... what causes a copper penny that's 'perfect' (mint state red) to change to brown? surprisingly I've had a hard time finding a related thread... technically written... thank you
It's a chemical reaction iwth oxygen. More here. http://answers.yourdictionary.com/science/why-does-copper-oxidize.html
Something like moisture can accelerate this process. That's why you should always keep fresh silica gel with your coins