For a good look into verdigris (technical name for the blue
to green stuff on copper, brass, and bronze) check out:

http://en.wikipedia.org/wiki/Ve
rdigris

Interesting history.  It seems that it isn't just one thing,
but can be copper carbonate, copper chloride or copper acetate.

Funny enough, the picture of the Statue of Liberty calls the
green color an example of oxidation.

Mike in Sacto