Make that Old Coin Shiny Again! - No WAIT!
The new widow brought the heavy box of coins into the coin shop. She told the owner that her husband had recently died and she wanted to sell the coins. She proudly announced that she had made them all real shiny so they would be worth more.
She did not really notice the look of horror on the face of the coin dealer. He looked through the box of coins that had been worth thousands. He kindly explained to the widow that the coins were now worth about half of what they were before she had cleaned them.
To the non-coin collector, a bright shiny coin can be a beautiful thing to see. And, indeed to the avid collector it can too - as long as that is the natural state of the coin. The problem is that one of the primary determinants in the value of the coin is its condition. You might think the condition is determined by how shiny it is. However, that would be a grave error. The condition is determined by the details still visible with a magnifying glass or microscope. Unfortunately, those details are many times completely removed in the process of cleaning a coin with polish or chemical dipping. Those processes 'clean' by removing the outer layer of metal.
There are two exceptions to this rule. The first is if a modern rare coin is actually dirty, not just tarnished. In that case, take the coin to a dealer and ask his opinion on the value of the coin and if it could be significantly enhanced by professional cleaning. A professional will be able to clean the coin without any significant alteration.
The second exception is with ancient copper coins. You may have seen these coins for sale on ebay or another online auction or coin dealer. They often show crusty, somewhat ugly coins that may sell for about a dollar each. These coins can be fun to get and actually much improved by cleaning methods you have at home.
The best way to clean those ancient coins is with olive oil. Yep, you heard right! Get a small jar or plastic airtight container. Put the coin(s) in and let them soak. The oil will dissolve all that crud on the outside. Then, several times a year take the coins out and gently rub them between your thumb and finger.
It may take as much as two years to get the coin into its best condition. However, it is quite exciting to see the change from the crusty mess to a finely detailed or worn coin.