Cupels

 Cupels are magnesia or bone ash cup shaped containers used in assaying to separate precious metals from base elements such as lead.  They operate as a “differential filter” and are porous to metal oxides, but not metals.  Used in an oxidizing atmosphere, base metals are oxidized and are absorbed into the cupel.  Since precious metals do not oxidize, they are not absorbed and are left on top of the cupel for weighing or further analysis.

The lead button for cupellation is typically generated in the crucible fusion from the fire assay process, but can also be produced by a scorification assay, or a bullion assay where the precious metal is wrapped in lead foil.

Typical cupellation furnace temperatures in gold and silver assays are 1650-1700°F, but can vary with technique, air flow volume, and cupel material.