arrastras

Arrastras are a traditional, primitive type of ore-crushing mill used in mining to grind ore by abrasion rather than by stamping. The device typically consists of a circular or oval stone bed, lined to form a grinding surface, and a heavy circular or rectangular drag stone that is dragged around the bed by animals or people. Ore mixed with water is placed on the bed; the drag stone is pulled around the circumference, crushing the ore against the bed to produce a fine slurry from which metal-bearing minerals can be recovered.

Operation and design vary regionally. In a common setup, the drag stone is attached to a chain or axle that runs around the edge of the circular bed, which may be set into a shallow pit or trench. A team of mules, horses, or sometimes humans provides the power to pull the drag stone along a fixed circular path. The crushed material is then washed, sluiced, or panned to separate the valuable minerals from the gangue.

History and usage. Arrastras were among the earliest mechanical means of ore grinding and were widely used in colonial mining in the Americas from the 16th through the 19th centuries. They offered a low-capital alternative to more complex mills, making them suitable for remote or small-scale operations. Variants include different bed shapes and configurations, but the basic principle—crushing ore by a dragged heavy stone in a circular pit—remained consistent.

Limitations include low throughput, significant labor requirements, and inefficiency with hard or finely disseminated ore. As more advanced crushing and grinding technologies became available, arrastras were gradually supplanted, though surviving examples serve as historical artifacts of early mining technology.