The older gold refining method is cupellation and is considered one of the most elegant metallurgical methods applied on impure gold and is based in the extraction of gold from lead. Basically, the method depends on the exposition of gold and silver in a state of fusion to the action of the air or oxygen. They neither give off perceptible vapors nor sensible oxidized, they remain in short, unaffected. Under similar circumstances, lead and almost all the other metals are oxidized with greater or less rapidity. There is also an important peculiarity connected with lead, which renders it the only metals applicable to the cupellation process. This peculiarity is related to the fact that lead oxide (litharge) is fusible at a bright red heat and in this state absorbs any other metallic oxide, which may be in contact. Without the presence of litharge, the metals would remain unaffected by the temperature of the process.
For instance, if copper is present as an oxide, it is taken up by the melted lead oxide, and if it is separated from the gold and silver, the copper oxide will be expelled along with it. Is has been established that the separation may be performed in several ways, but especially by means of bone ash or magnesite. The first one has the peculiar property of absorbing the melted lead oxide even when mixed with a certain proportion of other oxides, whereas it remains impermeable to the non oxidized metals.
The cupel is formed of pounded and compressed bone ash and into this vessel is introduced a mixture of lead oxide, copper oxide and Dore metal. If the mixture is exposed to a considerable heat, the lead oxide will promote the melting of copper oxide and both will be absorbed by the cupel, while the mixture of gold silver remain on the surface of the cupel melted, but no absorbed. In other words, the Dore metal may be stripped of copper or other metallic oxide by cupelling with lead oxide. For this purpose, it is not necessary to employ the oxide itself in the first instance. Essentially, when metallic lead is heated to a temperature above its melting point, it rapidly reacts with oxygen of the air, the product being in the first place yellow oxide and when the temperature is raised to about bright redness, the oxide is converted into litharge or the fusible oxide that immediately melts.
The porosity of the cupel is the key aspect of the process. Since base melting charge is composed by precious metals and base metals, the diffusion through the cupel is related to the viscosity of the phase. Since some compounds are melted first, they are absorbed easily and the precious metals remain on the cupel surface.