Hi Lolly (my aunt’s name!),
Is there a chance it's copper-plated...
Yes, in a way. What you’re seeing is firestain, the same stuff
that’s such a pervading problem when soldering ordinary sterling
silver. And just as with sterling, there are two main ways to get rid
of it: abrade it off or depletion gild it. I do the latter which
involves raising the fine gold content of the alloy to the surface.
What you’ll need to do is heat the entire piece (several times) and
pickle after each heating. The first time you heat it, you’ll see
that the gold areas turn black while the coppery areas do not. With
each successive heating, more and more of the piece will turn black
(or deep brown, which is fine). When the entire piece acquires an
even black/brown appearance when heated, you’re done: the piece
should then be evenly gilded (“plated”) with fine gold (and will look
yellower than the original material did!).
A few tips: Don’t overheat! Use an oxidizing flame (not one with a
tight blue cone). Keep the flame moving over the piece so you don’t
overheat (and unsolder) anything. Fan the flame on and off the piece
occasionally to better see the development of the black oxides. If
you can, try this first with a test piece of the two alloys you’re
using so you can see the differences between them.
Finally, understand that the depletion-gilt, fine gold surface is
not as durable as the original yellow alloy. This technique would be
questionable for a ring which is subject to a lot of friction, for
instance. But I’ve never had a problem with pendants, pins or