View single post by Keith Pashina
 Posted: Fri Feb 17th, 2017 09:20 am
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Keith Pashina


Joined: Sun Nov 4th, 2012
Location: Minneapolis, Minnesota USA
Posts: 781
Ore Processing: Amalgamation Tables and Blankets

A view of two stamps, with amalgamation tables in from of the stamps. I think parts of the mortar boxes have been removed here - normally, a narrow slot with a screen would be in front of the stamp heads
After crushing the ore in the stamp mill, a common practice was to run the mixture of finely crushed ore and water over first amalgamation tables, and then (but not always) blankets.
Amalgamation, at least as used in the Black Hawk mills, refers to the amalgamation of mercury with gold (and sometimes other precious metals).  This basically involved preparing the amalgamation table  and plates. As mentioned before, the stamp mortar box was used for both crushing and amalgamation. The stamp mortar box typically had copper plates at the front and rear of the mortar box. In front of the stamp mill, the amalgamation table was more or less a wooden or metal trough, with copper plates. Mercury would be applied by workers spreading it out with wooden paddles – the mercury would adhere to the copper plate.  Mercury was a consumable item – one source reports about 1/5 ounce of mercury was consumed per ton of ore stamped. Although most of the mercury was recovered and reused, some just “disappeared” out the mill – into stamp wastes or into Clear Creek.
When the slurry mixture of finely crushed ore and water ran across the mercury, the free gold, that is, gold not chemically bound to other minerals, would adhere to the mercury.

This amalgamation table is a display at the Western Museum of Mining in Colorado Springs

After passing over the amalgamation tables, some mills also had “blankets” – these were exactly what you think they were. A typical “blanket” was strips of 18” wide by 36” long strips attached to a sloping wooden table. The slurry ran across the blankets, and some of the amalgam which escaped from the amalgamation table was collected in the blankets. In the Hidden Treasure Mill, these strips were removed and washed of the amalgam every 4 hours. In a model, these would probably look similar to an amalgamation table, although I have never seen a photo of one.
Periodically, the stamp mill would be stopped, and workers would scrape off the amalgam – the mercury and gold mixture from the amalgamation table and mortar box.

An amalgamation table on display at the Argo Tunnel and Mill in Idaho Springs

The amalgam was collected, and then put into a retort oven or pot, where the amalgam was slowly heated until the mercury began to boil off. This was an extremely hazardous process – mercury vapors are poisonous. The sealed-off retort vented into a long tube, which was cooled by water. The water quickly cooled and condensed the mercury, which was collected and reused on the amalgamation tables and stamp mortar boxes.

Inside the retort oven/pot, was a “sponge”  was left inside the retort. The sponge was the gold, but with a spongy like structure, from where mercury had been boiled off. The heat was not sufficient to melt the gold completely. The sponges were collected and locked up, and later melted into bullion bars.

Next, we will look at ore concentrating tables: Gilpin County Bumping Tables, Deister tables, and Frue Vanners.


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