Cleaning coins is a subject most coin collectors tend to disagree upon. One side says, "Never clean a coin under no circumstances", while the other side says "You shouldn't clean a coin, but if you must, use a mild cleaner like soap or olive oil". Both sides have valid points, and most coins should never be cleaned. However, the author hopes to point out a few methods that may safely clean a mint state coin.
When talking about cleaning a coin, we are referring only to the removal of "surface only" dirt, grime, glue, tape, goo, etc. In other words, no method that involves the removal of any luster or metal is EVER recommended. What the author intends, by writing about coin cleaning, is the restoring a coin to its original state, as it came from the mint, and a cleaning that leaves the original luster intact.
Although cleaning a coin is not recommended by the author, there are instances where a mint state or a very slightly worn coin may be cleaned. For instance, any mint coin that has tape, goo, glue, or a copious amount of dirt, etc. can generally be safely cleaned using the right cleaner or method.
Furthermore, it's not cleaning, but restoration of a coin's natural surface that can be attained. It takes knowledge, caution and experience to restore a coin by removing unnatural surface contaminants off the surface of a coin.
SOAP AND WATER:
Most dirt and/or residue can be freed from the coin surface using soap and warm water. After soap and water is used, make sure the coin is completely rinsed off using warm water. If not, the residual soap may tarnish the coins luster. Also pat, not rub, the coin dry with a clean cotton clothe.
If the dirt is stubborn, and cannot be removed with soap and warm water alone, then one can use olive oil. The coin should be soaked in the oil for any where from hours to days and then gently washed off to remove unwanted crud. If these two methods fail to clean a coin or if their is glue, pvc or tape then another method is in order.
WARNING: Before we continue, I would like to warn anyone who follows the next step to be very cautious! Never use over the counter jewelry cleaner, polishing agents, or wire brushes, and/or tools with abrasives like a Dremel. The author cannot guarantee any coin, that is cleaned, will retain their value or be graded as uncleaned. Also, a coin that is *oxidized should be left "as is".
The author has successfully used two easily obtainable substances to remove stubborn dirt, greese, tar, glue and tape from a mint state coin, and the substance is called MS70 and. MS70 can be purchased on ebay or from certain online dealers. (Notes: I do not use this cleaner on copper coins, it produces undesirable results. Also, do not use it on coins less than AU58, and AU58, because some coins might look AU58 but are actually MS.)
MS70 is more the coin "restorer" than most debris cleaners on the market and is very similar to acetone. It removes dirt, grime, adhesive residue, etc., but will not remove natural toning and other stubborn surface debris like glue paste or fly specs.
Here are a few steps on how to use MS70:
Use a steady stream of warm water
Always wear a pare of latex gloves
Dip a Q-tip in the substance
Gently dabbed the substance onto the coins surface
Carefully rub away the grime, tape, glue, etc with the Q-tip
Then run the water over the coin for a few seconds
The "pat dry" the coin with a cloth
This cleaner is more for jewelry than coins and has some mild acid as part of it's contents. It can't be on a coins surface for more than a few seconds or it can permanently damage the surface of a coin. It should be dabbed-on and immediately removed and only used by experienced collectors!
*A warning about the E-zest: It is a stronger cleaner than the MS70 and should be used sparingly, and only as a last resort. If E-zest is needed more caution is involved; the coin should be wet before applying, and never leave the substance on the coin for more than 15 seconds. It can ruin the luster and natural surface of a coin.
Further note: If one decides to clean their coin using any of the above two coin cleaners, the collector should do a test run on a cheap coin to see if they obtain desirable results. Also, a coin should be looked over carefully before cleaning, because there might be a lot of contact marks under the toning or surface grime, and the coin may have less eye appeal after cleaning.
I have heard of collectors using vinegar on stubborn stains and debris but it's acidic and could harm a coin's surface. Example, if you leave an egg submerged in vinegar over-night it will dissolve the shell of the egg. So caution is in order for this substance and I don't recommend it.
This method, which probably leaves less chance for error, is an Ultrasonic Coin/Jewelry Cleaner. It requires the use of water and detergent, using ultra sonic sound waves to remove dirt in places even a brush will not get. Before you use this method, first clean a coin with either soap and water, or olive oil. This method is more expensive than the others, but it can remove stubborn debris on the surface of a coin. Do not confuse and ultrasonic cleaner with a rock tumbler, a rock tumbler will damage your coin.
It involves the use of an electric current to remove corrosion, or foreign matter from the surface of a coin. You will need a glass, 12V adapter, roach clips, salt, and a spoon. Here is a link that describes the entire process... Electrolysis.
Disclaimer: The author doesn't recommend cleaning any coin, and is not responsible for any results obtained, if one uses the steps outlined in this article.
*Oxidized coins have a hazy or white appearance, and is usually obtained by being stored in the old orange 2 x 2 envelopes.