I have this rather nice antoninianus of Herennia Etruscilla, but unfortunately the coin has some green and brown deposits one it. I know it's virtually impossible to clean these on bronze coins, but is it possible to get rid of them on a silver coin? I've tried picking with a dental tool, but the deposits are rock hard on the surface. Also, I know antoniniani aren't very high silver content, so is it possible to safely remove these deposits? Thanks!