Hopefully, that’s gold foil, not leaf. There’s a huge difference
in thickness. I bought gold leaf (elsewhere) one time & tried it in
enamels, and it’s not suitable. It’s designed for book pages, picture
frames, etc. When I used it in enamels, it was completely dull, even
under a clear enamel.
That said, once you fired down the gold layer, did you then cover it
with a clear or transparent coat of enamel? I have found any
discoloration during the gold firing stage magically disappears once
I have the next enamel layer on. Precious metal foils are not
designed to be part of the top coat…too easily abraded.
If this is not your problem, then I haven’t a clue as to what is