>>Suphur is not a great problem as it
>>produces a hard black oxide
>Sulfide, not oxide. Unexpectedly, silver has a greater affinity for
>sulfur than for oxygen.
Sorry, my mistake.
>>patina that protects the interior, but when
>>chemical interaction produces silver chloride it can be a very serious
>That must be some powerful stuff, if it converts metallic silver to the
>chloride. I suppose sodium chlorate might convert silver sulfide to
>silver chlorate and sodium sulfate, and then the chlorate could
>decompose to leave the chloride. But who would put sodium chlorate (a
>powerful biocide) on a field?
I don't know how it gets there, but I've seen lots of Celtic coins from
Britain with this type of corrossion. Mostly very small minims, but a few
units. Attempts at a chemical cleaning by collectors often results in a
grey sludge which, when wiped off, wipes off most of the design too. It
seems as if much of the metal has been converted to this material. Black
patinas clean off very well, but they should really be left on. I think it
much better not to touch these patinas at all as they seem to provide a
stable barrier for further corrosion. The chloride corrosion seems to
either continue while the coin is in the soil, or to be excessively thick.
Of course, I suppose what numismatists call chloride corrosion might be
something else. It is either grey and porous looking, or, rarely in
Britain, in the form of scaly flakes (called "horn silver" by numismatists.
Sometimes the latter can be removed mechanically with a non-metallic tool.
The grey porous corrosion might also be due to the presence of other metals
in the alloy -- there is often copper and tin, with very small amounts or
traces of other metals such as lead, iron and some other elements. Chemical
(acid) cleaning when there is a very high level of copper in the alloy
strips the copper from the surface and gives the appearance that the coin
is a higher silver content than it really is.
The leaching out of some of the baser elements might allow further
penetration, but this does not seem to happen if there is a black patina,
and the underlying metal always seems to be very sound.
Not being too familiar with metallurgy, or chemistry, this is just based on
observation, and what I have been told.
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