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Black residue with Ferric Nitrate


#1

Hi Folks, I’m stumped! I’m trying to do something relatively simple
I’ve done in the past, but am now having complications.

I have some 22k/sterling mokume gane sheet. I’m trying to lightly
etch the silver content with ferric nitrate to provide a texture
contrast… lightly frosted white next to the highly polished 22k
gold. A very elegant look when it worked before.

Now, on a the last couple pieces, a durable black substance is
developing on the silver. I have tried pickling, ultrasonic and ionic
cleaning and nothing has removed it. It appears that it did etch,
however. I mixed a fresh, stronger batch of acid using distilled
water. I have used metal from two different batches of mokume. The
last one was in the bath for only five minutes, and the black residue
was already forming. I put my vibrating tumbler on top of the
assembly containing the bath in order to provide some agitation.

Does anyone have any clue what is going on here, and/or how I can
avoid the blackening and just get a nice clean etch? I don’t have any
nitric acid at my disposal at this time.

Thanks in advance!

Dave
Dave Sebaste
Sebaste Studio and
Carolina Artisans’ Gallery
Charlotte, NC (USA)
dave@sebaste.com


#2

I asked Phil about your problem and here were his thoughts. Use
nitric acid, thus creating all soluble salts. If he doesn’t like
etching he could do a reverse plate using a silver plating solution.
The black is most likely caused by the copper in ther sterling
interacting with the iron in the Ferric Nitrate, but it could be
caused by chloride ion contamination as well. Bill


#3
    Does anyone have any clue what is going on here, and/or how I
can avoid the blackening and just get a nice clean etch? I don't
have any nitric acid at my disposal at this time. 

You are reducing the the ferric iron to ferrous iron as you oxidize
(etch ) the metal. What are the other metals in the Mokume? This
is the same sort of thing that happens in etching copper with
ferric chloride which makes the ferrous smut in that process only
you have more than one metal. Any silver nitrate formed will add to
the " black " . The mess may be just ferrous nitrate and silver
nitrate. I’m not really in my league at this point but you might
try adding a chelating agent such as EDTA or citric acid to the etch
solution… This will pick up the iron at least that is probably the
basis of the smut. Chelating agents such as these are in almost
all “patent” or secret metal cleaning- treating solutions. They
don’t show up in MSDS’s because they are both approved food and
drug components. Add maybe 100 grams per liter - should never need
more. start lower and build up if you want to go slow. The
citric acid addition to ferric chloride is what makes the Edenburg
etch used to etch printing plates . That chloride based etch won’t
work on silver but The principle is sound and probably will
improve the ferric nitrate etch as well – I don’t see any reason
why it wouldn’t. Jesse


#4

Sounds like silver nitrate to me Dave - you know, the stuff that
makes black and white negatives black. I have no idea how to remove
it. Is it common to etch silver with ferric nitrate? Tom


#5

Sounds like silver nitrate to me Dave - you know, the stuff that
makes black and white negatives black. I have no idea how to remove
it. Is it common to etch silver with ferric nitrate? Tom


#6

Commercial ferric nitrate is not very pure. I’d buy reagent grade
and use that instead. It’s a lot more expensive though.

Tony Konrath


#7

David we get the same thing with our nitric acid etches on our
mokume. I think it is a silver nitrate. We bead blast it off.

Jim


#8
    Does anyone have any clue what is going on here, and/or how I
can avoid the blackening and just get a nice clean etch? I don't
have any nitric acid at my disposal at this time. 

Dave, don’t know what it is or the cause, but brass bristle brush,
Bon Ami and dishsoap works for me. To get the frosted white finish on
the silver will be a problem. Perhaps relief etch the silver, brass
brush it, throw it back into a Sparex/peroxide pickle, then use a
cylinder polisher to touch up the gold?