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Old February 21st 10, 05:19 AM posted to rec.crafts.jewelry
Peter W. Rowe[_2_]
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Posts: 115
Default Modern silver goes black, old silver stays white

On Sat, 20 Feb 2010 19:56:39 -0800, in rec.crafts.jewelry Chilla
wrote:


My friend has a 10th Century Viking ring... the real deal, not a
replica. After 10 centuries it's just starting to take on yellow hue.

I see modern silver rings that simply go black within a relatively short
time.

The explanation that was given to me was that the silver blackening is
due to modern suphides pretty much everywhere.

Your thoughts?


There are two issues here.

One is the metal itself. Modern silver is generally sterling silver, which
contains 7.5% copper. Old silver may have less copper, or even traces of gold,
since the two metals are often found together, or one impure with the other,
etc., and ancient refining methods were often somewhat less than perfect. As
well, if the ring was buried for a time, there may well have been a good deal
of leaching of the copper, if any, that might have been near the surface, out of
the metal, leaving the surface area purer silver than the main body of the
metal. And ancient metalworking methods, principally heating and working
without the benefit of good firescale preventing fluxes, would also tend to lead
to a final object who's surface was copper depleted. Since the copper in
sterling tarnishes more rapidly than the silver (though both form black
sulphides), items with that sort of close-to-fine silver surface, would tarnish
more slowly. Traces of gold in the alloy, if any, would make it tarnish even
more slowly.

Modern silver items, by contrast, have not generally been surface depleted of
their copper. Some working methods might even leave significant levels of
copper oxide at or near the surface,(called fire stain) which will make these
items tarnish even more rapidly than an item who's surface is clean sterling
silver. As well, modern sterling silver jewelry is often made by casting,
which can produce metal with a degree of porosity. These tiny pores in the
metal surface increase the degree to which the silver can react with atmospheric
sulphides, so they can tarnish somewhat faster. Ancient jewelry was often
hammer forged, and even if cast, was often polished by means of burnishing, or
rubbing the surface to a shine with a hard steel or stone tool. That compresses
the surface, removing any porosity at the surface in a way that modern buffing
processes do not do (though tumbling with steel shot does do this, one of it's
benefits, if it's not then subsequently machine buffed.). So the differences in
the rate at which some ancient jewelry will tarnish can be attributed to
differences in overall alloy, differences in the working methods that change the
alloy at the surface only, and differences in working methods tha change the
density/reactivity of the surface.

The second issue is what you allude to, modern environmental issues. Sulphide
tarnish is caused by reaction with sulphur compounds in the air. There are two
primary sources of this that come to mind. One, of course, is volcanic
activity, and there's no reason why that componant of atmospheric sulphides
would be different now than in 10 century europe, or elsewhere. However, the
second major source of sulphur compounds in the air are fossil fuels, especially
coal. Since the industrial revolution, the contribution to atmospheric sulphur
levels has been on a steady increase. This is the same basic source as much of
the greenhouse gas worries behind global warming, and in fact, some of those
sulphur compounds are themselves, powerful greenhouse gasses. These are the
same compounds responsible for "acid rain", as well. And continuing air
pollution from industrial processes, automobile exhaust, and the rest, continue
to raise atmospheric levels of sulphur compounds.

The result is that all silver items are likely to tarnish more quickly today
than they might have done in earlier times. Whether your Viking ring is
tarnishing more slowly than modern silver is doing now, would then relate to the
first topic, the metallurgy of the ring.

Peter Rowe
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