WHAT GOES WRONG WITH METAL SCULPTURES?
Often as not, the alloy grade used for casting and fabricating a sculpture is ultimately a compromise between the inherent visual qualities if offers (or at least the potential to accept an applied finish such as a patina), and its mechanical properties (the metal’s hardness, plasticity, ductility, elasticity, tensile strength and so on - see METALLURGY). Although artists are principally concerned with the visual quality of materials, there is no escaping the influence of an alloy’s mechanical properties has on the structure and longevity of the sculpture itself.
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 the gold artifacts displayed in museums are of exceptional quality. Other metallic materials, iron for example, are less able to withstand exposure to the elements and therefore recovering iron artifacts in an excellent conditon is a relatively rare event (unless the artifact has been situated in an exceptioanlly dry or airless environment).
In terms of having the ability to resist to attack by acids, alkalines, chemical compounds, organic materials and organisms; copper and copper alloys generally fall into a category somewhere between that of noble metals and those metals exceptionally prone to corrosion - in other words, copper's inherent corrosion resistance is average.
Oxidising acids (especially nitric acid) and some salts (ferric chloride is typical), 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 (that this suggestion does not imply that leaded gunmetals and other alloys are inherently inferior materials).
Despite the relatively good resistance of most copper alloys to corrosive attack, copper and copper alloy 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 compounds, in addition to others (eg copper carbonate), 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], also chlorides [ammonium chloride], a common patina inducing compound). Likewise, if a 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 (see also PATINATION).
Typically, the sulphuric and nitric acid deposits found on out of doors 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, reacting with the metal to develop layers of copper chlorides and sulphates, ultimately resulting in the formation of heavy green and black deposits and 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 ELECTROCHEMICAL activity, all of which can be devistating to the scuplture’s appearance and longevity.
Ideally, some form of regular MAINTENANCE programme, 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.
Heavily corroded bronze surface.
Note heavy pitting & green 'patina'
of copper chlorites.
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