Patio Process for Refining Silver

For some reason, I have been running across references to the “Patio Process” for refining silver a lot lately. I hadn’t given much thought to exactly how the metal is refined, once the ore is out of the ground, but it’s frequently a noxious and polluting one (cyanide is one chemical that has seen much use in refining, for instance), but the “patio process” is a really nasty one. It’s a boon in that it can take relatively low-grade silver ore (including that containing silver sulfide, which can’t itself be amalgamated) and drawing out the silver. I wondered why this was called a “patio process”. Did it involve patios?

It turns out that it did. In the original Spanish method (used for centuries in the Americas), they mixed the creusged ore in a shallow pond on a rock "patio) with water, salt, mercury, and other things, mixing constantly, and letting the sun help the chemical processes along (the salt helps turn the sulfide into silver chloride, which them amalgamates and can be drawn off and have the mercury driven off by heating) This takes up to sux weeks. But the process is pretty nasty. Not only does it involve working with mercury (which is then baked off, fer cryin’ out loud!) with all the risks of mercury poisoning, but the mere stirring of the gunk in the patio is pretty brutal. From one source:

http://www.idahohistory.net/reference%20series/0380.pdf
Italics mine.
I suppose if you’re already working people to death in the silver mines (and in the mercury mines, which reportedly the locals wanted nothing to do with) it wouldn’t bother you too much if your wild burros had their feet dissolved off them. (Unless these were Wonder Burros capable of growing new feet, I have to assume that what was dissolving off were the hooves, which I could see regrowing. Still majorlu icky). I’ll bet the silver plant operators, always concerned for profits, substituted people when they couldn’t get burros.

Evidently I was right:

The reference is to

^ Fernand Braudel, The Wheels of Commerce 1982, vol. II of Civilization and Capitalism illustrates the process (p. 326) in an eighteenth-century drawing in the library of the Hispanic Society of New York.
http://en.wikipedia.org/wiki/Potosí

Oh my! Poor burros and slaves! :frowning: