As copper experiences natural or human-induced corrosive attack, its color changes from the iridescent, golden red normally associated with pure copper to a deep brown and, finally, to hues of blue and green.
The chemical reaction that produces patina occurs as cupreous and cupric sulphide conversion films develop with cupric oxide on the metal, thereby, darkening its surface.
Continued exposure to sulphurs and oxides, converts the sulphide films to copper sulphate, which is distinctive blue in colour. In saline, or maritime, environments, the surface patina may also contain copper chloride, which is a shade of green.
The evolution and colour of the patina is ultimately determined by a number of variables, including the temperature, length of exposure, humidity, chemical environment and surface condition of the copper. However, in general, the evolution of blue-green patina in different environments can be summarized as below:
- Salt water environments: 7-9 years
- Industrial environments: 5-8 years
- Urban environments: 10-14 years
- Clean environments: up to 30 years
Save for in controlled environments, the development of patina cannot be effectively prevented with varnishes or other corrosion resistant coatings.
Famous buildings that display patina's blue-green tones include the:
- Statue of Liberty, New York (USA)
- Canadian Parliament Buildings, Ottawa (Canada)
- NEMO Science Center, Amsterdam (Netherlands)
- Kresge Auditorium, Massachusetts Institute of Technology (USA)
- Minneapolis City Hall, Minneapolis (USA)
- Peckham Library, London (UK)
- Capital Museum, Beijing (China)
As a desired architectural property, the development of patina is often encouraged through chemical treatment of copper cladding or roofing.
According to the Copper Development Association (CDA), the following treatments have been used to induce chemical reactions leading to an early development of patina:
For deep brown finishes:
- Ammonium sulfide base
- Potassium sulfide base
For green patina finishes:
- Ammonium sulfate base
- Ammonium chloride base
- Cuprous chloride/hydrochloric acid base
To see examples of patina in architecture, see the Patina Photobook.
The Copper Development Association (CDA). Colour and Patina.
The Copper Development Association (CDA). Architecture.