner of feeding it, have a 

 great influence on the success of the grinding as well as on the amount of gold to 

 be found in the sediment ; and much depends upon the capitan de gal era, or foreman. 



When the ore is reduced to fine mud, after twenty-four hours' work, — that is to 

 say, when at the bottom of the arrastre no sand is found, — the arrastre is dis- 

 charged, provided the mud be not too curdy, which is shown by the hand emerging 

 from it as if it were japanned. .Such being the case, more water is to be added, 

 and the mud to be tried again, after having worked it a little while. 



When the sand is perfectly fine, the upper part of the contents of the arrastre is 

 taken out, and new ore put into it to complete its load. 



To discharge the arrastre, the mud is taken from it to large reservoirs, called 

 lameros, where the mud worked every day is put together until having the necessary 

 quantity to form a cake. The arrastres must not be totally emptied, because, how- 

 ever perfect the grinding may be, a certain portion of badly ground ore always 

 remains at the bottom, besides running the risk of throwing into the reservoir 

 a part of the amalgam which always exists at the bottom of the arrastre. 



Extraction of Gold. — On commencing the grinding in an arrastre, either by 

 having provided it with a new bottom, or by its having been scraped, the interstices 

 between the bottom flags are filled up with fine sand ; then, in order to level the 

 arrasfre, a small quantity of poor ore is ground. If the arrastre be not a new 

 one, but only scraped, the fine sand may be dispensed with, as the grinding of poor 

 ore serves the purpose. The arrastre being levelled, a quantity of quicksilver 

 amalgamated with some other metal, as silver, copper, zinc, etc., is thrown to the 

 bottom, — an operation which is called empellar, — taking care to evenly spread the 

 amalgam on the bottom. The quantity of amalgam to be put into an arrastre 

 depends upon the amount of gold contained in the ore, as well as upon the quantity 

 of ore to be ground within the time elapsing between the scraping operations, 

 according to custom or convenience. 



In general, when eight hundred weight of ore are daily ground in an arrastre, 

 twelve pounds of amalgam are employed, in which there are about nine and a half 

 pounds of quicksilver. The quantity of amalgam put at the beginning of the opera- 

 tion is insufficient to gather all the gold and a portion of the silver contained in the 

 ore, as in the course of the operation the -amalgam loses its gathering propertv. 

 Hence it is necessary to gradually add small quantities of quicksilver; to ascertain 

 which, trials of the arrastre are made two or three times a week, or oftener if the 

 ores be very rich, in order to know their condition. The trials are made as follows : 



