73. Stamp Mill / National Mining Museum, Leadville, Colorado

After walking through the excellent National Mining Museum in Leadville, Colorado, two weeks ago, and discovering an exhibit -- a diorama entitled, "Stamp Mills and Amalgamation" -- I wanted to incorporate the vignette of American gold mining into this blog in order to help us better understand the history of Swiss black powder milling as it pertains to the Treuthardt family (which will be described in forthcoming posts).  The display at the National Mining Museum was written exactly as follows:

"Stamp Mills and Amalgamation"

"As the mines went deeper and brought up larger quantities of ore, the arrastras* could no longer keep up.   Businessmen realized that stamp mills would be profitable ventures.   Stamp mills became the primary means of breaking and processing ore, and owing to the introduction of the amalgamation process, were a step forward in gold mining technology.

"At the top level of a stamp mill, larger metal grinders pound the ore into fist-sized chunks.   A step below, the chunks are smashed in water by half-ton stamps.   The resulting slurry passes through screens and drops into amalgamating pans.   How amalgamation works on a physical and chemical level is not clearly understood to this day, but it is known that if mercury is brought into contact with gold, the gold is 'drawn into' the mercury.   This results in an alloy of gold and mercury called amalgam.   After the mercury has gathered in the gold, the mercury can be removed by dissolving it in nitric acid or heating it to a vapor." 

*ARRASTRAS
Here (since we were wondering) is a description of arrastra, below, written by Kenneth Jessen.   For the rest of the interesting article, see this webpage.  

http://www.reporterherald.com/ci_21960868/arrastra-remains-near-buena-vista

"An arrastra, in its simplest terms is a grinder and dates back to the fifth century B.C.   The grinding surface is typically flat bedrock situated near a stream. A vertical pocket is drilled into the rock and a pole is placed in the pocket. Attached to the pole near its base is an arm and attached to the arm, usually by chains, are heavy drag stones. Farther up the pole is a long horizontal beam that is used to turn the center pole.  As the pole is rotated, the heavy drag stone do the grinding against the bedrock surface. Small arrastras could be human powered and draft animals were used to turn larger examples. The most common use in Colorado was to grind ore containing gold flakes. The ore was placed on the grinding surface and after hours the ore is pulverized into a fine powder. A small amount of mercury is added during the process to amalgamate the gold. The gold-mercury amalgam settles to the bedrock while the worthless rock is washed away using water from the nearby stream. The amalgam is then collected and strained through a cloth to remove most of the mercury leaving behind a gold button. The remaining mercury is driven off in a retort, condensed and reused. The gold requires further refining to remove the remaining impurities. Arrastras were used in the absence of large, efficient gold mills or in remote locations. They are very limited in the amount of material that could be processed.

"Arrastras in Colorado are rare. Many have been cut out of the bedrock and moved to museums. One example, still in its original location and relatively easy to reach, is north of Buena Vista on Fourmile Creek. It requires only a short walk from a four-wheel drive road."

"Kenneth Jessen has been a Loveland, Colorado, resident since 1965. He is an author of 18 books and more than 1,300 articles. He was an engineer for Hewlett-Packard for 33 years and now works as a full-time author, lecturer and guide."

(See also my related post #72.)