Carol made a piece in her jewelry class from sterling silver, soldered onto
a small piece of copper sheet.  Unfortunately, the silver accidently became
plated with copper when she put it in the pickle after soldering.  (Her
"Complete Metalsmith" book explains why this happened - copper ions get in
the pickle and then if someone accidently gets steel in there, it becomes a
copper-plating solution.)

So Carol would like to have a method for getting the copper plating off her
silver, without damaging the copper plate it's soldered to.  Preferably
low-tech and using household chemicals.  Can anyone suggest anything?

12 responses total.

I presume you mean the copper sheet was touching the steel. Copper will
not spontaneously deposit on silver - it will occur the other way around,
as copper is more electropositive than silver. However if the copper
sheet (with silver attached to it) is in touch with steel in the bath,
it forms a shorted battery, with oxidation at the steel (dissolving it)
and depositing copper upon the piece. You *might* be able to reverse this
by reversing the current: attach the piece to the + terminal and
the steel to the - terminal of, say 4.5 volts volts. (You should get
hydrogen at the - terminal and etch the piece a little.)

Do you mean to do that in some acid solution, Rane?  Is that the best way
you can think of to remove the copper?

Yes, in an acid solution. But apply the voltage first and then dip it
for a moment to observe the consequences. Without the voltage, and with
copper in the solution, you will redeposit copper on the silver (if
the piece is touching steel). 

Silver is somewhat similar to copper in its reactions, but it is less
reactive. This is why a simple chemical way to remove the silver coat may
be difficult to find. Certainly a quick dip in concentrated nitric acid
will remove a layer of both copper and silver (and produce some nasty
brown gnitric oxide as - and the nitric acid is quite hazardous).  This is
probably what I would do - but I'm a chemist, and have handled chemicals
like these (I wanted some carbon rods recently, so stripped the copper off
carbon welding rods with nitric acid - outside).

You can also strip off some silver with ammonia or cyanide, but both
require time to act, and will be working on the copper too. Both are nasty
stuff too, cyanide the nastiest. 

Try the electrochemical method first - with the voltage applied you
will not deposit more copper on the silver. 

STeve says to use ferric chloride.  He remembers getting it at Radio Shack.
It's used in the computer industry to remove copper shielding from PC boards.

Good thought. It will indeed attack the copper, but will also attack the
silver. However it is safer to use than nitric acid. 

A little checking  on properties of copper and silver suggests that
just dipping in strong ammonia might work! Again, both copper and silver
will be attacked (though very slowly), but copper is more reactive.
It is also a "household chemical" you probably have. It would  be much
less aggressive than nitric acid or ferric chloride, so it would not
happen very rapidly and you can watch the process. (Do this outside
too, as the fumes of strong ammonia are pretty potent!)

OK, thanks Rane and Glenda.  Carol says she'll try the ammonia frst and see
if that works.  We'll let you know.

If the copper is being dissolved, you will see a deep blue color. This
is a copper-ammonia complex that can be used to test for very low
concentrations of copper.

What is the green color that copper roofs eventually turn, after brown?

Carol's report:

I left the piece in full-strength clear ammonia for about 15 minutes and
then rubbed it with a toothbrush. Nothing happened--no blue, no polishing.

Reading response #1 again, however, I wonder if my assumption about why
this happened is even correct. I can't imagine that steel actually touched
the piece (unless someone dropped the steel tongs into the pickle, which
seems unlikely). There's also the fact that the bezel, which is also silver
and also soldered onto the copper, wasn't affected. In fact, the part of
the silver wire that wasn't touching the copper wasn't even affected. Could
there be some other explanation for why my silver wire is now
copper-colored?

All that does present a problem for all theories so far - especially that
silver bezel that was also soldered to the copper (new data often cause
problems....).

Copper is more reactive in acid than silver (as was stated somwhere
above).  So with them attached, even that forms a 'battery', but with the
low 0.42 volts. Nevertheless, it is possible that even this will cause the
copper to 'flash' on the silver to make essentially an atomic layer, when
the silver surface will 'look' like copper, and the reaction would cease.
But that should apply to the bezel too... 

Since that didn't work, I suggest my first, electrolytic, method. That
doesn't involve any noxious chemicals, and you can also easily watch what
happens. However if it works, get the piece out and rinsed quickly. You
could try the electrolytic method in ammonia, as that will complex the
metal that dissolves and reduce any chance of it redepositing.

Are you sure the copper *wire* was also sterling? Copper is a common
alloying metal with silver, to harden it.

Report: Tried applying 4.5 volts in ammonia bath as recommended. We
watched the reaction for maybe 10 minutes...it definitely etched all the
metal in the piece--copper and silver alike--and deposited copper on the
steel. But the silver wire became, if anything, "copperier" as a result.

(We attached the positive terminal to the piece and the negative terminal
to three steel nails.  A lot of bubbles appeared by the nails, when the
power was on and the piece was submerged, and not otherwise, so it seems
to have worked as Rane suggested it should.  Blue stuff seemed to be
coming off of the copper sheet, too, and the ammonia was distinctly blue
by the end.  The part of the nails that was submerged was coppery when we
were done.  And the bezel, which is silver and didn't seem to have been
affected by whatever deposited copper on the wire, seemed darker after the
experiment.)

Are we forced to conclude that the wire is not in fact silver?? It
certainly looked like silver before its fateful bath in the pickle...

That's what it looks like. It may now come to just polishing the whole thing.
But that was fun, wasn't it? 8^?

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