Fire assaying samples in a muffle
furnace at the Lupin mine in Canada
(Credit: Timothy Green)

In fire assaying, or cupellation, a small sample of the gold under test is weighed on a special balance. This is placed with a quantity of lead in a small crucible – or cupel – made of bone ash, which is heated in a muffle furnace with a draught of air flowing over the cupel. The lead and any base metals are oxidized and absorbed into the cupel, while the gold and any silver remain as a small button. The silver is then dissolved out with nitric acid, leaving a pure gold ‘cornet’, which can be weighed and the gold content calculated by comparing with the original weight of the sample. A third method of assaying, also known since antiquity although only accurately applied in modern times, involves Archimedes’ principle. This depends on the fact that the specific gravity of gold is nearly twice that of silver and more than twice that of copper. Thus, if gold is debased with either of these metals, its specific gravity is progressively reduced and its gold content can be approximately calculated. It is not an accurate method for fineness determination. This method is used primarily by museums and numismatists in checking ancient coins from which samples for fire assay cannot be taken. Instrumental techniques such as inductively coupled plasma spectroscopy, glow discharge spectroscopy and X-ray fluorescence spectroscopy are now accepted methods for assaying with varying degrees of accuracy. See also Assay Office, Hallmark, Refining.