I cast some sterling charms over the weekend. After putting the
castings through the ultrasonic to get off the last spots of clinging
investment, I put them into the pickle pot with citric acid for a bit
of cleaning before despruing, etc. I got distracted by the arrival of
unexpected drop-in guests, and I forgot about the castings in the
pickle pot. They were in the hot pickle for about 90 minutes before I
remembered them. When I went down to the shop, the pickle was nearly
boiling. I pulled the charms out, and they were pure white, except
the bottoms of the casting sprue buttons, which were pink. The charms
looked like they’d been painted with flat white paint. I cut some of
them off the sprues, and the insides of the sprues are still shiny
silver. I ran a few of them through a rotary tumbler with some worn
ceramic abrasive for about one hour, and the white surface turned
into a very bright but whiter than normal silver color.
What did I do to these charms, and are they still OK to use?
(please be gentle if this question seems dumb…remember, I’m new to
casting, self-taught, and I’m learning as I go along.)
I pulled the charms out, and they were pure white, except the
bottoms of the casting sprue buttons, which were pink. The charms
looked like they'd been painted with flat white paint. I ran a few
of them through a rotary tumbler with some worn ceramic abrasive
for about one hour, and the white surface turned into a very bright
but whiter than normal silver color. What did I do to these charms,
and are they still OK to use?
The charms are perfectly fine to use.
What you did was depeltion gilding! It’s very difficult isn’t it?
What happened is this.
Because the items were in hot pickle for an extended period of time,
the acid had a longer time to dissolve the copper in the outer
portion of the casting. This resulted in a thicker layer of fine
silver being left on the surface. Fine silver is a whiter color than
sterling after it comes out of the pickle. The reason it’s whiter is
the lack of copper.
When you tumbled it, you polished more fine silver than sterling,
hence the difference in color.
Depletion gilding is the raising of a fine metal surface (silver in
this case) coating by dissolving the alloying metal (copper in this
case). Gold items can also be depletion gilded using the same method.
Hi Kathy; Sounds to me like you “depleted” the copper out of the
sterling at the surface of the castings. What you now have is a
layer of fine (pure, almost) silver on the surface. This may not be
a bad thing at all. They’ll be much more resistant to tarnish, and
that bright white color is very nice in to my tastes. The problem
will come when people expect that effect as the normal surface of you
product, then they’ll complain if the next batch look “grayer” and
you’ll have to boil all your silver in citric acid. You could
probably tumble them for a while in a light abrasive then
burnish/tumble with steel shot if you wanted to get back to the
sterling.
I cast some sterling charms . They were in the hot pickle for about
90 minutes and the white surface turned (after polishing) into a
very bright but whiter than normal silver color. What did I do to
these charms, and are they still OK to use?
You just pickled 'em properly Kathy - the white surface will be pure
silver (unless there was more than citric acid in the pickle). You
will have removed all of copper oxides it is possible to remove with
this sort of process leaving you with a beautiful (but softer) quite
substantial, silvery-white layer of pure silver on the casting.
From what I understand, the pickle (acid) will dissolve all the
copper on the surface over time leaving fine silver which in the
"as-cast" rough state is that very pure white color. In your case the
layer is very thin and shouldn’t affect the strength or your casting.
As a technique I have seen people labor for hours working the fine
silver to the surface of a piece:
heat/pickle/heat/pickle/lather/rinse/repeat…Very hot pickle (not
boiling!) and putting the pieces in hot helps speed the process up.
Every time I’ve tried it I just end up with ugly uneven reticulation,
but YMMV.
Basically it’s up to you, does the whiter color work for you? If so
it can add value to a piece, also as you’ve noted the fine silver
takes a different polish than sterling, I really like it personally.
If you do not want it, such a thin layer can be buffed off with
bobbing compound, or filed/sanded off. Keep in mind, it’s probably a
VERY thin layer, and it also may not be perfectly even
Have fun, hope that helped =)
-Doug Harroun
Albuquerque, NM
Citric acid was used in ancient times to remove oxides from precious
metals instead of the sulfuric acid commonly used by jewelers in
modern times. Heating increased the chemical reaction, as well as
causing the water in the pickle to evaporate, thereby increasing
your pickle’s strength. Your pieces were in effect blanched (same
process as depletion guilding, but for silver)—that is, everything
other than pure silver (i.e., copper and silver oxides) was totally
removed from the surface of your work, leaving a layer of pure
silver, which would be white. Thus, you ended up with sterling
silver pieces with a pure silver surface. Burnishing or very light
buffing this surface will give a high shine (if the pieces were
shiny to start with) of good color which will be more resistent to
tarnishing than sterling because of the higher silver (lower copper)
content. (I do this intentionally on all silver pieces.) If you
polish too much however, you’ll remove the pure silver layer and get
back to your original sterling, which nevertheless will have the
true, even color of unoxidized sterling (assuming the original
pieces didn’t have super deep firescale)—whiter than heat-stained
sterling, but not as white as pure silver. Either way, you’re a
winner…:o)
Perhaps we should all chuck our unhealthy sulfuric acid and return
to citrus…?
I just used a brass wire brush, in a dremel, It came off quickly,
without any scratches or scaring, and appeared almost mirror bright. I
buffed with some Green Zam and it glows.
Didn’t realize it was depleting the copper at the surface, learn
something new when you least expect to.
You could