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!