In addition to trying to prevent the fire stain from occurring, as
others have noted, there is a way to deal with it after-the-fact by
depletion gilding the piece. This involves heating to about annealing
temperature, until the piece turns an even black, and then quenching
it in pickle to remove the black copper oxide. This procedure leaves
a layer of pure gold on the surface, covering the fire stain below.
The same procedure is used to raise fine silver to the surface of
If you repeat the process a few times, you'll build up a thicker
layer of fine gold. This is important because the surface layer
(like plating) can wear off, which makes this an unsuitable procedure
for rings or other items subject to constant friction. I do this
extensively and successfully with earrings, pins and pendants.