I bought a big bucket of South American coinage at an auction a few years ago. The coins are all in rough shape, I imagine they had been buried somewhere for a while.
I put some of these coins in a sonic washer with water and CLR (A calcium, lime, and rust remover brand) – DO NOT HEAT the water when using CLR- right away there was smoke, not sure if it was particularly toxic, but why risk it?
So I put them in tepid CLR- water and ran it for 5 minutes. I wiped the coins off. Probably would have gotten better results by cleaning the coins further with straight CLR on an old toothbrush? Maybe next time.
Some say that cleaning coins will ruin the patina bestowed on them by time. Maybe so, but my eyes aren’t what they once were; I need all the help I can get.