How fire assay measures gold

A carefully weighed sample is mixed with fluxes and lead oxide and melted in a clay crucible at around 1,100 °C. The precious metals dissolve into the lead, which sinks to the bottom.

The rest of the sample becomes a glassy slag that is thrown away.

The lead button is heated in a small porous cup called a cupel. The lead oxidises and soaks into the cupel, leaving a tiny bead of gold and silver.

Silver is dissolved out with acid and the remaining gold is weighed on a microbalance. Comparing its weight with the original sample gives the gold content, accurate to parts per million.

Because it is so reliable, buyers and sellers often agree that a named laboratory's fire assay is final.