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Drag-stone mill for pulverizing ores to isolate silver by the patio process, apparently used in pre-Columbian America. The silver ore was crushed and ground by mule power in the arrastras (shallow circular pits paved with stone). Large blocks of stone attached by beams to a central rotating post were dragged around the arrastra, reducing the ore to a fine mud. Further steps resulted in the isolating of the silver.
This entry comes from Encyclopædia Britannica Concise. For the full entry on arrastra, visit Britannica.com.