Soaking in soda solution and/or demineralized water plus detergent (with a weak basic/alkaline reaction in water) a few hours or even days. Afterwards brushing with a soft nylon brush until you see a satisfying result.
And/Or: soaking in alkaline oil (eg.
weapons oil) a few hours or days, then brushing with a soft nylon brush, again until you see a satisfying result.
If necessary repeat these steps in various combinations, at higher temperatures or with harder brushes (with a soft
brass brush as the extreme solution for hard encrustations).
But be careful: you have to try out the reaction of the surface and watch the result continuously.
Decide early enough when you have to stop this process - causing more damage than improving the coin!
