< INTRODUCTION TO METALS
METALS AND ALLOYS
Pure metals are ELEMENTS, which means they cannot be further broken down into component substances. ALLOYS, on the other hand, are a combination of two or more elements, which are united to create a new and often quite distinctive metallic material. Theoretically at least, most alloys can be reverted to their principal elements, an example of this is lead and aluminium which mix when molten but separate out like oil and water on cooling.
In practical terms though, separating out the different elements of some alloys is simply not possible. This is partly because the elements combine in different and sometimes complex ways – they may become fully soluble with each other in all proportions to become a HOMOGENEOUS COMPOUND (copper/nickel alloys), in other instances the elements form a SOLID SOLUTION (zinc in copper/zinc alloys [30/70 brasses]). Other elements are described as INTERMETALLIC COMPOUNDS (60/40 brasses, and iron/carbon); or may be only PARTIALLY SOLUBLE, in which case they are referred to as a EUTECTIC SYSTEM (selected aluminium/silicon alloys). Of course the average sculptor does not need know about metallurgy in any detail to appreciate the visual qualities offered by a particular alloy, but it is always worth having an awareness of the rich complexity and inter-relationships of the materials one works with.
COPPER is the metal of greatest interest to the sculptor and bronze founders simply because the vast majority of cast art, design and decorative work is produced in alloys derived from this PARENT metal.
Copper is rarely extracted naturally to any degree of purity. As a result, high purity copper products will always have undergone a protracted ore refining process to achieve acceptably low levels of foreign materials contamination. Whilst all the metals and alloys used in art founding undergo some degree of refinement, it is important to realise that high purity metals are not by definition ‘better’ for use. Indeed some level of ‘contamination’ (natural or manufactured), is often beneficial to the cast material’s characteristics and working properties. An example of positive contamination can be found in the presence of ARSENIC in copper ores. Whilst extracted from copper for many applications, arsenic can in some instances be beneficial, especially in brasses (the cast grade SCB6, and wrought grade CZ126 [ISO CuZn30As] are typical). In this case, arsenic provides the alloy with an improved resistance to corrosive attack.
More commonly, alloying involves deliberately adding in quantities of one or more elements. These elements are either completely absent in the natural ore, or else present in amounts too small to be of any notable influence (sometimes known as TRACE elements). Added elements need not necessarily be metals, for instance oxygen (added to copper), silicon (to aluminium) and carbon (to iron) are important examples of non-metallic additions to selectd alloys.
To distinguish each individual alloy from it’s parent metal and other alloy versions of a broadly similar composition, each product is given a specific name and/or code. For example copper, alloyed with tin, produces the copper/tin product known as BRONZE. Similarly, copper/zinc alloys are referred to as BRASSES, and copper/tin/zinc/+lead alloys as GUNMETALS and LEADED GUNMETALS respectively. An alpha/numeric code is also used to identify each further variant of the alloy, for instance (in the UK), LG1, LG2, LG3, and (US), 844, 836 are all designations for leaded gunmetal (each indicating different proportions of the same alloying elements). Stock material, either in the form of a foundry ingot or a preformed wrought shape, is also often marked with a paint indicator code for easy visual recognition – for instance, an LG3 (UK standard) gunmetal ingot is usually identified visually by a blue and white stripe paint marking, silicon bronze ingot is identified by a red and yellow stripe mark.
Bronzes, brasses and gunmetals are not metallic elements, instead they are each an alloy variation of their common parent metal – copper (see COPPER ALLOY FAMILY TREE). When describing the composition of a particular alloy, the ‘parent’ element (that which forms the greater part of the alloy) is normally given first, with subsequent alloying elements stated in order of decreasing quantity. Copper/nickel alloys are therefore composed mostly of copper with lesser nickel additions, by the same token nickel/copper alloys are composed mostly of nickel with a lesser quantity of added copper. Finally, copper/nickel/chromium alloys are mostly copper, with a lesser quantity of nickel, and an even smaller amount of chromium.
PROPERTIES OF METALS >
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