There is another "bright dip" for Sterling and 14 and lower
Karat golds I tried years ago with moderate success for removal
of firescale. The dip consists of 50% nitric acid ADDED TO
WATER. (Sorry to shout there, but acid is always added to water,
not the other way about, to avoid dangerous splatters). The
solution is used cold. (When mixing and using, wear appropriate
eye protection and clothing).
The work is carefully scrubbed with pumice (available at
drugstores) and is fastened to a length of stainess steel wire
(not copper, since the solution will dissolve it after a few
uses). The work is dipped into the solution for a few seconds
(longer if the solution has been used frequently). Almost
immediately areas affected with cupric oxide will turn black
(unaffected areas remain cream or light gray in color). If the
work is left in the solution too long, bubbles will form on the
surface and etching of the silver itself can occur, pitting the
surface and ruining it.
Rinse generously with water, then re-brush with pumice until the
black areas are removed. Re-dip and repeat the process until no
black appears and the entire surface is a uniform light gray.