148 Commentaries on the 



This process has the advantage of wasting very little quick- 

 silver, and is thus described : 



* Reduction by the cazo (pan). This method of reduction affords 

 the most speedy means of extracting the silver. The ore being tho- 

 roughly ground, and a quintal being taken, the proper quantities of 

 salt, water, and quicksilver, are mixed in, according to the nature 

 of the ore. The mixture is then placed over the fire, and must be 

 kept constantly stirred, and the act of ebullition further assists in 

 keeping it in motion. It is tried from time to time, to ascertain 

 whether it requires any further addition of quicksilver or salt. Each 

 pan will reduce three charges per day. If the ore be rich, it will 

 often yield a marc, a marc and a half, or two marcs per quintal : 

 and provided the quality be not lower than six ounces, this mode of 

 reduction is very advantageous ; but if the produce of silver be 

 below that rate, it will not answer, from the great consumption of 

 wood, quicksilver, and salt, together with the cost of the pans and 

 coppers. The latter must be closely attended to, to see that there 

 are no chinks or cracks in the bottom, through which the quicksilver 

 might escape; to prevent which, they should be varnished with 

 several coats of lime, slag, iron, and white of egg, well beaten up 

 together. Barba expresses himself in highly approbatory terms of 

 this method of reduction, both on account of the saving in quick- 

 silver, and because fuel may be supplied from various trailing plants, 

 which abound in the Indies, and may likewise be much economised 

 by making one furnace heat four pans, as we have seen in several 

 sugar mills in the kingdom of Mexico. 



4 The assays in the small way will indicate, exactly, what 

 quantity of silver the boiling should yield ; but this is more readily 

 ascertained by inspecting the substance itself, which, being taken 

 out with a ladle, and the slime being separated, the metal remains. 

 The slime is separated by washing, in vats of water, supplied from 

 a cistern appropriated to the purpose. This operation removes all 

 the earthy matter and slime ; which, when a sufficiency is collected, 

 are worked over in the process of reduction by cold amalgamation. 

 The quicksilver settles, and is found at the bottom of the vat, com- 

 bined with the silver. The quicksilver is then separated, in the 

 manner described under the head of reduction by the patio ; but 

 it always requires refining, never turning ont pure, like that from 

 the patio. 1 



A third method depends on the employment of sulphate of 

 copper, or colpa. This process, called the bcneficio por colpa, 

 is as follows : 



* Of the reduction by colpa (sulphate of copper.) The plan 

 or sketch of the new method of reducing the silver from all 

 classes of ore, whether cold or warm, by means of colpa, or white 

 or yellow copperas, was described by Don Lorenzo Phelipe de 

 la Torre Barrio y Lima, a proprietor of mines in the district of 



