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Why does copper turn green?


I need to write a paper about a chemical reaction. Can you give me any information about why copper turns green? What would the chemical reaction be? Or advice where to look?

Thank you,

Wanda K. [last name deleted for privacy by Editor]
student - Iowa City, Iowa


Copper turns green when it oxidizes. Copper (oxide) as found in nature is green - ask a chemistry guy or check a textbook for the complete reaction.

Doug Corbett
- SLC, Utah


I presume you are interested in the blue-green patina eventually formed on copper in outdoor atmospheres. It's a rather complex, lengthy sequence as shown in the Copper Development Association's Weathering Chart: https://www.copper.org/applications/architecture/weathering-chart.html [link no longer works]

Initially, bare Cu metal atoms react with air to form the pink oxide, cuprite, Cu2O, which has Cu+1 cations. This gradually oxidizes further to the black oxide, tenorite, CuO, with Cu+2 ions. The black sulfide CuS also sometimes forms. In the presence of moisture, the blackish layer slowly reacts with sulfur dioxide and carbon dioxide from the air to eventually form the patina, which is a mixture of 3 minerals:

brochantite, a green, hydrated copper sulfate, Cu4SO4(OH)6
malachite, the green, hydrated copper carbonate Cu2CO3(OH)3
azurite, the blue, hydrated copper carbonate Cu3(CO3)2(OH)2

In these compounds, copper atoms from the metal surface have been fully oxidized into Cu+2 (cupric) ions. The rate of patina formation and the proportion of the components depend upon humidity and the amount of air pollution. In European cities, where sulfur-containing coal has been burned for centuries, brochantite predominates on older copper roofs and statuary. In rural environments elsewhere, malachite may predominate. https://www.copper.org/applications/architecture/finishes-natural-weathering.html [link no longer works]

Do Internet searches for copper patina, blue green patina, malachite, copper oxidation, etc.

Ken Vlach [deceased]
- Goleta, California

contributor of the year Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.

September 14, 2009

The word for this action is verdigris.

Tom Sowell
- Dallas, Texas

January 4, 2011

What material or liquid can I spread onto copper sheeting to speed up the greening process?
Thank you

Agi Shani
- Haifa, Israel

Modern Masters Green Patina

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January 4, 2011

Hi, Agi.

Many different things will color copper, from proprietary patinating solutions to acid to ammonia to urine. We have numerous threads on that subject. Please use "patina on copper" in the search engine. Good luck!


Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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