During cupellation, the lead is oxidized to litharge (PbO), most of which is absorbed by the cupel. Gold, silver, platinum and other precious metals form a bead in the cupel. The bead is subsequently treated to determine its precious metal content. Cupels are typically made from magnesia MgO ( sometimes mistakenly referred to as magnesite), bone ash, Portland cement and other materials. Typical cupellation furnace temperatures in gold and silver assays are 900-1000°C, but can vary with technique, air flow volume and cupel material. Cupellation is the last and most important step of the fire assaying process and as such, the quality of the cupels is extremely important.

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