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Why is lead oxide used in a fire assay for gold?

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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 fire assay process, lead oxide—commonly known as litharge—is used as a critical agent during the fusion stage to collect precious metals from an ore sample.1

When the sample is melted with fluxes and a reducing agent (such as flour), the lead oxide is reduced to form fine droplets of metallic lead dispersed throughout the molten mixture. These lead droplets act as a solvent, effectively dissolving and capturing gold, silver, and platinum-group metals. As the mixture settles, these droplets coalesce and descend through the molten charge to form a metallic "button" at the bottom of the crucible, separating the precious metals from the waste slag.12

Following this, the lead button is subjected to cupellation, where it is heated in an oxidizing atmosphere. This process oxidizes and eliminates the lead, leaving behind a bead of concentrated precious metals for further analysis.321