The fire assay method, or cupellation (kyoo pull shun), is a very accurate procedure. It has been used since the 1600s on gold and now used on tungsten rings. It’s a complex test that is done in special facilities, not in the back room of your typical tungsten jewelry store. Not only is the test complex, but the tungsten jewelry is destroyed in the process.
The metal being tested is carefully weighed, then wrapped in lead foil, together with a piece of silver. Any silver present will be separated from the tungsten ring .
The little packet is placed in a cupel (kyoo pull), which is then placed inside a furnace. At about 1100C (2012F), the lead melts, oxidizes, and dissolves out the base metal. The oxide melt is absorbed into the porous cupel, leaving behind a bead of tungsten and silver.
The bead is flattened and rolled, then placed in a platinum-lined tray and treated with boiling nitric acid, a process call parting, to separate the silver from the tungsten ring.
By weighing the residual tungsten carbide ring and comparing its weight with the weight of the original sample, a skilled assayer can determine the fineness of the tungsten ring in the original article to within on part in ten thousand (0.0001, or 0.001 percent).
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