For pin guys, a question
 

Why are so many pins that are virtually perfect on the front covered with rust on the back? Where did the moisture come from to rust the back that it didn't also compromise the fronts? Does the tin or whatever these old pins were made of absorb moisture from the air? It's always been a mystery to me, maybe a net54 chemist or other smart soul has an explanation.

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.

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.