This is taken from “Basement Chemistry for the Prospector” by Dr. A.
K. Williams, Ph. D
Furnace smelting is usually carried out using a crucible made of
graphite (acrystal form of carbon). Of course I don’t have to tell a
basement chemist why we prefer to use graphite, but for any casual
visitor I should explain that at high temperatures the graphite
(carbon) becomes a reducing agent that helps keep gold and platinum
metals in their reduced or metallic forms. Some silica sand or
ground glass is usually added so that there will be aglass matrix
that floats on top of the metal. Some sodium nitrate (Chilean
nitrate or Saltpeter) is added. This nitrate is a rather strong
oxidizing agent. When hot it will oxidize almost any metal, except
for gold and the platinum metals, to its nitrate salt. These salts
combine with the molten sand. Usually some borax is added to thin
the viscous, molten glass.
This mixture is heated until the melt becomes “quiet” with no
bubbles, foam, or lumps in it. The precious metals are now poured
into a mold and the glass or other “gangue” is removed. The black
sand that was in our melt was oxidized to iron nitrate by the sodium
nitrate and is now dissolved in the glassthat we discard. Your gold,
at his point will not be 100% pure. It probablycontains small
amounts of copper, silver, tellurium, etc. The good news is that it
looks like gold and should be of fairly high quality. Plenty good
enough to sell.
I use potassium Nitrate as my oxidizing agent, it does not take very
much, plus Borax Glass from A&B prospecting supply. I also use
standard silica claycrucibles.