Method of isolating silver from its ore, apparently dating from pre-Columbian times. The ore was crushed and ground by mule power in arrastras, reducing it to a fine mud. This was then spread over a courtyard or patio, sprinkled with mercury, salt, and copper sulfate, and mixed by driving mules over it. Chemical reactions caused the silver to dissolve in the mercury. When the amalgamation was complete, the material was agitated with water in large tubs and the mud run off. The amalgam at the bottom was collected and heated to drive off the mercury. Used for much of the world's silver production for 350 years, the process was replaced by the cyanide process early in the 20th century.
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Silver ores were crushed (typically either in arrastras or stamp mills) to a fine slime which was mixed with salt, water, magistral (essentially an impure form of copper sulfate), and mercury, and spread in a one- to two-foot thick layer in a shallow-walled, open enclosure or patio. Horses were driven around on the patio to further mix the ingredients, and, after weeks of mixing and soaking in the sun, a complex reaction converted the silver to native metal, which formed an amalgam with the mercury and was recovered.
The patio process solved a crisis in the silver-mining districts of the Spanish colonies in the western hemisphere, where the high-grade silver ore that could be economically smelted (direct-smelting ore) was being rapidly exhausted. By recovering silver from the more common lower-grade silver ore, the patio process and later amalgamation processes allowed silver mining to continue for centuries in the great silver mining districts of Mexico, Peru, and Bolivia. However, the high cost of mercury became a limiting factor in treating marginally economic silver ores.
The amount of salt and copper sulfate varied from one-quarter to ten pounds of one or the other, or both, per ton of ore treated. The decision of how much of each ingredient to add, how much mixing was needed, and when to halt the process depended on the skill of an azoquero (English: quicksilver man). The loss of mercury in amalgamation processes is generally one to two times the weight of silver recovered.