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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 :)

Bea

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:
>>Hi John,
>>
>>>>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.

          CORRECT
>>
>>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
>>>>problem.
>>>
>>>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
>>too.

     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).

     correct
>
>>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
>present.
>>
>>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.
>
>Horn silver is the old alchemical name for silver chloride, I think, but
>that doesn't necessarily mean that the substance has been correctly
>identified.
>
>> 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.

     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
>Interested in professional sound reinforcement and distribution? Then go to
>http://www.isce.org.uk
>PLEASE do NOT copy news posts to me by E-MAIL!