Judy,
I'd love to hear someone who works with silver say that they have
tried bright-dipping and they still got the firescale when
finishing their jewelry piece.
your bright dip methods, as you say useful with brass, don’t work
for fire STAIN, which is the name used for the penetrating red oxides
of copper that are found under the layer of black oxides. A bright
dip will remove oxides from the surface, yes, but so would ordinary
pickle. The problem is that, unlike what you deal with in brass, the
red oxides penetrate into the metal, sometimes to a significant
distance. Bright dipping removes it from the outer surface, but the
acids cannot affect what’s under the surface.
Again, fire scale is not the same as fire stain. Close relatives,
but the first, the black surface oxides, are easily removed by
pickling, without even needing peroxide. The reddish oxide of copper
is almost impossible to remove without removing the whole layer it’s
in, silver and all, simply because you cannot reach it chemically.
It’s in the metal, not on it. This happens because with silver, when
heated, oxygen is able to not only react with the surface, but it
penetrates into the silver, and can oxidize copper that’s within the
crystal structure near the surface too. That shows up as a sort of
“creamy” slightly pinkish toned shadow on the metal when you polish,
usually only easily visible when you get to the rouge stage, where if
you’ve cut through that layer in some places with the buff, and not
in others, it creates a really blotchy look. Bright dipping helps
slightly, but then you have to repolish, and the problem reappears,
because the acid couldn’t get below the actual surface. You can use a
more aggressive bright dip, an actual etch, to remove enough of the
surface to get below the fire stain, but this is difficult to
control, and still leaves you with a surface that then needs a good
deal of polishing.
The main solutions to fire stain are either to plan on completely
removing all of it with aggressive buffing, which can be destructive
to good work), or by carefully preserving that layer intact, so the
whole surface is that color. Or better, proper use of anti fire stain
fluxes, like Prips flux or Cupronil, prevent it’s formation in the
first place, which is much easier than dealing with it. Or the
easiest modern solution, the use of different silver alloys, like
Argentium, which simply don’t form this subsurface oxide layer. Fire
stain problems were the entire driving force behind the development
of Argentium and similar alloys. If fire stain were as easy to deal
with as a simple bright dip, nobody would ever have bothered to
develop Aregntium.
Your peroxide pickle, by the way, is not fixing your brass by
removing red oxides of copper, it’s removing an actual surface layer
of metallic copper. The red color is because your heating has removed
the zinc from the surface layer of brass, leaving a slightly
porous/dull copper rich film, which is then reddish. If you pickled
it, the copper oxides on the surface were removed, but that red
copper layer stays. The peroxide enhanced pickle gets that copper
layer off, so you’re then back to the brass color. This is not the
same as fire stain. Perhaps the references to it as being caused by
the red oxides of copper are confusing. That’s true, but fire stain
is diffuse enough in the silver that the color is not actually red.
More a faintly warm or pinkish creamy colored shadow, distinct from
the clearer, brighter, darker polish you get on clean sterling
silver.
And yes, back to your “I’d love to” statement. I did try all sorts
of acids and bright dips at one point, long ago, just to see, since
Oppi Untracht also mentions bright dipping as a possible solution.
but I never got anything that approached a good result unless I let
the acid dissolve enough of the silver to actually get all the way
through the whole layer, copper oxide and silver both. That wasn’t a
good solution at all.
Peter
