The basic standard method
of assaying gold ore, bars
or scrap by placing a small sample
of the gold under test, which has been weighed on a special balance, with a quantity
of lead in a small cup or cupel made of bone ash. This is heated in a muffle furnace
with a draught of air flowing over the cupel. The lead and any base metals are
oxidised and absorbed into the cupel, while the gold and any silver remain as
a small metallic 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. The fire assay technique is the most accurate method for
gold assay and is the standard reference technique
against which other instrumental techniques are compared. Also known as
cupellation.