Arrastra

Arrastra diagram 1886

Arrastra (or arrastra) is a simple type of mill used to crush ore and extract gold, silver, and other precious metals. It is a primitive but effective technology that dates back to the Spanish introduction into the Americas during the 16th century. The arrastra's basic design consists of a large, flat circular stone or wooden base, known as the arrastra floor, with a central post for rotation and one or more large stones attached to arms or drag stones that pulverize the ore.

History

The arrastra's history begins with its introduction by the Spanish in Latin America for the purpose of gold mining. It was a technology that was readily adopted by local miners due to its simplicity and low cost of construction. Over time, the arrastra became a widespread tool for ore crushing and extraction across the mining regions of the Americas, with its use persisting even into the 19th and early 20th centuries in some areas.

Design and Operation

An arrastra consists of a circular area paved with river rocks, which serves as the foundation. A horizontal beam, attached to a central post, acts as the pivot for the dragging stones. These stones are usually made of granite or a similar hard rock to maximize efficiency in crushing the ore. The ore, mixed with water, is fed into the center of the arrastra. The drag stones are then pulled around in a circle, crushing and grinding the ore against the bedrock floor of the arrastra.

The process is slow and requires manual labor to operate. The crushed ore is then gathered and further processed to extract the precious metals. Despite its inefficiency and labor intensity, the arrastra's simplicity and low cost made it an attractive option for small-scale miners.

Variations

While the basic design of the arrastra is quite uniform, variations exist in terms of the power source used to rotate the drag stones. Initially, human or animal power was used, with animals walking in a circle to turn the central post. Later adaptations included the use of water wheels or even steam engines to automate the process, significantly increasing the efficiency of the arrastra.

Modern Usage

Today, the arrastra is largely obsolete, replaced by more modern and efficient milling technologies. However, it remains a symbol of the ingenuity of early miners and is preserved in some locations as a historical artifact. In certain remote or small-scale mining operations, variations of the arrastra may still be used, demonstrating its enduring utility.

See Also

• Gold mining
• Silver mining
• Mill (grinding)
• Mining in the Americas

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