I actually did this recently by mistake, so this is my experience
only. I’d never read anything about it, it just happened! I had been
working on experimenting with fusing a ring using some extra 14k
peach gold from some sprues. Whenever I have been trying to fuse 14k
gold, I use flux. So this was my process - flux, fuse, pickle,
forge, anneal, forge, anneal, forge, little extra flux to fuse
again, pickle, forge… some more of the above - then then last
round of flux - fuse and pickle, I took it out and it had turned
yellow - no longer had the peachy glow. it was really just all of a
sudden and I was pretty shocked by it at first.
That was my process, and if I repeated those, I don’t know for sure
if it would work again, but I tend to think it would. As far as
bringing the copper up, are you sure your pickle is fresh and
untainted? I have found that if my pickle is even slightly off the
14k copper plates before my sterling does.
Hope that helps!