The coloured surface layer a material develops as it ages — the green on old copper, the brown on bronze, the soft grey of weathered timber.
Usually a thin film of reaction products formed with air, water and pollutants: copper carbonates, sulfates and chlorides on copper alloys, oxide layers on steel, photo-degraded and leached cell-wall material on wood. Its colour depends on the environment as much as the material.
Patina is not always damage. On copper and weathering steel the layer protects the metal beneath; removing it restarts the process.
Copper roofs brown within months and can take years to decades to turn green.
A material chosen for its new colour will not keep it outdoors; the patina is the colour it will actually be.
A patina is characterised by what it is made of and how its colour has moved. Analysis of historic copper roofs finds a thin inner layer of cuprite under a thicker outer layer that is mostly brochantite, with other copper minerals where the air is rich in sulphur or chlorides. Colour change over time is tracked as CIELAB readings on exposed panels. Weathering steel is judged by eye and by time: it normally reaches its even dark brown in two to five years, provided the surface wets and dries in turn.
Copper or weathering steel is specified for a façade from a sample of its final colour.
The building spends years in the intermediate stages, and sheltered areas such as soffits may never reach the sample colour because they do not get the wetting and drying the reaction needs. Run-off in the early months can also stain paving and masonry below.
A bright brass or bronze fitting develops dark, sharp-edged patches.
It is usually a clear lacquer failing in places, letting the metal tarnish there and not elsewhere. The remedy is to strip and relacquer, or to remove the lacquer and let the whole piece age.