Why Does Copper Turn Green?

A surface that is pure copper or contains a large amount of copper will develop a green patina with a bluish tint after an extended period of exposure to the elements. The simple explanation for this is that the metal weathers in a specific manner when exposed to sunlight, wind and moisture.

Atoms of copper (also known by the elemental symbol Cu) react to the surrounding air. The combination creates a slightly different form of basic copper, usually with a mix of pink and black colors. Moisture in the air will gradually turn the black layer to forms of copper that are green and blue. These three minerals are azurite, malachite and brochantite. The final hue is determined by the predominant chemicals in the air and in the copper.

The surface layer, the patina, is said to be completely oxidized when the color doesn’t change significantly over a long period. The time it takes to make the initial change from black to green/blue varies depending on how polluted the air is (which chemicals are there to react with copper) and how moist the air is (relative humidity).

Color Change

One of the best examples of copper changing color with the passage of time is the Statue of Liberty in the harbor at New York City. This famous landmark contains a huge amount of copper. Rain and other elements have caused any protective layer on the statue to wash away or wear away over the decades. Air temperature and time have produced a color that is now constant, for all practical purposes.

We all know that copper starts out with a natural color that is a combination of brown and gold. The resulting color has become known as, you guessed it, copper. If you left an old penny (one that contains mostly copper) in the elements long enough it would change colors significantly.

We can also get a better understanding of the type of change taking place by thinking about another metal that changes color. When iron is exposed to moisture and other chemicals for a long time it changes color to a brownish-red. We know this as rust. Metals react to the oxygen in the air and to the effects of constant moisture. Cooper happens to stand up rather well (it doesn’t rust like bare iron). But it does change color.

Lost Metal

Close study of such large copper surfaces, such as the Statue of Liberty, has shown that only a very small fraction of the copper surface is actually lost because of the process called oxidation. In the case of the famous statue, the copper is actually just sheeting that covers the framework. It’s only about a tenth of an inch thick yet it stands up to the elements quite well.

Buildings in many cities have used copper for outside construction, for both appearance and for the metal’s ability to stand up over time. Quality copper piping used for water transport can last for decades, even in the presence of constant moisture. The brown/gold color may change to bluish-green but the metal will still be there doing its work.

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  1. What are the chemicals involved in this reaction?

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