Controlling the interaction between the natural world and the built environment is one of the most fundamental concerns for designers and architects. In most cases, this simply means ensuring the elements are kept out. Building facades, however, present an opportunity for much greater interplay and experimentation. Through the use of corten steel, architects are increasingly harnessing the weathering process, creating vibrant canvasses which naturally change over time. Composition for change Corten, or weathering steel, derives its unique character from a carefully balanced composition – incorporating low alloy elements, such as copper and nickel, during the smelting process. As with all steel, when exposed to the natural elements the surface of corten metal oxidises and forms a patina giving the sheets a distinctive “rusted” look. The oxide layer is porous, causing it to retain moisture. In standard steel products, this will inevitably lead to further corrosion over time. The alloys within corten steel, however, help to form secondary layers which are resistant to water ingress and help to limit further corrosion of the metal. As a result, it can be used in outdoor applications without the need for a protective paint layer. Much like the changes the autumn leaves are currently undergoing, when the patina first forms it takes on a yellow shade. If the corten is left uncoated, this colour will gradually shift to a vibrant orange before settling to a dark red after several years of exposure. The speed of the colour transition is dependent on the frequency of wet and dry cycles the steel undergoes.