I have a couple of large early imperial coins which are covered in a dull grey substance that seems resistant to soak/scrub/mechanical sessions. At
work so I don't have access to a scanner otherwise I'd upload them. I'm worried that this might be a form of corrosion although the hardened layer maintains much of the sharpness of the coins. So I guess I have 2 questions, what is it? and if it should be removed, what is the best method?
Thanks,
Grant
