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Message:: [QUOTE="Detecto92, post: 1790699, member: 75105"]This is just a thesis, or my idea. Feel free to correct me if I'm wrong here. Okay, so a nice MS silver coin will sometimes tone. But I rarely see worn silver tone, unless it has been cleaned. My hypothesis is this. Once the planchets are made, they develop a "scale", much like steel does. This is a VERY thin coating of oxidation. Once the coin is struck, and kept in MS condition, toning can occur. But once the coin enters circulation, grease, oils from skin, and other contaminants put a thin film over the surface of the coin, preventing it from toning. The more it wears down, the more gunk gets on it. Oil prevents steel from rusting, so maybe oils from skin keep silver from tarnishing? Then if you clean a worn coin, it removes that film, and brings it down to bare metal. This allows the oxidation (toning) process to occur much more rapidly, since the silver is now unoxidized, and exposed to air. Anyway, just my hypothesis, I sure do see a lot of cleaned, worn coins that have toned.[/QUOTE]