From the 7th to the 1st century BC, the ancient Greeks produced some 1,750, 000 tons of silver-bearing lead, with a 2 kg/t silver content. The cupellation process gave 3,500,000 kg of silver and 1, 750,000 tons of litharge. Later, when melting this litharge in fusion ovens, they produced 1,400,000 tons of commercial-grade lead. This work describes the cupellation process they used. It is admirable, given the circumstances of the time. It was carried out in two stages, in two ovens. In the first phase, the silver content obtained was 25 kg of silver per ton of lead, and the litharge reached a maximum silver content of 300 grams per ton. The final extraction of silver was carried out during the second cupellation. The ancient Greeks had invented a special process to remove the litharge, called here the submersion of cold iron rods process. In addition to the data and reasoning used by the ancients, the author presents the mathematic calculation involved in the evolution of the cupellation treatment in general. This calculation is based on measurements and observations done on silver content of the litharge in relation to the silver content of the silver-bearing lead in the bath in the cupel of the cupellation oven.