The beneficio de patio: silver amalgamation in New Spain
The cold amalgamation of silver ore with mercury, salt and roasted copper pyrites, worked in open yards in New Spain from the sixteenth century and still the standard method three hundred years later. It is the process behind the silver that reached China. Two technical witnesses describe it here — a Madrid lawyer of 1761 and a German mineralogist printed in 1825 — and both record its limits: mercury lost, gold left behind, and a chemistry its own practitioners misexplained.
Why this wins its question: The patio process is normally a clause in an economic history — "silver was refined by mercury amalgamation" — with no description, no chemistry and no people in it. Here it is described from two technical witnesses who worked with it, quoted from their own pages: the ingredient the trade called magistral and could not correctly explain, the assay cups turned from a tree the Nahua called jicalquahuitl and made in the village of Olinalán to a specification the assayers required, the gold the method visibly failed to recover, the twenty-four-hour process demonstrated before a viceroy in 1643 and then forgotten, the indigenous labour named in the sources only as an expense to be saved — and the European barrel apparatus that was tried against the Mexican yard and produced less silver at greater cost.
Claims
Every assertion below is bound to registered sources and carries its own confidence. Weight them; do not treat the page as uniformly authoritative.
The process is the incorporation of hard ores with quicksilver, salt and water. Gamboa's account of its origin gives the discovery to nature and the improvement to craft: nature guided men to it by the particles of quicksilver found among the ore, the operation being "grossera, y pesada en los principios" and costing months to yield the gold and silver, until art and experience perfected it by means of the magistrales and other mixtures, "de forma, que rinden la ley en veinte, ò menos dias, y aun se ha llegado à tocar en 24. horas".
Grinding was the precondition, and it was mechanised. The ore had to be milled as finely as possible so that the quicksilver would take up the silver, which was done, Gamboa writes, with water-driven engines of six wooden stamps a side with heads of about four arrobas of iron, or, where there was no water, with horse-driven arrastres of five, nine or ten stamps.
The two working ingredients were common salt and the magistral, and Sonneschmid calls the second "el ingrediente maestro de este beneficio", the success and the speed of the method depending on its quality and on the right dose, an excess causing much loss of quicksilver and notable delays. He identifies it as the yellow copper mineral — copper sulphide — ground and reverberated, and therefore a substance containing blue vitriol.
The craft ran ahead of its own chemistry, and the same page says so: the magistral is never used in a purified state, "muy pocos azogueros saben lo que es", and they attribute its virtue falsely to ground and reverberated copper. A process that supplied the world with silver was being run on an explanation its own practitioners had wrong.
The instrument of quality control was an indigenous manufacture. For the tentadura — the trial by which the state of the heap was judged — the assayer used cups called jícaras or chacuales, made from the fruit of a tree "que los indios llaman jicalquahuitl" found on the coasts of the kingdom; the halves of the rind, seven to seven and a half Castilian inches across and three to three and a half deep, were made by the Indians of the pueblo of Olinalán, varnished and coloured within and sometimes figured on the outside.
The specification for that cup was technical, not decorative: for the trials of the quicksilver process the jícaras had to be black or blue inside, since red and green ones deceive the eye in the colour that appears to accompany the quicksilver. A tool made in a Mexican village to a village pattern was thereby written into the quality control of the largest silver industry in the world.
The method was lossy in ways its own describers record. Sonneschmid notes that the amalgam scraped every eight or fifteen days from the cracks of the tahonas was cleaned, burnt and refined in a small furnace yielding silver of considerable gold content, but that "este método carece de exactitud por no extraer siempre todo el oro" — so visibly that a whole bead of virgin gold could be seen in a jícara when the residues of the patio were washed down.
Faster methods existed and did not survive. Gamboa records that no amalgamation works of his day rendered its silver within twenty-four hours, the ordinary run being twenty to twenty-five days or a month, and that the twenty-four-hour method discovered in New Spain in 1643 by Pedro González de Tapia and Captain Pedro de Mendoza Meléndez — tested in the presence of the viceroy Conde de Salvatierra and of ministers of the audiencia of Mexico — "era digno … de averse perpetuado" and had instead fallen out of use.
Indigenous labour is named in the source only as a cost to be saved. The advantage claimed for the twenty-four-hour method is that it dispenses with the repasos and the wasting of the quicksilver, "ahorrandose el gasto, y ocupacion de Indios, con los demàs que demanda el beneficio ordinario" — evidence that the ordinary patio process consumed indigenous labour in the treading of the heaps, given from the point of view of the man paying for it.
A European improvement was tried against the American method and lost. The editor of Sonneschmid's treatise describes the barrel-amalgamation apparatus set up in New Spain — barrels turning on their own axes through gearing, driven by horses or mules, charged with milled ore, salt, quicksilver, pieces of copper and water, and run without interruption for twenty-four hours — and records that from the first trial with known ores it "no solo dió menos plata que por el beneficio comun del patio, sino que la pérdida de azogue era enorme, y mayores todavía los otros gastos", the silver coming out so impure that after burning and refining it still did not reach the standard of eleven dineros.
The same editor turns the criticism on his own author: Sonneschmid "no quiso convencerse del mal resultado de este primer ensaye" and tried to attribute it to apparent or real accidents, on reasons the editor judges very weak, the experiments repeated always giving the same bad results. The strongest evidence for the American process in this object comes from the prologue to a book by the European expert whose alternative it defeated.
What is not evidenced here should be said plainly. Neither source is a record of health or of working conditions: the mercury exposure of the men and women who trod and turned the heaps, the mortality of the mining districts, and the terms on which indigenous workers were engaged do not appear in the pages read. Nor is the sixteenth-century invention of the process documented here — both witnesses write long after it, and neither of the pages read names its first inventor.