Amalgamation Process for Silver Extraction (Patio Process)

Timeline

1. 1554–1560Pachuca · MexicoSpanish merchant Bartolomé de Medina carried out the first industrial-scale amalgamation silver extraction trial in Pachuca, Mexico, in 1554. The core process involved spreading crushed ore on an open courtyard floor, mixing it with mercury, common salt, and a copper oxide catalyst, and letting sunlight and treading over several weeks allow the mercury to penetrate and bind with silver into an amalgam, which was then heated to distill off the mercury and yield pure silver.
2. 1571–1600Potosí · Andean regionIn 1571, Peruvian viceroy Francisco de Toledo led the introduction of the patio process to Potosí. Combined with large-scale mercury supply (from the Huancavelica mercury mines) and the mita forced-labor system, Potosí's silver output rose sharply. The patio process rapidly replaced the earlier blast-furnace smelting method as Potosí's standard technique.
3. 1621–1630Potosí silver mining district · BoliviaBetween 1621 and 1630, over a hundred refineries (ingenios) operating the patio process in Potosí still produced silver at considerable annual scale, despite declining ore grades. The patio process's heavy mercury consumption deepened the economic bond between the Huancavelica mercury mines and Potosí, forming a twin-pillar colonial mining economy of the Spanish Empire, with exported silver flowing to Asia and Europe.
4. 1880–1900Major silver mining districts of the Americas · AmericasIn the late 19th century, flotation and cyanide gold (silver) extraction matured successively, surpassing the patio amalgamation process in processing efficiency and environmental control. The patio process largely exited the stage of history in mainstream mining, but the long-term environmental legacy of mercury pollution still affects many mining areas in Latin America to this day.