< GUNMETALS
COPPER/SILICON (SILICON BRONZE, C87300-87800 CAST ALLOYS, CS101, EVERDUR®, DURONZE®, HERCULOY®)
In recent years copper/silicon alloys have been adopted by an increasing number of art foundries as their principal casting material. This is especially so in the United States, where EVERDUR (4%Si/1%Mn/Cu rem), has effectively replaced the so called US STANDARD BRONZE (a 90/7/3 gunmetal). Reasons for the silicon ingot’s popularity include the alloy’s ability to flow well into complicated moulds, the ability to maintain an excellent colour match in welded joints (with a matching filler rod) and the absence of an inherent lead hazard.
Quite distinct from traditional art foundry alloys, cast silicon ‘bronzes’ (C87300 & similar grades) contain no significant tin, zinc or lead additions (<1%), instead 3.25-4.5% silicon and up to 1.5% manganese make up the balance of this copper alloy (see also MATERIALS & WELD TESTING).
A fair range of preformed wrought sections are available in a variety of Imperial and A/F sizes (BS CS101, and US standard ASTM B96/98 C65500 grades are typical). Commonly stocked sections include bar (square, round and hexagonal), as well as sheet and plate in various gauges. Wrought silicon bronze, like the cast alloy, can be worked or forged whilst either COLD or HOT. The plastic behaviour of this material is quite unlike that of gunmetal – which fractures all too easily when hot worked. Silicon bronze also has an better tensile strength than gunmetal and is significantly more resistant to corrosive attack. As a result silicon bronze may be preferred to gunmetal if the sculpture is to be situated in an adverse environment, such as a coastal location or heavily polluted industrial district.
The joining of both cast and wrought silicon bronze sections is normally carried out by TIG welding with a compatible C9 type copper/silicon/manganese filler rod (similar to AWS 5.7 Cu S type fillers). Other fusion and non-fusion techniques, which include oxy-gas welding and brazing, are also suitable for joining silicon bronzes, all enabled with the added protection of a boric acid or borax based flux. As noted above, an excellent cast alloy/weld alloy colour match can be achieved in this material, especially advantageous where a polish or light patination is the intended finish on an extensively welded cast. Silicon bronze is one of the easier copper alloys to weld due to the material’s poor conductivity (thus concentrating input heat into the weld pool), also assisted by the inherent deoxidising effect of the silicon content.
One occasional problem encountered in welding silicon bronzes however, is that of stress cracking. Stress cracking in the area of a weld at best leaves unsightly hairline fractures in the cast’s fabric (these cracks can become more visually prominent after a patination is applied), at worst stress cracking significantly weakens a joint. Using a good welding technique is one solution (see FINISHING); it is also believed that the addition of a small quantity of zinc to the molten charge helps reduce the incidence of weld induced stress cracking in cast alloys.
The red-gold lustre of silicon bronze can make an attractive alternative to polished phosphor bronze or gunmetal alloys. In keeping with most other copper based alloys, silicon bronze readily accepts a good range of patina treatments and alternative finishes, though the alloy’s resistance to corrosive attack may inhibit the development of the less aggressive compounds.
Casts formed in silicon bronze are neither better nor worse than those formed in any other alloy, including GUNMETAL. Each alloy has it’s own merits, and some founders and artists simply prefer the working properties and/or finish offered by one casting alloy over another. Founders who do not normally stock silicon bronze ingot may impose a surcharge if this alloy is specified instead of their standard foundry alloy.
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