How do copper and copper alloy surfaces evolve in outdoor conditions?

The natural development of a patina is one of copper’s unique characteristics: exposed to the outside atmosphere, it protects itself by developing a patina layer over time which makes it weather-resistant over a lifespan of many generations. Changes are gradual and not entirely predictable – just like the weather, which is a major factor for copper’s continual changes. The prevailing concentrations of air pollution and the environmental conditions essentially determine the composition and protective properties of the patina. The interaction of the patina with local atmospheric conditions will determine the surface appearance and how it changes over time.

Typical surface appearance evolution of copper and copper alloys used in architecture

Copper - Within a few days of exposure to the atmosphere, the surface begins to oxidise, changing its colour to chestnut brown which gradually darkens over several years and later may become a typical green patina.

Brass - An alloy of copper and zinc. The original shiny surface changes gradually through initial matting gradually to a greenish-brown that further develops to greyish brown then dark brown/anthracite colours. Sloped areas ultimately may develop a patina surface akin to that of pure copper, yet quite clearly different.

Bronze - An alloy of copper and tin. The original warm reddish-brown surface develops in a distinctive manner through weathering. A brown-red surface oxidation with a brown-grey undertone is typical for this alloy; the material then gradually changes to dark brown anthracite throughout - the subsequent patina coating forms much more slowly than with pure copper.

Golden Alloy - This material is an alloy of copper with aluminium and zinc, which is very stable and keeps its golden shade over time. It behaves differently to pure copper in the environment as it has a thin protective oxide layer containing all three alloy elements when produced. As a result, the surface retains its golden colour indefinitely and simply loses some of its sheen as the oxide layer thickens with exposure to the elements, giving a matt gold-coloured appearance.

Further information

How Copper and Copper Alloy Surfaces Evolve - https://issuu.com/copperinarchitecture/docs/surfaceevolutioneng_6742e3ff45d3d9 (accessed October 3, 2023)

Hughes, Richard: The Colouring, Bronzing and Patination of Metals - https://copper.fyi/library

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Last update: October 3, 2023