My inorganic textbook says
Cupellation is a process, where crude gold is taken in a small bowl called cupell and melted in the presence of air. Due to high oxygen affinity of Pb, it is converted to PbO(volatile) and escapes from the system
Now, the affinity for sulphur and lead is explained by HSAB(soft acid-soft base). But how to account for the affinity of oxygen (hard base)and lead (soft acid) ?