A Chilean mill operated on the same basic principle as the arrastra. The technology travelled to other gold fields from South America. Instead of flat stones being pulled across the ore, massive wheels or millstones were used to crush the chunks of quartz into sand or grit. The millstones were pulled around a watertight pit, usually by a horse. As the quartz was crushed and ground into sand, water was added to wash away the ore. The heavier gold sunk to the bottom of the pit. Miners added mercury – they called it “quicksilver” – to form an amalgam with the gold. They then collected the amalgam and evaporated off the mercury, leaving behind the gold.

Chilean mills required more specialized labor than the arrastra. The millstones had to be cut precisely by a stonemason, and the machine was difficult to adjust or to repair if it got broken, which meant they were quite expensive to own, transport, and operate. The arrastra became much more popular than the Chilean mill for this reason.
Art Gallery of Nova Scotia, Dian Day, Susan Sellers, Rita Wilson

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