Maybe a furnace and a whole lotta heat? (Sarcastic of course).
Furnaces use to much fuel. A Butane Torch would be a better choice.
high power pressure washer?
Problem is who would hold it?
I have always found a wire brush works best
(disclaimer: that was sarcastic, do not use a wire brush)
WHY NOT? Works well on coins all the time.
If a Brass wire wheel, makes the coins look new.
I seriously wonder if Laquer thinner would work.