Cupellation exploits a straightforward chemical difference between metals: gold and silver resist oxidizing even at high heat in the presence of air, while lead and most other common base metals oxidize readily under the same conditions. The cupel itself, a shallow dish of bone ash or a similarly porous, absorbent material, is what makes that difference useful: a weighed sample of the alloy, typically gold or silver mixed with lead, is melted on the cupel's surface inside a muffle furnace, where it is exposed to a controlled stream of air.
As the lead oxidizes into litharge, it is drawn straight into the cupel's porous body by capillary action, the way a paper towel draws up a spill, leaving the surface clear rather than pooling as liquid oxide on top. The gold or silver, unable to oxidize under the same conditions, stays behind on the cupel's surface as a small, gleaming, pure metallic button once the process finishes. Weighing that button against the original sample's weight gives an assayer a precise, reliable reading of exactly how much precious metal the original alloy actually contained, which is why cupellation became the trusted standard for judging the purity of coinage and bullion for thousands of years.