Cupellation kick-started the Bronze Age and is still being used in refineries around the world. Even here at MGS, we utilize cupellation in our fire assays to ensure the most accurate results.
Native gold and silver are very rare – they are usually found blended into lead ores. The cupellation process is one of the best ways to extract these precious metals. It starts with the construction of a cupel, a small bowl that will contain the alloyed metals throughout the process. The cupel must be made of porous, calcified material so it can absorb lead without causing a reaction.
The alloy is placed in the cupel and heated. Lead’s melting point is much lower than that of precious metals. This, coupled with the fact that precious metals do not readily oxidize, is what makes cupellation so effective. Hot air is blown across the cupel, which causes the lead to oxidize. The newly formed lead oxide, or litharge, is absorbed by the cupel, leaving only precious metals behind.
Cupellation produces extremely accurate assay results because it fully separates gold from impurities. However, it takes much longer than other assay methods. For these reasons, it’s best reserved for larger lots of precious metals.