It is important to pre-heat the cupel at beginning of the cupellation process. This operation is performed in order to avoid the thermal shock of the cupel. The original procedure comprised the mixture of Dore metal with metallic lead and the charge had to be melted before charging the cupel. Once the cupel is loaded the oxidation process starts and the temperature is maintained at 1050-1100 oC with presence of air to oxidize the charge. At first lead becomes covered on the surface with a yellow or grayish dross, but on further raising the heat, the surface of the bath uncovers, and a film of melted litharge begins to appear. Part of this litharge sinks into the cupel.
The current of air introduced not only assist in clearing away the litharge, but supplies the amount of oxygen required for its rapid formation. In proportion as litharge flows away, more of the melted lead is gradually supplied to the charge, which is done by ladling it into a channel from the pot outside. After certain period, the operation is repeated is required. In dealing with a Dore with high content of copper, the refining is performed by first melting the Dore in a wind furnace in crucibles and ladling it into the previously heated cupel and adding molten lead in sufficient proportion to carry all the copper away. The proportions of lead necessary to purify the Dore are variable. Basically, 15 parts of lead must be used when the copper content is approximately 0.05-0.10 % of copper.
At the end of the operation some striking appearances are presented. For example, if the gold present is mixed with a considerable proportion of silver, the surface of the mass, as it cools, is suddenly thrown into agitation, cones or little craters are thrown up, from which oxygen gas escapes with violence projecting particles of silver with force. This phenomenon is called spitting or vegetation due to the arborescent forms which are presented, and arises from the circumstance that metallic silver when is melted has the property of absorbing six times its volume of oxygen, which is ejected with violence at the moment of solidification. After cupellation, the metal is poured into moulds. Platinum group metals remain with gold and silver until the final product of the cupellation process.