Copper Red to Red Brown

[MikeKing]

Copper is something I would definitely like to understand more of because I am clueless about how something goes from red to red brown and then to brown. I mean, I understand that the metal oxidizes, but often, when I look at these coins, it looks like the 'brown' over the 'red' is really a superficial coating of 'dirt'. I'm assuming I'm wrong, otherwise it would be easy to turn red brown coins into red ones. Could you explain this to me?