The natural development of copper 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 with a lifespan over many generations.
Changes are very gradual and not entirely predictable – just like the weather, which, in turn, is solely responsible 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 the 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 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 from 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 golden 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. 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.
A new feature on our website demonstrates the evolution of copper and copper alloy surfaces on buildings in different locations throughout Europe. To see how buildings change go to References and click on “surface evolution” filter or click here: Surface Evolution