< PRODUTION METALS & ALLOYS
COPPER & COPPER ALLOYS
Archeological evidence from excavations at Catal Huyuk in Southern Anatolia (now part of Turkey), suggests that copper was being refined by man as early as 7000 BC [ref]. The long availability of a workable copper material means that many surviving artifacts of historical and technical interest (as well as artistic merit), have been created in this metal and it's subsiduary alloys.
Today, casting in a copper alloy accounts for the greater part of any art foundry’s output, indeed some small workshops will cast exclusively in one copper alloy alone. From the founder’s point of view, copper alloys are generally forgiving to cast and straightforward to work with; a molten charge flows well into refractory moulds, and casts formed in the more common alloys are relatively easy to weld, chase and machine - soft enough to work, strong enough to form sound, long-lasting structures. The vast majority of copper alloys also allow for a fine finish to be worked onto the cast’s surface, usually this is in the form of a high mirror polish or chemical patina.
COPPER (HCC & C SERIES)
Copper is available in a highly purified form (>99.90% Cu), as both as cast and wrought material. Due to it’s low resistance to electricity and heat, the primary use of this metal is as a conductor, hence the designation HCC or HIGH CONDUCTIVITY COPPER (US Grade ‘A’). Few, if any cast artworks are produced in this highly refined metal, principally due to it’s relative expense and the difficulties encountered in correctly casting and working the material. Where a high copper finish is required on a cast work, it may be more practical to specify a more common casting alloy such as a gunmetal, then later deposit a fine copper electroplate over the design. Alternatively, copper GILDING leaf is readily available in both loose and transfer foils.
Most copper sculptures and designs are produced from wrought sheets and other preformed sections. Stock material of this sort is readily available from a good non-ferrous metal stockholder in a variety of hardnesses. Wrought sheet can be profile cut by a PLASMA or water jet process. Thin gauge copper sheet is well known for it’s suitability for creating raised REPOUSSÉ works by hammering. Unless a specifically ‘high copper’ finish is called for, phosphor copper (PHOSPHOR BRONZE), is generally preferred for use over copper, especially for straightforward fabrications.
Copper casts and fabrications are usually joined by a TIG, or OXY-GAS process (welding, soldering and brazing). The alloy version BS 2874 C106 (Cu-DHP) is deoxidised, and considered the most suitable grade for TIG welding (using a C21 type filler rod). Owing to the material’s excellent conductivity, high current settings (arc), or large bore heating nozzles (gas), are necessary for the formation of a weld pool; use of a HELIUM gas shield instead of ARGON when TIG welding can reduce the current level needed to form a good weldpool.
Thin walled copper section (such as plumbing tube), is normally soft soldered with an LPG or butane fuelled torch, a lead (or lead replacement) solder, and flux protection. Hard silver soldering is generally preferred for joining light gauge decorative and jewellery copper work. Copper sheet can also be rivetted, or secured with a self fixing ‘panned over’ joint. Copper will accept a wide variety of chemical patinations and other finishes, similar to those commonly applied to it’s derivative casting alloys (GUNMETALS and bronzes etc). If stock material for patination is supplied pre-polished, it is often first lightly keyed (SHOT BLASTED) to improve the adherence of the patina deposit – though care must be taken not to stretch the metal by using too high a blast pressure. The metal polishes to a high lustre/mirror finish and offers good resistance to corrosive attack.
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