I've found vinegar (spirit vinegar preferably, but other
types would probably
work) extremely effective at cleaning greenish copper deposits from the surface of silver coins.
It seems to only dissolve copper compounds, and not silver ones, so it has minimal effect on the
toning +
patina of silver coins. For the same reason, it's not much
good at cleaning very dirty/blackened/crusty silver coins (I use lemon juice for this)
I used this to clean some Victorian and 20th century British silver coins which
had been stored in
PVC coin
album pages for years, and
had nasty greenish and yellowish surface deposits; after a few hours soak in vinegar, then a very light scrub with a nylon brush followed with a brief soak in distilled water, they were completely transformed - all the green
had disappeared, but as the surface
toning of the silver was not affected, they did not have a "stripped" or "over-cleaned" look.
As I said, this probably won't be much
good for dirty silver ancients straight out of the ground, but if you have any silver coins which have gone green due to being stored poorly, a soak in vinegar should do the trick.
It should be OK on base silver/billon; many of the British coins I cleaned were 50%
fine silver (post-1920). I'd be careful with silvered/fouree coins though.