Depletion gilding does not remove fire scale, it only covers it up
as long as the layer of fine silver is not removed in polishing.
That's exactly right. Heating the metal causes the formation of two
copper oxides: cupric oxide and cuprous oxide. I can never remember
which is which but one of them forms the discoloration or black stuff
on the surface of the silver (or gold) and is removed by the pickle.
The other lurks underneath the depletion gilded (fine silver or fine
gold) surface as fire scale (and can only be removed by abrasion or
stripping if you want to get rid of it).
If the metal has been fluxed and not overheated, the surface oxide
is removed by the pickle leaving the original alloy (sterling silver
or 14K gold, for instance). If the metal was not fluxed and was
sufficiently heated (usually more than once), you can get a lovely,
even coat of pure metal covering the fire scale. But, if the metal
was fluxed and overheated or heated unevenly, you'll end up with
patches of firescale marring the surface of the original alloy and
possibly gilded patches as well.
Rachelle Thieves developed a wonderful technique to heat-patinate
sterling silver by taking advantage of this process. Briefly
described, it involves depletion gilding the sterling, then abrading
away the fine silver surface in selected areas only in order to get
back down to the sterling. Next step is to heat the metal one last
time so that the abraded areas turn black but DO NOT put the metal in
pickle. If done right, the result is a deep black pattern on a
bright white background (although the "background" is actually the
foreground). There are some tricks to this process which I don't
remember offhand but, if anyone's interested, I'll go dig out my
workshop notes and pass them on.