Why Does Copper Turn Green?

Statue of Liberty- Copper Oxidisation

Did you know that the Statue of Liberty is made from copper? Copper is that beautiful rich orange red metal that is used within plumbing, and what coins used to be made from. Then why is the Statue of Liberty green I hear you ask? Copper turns green because of a process called oxidisation, which forms a distinctive green coating named patina. The patina is a thin layer that forms on the coppers surface, so it’s this patina that gives the Statue of Liberty her beautiful green appearance.

What Is Oxidisation?

Simply put, oxidisation is when an atom, ion or molecule experiences the loss of electrons in a chemical reaction.  Oxidisation is a process whereby a substance gains oxygen. When copper is exposed to air and moisture over a period of time the oxidisation process begins, which will begin to slowly change over time the natural colour of copper. Copper starts from its usual deep orange hue turning into a duller brown colour, which becomes darker over time to then eventually turning from the brown to a green colour which lightens over the years.

Patina A Protective Layer

Copper - Patina Protective Layer

The green layer that forms is known as patina, and is the result of the chemical reactions that have took place over years with the copper exposure to oxygen and moisture in the air.

The patina is in fact a protective layer that sits on top of the copper- it protects the metal from further oxidisation and oxygen exposure, keeping the properties of the metal intact. 

This is why in architecture you will often find copper in use for roofs, as the patina protects the material from further exposure and is a good weather proof material.

When is patina not a good thing?

Whilst having this protective layer of patina covering the surface of copper is usually a good thing- it can in some cases not be useful.

For example, have you ever brought a piece of copper jewellery and had it turn green? The oxidation process can happen to copper jewellery as the copper reacts to the oxygen in the air and the sweat and oils from your skin which then forms a green layer on our skin.

Copper Wires

Copper is known for is high conductivity levels- however the patina coating is nonconductive, and so when copper is used for wires and electrical connections this protective coating disables one of the metals strongest qualities.

Copper saucepans are an extremely popular way of heating and cooking food, but the copper oxide is extremely dangerous if ingested as it is toxic- so proper care needs to be took when using copper saucepans to keep them from not forming the green patina. Patina can be stopped from happening by regular and through cleaning.

However, it can also be sped up should you wish. This can be achieved by ensuring that the surface of copper is completely free of any oils- should any oil residue remain on the copper this could impede the patina process as it would act as a barrier for the copper in protecting it. By exposing the copper to ammonia, the fumes from the chemical will react with the surface of the copper creating the green patina layer within a matter of hours.