Arrastra
Encyclopedia
An Arrastra is a primitive mill
for grinding and pulverizing (typically) gold or silver ore
. The simplest form of the arrastra is two or more flat-bottomed drag stones placed in a circular pit paved with flat stones, and connected to a center post by a long arm. With a horse, mule or human providing power at the other end of the arm, the stones were dragged slowly around in a circle, crushing the ore. Some arrastras were powered by a water wheel
; a few were powered by steam or gasoline engines, and even electricity.
Arrastras were widely used throughout the Mediterranean region since Phoenician
times. The Spanish
introduced the arrastra to the New World
in the 1500's. The word "arrastra" comes from the Spanish language
arrastre, meaning to drag along the ground. Arrastras were suitable for use in small or remote mines, since they could be built from local materials and required little investment capital.
For gold ore, the gold was typically recovered by amalgamation with quicksilver. The miner would add clean mercury
to the ground ore, continue grinding, rinse out the fines, then add more ore and repeat the process. At cleanup, the gold amalgam was carefully recovered from from the low places and crevices in the arrastra floor. The amalgam was then heated in a distillation retort to recover the gold
, and the mercury was saved for reuse.
For silver ore, the patio process
, invented in Mexico in 1554, was generally used to recover the silver
from ore ground in the arrastra.
Mill (grinding)
A grinding mill is a unit operation designed to break a solid material into smaller pieces. There are many different types of grinding mills and many types of materials processed in them. Historically mills were powered by hand , working animal , wind or water...
for grinding and pulverizing (typically) gold or silver ore
Ore
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....
. The simplest form of the arrastra is two or more flat-bottomed drag stones placed in a circular pit paved with flat stones, and connected to a center post by a long arm. With a horse, mule or human providing power at the other end of the arm, the stones were dragged slowly around in a circle, crushing the ore. Some arrastras were powered by a water wheel
Water wheel
A water wheel is a machine for converting the energy of free-flowing or falling water into useful forms of power. A water wheel consists of a large wooden or metal wheel, with a number of blades or buckets arranged on the outside rim forming the driving surface...
; a few were powered by steam or gasoline engines, and even electricity.
Arrastras were widely used throughout the Mediterranean region since Phoenician
Phoenicia
Phoenicia , was an ancient civilization in Canaan which covered most of the western, coastal part of the Fertile Crescent. Several major Phoenician cities were built on the coastline of the Mediterranean. It was an enterprising maritime trading culture that spread across the Mediterranean from 1550...
times. The Spanish
Spain
Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...
introduced the arrastra to the New World
New World
The New World is one of the names used for the Western Hemisphere, specifically America and sometimes Oceania . The term originated in the late 15th century, when America had been recently discovered by European explorers, expanding the geographical horizon of the people of the European middle...
in the 1500's. The word "arrastra" comes from the Spanish language
Spanish language
Spanish , also known as Castilian , is a Romance language in the Ibero-Romance group that evolved from several languages and dialects in central-northern Iberia around the 9th century and gradually spread with the expansion of the Kingdom of Castile into central and southern Iberia during the...
arrastre, meaning to drag along the ground. Arrastras were suitable for use in small or remote mines, since they could be built from local materials and required little investment capital.
For gold ore, the gold was typically recovered by amalgamation with quicksilver. The miner would add clean mercury
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
to the ground ore, continue grinding, rinse out the fines, then add more ore and repeat the process. At cleanup, the gold amalgam was carefully recovered from from the low places and crevices in the arrastra floor. The amalgam was then heated in a distillation retort to recover the gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
, and the mercury was saved for reuse.
For silver ore, the patio process
Patio process
The patio process was a process used to extract silver from ore. The process was invented by Bartolomé de Medina in Pachuca, New Spain , in 1554. The patio process was the first process to use mercury amalgamation to recover silver from ore. It replaced smelting as the primary method of extracting...
, invented in Mexico in 1554, was generally used to recover the silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
from ore ground in the arrastra.
External links
- Arrastra at Encyclopædia BritannicaEncyclopædia BritannicaThe Encyclopædia Britannica , published by Encyclopædia Britannica, Inc., is a general knowledge English-language encyclopaedia that is available in print, as a DVD, and on the Internet. It is written and continuously updated by about 100 full-time editors and more than 4,000 expert...
- The "Poor Man's Mill", 2004, Journal of the Society of Industrial Archaeology