The green patina ofbasic copper sulfate or carbonate on exposed copper gradually develops according to the environmental conditions. On roofs within a marine or industrial environment the green patina develops within five years; under heavy pollution it may eventually turn dark brown or black. Within a town environment, the patina on roofs will typically develop over a period of ten years. However, vertical copper cladding will normally remain a deep brown, due to the fast rainwater run-off, except in marine environments when the green colour will develop. On-site treatment to accelerate the patinisation process is unreliable, but pre-patinised copper sheet is available if the effect is required immediately. Green pre-patinised copper sheet should not be welded, brazed or soldered as heat treatment causes discolouration of the patina. The factory-generated bright green patina will weather according to the local environmental conditions, often turning quickly to a blue-green. Tinned copper, which is grey in colour, quickly weathers to a matt surface with the appearance of zinc or lead, but has the durability and workability of copper. The Urbis Centre in Manchester illustrates well the visual effect of a large feature patinised copper roof (Fig. 5.21).

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