< PROTECTING A POLISHED FINISH
PATINATION
A patina is a surface effect that can develop on many metallic materials through exposure (either naturally or artificially), to a variety of chemical compounds. A patina will often develop naturally on susceptible metals over a period of time, though the ultimate quality of the patina can be variable according to the alloy's composition and the local environment.
Naturally developed patina are collectively referred to as PATINA NOBILE. This type of patina develops as a result of interactions between the metal's surface and compounds present in the immediate atmosphere, soil or any other agent in contact. In the case of copper based alloys, the interaction between the metal and the environment invariably results in the development of a ‘hallmark’ grey-green or VERDIS GRIS colouring. In chemical terms, a verdis gris development can be broadly described as a hydrated copper oxide laid over a substrate of red and black copper oxides.
In addition to chemical compounds, the presence of OXYGEN is also critical in the development of patina, which is unlikely to form easily in conditions sustaining low oxygen levels. An every day example of this is the interaction of oxygen and rain water with the FERRIC compounds on exposed steel and iron structures. The resulting reaction produces an red oxide development known as iron oxide, or RUST.
Many factors influence the initial development of a patina and some of these factors can continue to be active throughout the life of an artwork. In this respect, a sculpture’s chemical patina can be considered a dynamic feature with the potential for significant change during the work’s lifetime. Unfortunately not all of these influences lead to a positive outcome, for example the deleterious effects on metal sculptures that are exposed to urban pollution and other corrosive environments. The surface condition and colour of these artworks often indicates evidence of prolonged contact with airborne pollutants – principally chlorides (sodium chloride which encourages the development of copper chloride), sulphates and oxides (especially sulphur dioxide). These chemicals have a damaging long term effect on a cast’s metal fabric, which may endure surface penetration by 0.01mm or more for every ten years of exposure to a polluted atmosphere [ref 1]. Long term attrition by pollutants inevitably requires affected artworks to undergo an extensive treatment process if the deterioration is to be arrested or repaired (see SUPPLEMENT).
Corrosive chemicals capable of degrading cast metals are also present in soil, primarily in the form of chloride and nitrate compounds. The surfaces of copper alloy casts which have been exposed to corrosive soils over a prolonged period (historically, as ritual artifacts for burial rites; nowadays, as designs installed with part of the cast set below the soil’s surface), are gradually converted to a green crust like material composed of oxychlorides and oxycarbonates of copper and tin. The underlying metal fabric of exposed casts eventually converts to a copper oxide, effectively destroying the metal’s cohesive metallurgical structure [ref 2].
A cast sculpture or design should never be installed in such a way as to directly make contact with soil. The usual method of preventing corrosion in a ground based work is to mount exposed portions of the cast onto raised piers. Constructed from 316L grade stainless steel and insulating buffers, these corrosion resistant piers can be fixed down onto a sub soil foundation, then used to raise the lower portions of the cast artwork up to ground level, thus preserving the fabric of the cast for the long term.
APPLIED PATINATION PROCESSES >
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