I don't burn my utilities to clean coins. As I get them, I steel wire or brass wire brush them using detergent....tedious and laborious I know. Better to clean them as you get them than to have to force yourself to clean a giant pile of coins at the end of the season...that's a bummer. I only do this to the quarters, nickels and dimes. The pennies, I soak in vinegar for awhile then hand agitate them in a slurry of salt, baking soda, dish soap and a little water in a sealed plastic container. Don't soak zinc pennies in vinegar too long...I ruined a dollar or so worth doing that...they just got eaten. I know it seems like a lot of work, however, I have come to realize that I only need mine clean 'enough' not brilliant looking. This year's clad...the quarters, nickels and dimes still were mostly blackened or very dark toned yet they were clean, but the machine only rejected one dime and a few pennies.
Sorry, i diverted from the OP. No, lol, i don't know really. Wish i did. I'm thinking other than defacing the coins using abrasives...the solution must have to be chemical. So, whatever happens to copper/nickel in the ground to develop that blackish/reddish impossible to remove tone or crud...whatever chemical with actually dissolved that without killing a person or melting the coin.