We get this question a lot so it may be time to address this as well as we can. It is a reasonable enough question. Hotels like the Westin St. Francis have had actual coin-washing machines since 1938. It is a tradition that began to keep the white gloves that women wore in the day from getting stained by dirty change. Let’s face it, when money changes hands, as it is designed to do, it gets all kinds of stuff embedded in all the nooks and crannies, or as we like to say in numismatics, in the reliefs and devices.
The first thing you must ask yourself is WHY do I want to clean this coin? I mean, when was the last time you went to a fancy dinner and saw someone with white gloves on? So I am going to assume that is not it. Is it because you found a coin that had been stored for some time in less-than-ideal conditions and it's looking pretty crusty? Maybe it belonged to your grandfather, and it was a well-worn silver dollar, or a buffalo nickel where you can hardly read the date. Perhaps a few scrub-scrubs could make it easier to read? Who knows why? There are no bad reasons to clean coins, except one. That is if it is a collectible key, semi-key, or conditional rarity. In other words, only coins that are not worth collecting should ever be cleaned.
If you want a recipe for cleaning the coins jingling in your pocket, you would be surprised what a little baking soda and a toothbrush will do. That will shine up just about any coin. However, please understand this. If you did the same thing to a 1909-S VDB Cent, or a 1901-S Quarter, or a Toned BU Morgan Dollar, you may hear a distant flushing sound as you do it. That sound is you flushing away thousands of dollars in value of the coin.
How do you clean collectible key, semi-key or conditional rarity then? The short answer is YOU don’t. In much the same way as antique toys, guns, and cars, collectors want their coins with all their originality intact. Anytime you clean a coin you remove a very small amount of metal surface from the coin. When you consider that coin luster is created in the striking process, and that luster is really just a few molecules thick on the surface of the coin, you come to realize that just about any physical contact with that luster can be enough to erode it. And in many cases will subject the coins to a “Details” (problem) grade.
So, if you have a valuable coin that you think would be worth more if you try and clean it so it will look better, please don’t. In the rare coin world cleaning, or more appropriately called “coin restoration” should be left to the professionals at NGC or PCGS. It's bad enough to have had so many coins ruined because of white gloves, let’s not add insult to injury with any coins that have been spared by the coin-washing machines of the past.