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What Causes Copper To Turn Green? Patina?

Updated on December 14, 2010

What Makes Copper Turn Green?

It is common to think of any metal as a strong and stable material. Yet environmental forces affect metals as well. Exposure to air will in time turn iron to rust often producing a dark orange color. Copper is also susceptible to these changes and this new state causes copper to turn green. The reason for this is the copper is unprotected from the oxygen found in the air.

WHAT IS COPPER? - Copper is a reddish brown chemical element closely related to silver and gold. It is highly conductive to both chemical and electrical energy. Copper is a softer metal so is quite malleable. It has been mined and used since ancient times by many cultures for many purposes. It also turns green.

WHY DOES COPPER TURN GREEN? - Copper turns green through a process called oxidation. When copper is left exposed to water and air a thin layer of a green substance forms on its surface. This substance is known as copper carbonate. The oxidation of copper does not weaken it unlike iron which weakens when it rusts. Some speculate the build-up of copper carbonate actually protects the metal. Oxidation takes time. It is a gradual process and copper may turn black or dark brown before any sign of green appears. The thin layers of green are often referred to as patina.

Patina Explained

HOW IS PATINA USED? - Copper patina has several applications. In history, impermanent patina induced by seawater was used in excess by Greek artists as a colorant. It was called verdigris which literally means "green of Greece". It was considered the brightest green pigment available. Because this patina would easily degrade, it was abandoned by artists by the late 1800's. Patina is also used by historians to date objects. It is popular in the antique trade where collectors believe it preserves the integrity of the object. Patina is also used in certain functions of electrical engineering. 

WHAT ARE SOME EXAMPLES OF PATINA? - The most famous example of copper patina is the Statue of Liberty. This notable New York harbor landmark is known for its bright green hue. Few remember seeing its original copper color. Over a long period of time the copper reacted to the local climate by oxidizing producing the protective green patina seen today. Patina can be found on the copper roof of an old house. Old pots and firearms will show signs of patina as they age. For some, it adds character and value to an object. But what if someone doesn't want their copper fixtures to turn green? Because copper patina is only a thin layer it can easily be cleaned. 

HOW CAN PATINA BE REMOVED? - Keeping copper dishes and pots out of direct sunlight will help prevent them from turning green. What causes copper to turn green is the oxygen in open air so valuable items like war medals could be placed in an airtight container. To remove patina a mixture of salt and vinegar will usually do the trick. Pennies can be soaked in a jar. Certain copper jewelry can be cleaned by boiling them in water, salt, vinegar and liquid dish detergent. Patina comes off quickly in most cases and the item only needs to be dried with a soft cloth. Here is a link to a video that will show you what to do -


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