While working in the Liberty Mining District of Central Washington sometime around 1975, I watched an old prospector named Leonard run this test for gold on some samples from a mine that he had dug around in for years. This was a long time ago, and I have probably forgotten some of the details. I found the notes that I made on this test while searching through some of my old field notes and decided to share it. I do remember that he did get a little gold button from one of the samples he ran using this test. He got the test from a book titled "Reference Book of Inorganic Chemistry - Commercial Edition", by Latimer and Hildebrand, published by McMillan, 1940. I copied this test from that book and have adapted it for use here.
I haven't tried to actually perform this test since then. Just haven't gotten around to it. If you should try it, please let me know how it turns out.
Grind the ore sample into a powder. Place about 1/4 pound of the powdered ore sample in a ceramic dish with at least 1/4 pound of pure lead and 2 tablespoons of borax powder. Heat until lead melts and oxidizes. This forms an easily fusible flux with the silicates in the ore and the borax. The rest of the lead not consumed in making slag serves to collect the gold and silver in the ore sample.
When the molten mass is allowed to cool, the slag is separated from the gold-silver button.
Separate gold and silver from lead: put the button in a small ceramic crucible (cupel) lined with lots of bone ash. Heat again. Bone ash readily absorbs the liquid lead oxide, and the molten gold and silver form another button in the bottom of the cupel. At this point, the button will consist of pure gold, silver, and platinum, if these metals are present.
Remove the gold-silver button and flatten it with a hammer until a sheet is formed. Do this because you want a large surface area for nitric acid to act on.
Now, in a clean cupel, add nitric acid to dissolve silver out of the gold. Keep working the sheet (with a glass rod, or the like) and adding fresh acid until there is no further reduction in size. Save the nitric acid solution to get the silver out later. What now remains should be either pure gold or a gold-platinum alloy.
This button may be further broken down in a lab using 1.2N CHl and hydroquinone to separate platinum metal from gold. The gold should precipitate out, and the platinum, if any, should remain in solution. At least, that's what I wrote in my ancient notes. I can't find the rest of the how-to for that part.
The silver dissolved in the nitric acid solution may be converted to free silver by simply burning the mixed solution of nitrate and silver at red heat in a cuple crucible. Pure silver should remain and the nitric acid should volatilize off into the air. The remaining silver should be black or silver-black in color.
Revised: 17 February, 2010