Copper producer Aurubis discusses the wide variety of copper surfaces available for cladding today and how they can be used in conjunction with other materials to add architectural quality to housing.
Copper was one of the first metals used by man and is one of our oldest building materials, with unique properties and characteristics. We have witnessed its transformation from an historic role as a durable roofing material to a flexible architectural skin over any surfaces, notably facades. Its growth in popularity amongst architects has been significant and it is often used extensively throughout facades as the predominant material, effectively defining the architecture of high quality buildings of all types.
But it can also be applied more sparingly in housing projects to complement other, perhaps lower-cost, materials, adding kerb appeal. Here, it can act as a highlight, perhaps defining entrances and vertical circulation areas, where the material will continue to develop its natural appearance over time adding distinctive character to developments.
The natural development of a blue or green patina is one of copper’s unique characteristics. Within a few days of exposure to the atmosphere, the surface begins to oxidise, changing its colour 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, defining the exceptional longevity of copper cladding and roofing.
A complex combination of factors determines the nature and speed of development of patina. Some rainwater is needed for the patina to form and its rate of development will depend on the water ‘dwell time’ on a surface. So, vertical cladding and sheltered surfaces will take much longer to patinate naturally than exposed roofs and might not turn blue or green over the lifetime of the building. Not surprisingly, factory applied surface treatments have been popular for some time to provide straightaway oxidisation and patination of copper surfaces, particularly for facades.
Some of the processes involved are very similar to those taking place over time in the environment and utilise copper mineral compounds, not alien chemical actions. Essentially, they bring forward the environmental changes without taking away the integrity of copper as a natural, living material. All these surfaces 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. They include pre-oxidised copper, where the thickness of the oxide layer determines the degree of lightness or darkness, and pre-patination to provide straightaway the blue or green patina that otherwise takes many years to develop in the atmosphere.
Pre-patination processes have moved on and can now enable designers to determine both the colour and intensity of 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. ‘Living’ pre-patinated copper was used very successfully on a mixed-tenure housing development of 280 apartments on an historically sensitive West London site.
Breaking up the elevations
The design aims to create good modern buildings that relate to the context of this sensitive site. External materials were selected that are durable and have integral, rather than applied, finishes with ‘natural’ hue and colour. The pre-patinated copper was specified to give the finished impression and to blend immediately with the surrounding older buildings. The use of high quality, subtle materials – brickwork, grey roofing and pre-patinated copper – aims to soften the visual impact of the buildings. Lift and stair cores are expressed as vertical stacks equivalent to traditional features such as chimneys, emphasised through the green copper cladding. They provide a rhythmic punctuation breaking up the street elevations.
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. Brochantite is a light blue colour but in many locations impurities and other components in the air add a yellow tint to give the naturally developed patina a green hue and this can be replicated with pre-patination processes as well. In marine climates, the natural copper patina contains some copper chloride giving it more of a blue colour and this can be emulated using 100 per cent brochantite.
Of course, copper alloys have also been used throughout history and coils or sheets of Bronze and Brass – which can also be pre-weathered – are available 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 or green patina.