[LostinLondon]

Dear all,

I am not a chemist by trade, but a numismatist with a practical problem in the form of a silver coin that has hard purple/beige deposits of silver chloride and silver sulphide on the surface. These deposits adhere firmly to the surface, and I wish to find a means by which to remove them without harming the surface of the silver underneath (or around) the deposits.

Ammonia is not suitable, as it affects the surface of the coin. Would sodium thiosulphate similarly attack the silver?

It has been suggested that I leave the coin in a solution of zinc powder and iron nails, and/or leave it in silver nitrate solution in order to 'soften' the deposits for removal by hand.

Does anyone have any ideas or suggestions on how to proceed?

Many thanks for any and all replies.

[Arkcon]

As I understand it, any alteration of the surface patina, no matter how unsightly it is, will damage the value of the coin.  But to summarize your chemical choices:  Ammonia will only slowly remove the silver compounds.  Chemically, I don't believe ammonia will attack coinage silver, so right away, we came face to face with the fact that removing the patina is bad for the coin regardless of method.  I don't know about sodium thiosulfate,  I've used similar things, like Tarn-X on silverware, but its probably too harsh for your coin.  I don't understand how silver nitrate will help with tarnish on silver, except to function as a weak acid to dissolve the tarnish.  The method you seem to have been given only part of -- the zinc powder/iron nails solution is so called electrolytic tarnish removal -- basically you put a tarnished silver article into a pan of hot sodium carbonate/sodium chloride solution with a piece of aluminum foil, and the tarnish transfers to the aluminum.

Just remember, and don't kid yourself, all of these methods strip some silver off the surface.  The removed tarnish takes some silver with it.  There's no way to reverse the tarnish process and get the silver metal to redeposit in a even surface equivalent to when the blank was stamped by the die.