< METALS & ALLOYS
PROPERTIES OF METALS
For sculpture casting purposes, the properties most demanded of an alloy include a good natural colour (or the basis for a PATINATION application), STRENGTH, workability, castability, corrosion resistance and wear resistance. All these characteristics and more (see METALLURGY) can be adjusted according to the selection of a parent metal containing appropriate alloying elements.
Taking the example of a typical leaded gunmetal alloy, which is often used for sculpture casting; the addition of tin to the copper parent enhances the alloy’s strength, wear and corrosion resistance, tin also has a marked effect on the metal’s colour. Zinc has a somewhat similar effect on the copper as tin whilst also functioning as a scavenging (DEOXIDISING) agent in the molten charge and later improving the metal’s machining characteristics. Lead additions are in many ways detrimental to the performance of the alloy (quantities of lead can make the welding of cast sections more difficult and diminish the ultimate quality of polished finishes), however, lead does improve both the flow of the alloy into a casting mould and later enhances the cast’s machineability. In this last respect, leaded gunmetals are significantly softer and easier to work (CHASE and MATT) than true copper/tin BRONZES, which are generally much harder alloys. It is the LEADED GUNMETAL combination – copper/tin/zinc/lead, which forms the basis of many so called ‘bronze’ casts (see following page).
The proportion of an ‘added in’ alloying elements required to influence the characteristics of a parent metal varies considerably. The so called BETA TYPE (or high tensile) BRASSES for example, require in excess of 40% zinc to be added into the copper base before the material aquires with it’s exceptional structural strength. At the other extreme, the presence of tin in some of the ZINC alloys used for DIE CASTING can detrementally effect the material when present in proportions as small as 1 part in 50,000.
Most of the COPPER ALLOYS used in art founding are composed of at least 80% copper, with a 85-90% copper content being typical. The balance is made up of alloying elements, of which either tin or silicon are the most important (variable according to the specific choice of alloy).
WHY YOUR SCULPTURE CAST IS PROBABLY NOT BRONZE >
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