each, and carried on mule-back to the hacienda or reduction-works by the arrieros. The sortings at the mines into several sizes are called respectively gavarro or broza, or stones as large as an egg or apple, granzas or tierras de labor, which are chips from blasting, or assorted. Their richness is also taken into account at the assortment, the gavarro or broza being the first and richest; the second follows, and the piedra comun and tierras de labor are the lowest. After the ore has been reduced to powder, it is made into a pasty substance, and then placed in the patio in great heaps of from one to twelve, and even twenty, tons, where it is subjected to the process of amalgamation by the old Medina system. The second step is to concentrate the sulphurets; the third to produce the plata fuego, or fire-silver, in cakes. The ores are worked very closely, 97 to 9812 per cent of their assay value being extracted, though consuming much time and losing a great deal of quicksilver. Another method in general use is that of smelting. The first and second groups of ores, known respectively as colorados and negros, are amalgamated; the ligas or polvillos, or sulphurets, are smelted. In later times lixiviation or leaching has come into use. The leaching process by means of hyposulphate has not been long practised in Mexico. Many mills in Chihuahua, Sinaloa, Sonora, and Lower California have taken out their barrels and pans, and replaced them with leaching tubs.[1]
- ↑ Mines have 10, 15, or 20 stamps. One has as many as 40. The rock is crushed dry, and passed through screens of from 20 to 30 meshes to the inch. The ore is then roasted in reverberatory furnaces with salt; after which it is placed in large tanks or tubs holding 8 to 10 tons, and a stream of clear water is turned on until the ore is covered, and kept running five or six hours. The water is then run off, and a cold solution of hyposulphate of soda is passed through the ore in the same manner, until it is seen that the solution carries no more silver. The precipitation of the silver is effected by adding a solution of quicksilver and sulphur, made by boiling lime and sulphur. This is done in the tanks by the ail of steam. After the precipitation, and the running off of the precipitating liquid, the silver remains in the forın of a sulphide. It is then put into canvas filters, and afterwards dried, when it is roasted in reverberatory furnaces to carry off the sulphur, and then melted into bars. When the operation is successfully performed, the bullion is 900 to 1,000 fine. The solution is pumped back into the tanks to be used again.
