Tarnishing

Welcome to the forum Casey, I hope you stay awhile and learn the answers to your questions. I'm sure there are those here who will be helpful and willing to take the time to respond.
The basic answer to this is first of all ...DO NOT CLEAN ANY OF YOUR COINS...especially to remove the so called tarnish. All metal tarnishes and they all do it in no matter what type of envirement you place them in, unless its a complete vacuum, temperature controled and with a lot of luck.
This is a good start to learn what you want, but please do not clean.

What you refer to as Tarnish some call toning. It is as already noted normal for almost all metals to tone or tanish. This is usually a chemical reaction with some other element. It varies due to the type, quantity, temperature of the situation. However, even in a vacuum a metal can change due to electrolytic reactions of dissimalar metals touching each other. Although the tarnishing or toning is greatly increased by the presense of heat, water, oxigen, etc. Now as to Copper being so highly reactive with numerous other elements tarnishing is normal. Since Brass and Bronze are mistures of Copper and other elements, they also are attacked similarly. Example is Copper in normal air. Bare Copper will react with Oxygen to form Cu2O. Now this substance likes to react with CO2 and H OH, otherwise none as Carbon Dioxide and water, to form [CuCO3-Cu(OH)2]. Other wise none as Copper Carbonate. Please note the 3 and 2 should be lower case representing amount of atoms or molecules. This is the very normal slight discoloration you find on all Copper substances including piping in houses. Now if you live in an area where there is any Sulfur in the air in the form of SO or SO2, this will form with the H OH in the air to form either Sulfurous or Sulfuric Acid (H2SO3 or H2SO4)pending the amount of SO, sometimes called acid rain when it rains. This also attacks the Copper Carbonate, not the Copper, to form greenish CuSO3 or CuSO4.
To sum it all up DO NOT CLEAN OFF THIS STUFF. You would be removing some of the original metal. Also, to sum up this answer you just were introduced to Chem 201.

Good morning folks,
Thanks for those replies. They make for interesting reading. Reminds me of a chemistry lesson from my school days. Now I've learnt something!
The reason I asked that question in the first place was I had just received a proof set of 1982 Falklands Islands Liberation coins & a few of the coins showed tarnish...which reminded me of some I'd picked up in Thailand a few years ago that have since developed tarnish/toning.
Your help is much appreciated. Thanks!!