I used to make a line of earrings which were assemblies of fused
sterling scrap - and as I ran out of scrap I’d make up components out
of sheet and wire. Fusing was done on a fire brick after laying out
the pieces carefully with tweezers to get a pair to sort of match,
with 3 or 4 picklings and wire brushing to raise a fine silver
coating. But after a 6 months to a year, the copper oxide would bleed
through the fine coating and darken the pieces. I have a rectifier
and am thinking about firing it up and seeing if a thicker fine
silver plating would retard, or even stop the migration of oxides to
the surface. Anyone out there have an experience or opinions?
But after a 6 months to a year, the copper oxide would bleed
through the fine coating and darken the pieces.
The copper oxides, to the best of my knowledge (which may be wrong
here. Jim Binnion, you know?), don’t tend to migrate much over time.
Larger molecules and all. Metals themselves diffuse, such as silver
or gold diffusing through a plating of the other. And when hot,
unattached oxygen migrates through the metal. But once bonded to the
copper, it pretty much stays put. Instead, I suspect what you’re
finding is simply tarnish, ie the formation of sulphides. Even fine
silver tarnishes, given some time, so copper oxides are not needed
for some darkening to take place. Fire stain itself, the imbedded
copper oxides in an improperly heated piece of sterling, is a faint
reddish to creamy color, not really all that dark. it’s
objectionable in part simply because on a piece of jewelry, it’s
usually blotchy, so it looks bad. However, a fire stained surface
tends to tarnish a bit more quickly in my experience, though I can’t
say I quite know why that would be so. But if you’re silver is fire
stained from the way you build the pieces, it could have that fire
stain right at the surface, but without being polished bright, you
might never know it. Still, it would tend to make the metal tarnish a
bit more quickly, even without any “migration” or diffusion taking
place. If this is the case, then electroplating the pieces with fine
silver will help. But remember that fine silver itself is not immune
to tarnish, so it won’t fully cure the problem.
Your oxides are not migrating, they are forming on the surface. When
you enhance the silver content of the surface by pickling and
brushing that “extra” silver content is actually standing proud of
the normal surface and has a larger surface area to contact with the
air and hence oxidise. The normal way of making the best of the
silver enriched surface is to burnish it to stop oxygen getting to
the normal metal underneath. Plating will give you a dendritic
coating on your silver but wont cure the problem, just give you a
slighlty dull silver finish because the silver will preferentially
attach to your raised whiskers still leaving the copper still exposed
to the atmosphere.
Your oxides are not migrating, they are forming on the surface.
When you enhance the silver content of the surface by pickling and
brushing that "extra" silver content is actually standing proud of
the normal surface and has a larger surface area to contact with
the air and hence oxidise.
It is not standing proud of the original surface, it is the original
surface. The process of depletion gilding or “raising up the fine
silver” is done by oxidizing the copper in the surface of the
sterling piece by heating and then dissolving the oxide by pickling.
The copper depleted surface becomes matte in appearance as the
dissolving of the copper oxide leaves a pit behind wher it once was.
This is why one often uses a brass brush to burnish the surface
between courses of depletion of the copper. Also silver does not
easily form stable oxides without a moderately complicated chemical
reaction. What he is seeing is the formation of silver and possibly
copper sulfide aka tarnish from the sulfur compounds in the air not
silver oxide.
The copper oxides, to the best of my knowledge (which may be wrong
here. Jim Binnion, you know?), don't tend to migrate much over
time. Larger molecules and all.
Peter, I am in agreement with you here.
Metals themselves diffuse, such as silver or gold diffusing through
a plating of the other. And when hot, unattached oxygen migrates
through the metal. But once bonded to the copper, it pretty much
stays put. Instead, I suspect what you're finding is simply tarnish
I didn’t say original surface and didnt intent to imply either. By
normal I meant compositionally. I should have said “react” rather
than oxidise. Easier to explain with sketches than just words. The
technique was used by the Romans on debased silver coinage where the
silver content was as low as 25%.
Nick, thanks for the explanation. I’ll try some experiments: tumbling
with stainless steel shot, pickling before, pickling after. And maybe
ss shot would leave a less 3-D surface upon which plating would leave
a more pleasing result. More abrasive tumbling I tried years ago, and
was not pleased with the resulting texture, liking the uniform matte
finish after repeated pickling. (I tell buyers they are ‘torch fused
and torch textured’, whatever that means.) Many patrons at shows were
uncertain as to the metal, the only pieces in my booth not polished
to a high luster.
Peter, thanks for the insights on surface metalurgy. That the CuO2 is
not ‘migrating’ give me hope a solution may be found, short of
dipping them in lacquer or something to keep the oxegen away. A
single earring is comprised of 15 - 20 individual pieces of all
shapes, sometimes silver sawdust too, maybe covering a square inch,
and I try to keep them around 3 grams - so making them out of fine
silver doesn’t yield a result with any resistance to bending. I never
bought enough argentium alloy to play with (I just used it for bezel
cup backing for translucent cabs), and with the current price of
silver, this option is iffy. I will run a test with hot salt water
and aluminum foil and see if the darkening is tarnish/sulphides.
Thanks for the advice.
I didn't say original surface and didnt intent to imply either. By
normal I meant compositionally.
Ok I see, I read it to mean original.
I should have said "react" rather than oxidise. Easier to explain
with sketches than just words. The technique was used by the Romans
on debased silver coinage where the silver content was as low as
25%.
Never has been a shortage of scoundrels has there Most metal
working cultures have some variation on the depletion gilding trick.
Most metal