Ageing

In dry air, bare copper forms an oxide film within a few hours that the eye cannot yet perceive. It already stabilises the surface noticeably. Under moisture, atmospheric oxygen and aggressive airborne substances that layer keeps growing, the metal loses its shine and turns an even brown.

Over the years the brown darkens into an anthracite brown. On vertical building surfaces such as external wall cladding that is as a rule the final colour state. On pitched roof surfaces the layer develops further until the copper-typical patina green is reached. The reason is simple: precipitation stays on a pitched surface longer, and that favours the formation of basic copper compounds.

Where pitched and vertical copper surfaces meet, a drip edge therefore belongs in between. Otherwise water runs from one surface onto the other and carries the colour development with it.

The composition of the patina depends on the air at the location. It consists mostly of basic copper sulphates and carbonates, near the coast chlorides are added. That is why it turns out differently from region to region: often blue green on the coast, dark brown for longer in cities. Whatever its composition, it protects the metal reliably against further attack.

The pitch and orientation of the surface also play their part, as does the amount of rain that actually reaches it. A sheltered surface ages differently from a fully weathered one, even when both sit on the same building.