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Message:: [QUOTE="-jeffB, post: 2622208, member: 27832"]Collectors call this "toning" when they like it, "corrosion" or "damage" when they don't. <img src="styles/default/xenforo/clear.png" class="mceSmilieSprite mceSmilie1" alt=":)" unselectable="on" unselectable="on" /> Copper combines with oxygen from the air to form copper oxides. There are two kinds of copper oxide. Cuprous oxide has two atoms of copper combined with one atom of oxygen; it's red, or sometimes yellow. Cupric oxide has one atom of copper for each atom of oxygen; it's black. Depending on what the cent gets exposed to over time -- temperature, moisture, light, and so on -- it can form both kinds of oxide. This means it can turn red (like yours), brown, or black. Copper reacts with other chemicals, too, so it can also turn green or blue, but that's usually corrosion -- it makes a powdery or slimy deposit that wipes off, and eventually leaves pits or holes in the coin. Finally, if you get a very thin layer of copper oxide or copper sulfide, it can create a rainbow of colors. This is the kind of "toning" that some collectors like, and they'll pay extra for it (but only if they're convinced it happened "naturally", instead of being produced on purpose, "artificially"). That's me pretending to be a chemist/metallurgist again. There are real chemists and metallurgists here who can tell you a lot more if you want.[/QUOTE]