Page:Vol 6 History of Mexico by H H Bancroft.djvu/530

510 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 98 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.