I've had a 1690's Italian silver coin soaking in pure Acetone for over a month and it has not harmed the coin in any way. This coin has some kind of adhesive/epoxy residue on the reverse and I'm trying to remove it. I change the Acetone bath every few days but it's not doing much to remove the residue.
Thanks for the cleaning tip, unfortunately xylene can't be sold here in Calif. Next time I go to Nevada I'll pick some up.