Toning On Pennies

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I can only give you my opinion as an answer to your question. Most of the toning is from different sorts of grease, chemicals, or heat. Sometimes is't a combination of several of them. Copper responds to paper as well and is the most chemically active coin metal. I have noticed when coin searching that when I remove some of the various oily or gummy substances that what it left under them are the multi colors that appear as toning on so many others. In any case it seems to be a chemical reaction to various contact elements.

I hope this helps a little but perhaps someone with a bit more knowledge will respond as well.