CASSETTES Cassettes of golden copper alloy with different levels of perforation at The Deptford Lounge, London (Pollard Thomas Edwards architects) Photo: Chris Hodson

The processes bring forward the environmental changes without taking away the integrity of copper as a natural material – they form an integral part

Copper’s unique architectural qualities are defined by its naturally developing patina – which cannot be replicated successfully using other materials with surface coatings. Within a few days of exposure to the atmosphere, a copper surface begins to oxidise, changing from the ‘bright’ mill finish to a chestnut brown, which gradually darkens over several years to a chocolate brown. Continued weathering can eventually result in the distinctive green or blue patina seen on older roofs.

The patina film provides impressive protection against corrosion and can repair itself if damaged, giving exceptional longevity. A complex combination of factors determines the nature and speed of development of patina over the years. So, it is not surprising that factory-applied surface treatments are popular to provide ‘straightaway’ oxidisation and patination of copper surfaces to a selected level, particularly for facades.

Natural processes

Some of the processes involved are very similar to those taking place in the environment and utilise copper mineral compounds, not ‘alien’ chemical techniques. Essentially, they bring forward the environmental changes without taking away the integrity of copper as a natural material. They form an integral part of the copper and are not coatings or paint, and ongoing changes will continue over time depending on the local environment. These surface treatments include pre-oxidised copper, where the thickness of the oxide layer determines the colour


lightness or darkness. Then, pre-patination enables designers to determine both the colour and intensity of blue/green patina for each project with ‘living’ surfaces. As well as a solid patina colour, other intensities can be created revealing some of the dark oxidised background material. The most common compound found in natural patinas all over the world is the copper sulphate mineral brochantite, and factory-applied patinas have been developed with properties and colours based on the same mineralogy. In marine climates, the natural copper patina has more of a blue colour and this can be emulated using 100 per cent brochantite. But in many locations impurities and other components in the air add a yellow tint to give the naturally developed patina a green hue, which can also be replicated with pre-patination.

Copper alloys Of course, copper alloys have alsobeen used throughout history and bronze and brass – which can also be pre-weathered – remain popular for architectural applications. In addition, a recently developed alloy of copper with aluminium and zinc gives a rich golden through-colour. Its surface retains the golden through-colour and simply loses some of its sheen, as the oxide layer thickens with exposure to the atmosphere to give a protective matt finish. This golden alloy behaves differently to other copper products over time and does not develop a blue/green patina.

Graeme Bell is sales & marketing manager at Aurubis


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