Why Does Copper Turn Green?
Andrea Thompson, OurAmazingPlanet Managing Editor | February 09, 2013 02:17pm ET

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metal, metals, copper, patina
Pin It Copper turns green because of chemical reactions with the elements.
Credit: USGS.
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For the same reason that iron rusts.

Just as iron that is left unprotected in open air will corrode and form a flaky orange-red outer layer, copper that is exposed to the elements undergoes a series of chemical reactions that give the shiny metal a pale green outer layer called a patina.

The patina actually protects the copper below the surface from further corrosion, making it a good water-proofing material for roofs (which is why the roofs of so many old buildings are bright green).