Verdigris is a blue-green pigment or patina that forms on the surface of copper, brass, or bronze when exposed to air or moisture over time.
Copper turns green due to a natural process called oxidation. When copper is exposed to the elements, particularly air and moisture, it undergoes a series of chemical reactions that result in the formation of a greenish layer on its surface. This green patina is primarily composed of copper compounds, such as copper carbonate and copper hydroxide.
The oxidation process begins with the formation of copper oxide, a brownish layer that develops when copper reacts with oxygen in the air. Over time, this copper oxide further reacts with carbon dioxide and moisture in the air to create copper carbonate. The copper carbonate is the green patina that gives aged copper its characteristic colour.
The chemical reactions involved in the formation of the green patina can be summarised as follows:
The green verdigris patina not only changes the colour of the copper but also provides a protective layer that helps prevent further corrosion. This layer acts as a barrier, shielding the underlying copper from more extensive oxidation. In this way, the green patina serves a dual purpose by both altering the appearance of the copper and preserving its integrity.
The speed at which copper turns green depends on various factors, including environmental conditions, humidity, and the presence of pollutants in the air. In areas with high humidity and exposure to salt air, such as coastal regions, copper tends to develop the green patina more rapidly.
While the green patina is a natural and protective aspect of copper, some people prefer the shiny, reddish-brown appearance of freshly polished copper. To maintain this look, protective coatings or sealants can be applied to slow down the oxidation process and preserve the original colour. However, allowing copper to develop its natural verdigris patina is often embraced for the character and unique aesthetic it brings to outdoor copper elements, such as roofs, statues, and decorative items.