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Why Does Copper Turn Green?

metal, metals, copper, patina
Copper turns green because of chemical reactions with the elements. (Image credit: USGS.)

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).

In fact, the weathering and oxidation of the Statue of Liberty's copper skin has amounted to just .005 of an inch over the last century, according to the Copper Development Association.

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Andrea Thompson

Andrea Thompson is an associate editor at Scientific American, where she covers sustainability, energy and the environment. Prior to that, she was a senior writer covering climate science at Climate Central and a reporter and editor at Live Science, where she primarily covered Earth science and the environment. She holds a graduate degree in science health and environmental reporting from New York University, as well as a bachelor of science and and masters of science in atmospheric chemistry from the Georgia Institute of Technology.