I've got a 1830 10 kopecks Novodel, that has ugly crusty black spots. I've had it soaking in olive oil, for many months now, with no change in color either in coin or oil.
Not sure what to do with it, but when I looked online, I did come across this statement on the NGC website: "NCS can remove contaminants such as PVC, soils, tarnish, carbon spots, verdigris and encrustation from the surface."
If they can help I will gladly send it over. Question is can they really help? Has anyone tried NCS services for copper coins with spots or encrustation?