i made a piece for a client with Argentium Sterling and they got a
bad reaction. their finger turned black then green. after we
recast it in traditional sterling it was fine. does anyone know
what causes this?
I see that you haven’t received an overwhelming response to your
question so I’d like to chip if I may. I apologise in advance for
the length of this post. I hope you find some of it useful.
Let me begin by saying that what you’re seeing is very unusual. I’ve
never seen this kind of severe discoloration in over 100 pieces that
I’ve made and put in into people’s hands, or ears or their wrists as
the case may be. And there a lot of others who report pretty much the
same thing. Quite the contrary in fact: most people, including
myself, who are familiar with this kind of problem with regular
sterling (RS) silver report that they haven’t seen anything like it
with Argentium Sterling (AS). I’m talking about people who seem to
have a metabolism that causes this problem to occur frequently with
RS. I happen to be one of those people: I simply couldn’t wear
sterling jewelry because it was constantly turning black, often
within a day or so of cleaning and/or polishing it. That’s one of the
reasons that I turned to AS in the first place and it almost never
happens to me with AS. Others report similar experiences.
You’ll notice I said “almost never”. The truth is that I have seen
slight discoloration problems with some of the test pieces I make
and wear, often to try out a design for comfort and everyday
wear-ability. I can’t say for sure yet but I believe that this occurs
with my test pieces and not my final pieces because the test pieces
haven’t yet been run through the complete make-and-finish cycle (more
on this later).
So just to put a number to this, I have seen slight tarnishing in
less than 1% of my AS pieces, probably closer to 0.5% if I had to be
more specific about it.
However, that said, it doesn’t really address you question does it?
You asked “what causes this?” and the only honest answer I can offer
at this time is “specifically, I don’t know, yet”. I think it’s a
good guess that it’s the copper in the AS that’s reacting (the green
you mention, which I’ve never seen, is a good tip-off here) but
that’s not exactly news. Again, why?
Ok, there are a few avenues that might be worth persuing here. The
first is the issue of casting. I believe it is common industry
knowledge that when one casts objects in sterling silver (RS or AS)
you end up with an object that has a disproportionately high
percentage of copper on the outer surface, a copper-rich skin if you
will. I don’t know the metalurgy here but as I understand it a
percentage of the copper in the alloy tends to come out of solution
as the liquid metal is cooling and it migrates toward the surface.
All producers of sterling know this so one of the standard steps in
producing RS (and AS) stock is to scrape or grind off that
copper-rich skin once the alloy has been cast into rough sheets or
ingots. The idea is that you are thereby getting rid of the surface
material that is, technically, not really sterling anyway because
it’s got a disproportionate percentage of copper in it. That and the
fact that copper-rich alloys have a habit of tarnishing. Sound
familiar? So, the bottom line is that cast objects have a greater
tendancy to tarnish because of all that copper on the surface. If you
are casting then that might be part of your answer.
Another known cause of this tarnishing and discoloration in finished
sterling silver goods is the polishes that we are all so familiar
with. I wish I had specific references for this but “studies” have
shown that things like tripoli and rouge have a lot of sulphurous
compounds in them. And of course we all know that sulpurous stuff,
like Liver of Sulphur for instance, is great at turning sterling
black. I know I’ve heard testimonials that other polishes produce
better results but I can’t remember just now what those were.
Personally I just use my tripoli and rouge and count on the finishing
to clean things up.
And that brings us to the third and final item I wanted to touch on:
finishing. As I mentioned earlier I have seen a little discoloration
from unfinished AS items, rings in particular. Ok, why "unfinished"
in particular? This gets us down to AS specific issues, namely
treating the metal in order to “bring up” the germanium which is the
active element in AS that gives it it’s no-firescale and
low-tarnishing properties. For lack of a better term I have called
this “activating” the AS.
The purpose of “activation” is pretty basic: without it you’re not
getting your money’s worth out of AS simply because you haven’t done
what needs to be done in order to let the AS do it’s thing.
Fortunately the “activation” process is pretty simple. You warm or
gently heat the metal which allows the germanium to migrate to the
surface, much like the copper migrates in molten sterling as it
cools. I say “warm or gently heat” because germanium is quite active
within the alloy and will migrate to the surface much more readily
than other things, like the copper for instance. In fact it’s been
shown that room temperatures alone will allow some germanium to
migrate but since we’re trying to get the most out of this we apply a
little heat to expedite the process and more fully achieve the
Once the germanium is near the surface it readily reacts with the
atmosphere to form germanium oxide (which is largely transparent).
And that oxide layer is what acts as a barrier between the oxygen in
the atmosphere and the copper in the alloy. In other words it’s the
thing that keeps the object from tarnishing, or at least
significantly reduces the progress of the tarnishing process.
Ok, we’re just about done here. How much heat and for how long in
order to “activate” the AS? That depends on who you talk to but I use
the precipitation hardening process as my benchmark. During the
precip hardening you heat the AS object in a domestic over to as
close to 300 C (500 F) as you can get it and you hold it there for
between 30 and 60 mins. Since it’s a cumulative process a cooler oven
would necessitate a longer soak time. Let the object air cool,
pickle, and “bob’s your uncle” you’ve got a hardened object.
I use the precip hardening as my “activation” benchmark for the
simple reason that if that amount of heat has allowed the germanium
to move around enough to change the internal structure of alloy then
it will certainly have given the germanium it’s chance to migrate to
the surface and form the protective germanium oxide layer. Some say
less heat will accomplish the same “activation” results. Possible
true but I’m not taking any chances. Besides, I like hard finished
goods so I get both in one easy oven soak.
And now we have a fully heat treated item that has it’s protective
germanium oxide layer. (Lest you worry that this is a delicate layer
fear not, tarnish tests have shown that the layer can be as thick as
1 mm or more on the surface of the object.) Anyway it is objects
thusly treated that have produced, to the best of my knowledge, the
discoloration-free results we’ve seen in the wearer testimonials. So
that’s what I would do if I saw skin discoloration from an AS
object, I’d clean it thoroughly and heat treat it. (Since I don’t do
casting I’ve never had to deal with the copper migration problems
that might result from it.)
Finally I offer this: in order to isolate the conditions under which
skin discoloration might occur with an AS item a couple of us AS
makers are running a simple test. We’re taking a simple ring and
wearing it after each step of the fabrication process to see if and
when discoloration occurs. Our prediction is that we might see it
before the ring is finished but with proper cleaning and the heat
treatment process any such problems will be eliminated. We’ll keep
you posted on our results.
in The City of Light
Visit TouchMetal.com at http://www.touchmetal.com