Alloys of copper with tin, zinc and lead have been used for at least 2,000 years due to their ease of casting, good strength and corrosion resistance. Early uses were for brooches, mirror cases, church doors, fonts and statues. The use for cannon in Medieval times led to the term ‘gun metal’ being adopted; this use is now obsolete with the manufacture of steel ordnance.
Today, in the UK, the term ‘gunmetal’ is applied to a family of copper-based casting alloys containing between 2-11% tin and 1-10% zinc. Modified forms may contain, in addition, such elements as lead (up to 7%) and nickel (up to 6%) when the alloys are classified as ‘leaded gunmetal’ and ‘nickel gunmetal’.
Gunmetals are noted for the manufacture of intricate castings required to be pressure tight such as valves, pipe fittings and pumps. They are also frequently used for bearings where loads and speeds are moderate.
Gunmetals are the first choice for modern statues, an example being the 2.1 metre high sculpture of Sir John Betjeman by Martin Jennings in St Pancras International Station, London.
This alloy has been widely used for some notable contemporary statues since it patinates easily and is capable of being deformed to remove any distortion produced on casting. However, since no current standard exists for this alloy there might be a problem since the public has access to the structure, so it is suggested that leaded gunmetals CC491K (LG2) or CC492K (LG4) are used.
This is a Chinese Specification composition 57-60 copper, 0.8-1.9% lead, balance zinc. It is a leaded Muntz Metal. A more suitable alloy for this application would be the gunmetal LG2 (CC491K) 85%copper, 5% tin, 5% zinc, 5% lead. It has better corrosion resistance and is easily cast.
Leaded Gunmetal is a cast alloy which is not designed to be cold worked. Crimping involves extreme deformation which will not usually be tolerated by the gunmetal without cracking. However, castings do not have a uniform structure and in the areas free from lead and possibly a smaller grain size some ductility is present. This will account for some of the castings surviving the crimping without cracking.
Suggest that you use continuously cast CC491K (LG2) bar or a wrought free-machining phosphor bronze.
Gunmetal or dezincification resistant brass.