What Is Patina?

"Patina" is a term that refers to the blue-green layer of corrosion that develops on the surface of copper when it is exposed to sulfur and oxide compounds.

The word is derived for the Latin term for a shallow dish. While it usually refers to a chemical process, a patina can mean any aging process that causes natural discoloration or fading. 

Chemical Reactions in Patina

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 sulfurs and converts the sulfide films to copper sulfate, which is a distinctive blue in color. In saline, or maritime, environments, the surface patina may also contain copper chloride, which is a shade of green.

The evolution and color 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:

• Saltwater 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.

Patina in Geology

In the field of geology, a patina can refer to two possible conditions. It's the discolored thin outer layer or film that forms on a rock's surface, due either to desert varnish (an orange coating) or a weathering rind. Sometimes the patina comes from the combination of these two conditions. 

Patina in Architecture

Because of patina's aesthetically pleasing appearance, copper and copper alloys, including brass, are often used in architectural projects.

Famous buildings that display patina's blue-green tones include the Statue of Liberty in New York City, the Canadian Parliament Buildings in Ottawa, the NEMO Science Center in Amsterdam, Minneapolis City Hall, the Peckham Library in London, the Capital Museum in Beijing, and Kresge Auditorium at the Massachusetts Institute of Technology

Uses for Induced Patina

As a desired architectural property, the development of patina is often encouraged through chemical treatment of copper cladding or roofing. This process is known as patination. 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