< COPPER SILICON (SILICON BRONZES)
PHOSPHOR BRONZE (PB SERIES, PHOSPHOR COPPER, EN CC480K-CC484K [CAST ALLOYS])
Metals in the phosphor bronze group are alloyed with between 8-12% tin and up to 0.7% phosphor. Phosphor bronze is considered a superior material for casting or fabricating sculpture than those copper/tin alloys which are phosphor free (the CT series). Phosphor bronze has a bright gold lustre, often with a slight pink tint which is most evident on newly KNOCKED OUT casts and welded joints. The metal is very fluid when molten and is excellent for filling complicated refractory moulds. However, care must be taken to ensure that the tin content of the alloy does not separate out from the parent metal as the cast chills. If the tin does separate, it will usually solidifies in clusters which are clearly visible as grey patches on the surface of the cast. These concentrations of tin, which are referred to as TIN SEPARATION or ‘tin sweat’, can visually disfigure an otherwise sound cast.
One of the important industrial uses of phosphor bronze is as a bearing metal, this is due to phosphor bronze being one of the HARDEST, wear resistant copper alloys available. This feature makes phosphor bronzes considerably more difficult to hand work than the softer, more MALLEABLE gunmetals. Phosphor bronzes typically evolve shards and splinters when cut or machined and the alloy is somewhat resistant to manipulation by hammer driven matting tools. For these reasons, cast phosphor bronze alloys are not usually selected for use in the art foundry without good reason; for example, when filling complex refractory moulds, or casting simple and regular forms intended for a high mirror polish. Phosphor bronze may also be chosen if an especially hard copper material is demanded, though in many such circumstances the use of an alternative material, such as high tensile brass, may be equally appropriate.
A good range of wrought phosphor bronze preformed sections are available under BS standards 2870, 2874, 2875 (US C54400, NES 838), including sheets and plates. Principally supplied in both IMPERIAL and WHITWORTH stock measurements, phosphor bronze (in the HALF HARD condition), is a preferred material for use in constructed art fabrications. The popularity of wrought phosphor bronze for art and design fabrications is enhanced by the material’s excellent weldability, high structural integrity, and good colour match between parent metal and filler. Suitable welding processes include TIG and MIG, both done with matching C10 or C11 type filler rods or wires. Like SILICON BRONZE alloys, phosphor bronze is resistant to corrosion, but will readily take a patina via an application of one of the common chemical colouring compounds (polished stock material is best be prepared for patination by a light abrasive sanding or blasting to key the surface).
COPPER ZINC ALLOYS (BRASSES) >
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Phosphor bronze takes on a fine polish. By eye, it’s colour can be difficult to distinguish between that of other copper alloys. Similarly, when shot blasted, the surfaces of many copper alloys take on a dull sandy tone. (Photo R. Moule). |
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TIP: To test if a metal is copper or a copper-tin alloy, dissolve a small sample in a 45% nitric acid solution. Metals containing tin (bronzes & gunmetals for instance), will destansify, leaving a white precipitate of tin at the bottom of a test tube. |
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