How it began
The patio process was first used at scale by Bartolome de Medina at Pachuca, in New Spain, now Mexico, in 1554. Some knowledge of amalgamation was probably available since the classical era, but it was in the New World that it was first applied on a large industrial scale. Medina's method used mercury to recover silver from ore in open yards, the patios that gave the process its name.
Before it existed
Silver is rarely found as a native metal. In the Americas it usually occurred as compounds such as silver chloride and silver sulfide inside a larger ore body. Before 1554 the Spanish colonies extracted it by smelting. The patio process replaced smelting as the primary method of extracting silver from ore in Spanish America.
How it works
Ore was broken up and ground to a fine sand, called harina, in an arrastra or stamp mill. The harina was piled in heaps of 2,000 pounds or more, and salt, water, magistral (an impure copper sulfate) and mercury were added. Laborers, horses or mules mixed the mass and spread it one to two feet thick in a patio. After six to eight weeks of mixing and soaking in the sun, it was washed, strained through canvas and heated to drive off the mercury and leave the silver.
How it spread
Amalgamation was introduced to the Andes in 1571 and made the mines of Potosi viable. Mercury for the process came from deposits found at Huancavelica, Peru, in 1563. Other amalgamation methods followed, notably pan amalgamation and its variant, the Washoe process. Spanish America produced three fifths of the world's silver supply before 1900.
Who pushed back
The process depended on mercury, and one to two times the weight of the silver recovered was generally lost. Expanding mercury and silver output rested on forced labor. From 1572 Viceroy Francisco Toledo organized a labor draft, the mita, under which thousands of Indigenous people worked in silver and mercury mines for less than subsistence wages. Many tried to avoid it, and villages were abandoned.
