What Causes Copper to Turn Green?
What causes copper to turn green? Copper is a chemical element, a metal that is ductile and is highly conductive for both chemical and electrical energy. It is peachy-bronze in color and is a softer metal, making it quite malleable. Copper has been mined and used by humans for thousands of years, its history with humankind dating back to the times of the ancient Romans. It is considered one of the most useful metals in existence, one thing it is commonly used for is wire—copper can be drawn into wire that is exceptionally strong. It is used in making bronze and brass and forms a number of useful alloys. It can also turn green.... but why?
Like other metals, copper will oxidize when exposed to the air. This is seen with iron when the metal rusts into an orangey maroon color, as the metal has gone through a series of changes to result in this new state. So, what causes copper to turn green? The fact that it has been unprotected from the oxygen found in the open air.
Copper's Patina
The layer of green material that forms in the metal is called patina and protects the rest of the metal from decay. The word “patina” has Italian roots and loosely translates to “shallow dish.”
Patina will either build up over a period of time or patina can be formed more quickly in copper and similar metals by applying vinegar to the surface. However, this form of patina is much more temporary and will not last in the elements for a long time the way a true acquired patina will last.
Uses of Patinas
Verdigris is the name for short-wearing patina, and can also be induced by seawater. The name literally means “green of Greece,” and is called so because this material was used in excess by Greek artists in painting and other media as a colorant. It was known for centuries as the brightest green pigment available, and was widely used until the 1800s.
However, the pigment would degrade over time and the bronze color of grass and foliage seen in older paintings today is attributed to that fact. It was also difficult to use—unless the paint was oil based, the painting had to be sealed immediately to prevent the color from being tarnished. As soon as more uniform and sturdy green pigments were available, artists ditched using verdigris.
Electrical Engineering
Patina is also used in electrical engineering. Slip rings, a type of electrical connection via rotation, and commutators, a type of electrical switch on motors and generators both utilize patina in their respective functions.
Dating Objects
Historians utilize patina in dating objects and in antique trade. Patina that is intact is highly valued, often helping raise the price of valuable antiques pieces. If the patina is compromised and flakes off or is damaged, re-patination of the piece is a common tactic to preserve the integrity of the object.
Patina is often referred to as “natural toning” in furniture and other items. Synthetic patinas are now available to preserve outdoor appliances or decorations that contain copper.
Preservation
The long-wearing patina acquired from oxidation over a long period of time is highly useful as it preserves the metal from water, wind and other elements. Many older rooftops are copper with layers of patina on top.
The Statue of Liberty, a notable landmark of the United States is a statue made out of copper that has oxidized. Known for the brilliant sea foam green hue, the patina is only about .005 inches thick on the statue’s surface. The statue is located on Ellis Island in the New York harbor, and the patina keeps it preserved in the temperate climate of New York, which experiences rain and snow regularly.
So, what causes copper to turn green? Naturally, time and open air will produce the green effect. The process can also be sped with vinegar, which will produce verdigris and synthetics aimed at preserving an object.