Pierre Joseph Macquer,
Elements of the Theory and Practice of Chymistry…
“ The operation of the cupel may be considered as the sequel of that process, being intended only to reduce into a scoria whatever is not Gold or Silver. Lead being of all metals that which vitrifies the most easily, which most promotes the vitrification of the rest, and the only one which, when vitrified, penetrates the cupel, and carries along with it the other metals which it hath vitrified, is consequently the fittest for that purpose. ”
