Sorry, yes, the copper in the top layer oxidizes with heat. That copper
oxide is what dissolves in the pickle to create the depletion gilding
Good firecoat helps prevent the oxidation, but it's hard to be perfect.
That's why accidental depletion gilding is usually splotchy instead of even
across the piece. Uneven flux, or maybe a hot spot where the flux burns
off, lets more copper oxide form on the surface.
Thanks for catching that