Quote:
Originally Posted by MK
I’m no chemist, and no expert, but here’s my guess. Celluloid pins have the plastic coating over the paper photo on the front to protect it. Lithograph pins are painted on the front which protects them until they are scratched. Just the first thing that came to my mind.
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That does make sense, and I guess the rest of the explanation has to do with oxidation, some metals just take moisture out of the air and react with it to make rust.