Description

Water Wheel, Big Bug, Yavapai County, Arizona Territory. An Arrastra for gold ore, used in an "early day," I.e. in 1865 or '66.

Separating valuable metal from the waste rock was seldom an easy task. The earliest and simplest method of crushing ore was the use of arrastras. An arrastra was the most primitive method used to crush ore into a fine enough powder so that precious ores could be separated from host rock. Arrastras (also “rastra” or “arastra”) had their origins in Mexico. They were the earliest type and used during the California Gold Rush in the 1850s. These devices consisted of a flat track of stone inside a low wall of rock or wood, a retaining basin to capture and hold the ore, and a center post and horizontal shaft made of wood or from small tree trunks. The ease by which they were built and operated made them ideally suited to small mining operations. Powered by horse or mule, the shaft would drag one or more large stones in a circular path on top of the flat stone base, crushing the ore that has been placed in the basin. Mercury was added to the resulting mix to help capture the gold as it was separated from the pulverized rock. When water was plentiful, flumes and water wheels were often used instead of animal-power. More elaborate arrastra mills that were often housed in a building and powered by water from flumes or even steam power. Arrastras were replaced by much larger and more complex milling methods and machinery and stamp mills replaced arrastras at most mines when adequate capital was available.