To the two John's :)
My husband knows something of chemical reactions and was trained as a
ferrous metalurgist but this needs a non-ferrous metalurgist, he says.
Otherwise he agrees with all the reactions described below.
Nice to have all the confirmation you can get :)
On 7/21/03 11:52 AM John Woodgate writes:
>John Hooker <[log in to unmask]> wrote (in <[log in to unmask]
>com>) about 'Detectorists : News travels fast', on Mon, 21 Jul 2003:
>>>>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.
>Don't apologize. I was intent on informing, not criticizing.
>>>>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?
CORRECT. Sodium chlorate is a weed killer
>>I don't know how it gets there, but I've seen lots of Celtic coins from
>>Britain with this type of corrosion. 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
PROBABLY AN ACID OF SOME KIND
>Collectors should never attempt to clean coins without expert advice
>from a real chemist as well as advice from a coin expert as to whether
>it should even be attempted. 'Grey sludge' doesn't sound too much like
>silver chloride, but it might be: it's been well exposed to light so it
>may be grey (it's white when freshly made). It is soluble in sodium
>thiosulfate solution (photographic 'hypo': indeed this is how hypo
>'fixes' chloride emulsion).
>>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.
>I agree: the sulfide comes off with sodium thiosulfate solution or
>similar ('silver dip'), but if it's not just a very thin layer it takes
>a significant amount of the coin with it.
>>The chloride corrosion seems to either continue while the coin is in the
>>soil, or to be excessively thick.
>H'mm. It seems odd, without some strongly-oxidizing substanbce being
>>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
>Horn silver is the old alchemical name for silver chloride, I think, but
>that doesn't necessarily mean that the substance has been correctly
>> 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
makes sense if the cleaning agent is selective in attacking the coer
and not the silver. If nitric acid is used it would attack the silver.
>As you observe, the other metals are attacked first; silver is the least
>'noble' of the noble metals but the others are base metals.
>>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.
>Some of the black could be copper oxide, co-existing with the silver
>sulfide. Both are protective, but the copper oxide definitely won't
>stand up to chloride for long, even in the form of sea-salt.
Dont know. Percentage of the alloys present might also be a factor.
>>Not being too familiar with metallurgy, or chemistry, this is just based
>>on observation, and what I have been told.
>Your observation is on the ball: some of what you have been told may not
>be. Do we have any real chemists here?
>Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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