I recently happened upon some old European gold bullion that is really dirty and grimy. I initially tried soaking in acetone for a couple of days and some of it came off, but it's pretty dirty still. As an experiment, I tried to just roll a q-tip soaked in acetone back and forth lightly, and that removed more of the gunk, but it definitely left hairlines. I think this is going to require some sort of a detergent to remove chemically and I don't want to use any mechanical force. I'm also not interested in messing with the patina...I just want to get the black off.