I brought up the potato cleaning method in the old forum, after a freind of mine in another forum recommended the "tater treatment" for a crusty old buffalo of mine and some wheats.
I tried it [just stick the coin in and let it sit for a day or two], but didn't like the results.
Out of 3 wheats, it did remove alot of the gunk, but left two a sickly pumpkin orange color. The third was very good.
I forgot the results with the buff, or if I even tried it, but I would not recommend it for copper.