Colour and Patina

Colour

The natural development of a patina, with colours changing from salmon pink to chocolate brown, and eventually to the distinctive light green seen on older roofs in our towns and cities, is a unique characteristic of copper. A full understanding of this process is important for building designers.

Click the play button to see the sequence of colours during the patination process.

Patina

When exposed to the atmosphere, copper oxide conversion films form, changing the surface colour of copper from salmon pink to russet brown within a few days. As weathering progresses over a number of years, cupreous and cupric sulphide conversion films intersperse with the initial oxide film increasingly darkening the surface to a chocolate brown.

Continued weathering results in conversion of the sulphide films to the basic copper sulphate patina which, when complete, gives the distinctive light green colour of older copper roofs. In marine climates, the surface patina will also contain some copper chloride. The eventual development of the light green patina can take 7 to 9 years in saline climates, 5 to 8 years near heavy industry, 10 to 14 years in urban surroundings and up to 30 years in clean environments.

A certain amount of rainwater is necessary to form the green patina and the process takes much longer for vertical surfaces, due to rapid run-off, except in coastal areas. Apart from internal applications, the natural progression of patina cannot be successfully prevented with varnishes and other coatings: this characteristic of copper is one of its unique features - not just visually, but also in terms of exceptional longevity - and should be anticipated in a building’s overall design.

Pre-patinated Copper

Different systems are now available which create a similar process to 'natural' patination, not just a coating which could be eroded with time. These use either special processes to produce pre-patinated sheets in the factory, ready for installation in their green state, or treatment of finished copper cladding to produce an immediate patina on site.