Patina is the layer that forms on a material surface over time through the action of air, water and light. On metals it is a corrosion product that actually protects the material beneath; on timber and stone it is a matter of discolouration, soiling and surface erosion. Patina is therefore not damage but the visible result of ageing.
Patina by material
- Copper — first red-brown, then brown-black, and after decades blue-green. That green layer consists mainly of basic copper sulphates and carbonates and is exceptionally durable; copper roofs last a century and more.
- Zinc — forms a matt grey zinc carbonate layer within a few years that protects the bright metal beneath. Zinc that cannot dry out forms white powdery zinc oxide instead, and that is genuinely damaging.
- Lead — develops a light grey, dense oxide and carbonate layer.
- Bronze — dark brown to green depending on the environment; on outdoor sculpture it is an essential part of conservation.
- Weathering steel (Corten) — a dense rust layer that strongly slows further corrosion, provided the steel can dry out regularly.
- Timber — untreated wood silvers under ultraviolet light and rain washing; the greyed layer is only tenths of a millimetre deep.
- Stone and brick — dark deposits on sheltered surfaces, washed pale areas on the weather face.
Applications
- Material choice — with copper, zinc and weathering steel the patina is designed in from the start. Show the client in advance what the material looks like after five and after twenty-five years; the difference from new is considerable.
- Run-off — water running off copper or zinc carries metal ions with it and can permanently stain masonry, stone or concrete below. Design the water shedding accordingly.
- Unevenness — patina only forms where the material is exposed. Under overhangs the surface stays bright for years, so expect a patchy appearance early on.
- Restoration — patina forms part of a building’s historic value. Aggressive blasting or chemical cleaning removes not only the dirt but the protective layer as well, and accelerates decay afterwards.
Related terms
- Patination
- Weathering
- Copper
- Zinc
- Weathering steel
- Corrosion
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