Larry (IL) said:
Pete, any coin that is valuable, probably should not be cleaned at all. The problem is most of the coins we find is not worth much over face value considering the environmental damage. You couldn't get nowhere near "book value" for most dug coins.
Since I do not sell my finds, I clean to please myself, wire brush to some, toothbrush to others and soap and water and my fingers for the ones worth more.
As far as tumbling medium goes, sand, aquarium gravel or, I found out from necessity, small rocks out of my driveway works just fine.
The mantra of "never clean a coin"

is familiar to anybody involved in coin collecting as I have been most of my life and in most cases its good advice. As with any good advice, however, there are qualifications and limitations to its applicability. Two of these are coins dug from the ground and those recovered from the ocean.
Silvers dug from the ground usually look pretty nice and wouldn't need anything more than a little soap and water and maybe a soft toothbrush if the crunk is particularly stubborn. But have you ever seen a silver coin recovered from the ocean? Black as night and ugly as sin! I wouldn't give you jack for a coin like that! There was a story in a treasure mag back in the 80's about a guy in Florida that loved to walk the beach. He loved one beach in particular because it had these wonderful skipping stones. He would walk along and periodically stop to skip a stone into the water. Later he found out that these weren't stones at all but Spanish cobs, and he'd been throwing them back into ocean for years! Silvers recovered from the ocean including but not limited to those from Spanish treasure wrecks like the Atocha and Concepcion are routinely cleaned (usually by electrolysis) before being offered for sale. Even rare gold coins recovered from historic wrecks like the SS Republic and SS Central America are cleaned in an acid solution

before being appraised for sale. These treatments restore their beauty, desirability and ultimately their value.
Coppers from the ground or from the ocean are a special case because the encrustation is often not a simple protective patina (and nobody loves a nice patina more than me) as you would find on a piece that was removed from circulation and kept safe for many years. There is something called "bronze disease"

which will continue degrade the piece
even after it is recovered. The chemical reactions of bronze disease require oxygen, water (humidity in the air is sufficient) and chloride ions which are frequently found in soil as well as in the ocean. Bronze disease can't be reversed but it can be halted: http://en.wikipedia.org/wiki/Bronze_disease Enter the professional conservator. Museums employ them to treat and preserve copper coins and other artifacts dating from Greek and Roman times and older. Most coin collectors would be horrified

if they knew what methods conservators used to save these pieces but the results speak for themselves. Ancient copper coins and other artifacts not only have their appearance restored but are stabilized in the process. The methods they use are not magic or secret but are available to the public in the form of published academic papers posted on the websites of various colleges and universities with decent archaeology departments such as Texas A&M.
If I had a particularly valuable piece due to its rarity or otherwise, I would seek out a professional conservator who would know the best method(s) to restore/preserve its value.

For most of the junk we dig up it just doesn't matter, so there's no harm in practicing on it - you can learn something in the process and if it had no value to begin with there's really nothing to lose. The big question mark is the stuff in between - things with moderate value. The more practice you have on the junky coins the more equipped you'll be to deal with these pieces effectively and intelligently. If it has bronze disease, something has to be done or you will lose the piece - these are the toughest cases. For the rest, go slowly and be conservative with the treatment and be especially cautious about things that may produce an undesirable effect that can't be undone. The standard here is "am I enhancing/restoring/preserving the beauty/desirability of this piece?". To the extent that you can honestly answer yes to this question, proceed cautiously - otherwise stop.
On the subject of tumbling media, I would never use sand gravel or rocks on anything but clad/memorials - its too harsh.

Where there is a chance to improve wheat cents, Indians and other collectible coppers a dry tumble in a softer organic medium is more conservative. I recently tried (black) walnut shells but found even they were a bit too aggressive and tended to expose bright metal on some of the high points like the rim. Back in the 80's I had pretty good luck using Grape Nuts (yes, the breakfast cereal). Only downside to them is they wear out - they get pulverized into a powder and lose their effectiveness. There are some commercially available walnut shell media made from the English/Carpathian walnuts. These are popular with folks who do their own ammunition reloading - they clean their brass with it. I would like to give this a try - perhaps they'd be gentler than the black walnut shells - probably smaller pieces anyway. If anybody has experience with this particular one, give me a holler and tell how it worked out.
- pete