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How does nitric acid help identify gold content after a fire assay?

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A worker in safety gear walks beside a conveyor belt carrying ore at the Tanami gold mine in the arid, expansive landscape of the Tanami Desert. — gold processing The Tanami gold mine, Northern Territory, Australia.

In the context of a fire assay, nitric acid is used in a process called parting to isolate gold from other precious metals.123

After the fire assay produces a metallic bead containing gold and silver, the bead is boiled in nitric acid. The acid dissolves the silver, converting it into silver nitrate, while leaving the gold as a solid residue. By weighing the remaining gold residue and comparing it to the initial weight of the bead, metallurgists can accurately determine the gold content of the original sample.1243

If the gold content in the bead is too high, it can protect the silver from the acid, preventing complete dissolution. In such cases, additional silver is added to the bead before the parting process—a technique known as inquartation—to ensure the silver content is sufficient for the nitric acid to effectively separate the metals.13