But after a 6 months to a year, the copper oxide would bleed
through the fine coating and darken the pieces.
The copper oxides, to the best of my knowledge (which may be wrong
here. Jim Binnion, you know?), don’t tend to migrate much over time.
Larger molecules and all. Metals themselves diffuse, such as silver
or gold diffusing through a plating of the other. And when hot,
unattached oxygen migrates through the metal. But once bonded to the
copper, it pretty much stays put. Instead, I suspect what you’re
finding is simply tarnish, ie the formation of sulphides. Even fine
silver tarnishes, given some time, so copper oxides are not needed
for some darkening to take place. Fire stain itself, the imbedded
copper oxides in an improperly heated piece of sterling, is a faint
reddish to creamy color, not really all that dark. it’s
objectionable in part simply because on a piece of jewelry, it’s
usually blotchy, so it looks bad. However, a fire stained surface
tends to tarnish a bit more quickly in my experience, though I can’t
say I quite know why that would be so. But if you’re silver is fire
stained from the way you build the pieces, it could have that fire
stain right at the surface, but without being polished bright, you
might never know it. Still, it would tend to make the metal tarnish a
bit more quickly, even without any “migration” or diffusion taking
place. If this is the case, then electroplating the pieces with fine
silver will help. But remember that fine silver itself is not immune
to tarnish, so it won’t fully cure the problem.