Was just looking into this myself. I have been going through a bunch of my hoard of clad, looking for special coins and found a handfull of pennies and a couple dimes that were covered with green corosion and other crud. These were not MDing finds. Just pocket change that I picked up here and there.
I started by just soaking them the last couple days in dish soap to take the crap off but no real good. I then thought of when I clean our pots and pans. I have a set of Emeril Ware that is stainless and has a copper rim (Plate) in the bottom of the pans. I recalled how Barkeepers Friend cleans the copper very bright, almost on contact. So, I poured a little of the powder in a small pint sized storage container and threw the pennies in there. As I suspected, in a matter of several minutes, they started to get very bright. Some of the ones that were a little corroded but not too crusted over, cleaned up real good. Almost like new. Others, I still have soaking a little as the crust is a little heavier.
Now, I should say, I would never use this on any coin that I know is somewhat precious. I don't know if it is acceptable or will harm the coins. After cleaning the first batch that came out, I rinsed them real well under cold running water and dried them off. Among the coins I found one penny that was, originally, so bad that I couldn't find a date. It turned out to be a 1962. Another had sort of a reddish tone to it and turned out to date to the 60s also. A later year though. As the reddish color started to dissappear, I rinsed it to not clean it any further. The 1962 came out bright as new.
As I said, I did this as a test as I figured the coins just would get cleaned to be put back into circulation. I wouldn't think of it if I knew the coin(s) were collectable. I then went online to find what was acceptable and if there was any mention elsewhere on Barkeepers. I stumbled accross a couple sites that mentioned using an acid and salt. The sites I found seem to be directed to school kids and science experiments. Vinegar and Lemon Juice turned out to be the acids of choice. I didn't see any mention of the results. Perhaps I will try that. There was no mention, at least that I saw, about Barkeepers Friend.
