Typical arrastra construction. From Mining and Scientific Press 52 (1886): 237.
An Arrastra (or Arastra) is a primitive mill for grinding and pulverizing (typically) gold or silver ore. The simplest form of the arrastra is two or more flat-bottomed drag stones placed in a circular pit paved with flat stones, and connected to a center post by a long arm. With a horse, mule or human providing power at the other end of the arm, the stones were dragged slowly around in a circle, crushing the ore. Some arrastras were powered by a water wheel; a few were powered by steam or gasoline engines, and even electricity.
Arrastras were widely used throughout the Mediterranean region since Phoenician times. The Spanish introduced the arrastra to the New World in the 1500s. The word "arrastra" comes from the Spanish languagearrastre, meaning to drag along the ground. Arrastras were suitable for use in small or remote mines, since they could be built from local materials and required little investment capital.
For gold ore, the gold was typically recovered by amalgamation with quicksilver. The miner would add clean mercury to the ground ore, continue grinding, rinse out the fines, then add more ore and repeat the process. At cleanup, the gold amalgam was carefully recovered from the low places and crevices in the arrastra floor. The amalgam was then heated in a distillation retort to recover the gold, and the mercury was saved for reuse.
For silver ore, the patio process, invented in Mexico in 1554, was generally used to recover the silver from ore ground in the arrastra.