arrastra

An arrastra, also known as arrastre, is a simple, primitive mill used for grinding ore to extract metals such as gold and silver. It consists of a circular trough or pit with a smooth bed stone, in which ore is crushed by a heavy drag stone dragged around the circle.

In operation, ore is placed in the shallow pit with water to form a slurry. A heavy drag stone, often shaped like a disk or wheel, is pulled around the circumference of the pit by a mule, horse, or sometimes a human-powered drive. The friction between the drag stone, the bed, and the ore breaks the material into finer particles. The resulting ground ore is then processed further, commonly by amalgamation with mercury or, later, by other mineral-processing methods.

Arrastras were widely used in Spain’s colonial territories and across the Americas, especially in remote mining camps where capital and skilled labor were scarce. They are inexpensive to construct from locally available materials and can operate without complex machinery, making them suitable for small-scale or early-stage mining operations in the 18th and 19th centuries.

The use of arrastras declined with the advent of more efficient milling technologies, such as stamp mills, ball mills, and modern cyanidation and flotation processes, which could process larger quantities of ore more quickly. Today, arrastras are primarily of historical and archaeological interest, with remaining examples serving as reminders of early mining practices in various regions.