Copper – the real thing

Despite an unrivalled heritage including use on some of our oldest buildings, continuing developments make copper and its alloys thoroughly modern and sustainable options for contemporary architecture, as Graeme Bell of Aurubis explains

Copper has seen a dramatic shift from its historic place in roofing to a leading role in today’s facades. Its flexibility offers exciting potential for a flexible skin covering architectural elements of all shapes with minimal constraints. Architects are experimenting with this design freedom, fired by the potential for complex forms enabled by CAD and BIM techniques.

Recycling & sustainability

Adding to an exceptionally long-life, demonstrated over hundreds of years, copper requires no maintenance or decoration. As a lightweight and flexible covering, structural demands are reduced with lower carbon and ‘whole of life’ costs. Copper is also fully recyclable, utilising long-established practices – 97 per cent of copper in construction comes from recycling – and has other impressive sustainability and environmental credentials. With a melting point of 1083˚C and A1 (non-combustible material) fire classification to EN 13501-1, copper is suitable for cladding tall buildings, using appropriate constructions. Low thermal movement also makes it appropriate for any climate and location, and it is non-toxic and safe to handle, as well as non-brittle and safe to work. Its inherent antimicrobial qualities make it ideal for ‘touch’ surfaces internally.

New techniques

Traditionally, copper cladding and roofing utilised thin sheets or strips, with formed joints and fully supported by a substrate. But other techniques are also growing in popularity, such as copper shingles, panels pre-formed on two sides, and cassettes in squarer proportions with folded edges to all four sides. Copper is also being used to clad distinct elements such as fins, screens and brise-soleil. One of the most exciting developments today is experimentation with diverse forms, apart from flat rolled material. For example, copper can be pressed to provide surface textures and modulation, and perforated, expanded or woven as mesh for transparency. 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