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WHAT GOES WRONG WITH METAL SCULPTURES?
One of the primary aims of this site has been to provide an insight into the the various metals and metal alloys used for sculpture making. Often as not, the alloy used for creating a cast artwork is a compromise between it’s visual qualities (either the natrual colour of the metal, or it’s ability to take on a finish such as a patina), and mechanical properties (the metal’s hardness, plasticity, ductility, elasticity, tensile strength and so on). Although the artist is principally concerned with the visual quality of the material, there is no escaping the influence of the casting alloy’s mechanical properties on the structure and longevity of the sculpture itself.
As we have seen from the METALS section of the site, the noble metals (especially gold and platinum), are exceptionally resistant to corrosive attack. This abiltiy to resist chemical activity is partly why a relatively large number of high quality gold artifacts can be found in our museums and historical collections. Other metallic materials, such as iron, are less able to withstand exposure to the elements, therefore recovering iron artifacts in an excellent conditon is a relatively rare event (usually any quality item has been uncovered in an exceptioanlly dry or airless environment).
In terms of their ability to resist to attack by acids, alkalines, chemical compounds and organic materials or organisms; copper and copper alloys generally falls somewhere between that of noble metals, and those metals exceptionally prone to corrosion.
Oxidising acids (especially nitric acid), and some salts (ferric chloride for instance – used for some copper etching processes), are especially damaging to copper and most of it’s alloy variations. On the other hand, a select number of copper alloys (aluminium and nickel bronzes for example), are extremely resistant to corrosion if fact these alloys have been specifically developed for use in marine or non-oxidising acid environments. Of of the two common sculpture casting alloys (silicon bronzes and leaded gunmetals), silicon bronzes generally offer the greatest corrosion resistance. The zinc and tin components of leaded gunmetals can be prone to corrosive attack, resulting in a dezincification (of zinc), and/or destanification (of tin) in the alloy. Where available, a silicon bronze casting alloy is advised for sculptures likely to be situated in poor environmental conditions (bear in mind though, that this suggestion does not imply that leaded gunmetals and other alloys are by definintion, inferior materials).
Despite the relatively good resistance of most copper alloys to corrosive attack, copper and copper casts can still be structually damaged by chemical agents in the environment. In addition, casts can also suffer mechanical damage (from accidental shock or impact, vandalism, fatigue, stress cracking and other factors). Sculptures cast in copper alloys are by no means indestructable. The chemical agents which typically do the most damage to copper alloy casts and ultimately lead to the effect widely referred to as BRONZE DISEASE, are copper sulphates and copper chlorides. These, as well as other compounds (copper carbonate for instance), develop on the cast through environmental exposure, though some examples may also be attributed to artificial patination processes (typically deposits of malachite [green carbonate] and azurite [blue carbonate], but also chlorides, possibly as a result of using ammonium chloride, a common patina inducing compound). If the cast sculpture is exposed significant levels of sulphurous or nitrous compounds, chemical reactions can be initiated, which if left unchecked, can convert copper structures into cuprous chloride and other unstable compounds.
Typically, any sulphuric and nitric acid deposits on sculptures derive from airbourne sulphur dioxide and nitrous oxides which have been expelled into the atmosphere as industrial waste. Harmful particles combine with water vapour to form ACID RAIN. The precipitate falls onto exposed sculptures and other copper structures, which go on to develop layers of copper chlorides and sulphates, ultimately resulting in the formation of heavy green and black deposits, promoting corrosion of the underlying metal fabric. The corrosive effect is intensified with the presence of salts, so sculptures situated in a polluted marine environment may suffer accelerated deterioration from the combined effects of corrosive acids, salts, oxygen and electochemical activity, all of which can be devistating to the scuplture’s appearance and longevity.
Ideally, some form of regular maintenance programme (see above), will help preserve the cast, or at least inhibit the effects of corrosive agents on the metal’s fabric. Unfortunately, for many sculptures, the damage will have gone unchecked – possibly for many decades, requiring the cast to undergo active restorative treatment.
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