804 



SILVER 



1793 



After the breaking and dressing by hand, the ore is crushed, either by crushing- 

 rollers or more generally by stamps, called in Mexico, molinos. The stamps are 

 similar in principle to those used in the tin mines of Cornwall, but not so powerful, 

 and are worked either by water-power or by mules. As the ore is crushed, it falls 

 through small holes of about the size of a pea, perforated in strong hides stretched in 

 a slope on either side of the machine placed over a pit which receives the fine ore, 

 from whence it is conveyed to the arrastres or grinding mills. 



These stamping mills are sometimes driven by a small breast water-wheel, of five 

 feet diameter and one foot broad. Fig. 1793 will give a sufficient idea of their con- 

 struction. The long horizontal shaft, fixed on the axis of the wheel, is furnished with 

 5 or 6 rams placed at different situations round the shaft, so as to act in succession 



on the projecting teeth of the 

 upright rods or pestles. Each 

 of these weighs 200 Ibs., and 

 works in a corresponding oblong 

 mortar of stone or wood. 



The arrastre, or ' takona,' as 

 it is called in the northern dis- 

 tricts, is exceedingly simple, but 

 for so rude a machine is very 

 effective. Baron Humboldt, in 

 alluding to it, says that he never 

 saw ore so finely pulverised as 

 he saw it in Mexico. In Guan- 

 axuato, where there is much gold 

 in the ore, this is particularly 

 observable. 



The arrastre consists, in the 

 first place, of a strong wooden 

 post moving on a spindle in a 

 beam above it, and resting on an iron pivot beneath, turning in an iron socket on 

 the top of a small post of hard wood which rises about a foot above the ground in the 

 centre of the arrastre. See ORE DRESSING. 



These arrastres are usually arranged in rows in a large gallery or shed, as will be 

 seen by reference tvfig. 1794, which represents the gallery of the Hacienda of Salgado. 

 A machine has been introduced at Eeal del Monte which has superseded the old 

 Mexican arrastres. This machine is similar in principle to some of the grinding 

 mills of this country, and to the trapiche of Peru. It consists of two large 

 circular edge stones faced with iron, and moving over iron bottoms, the ore being 

 crushed and ground with water between the two metal-surfaces. The machine is 

 turned by twelve mules in the twenty-four hours, four mules working at a time, 

 .ind the quantity ground to a fine slime is sixty quintals, or about ten times the 

 quantity ground by a common arrastre; and there is reason to believe that the 

 quantity might be doubled by the use of water- or steam-power, as the number of re- 

 volutions would be increased. 



The ore being brought into a finely-divided state, is allowed to run out of the 

 arrastre into shallow tanks or reservoirs, where it remains exposed to the sun until a 

 larger portion of the water has evaporated, when it has the appearance of thick mud ; 

 and in this state the process is proceeded with. 



The lama as it is called, or slime, is now laid out on the patio, or amalgamation- 

 floor (which is in some places boarded, and in others paved with flat stones), in large 

 masses called tortas, 40 to 50 feet in diameter and about a foot thick, consisting fre- 

 quently of 60 or 70 tons of ore ; and so extensive are the floors that a large number 

 of these tortas are seen in progress at the same time. Thus, at the Hacienda de Kegla, 

 the patio, which is boarded and carefully caulked, to render it water-tight, is capable 

 of containing ten of these tortas, of about 60 tons each and 50 feet in diameter. The 

 Hacienda de Barrera in Guanaxuato will hold eighteen tortas of 70 to 75 tons each. 

 The Hacienda Sauceda at Zacatecas will contain twenty-four tortas of 60 tons each ; 

 and the patio floor of the Hacienda Nueva at Fresnillo is still larger, being 180 varas 

 in length by as many in width, and capable of containing sixty-four tortas of 70 tons 

 each! 



Having laid out the masses of ore in the patio, the operations necessary to produce 

 the chemical changes commence. The first ingredient introduced is salt, which is 

 put into the torta in the proportion of 50 Ibs. to every ton of ore (but varying in 

 different districts), and a number of mules are made to tread it, so that it may become 

 dissolved in the water, and intimately blended with tho mass. On the following day 

 another ingredient is introduced, called in Mexico magistral. It is common copper 



