Gold miners of the 19th and early 20th century had a processing advantage over todays gold miners despite all of their modern diesel powered trommels, pumps, and sluices. The early placering miners had access to mercury or quicksilver. Auriferous fines could be concentrated is a small container with water and a few ounces of mercury would be added to extract the gold as an amalgam. Or, the concentrates could be contacted with mercury-coated copper for the same effect. Copper coated pans or flat plates were often used to scavenge gold and isolate it as the amalgam.
Today, the use of mercury is strictly forbidden in mining operations around the world. But there was a time when mercury was a key part of the miners toolkit. Many extraction schemes were developed to concentrate gold into a small package. Panning or the use of a shaker table would provide nuggets and dust as concentrate. But often there was heavy black sand comingled with the gold dust. Isolation by amalgamation followed by distilling off the mercury (retorting) would provide moderately pure gold.
For example, a simple retort may be made from a pipe nipple with a cap on the bottom and a top connector attached to a long condenser tube that could be cooled with stream water. The retort vessel was set into a campfire and perhaps a cloth was wrapped around the condenser tube and wetted to knock down the mercury vapors so that they could be collected in a receiver.
Curiously, there is lore about the potato retort. My source is Eldred D. Wilson, Gold Placers and Placering in Arizona, Bulletin 168 (1961), State of Arizona, Bureau of Geology and Mineral Technology, Geological Survey Branch. In the potato retort method, a potato is cut in half and one half is hollowed out enough to accomodate an ounce or so of amalgam. The auriferous spud is wired back together and placed in the ashes of the campfire for 30-60 minutes until done. The potato is then opened to reveal a gold button in the middle. Or, so the story goes.
There were variations. Analogous to the preparation of hoe cake, digging implements were put to use in retorting duty. A potato amalgam package could be placed in a frying pan or in a shovel which would then sit in the campfire.
It’s hard to say just how effective potato retorting was compared to other methods. Admittedly, I have trouble believing that the internal temperature of the potato was high enough to do the job. It’s conceivable, perhaps, that enough Hg was cooked away to leave behind a metallic mass with some gold color. It would be interesting to try this and then get an assay.
Wilson offers this advice- don’t eat the potato.