The patio process is a process for extracting silver from ore. Smelting, or refining, is most often necessary because silver is only infrequently found as a native element like some metals nobler than the redox couple 2 H+ + 2 e−⇌ H2 (gold, mercury, ...). Instead, it is made up of a larger ore body. Thus, smelting, or refining, is necessary to reduce the compound containing the Ag+ cation into metallic Ag and to remove other byproducts to get at pure silver. The process, which uses mercury amalgamation to recover silver from ore, was first used at scale by Bartolomé de Medina in Pachuca, Mexico, in 1554. It replaced smelting as the primary method of extracting silver from ore at Spanish colonies in the Americas. Although some knowledge of amalgamation techniques were likely known since the classical era, it was in the New World that it was first used on a large industrial scale. Other amalgamation processes were later developed, importantly the pan amalgamation process, and its variant, the Washoe process. The silver separation process generally differed from gold parting and gold extraction, although amalgamation with mercury is also sometimes used to extract gold. While gold was often found in the Americas as a native metal or alloy, silver was often found as a compound such as silver chloride and silver sulfide, and therefore required mercury amalgamation for refinement.
Development of the patio process Bartolomé de Medina was a successful Spanish merchant who became fascinated with the problem of decreasing silver yields from ores mined in Spanish America. By the mid-sixteenth century, it was well known in Spain that American silver production was in decline due to the depletion of high-grade ores and increasing production costs. The New Laws, prohibiting the enslavement of Indians, had resulted in higher labor costs as miners turned to wage labor and expensive African slaves. These higher production costs made mining and smelting anything but the highest grade silver ores prohibitively expensive, just as the availability of high grade ores was in decline. Bartolomé de Medina initially focused his attentions on learning about new smelting methods from smelters in Spain. He was approached during his research by an unknown German man, known only as "Maestro Lorenzo", who told him that silver could be extracted from ground ores using mercury and a salt-water brine. With this knowledge, Medina left Spain for New Spain (Mexico) in 1554 and established a model patio refinery in order to prove the effectiveness of the new technology. Medina is generally credited with adding "magistral" (a type of copper sulfate CuSO4 derived from pyrites) to the mercury and salt-water (H2O · NaCl) solution in order to catalyze the amalgamation reaction. Some historians assert that there were already sufficient copper sulfates in the local ores and that no additional magistral was needed, though others argue that while naturally occurring copper sulfates allowed for potential amalgamation sans magistral, the erratic results of this method made magistral a key component of the effective refinement of ore on a large scale. Regardless of whether or not Medina's contribution was entirely original, he promoted his process to local miners and was able to obtain a patent from the Viceroy of New Spain. As a result, he is generally credited with the invention of silver amalgamation in the form of the patio process. The effectiveness of this method was such that when German experts arrived in America in the late 1700s to teach the newest amalgamation technology, they admitted they did not believe they could improve upon the American method in its context. Friedrich Sonneschmidt, a German refiner, stated: "It is not to be expected that there will ever be found a method by which all varieties of ore can be refined, having expenses lower than or even equal to those required by the patio beneficiation." The amalgamation was so efficient that a refiner could turn a profit even if the ores were poor enough to yield only 1.5 oz of silver per 100 lbs of original material.
Basic elements of the patio process
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