Should I clean the “greenish substance” from my coins?

Azim K. writes: Question: I have some ancient Islamic coins of copper. They are been attacked or have greenish substance attached to them, which is either copper sulphate or copper oxide. Do I clean them? If so how to clean them or let it be as it is.

The green substance is “verdigris”. There are methods to remove verdigris but this is best left to a professional conservator. I suggest leaving the coins as they are.

See: Why aren’t you supposed to clean your coins?