Patio Process

The patio process is a method of extracting silver from ore by mercury amalgamation rather than by smelting, developed to treat the low-grade silver ores common in colonial Spanish America. The Spanish merchant Bartolome de Medina first put the process into large-scale use at Pachuca, Mexico, in 1554, grinding silver ore into a fine sand and spreading it in a shallow open courtyard, or patio, mixed with salt, water and mercury along with magistral, a copper sulfate compound Medina added to catalyze the reaction between the mercury and the silver. Workers trod the resulting paste to mix it thoroughly, and the mixture was left to react for six to eight weeks while metallic silver formed and combined with the mercury into an amalgam; the amalgam was then washed free of waste rock and heated to vaporize the mercury, leaving behind pure silver. The patio process required far less fuel than furnace smelting and could profitably work the poorer ores of the Mexican and Peruvian silver districts, making it the dominant method of silver refining in Spanish America for roughly three centuries.