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How does fire assay measure the amount of gold in an ore sample?

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A diagram showing the periodic table entry for gold, with the symbol Au, atomic number 79, atomic weight 196.96657, electron configuration, and labels for its properties as a solid transition metal with a face-centered cubic crystal structure. — gold The precious metal gold is a chemical element with the symbol Au. Its square on the periodic table details some of its properties, including its atomic weight, atomic number, and crystal structure.

Fire assay is a reliable chemical analysis technique used to determine the concentration of precious metals in ore samples through a six-step process.12

Sampling: A representative portion of the ore is collected.1
Fusion: The sample is melted in a crucible with fluxes and lead oxide. The lead collects the precious metals and settles as a metallic "button" at the bottom, while the slag is discarded.12
Cupellation: The button is heated in an oxidizing atmosphere to remove the lead, leaving behind a bead of gold and silver.12
Weighing: The bead is weighed to determine the combined mass of the precious metals.1
Parting: The bead is treated with nitric acid to dissolve the silver. If the gold content is too high, silver is added beforehand—a process called inquartation—to ensure complete separation.132
Weighing: The remaining gold residue is weighed to calculate the final gold content.12

This method is preferred for ores because it allows for the use of large samples, which is necessary due to the random distribution of precious metal particles.1