Over the years of working in a different field, I have noticed that
silver seems to be especially susceptable to sulfer bearing
compounds. I once had a customer where the 15% silver brazing would
get eaten out of the joints. They did casting of special magnets.
Also the silver, copper, and aluminium get eaten up quickly in the
coils of little refrigerators where they store salads; most
especially onions. Many of those coils are expoxy coated for that
reason.
Perhaps one of the chemists will explain this further.
Well you’ve sort of answered your own question really. Silver will
react with sulfur in the atmosphere to form silver sulfide - tarnish.
I’m not sure about the casting of special magnets scenario, but with
the fridge coils, I would imagine that the onions are releasing
sulfur compounds which are reacting with the metals to produce
aluminium sulfide, silver sulfide and copper sulfide. Onions contain
volatile sulfur compounds and it’s those that make you cry when
cutting them.
What exactly does the sulfur do? I’ve always wondered about this too.
I took a trip to Italy once, bought a beautiful silver bracelet on
the Pont de Vecchio in Florence and then (regrettably) wore it
swimming in a public pool that smelled badly of sulfur (apparently,
so I was told, something sulfurous was put in the water to keep it
disinfected?). The bracelet ended up coated with a thick layer of
gray that penetrated deeply enough it took a lot of doing to get back
the original silver luster. I always figured it was in some way
related to what liver of sulfur does to silver…
the answer to that is in part to explain that we use incorrect terms
to casually describe the black “antique” finish we often put on
silver. We call it oxidized or antiqued. But this is not a silver
oxide (which is not black, among other things). The black color on
silver that’s been treated with liver of sulphur is a mix of silver
and copper sulphides. Silver simply reacts with sulphur compounds
(like in eggs, in your pool water, or in liver of sulphur solution)
to form black sulphide compounds of silver and copper. It’s that
simple. The sulphur is the active agent in turning the silver black
because the black color is sulphur compounds, rather than, as might
be assumed by what we call it, oxides. You can demonstrate that
these are not simple oxides by putting silver like this in your
pickle solution. If you heat sterling silver, such as in annealing or
soldering, without flux, it turns black. The black color there is
copper oxides. It’s also forming silver oxides, but much less so, as
as I noted, they’re not black. All those oxides are soluble in the
acid pickle solution, so after pickling the silver is then nice and
white again. But after “oxidizing” with liver of sulphur, pickling
doesn’t take off that black color. The reason is because it’s
sulphides, not soluble in sulphuric acid based pickle… The easiest
way, by the ways, by the way, to remove that black chemically, are
thiourea based silver cleaners like “Tarnex”, or with cyanide
solutions, or simplest of all, electrolytically with a solution of
washing soda in an aluminum container, with your silver in contact
with the aluminum.
the answer to that is in part to explain that we use incorrect
terms to casually describe the black "antique" finish we often put
on silver. We call it oxidized or antiqued. But this is not a
silver oxide
All that Peter says is true, except that chemically there is
oxidation and reduction. Sulfur is an “oxidizing agent” as are the
halogens and others. No, it’s not “oxide”, but it’s chemically
accurate to call it “oxidation”. Just a detail - works the same no
matter what you call it. And I’ll chime in that we use a Tarn-X type
thing, too. Ours is from Otto Frei and I’m too lazy to get up and go
get the name in the other room… ;} Real handy stuff…
Do you know any other compound (than sulphur) that produces the
"oxidizing" (what term do you think would be more accurate?) effect
on silver? And thank you for your so very clear explanation of what
happens when the silver turns dark.
Peter, thank you for the BEST description I’ve seen for the
phenomenon we mistakenly call “oxidization”, and the distinction
between sulphide/oxide. I’m saving this one!
Do you know any other compound (than sulphur) that produces the
"oxidizing" (what term do you think would be more accurate?)
effect on silver? And thank you for your so very clear explanation
of what happens when the silver turns dark.
Actual pure sulphur is usually not used. Not effective. Mostly, we
use various compounds of sulphur. There are several. “Liver of
Sulphur” is a sort of impure mix of several potassium/sulphur related
compounds, and it’s fairly cheap. But other sources of the sulphur
can be found. Pollution does it with sulphur dioxide gas in the air,
and egg yolks have enough sulphur in them to do it.
Commercially, there are a number of silver oxidizers that blacken
silver by related, but not quite the same means. Some of these
proprietary mixes are quite dangerous, including hydrochloric acid,
teluric acid, and other nasty stuff. I don’t know if these actually
produce a sulphide, but I suspect it may not be, since the colors are
slightly different. Not sure what they actually produce. And there
are a range of plating solutions that can give a black finish. Black
nickle and black rhodium are the most commonly seen. They work on
silver, gold, platinum, etc.
On silver, of the simple chemicals used to antique items, the only
other common agent that can be of use that I’m aware of is household
bleach. This does not produce a black color, but instead, can take a
nice clean silver surface, and give it a grey dingy smutty corroded
sort of look. I personally don’t like the effect, but some folks find
it useful. There may be many other simple ways to chemically patina
silver, and I know there are many other more complex mixed chemicals
and procedures that can be useful, but I’m not familiar with them.
You can find in various book offerings a number of fine books on
patinas. Many of these will work on silver as well as the more
commonly described copper based alloys.
thank you for the BEST description I've seen for the phenomenon we
mistakenly call "oxidization", and the distinction between
sulphide/oxide. I'm saving this one!
You’re welcome, Allan. But it should be noted, and John kindly did
so, that I did make one technical mistake. In truth, the formation
of sulphides on silver is still properly called an oxidation
reaction, as in oxidation/reduction mechanisms. My point was to
explain that what forms, when we “oxidize” or “antique” silver, are
sulphides, not oxides. But to the chemist, this is still considered
an oxidation type of reaction, even though it forms sulphides, not
actual oxides.
I’m not Peter, but I know that you can buy washing soda in the Gaiam
catalog (used to be known as Seventh Generation). Gaiam is a company
that concentrates on low impact and sustainable living. I work at a
health food store and that is about the only place I have seen it.
Even our main destributors don’t carry it. Good Luck! I believe the
web site for Gaiam is just gaiam.com.