I have this nice piece that has a bunch of something on it, doesn’t seem gooey at all and doesn’t come off. I have tried vinegar and salt on an old beat up Indian head for practice a year or so ago and that coin got obliterated so I never even thought to attempt this again, thank god it was a junk Indian head. I heard about acetone doing some good things some times but this is NOT a penny I would want to chance anything with! Do anyone have any ideas or suggestions or should I just leave it be? I just now it would be sell a bit better without it if possible But if there’s nothing I can do I’m fine with that as well! Thank you in advance.