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ZINC (BS 1004 A/B, BS 849:1939) & SPELTER
Despite a vogue for zinc (SPELTER) artworks in 18th and 19th century continental Europe [ref], few sculptures are nowadays produced in this metal. The main use of zinc in modern foundries is as a die casting alloy, especially BS 1004 A which has additions of 4% aluminium and a small quantity of magnesium (copper is added to the ‘B’ version). This die cast alloy is sometimes used as an alternative to aluminium, and copper/zinc die alloys. Spelter is in fact REDUCED ZINC ORE, a material supplied to the founder in an ingot form.
Silver-white in appearance, high purity zinc has a low melting point (419.6°C), and so easily ‘burns off’ if heated beyond the metal’s boiling point of 911°C. Zinc is best known in art founding for it’s use as an important alloying element, notably in BRASSES and GUNMETALS. Zinc is also the protective coating on carbon steel imparted by HOT DIP GALVANISATION. Some wrought zinc material is available, this is usually obtainable through building trade outlets as sheets which are normally used as ‘flashing’ for domestic roofing. Fusion welding of zinc is not considered practical, and joining methods are usually restricted to soft soldering or mechanical fastening techniques (bolting, rivetting and so on). Finishing is normally to an ‘as is’ surface, or else the material is electroplated with other metals. Exposed zinc tarnishes in air, so out of doors designs in this material (including those in galvanised steel material) are often painted, although the tarnish can be pleasing to some.
MAGNESIUM (A, AZ, AM, RZ SERIES)
An exceptionally light and useful engineering metal, the casting and working of magnesium is a specialised (and expensive) process, usually requiring specialist casting equipment and techniques not common in the art foundry. Enquiries about casting in magnesium are best directed to specialist foundries, in particular those whose output centres on aerospace and other specialist industries. Cast alloys such as RZ5 and the wrought alloy AM503 are weldable by a TIG process.
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