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Milling the Ore

by Todd Underwood

When a full size stamp mill was not available, arrastras were used to crush the ore. Arrastras were small circular flat areas of land usually about 10-20 feet in diameter with a pole in the center.

Early Arrastra

Attached to the pole was a rod or line running out to a large heavy wheel. A horse or mule was usually attached to the end of the wheel area and would walk around in circles. As the animal walked, the heavy wheel would crush the ore underneath it. Arrastras were a crude way to crush large pieces of rock into much smaller and more manageable sized bits. In later years, iron arrastras (pictured on the right) replaced the wheel method.

Stamp Mills
Stamp Mills were far more advanced than the early arrastras although they both performed the same function. Stamp mills ranged from one stamp on up to twenty or even fifty stamps all operating together.

5 Stamp Mill

Each stamp was a large piece of solid iron or other metals attached to a long shaft. These shafts were usually attached to a cam with the other stamps if there were more than one. This cam had usually had a wheel on its end that was driven by a belt system. A steam engine was usually used to turn this wheel which lifted the stamps and dropped them with all their weight on the rocks that were to be crushed. Stamp mills would run 24 hours a day and as one can imagine, are extremely

Small 3 Stamp Mill

loud. They also tend to shake the ground as they are dropped which can make it feel like there is a continuous earthquake.


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5 Stamp Mill

Close Up of stamps on a 2 Stamp Mill

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