In the case of higher-end silver coins, they generally become ineligible for professional grading, lose their wagonwheel luster, and drop enormously in resale value. So this discussion should not be interpreted as a promotion for cleaning coins (especially not the way I proceeded here!). But my goal here is to produce "socially acceptable" coins that I could either use in circulation or bring to the bank. If a coin has been sitting in the ground for 1-50+ years, it's usually in no condition to circulate. Inspiration was taken from Michael Chaplan's excellent Urban Treasure Hunter (2004, Square One Publishers): http://www.amazon.com/Urban-Treasure-Hunter-Practical-Beginners/dp/0757000908.
Unfortunately I didn't take photos of the coins before I cleaned them. It doesn't take much imagination to visualize a coin as it is dug out from years of immersion in soil, clay, etc. Other than brushing them off, or giving a rinse under the faucet after they were found, I didn't make an effort until this winter to remove the hardened grime en masse. So the photos here are all "after" photos.
I ran all of the common coins through various cleaning experiments. Soaking for an hour or so in white rice vinegar and salt, or running them in batches in my rock tumbler. In the tumbler I added some rounded pieces of gravel and a batch of either vinegar and salt, and/or hot water and a mild non-phosphorous dish detergent. The typical batch ran 1-3 hours. Afterward I rinsed them in a pasta strainer. The coins were left to dry on paper towels, and were then put into open-lidded plastic containers. I've since located a discussion that may have been useful for me to know earlier: http://metaldetectingworld.com/cleaning_coin_p10_tumbler.shtml.
First observation: Don't mix copper coins with silver-colored clad coins. This resulted in copper quarters, dimes and nickels. This is reversible, though, by more tumbling. To avoid this problem, simply tumble the pennies separately. Others have suggested soaking coins in olive oil, but I find that this takes a long time and doesn't usually yield great results. Another point on copper -- even rinsing copper coins in plain water can turn them black.
Apparently this is the chemical equation that can remove it. I've long since forgotten my stoichiometry, so I leave it to others to verify.
"Black copper oxide CuO reacts with acetic acid CH3COOH (inside vinegar) forming copper acetate Cu(CH3COO)2 and water:"
CuO + 2CH3COOH --> Cu(CH3COO)2 + H2O
I learned with a different batch of pennies that lemon juice is too powerful. After a few hours it actually began to eat through the coin causing permanent damage. So if it's an old wheat penny, Indianhead penny, etc. I'd resist the temptation to clean them in general.
