I call this one blazing rainbow because apparently it was in a fire probably 150 years ago-just an estimate.
Acetone or similar will only be effective to some degree on organic substances. The verdigree is basically corrosion
I was referring to the “gunk” at 10:00 o’clock on the obverse. I had assumed that at some point someone applied varnish or similar.
If it is varnish then try acetone, 10-15 minutes and then a rinse of flowing acetone or distilled water. Keep repeating as necessary