The first type is a lightly soiled coin which is often fully attributable even when “dirty”. This type of coin most likely came from a hoard along with many other coins. Because they were buried in clay pots or similar containers these coins were largely shielded from the ground itself. Rainwater, dust and the detritus from the vicinity accumulated on them over the ages and this often were ideal conditions for the coins to develop a patina along with a light coat of dirt.
Hoard coins should be handled with relative care and gentle cleaning procedures should be used. Attempt to clean them first with nothing but water and brushing. Failing this, soak them in light mineral oil for a week or two, scrub it and see if any progress is made and the process can be repeated.
It’s often the case that hoard coins come in an essentially clean state with just occasional light deposits on them. Many collectors prefer to leave some of this dirt on the fields as it gives a pleasing "ancient" look and has the added benefit of increasing contrast. The increased contrast helps bring out faint lettering and portrait details.
Next come the typical uncleaned coins.
These types often have some discernible features showing through a thicker
layer of dirt than hoard coins. Unlike hoard coins however, these were found singly or
in groups in the ground in which they remained buried for hundred or thousands
of years. Because they were in direct contact with the soil the coin’s metal
reacted with its surroundings in unpredictable ways. Although this type of coin
sometimes shows a patina, often there is no even,
attractive metal oxide layer underneath.
Because of the unpredictable nature of the average uncleaned coin, you will need to assess its condition and determine what course to follow in cleaning it. As with the hoard coins, it’s best to try the milder cleaning techniques first to see whether these are effective. If soaking and scrubbing fail to dislodge the dirt you may have no option but to use more invasive means. It is important to realize that patience is the key factor to success in restoring these antiquities, rushing the job in desperation will disappoint you in the long run.
Finally we have those coins
which are so dirty that the dirt has become a cocoon. While attempting an
initial soaking in water is recommended on the off chance that it can dissolve
some of the dirt, it is much more often the case that these coins will
respond poorly to soakings of any type.
Electrolysis or extended periods of soaking in oil are the most likely routes of dealing with the crust. While soaking and brushing and electrolysis are the main methods for cleaning coins these are by no means the only ones. Some hobbyists have reported success by freezing the coins then throwing them into boiling water. The principle behind this strategy is that as metal expands during “thermal shock” the dirt is unable to hang on to the coin’s surface. While this is an interesting approach you should expect different levels of success depending on the type of dirt that is found on the coin to be cleaned.
For those coins which have persistent thin layers of dirt which scrubbing fails to remove you may try treating by means of adhesive. Simply apply some glue such as Elmer’s to form an even coat and let dry for a few minutes then peel it off. You will notice that the dried glue has come off with some of the dirt and this process can be repeated several times. The main disadvantage of using this method is that it often damages the patina, if there is one, giving it a “matte”, low-contrast look. Experimenting perhaps with a lower grade coin first may be advisable to learn how this process works and where it may most efficiently be used. Others have resorted to using rock tumblers or jewelry cleaners. While these are worth considering, they take special care and equipment and, at best, seldom yield better results than the more traditional means.