Wish I could figure this thing out. Don't really think its
firescale. Does anybody else out there know?
It IS firescale, i.e., copper oxide, and as with firescale on silver
there are a few ways to prevent it and a few ways to remove it after
the fact. Here are some of those ways:
Prevention: Apply a good oxidation-inhibiting flux, use a neutral
flame, and try not to overheat the metal.
Removal: Abrasion or stripping.
Other possibilities: Depletion gild the metal (raise a layer of
fine gold over the firescale by repeated heating and pickling) or
switch to an 18K alloy.