The Patio and Oazo Process. ? 



At Chihuahua the third class ore is carried to the stamps; 

 morteroa. the best ore, is carried to the store-house, from which it 

 is weighed out. The ore is crushed with small stamps, weighing 

 about 150 kilos each, with a fall of 0.20 m. to 0.25 m. The 

 slots of the screens are 0.15 m. wide. The coarse sand which 

 passes the screens is called grama. The lumps of native 

 silver which do not pass through the screens are cleaned 

 by hand. The stamps are run by a horizontal water-wheel. 

 From the stamps the "ore is taken to the arras tra. At San 

 Dimas, and generally in the whole country, the ore which 

 is not small enough is broken by hand, until it is small 

 enough to be stamped in old-fashioned German stamps, 

 with wooden stems and iron shoes. Three stamps, weighing 

 0.50 kil. each, with a drop of 0.22 m., are capable of crushing 

 and forcing out of a screen with 0.15 slots, about 8 tons in 24 

 hours. These stamp mills, molinos, are sometimes run by mule 

 power. In some works rolls are used. 



The Chilian mill, trapiche, is also still in use. This was 

 formerly a large stone, generally granite, about two meters in 

 diameter, with an edge of 0.40 m. wide, weighing between 

 three and four tons. It was made to revolve on the circum- 

 ference of an enclosure formerly built up of stones, on 

 a long horizontal arm, which pivoted on a heavy piece of 

 metal driven into a post placed in the centre of the grinding 

 space. When metal was not easily procured, the beam was made 

 to turn on a piece of tough wood. The stone revolved on 

 one end of this beam. The other end projected beyond the 

 outer edge, and to it a horse or mule was attached. The inside 

 diameter of the stone is slightly smaller than the outside, so that 

 it inclines somewhat toward the centre. Sometimes, instead of 

 having one stone on\y, two stones are placed on the same arm, 

 on opposite sides of the circle, and at different distances from 

 its centre. These wheels run on a bed of hard stone. Some- 

 times the crushing is done dry ; but it is generally done 

 wet — the ground ore being washed out and allowed to settle. 

 The more modern mill* consists of a large wheel, of iron or 



* Eng. and Min. Jour., Vol. XXXIII, p. 104. 



