Introduction | Essay | Paintings | Paper | Preventive | Textiles | AICCM conference papers 2008

Preventive Conservation


Outdoor bronze sculptures

Is green patina a sign of age or sickness?

Corrosion or patina?
Most metals are found in nature as composite mixtures of many compounds – they require a special smelting procedure to obtain the pure metal, that is, to produce its ‘unnatural’ form.

Corrosion is a natural process during which the purified metal attempts to return to its more complex form. Corrosion describes a natural, and often quite beneficial, process during which a corrosion layer or patina is formed on the metal’s surface, bringing the metal into equilibrium with its environment. In many cases this layer has a protective character; it slows down further reactions.

Different environments and the presence and concentration of corrosive or aggressive agents will determine the type, colour, and degree of the patina. The corrosion will also be dependent upon the nature of the surface, ventilation, temperature, microclimate, etc. In addition, slight variation in the alloy structure and composition may have a great influence upon the type and speed of patina formation.

Patina formation on bronze objects
Shortly after a bronze object is produced, a very thin layer of a brown protective patina forms on the surface. Subsequently, depending on accessibility of moisture and other agents, this patina may slowly become evenly blue/green (this may take 30 to 50 years). Prior to the Industrial Revolution in the nineteenth century, outdoor bronzes slowly developed natural patinas – thin, translucent, red/brown or brown/black with slight green tinges depending on the accessibility of moisture.

Modern environments are much less friendly to outdoor bronzes. Pollutants, particularly sulphur compounds (products of solid and liquid fuel burning) in conjunction with moisture, cause rapid corrosion. This process can be very fast and aggressive, and produces visually unpleasant results: A mottled, streaked, pitted, or powdery green surface. In addition, pollution also causes deposits of industrial dirt, which facilitate further corrosion on the metal’s surface.

Marine environments can cause a very rapid and destructive form of corrosion named ‘bronze disease’. The beautiful bright blue/green colour of this type of corrosion can be very deceptive: Bronze disease is particularly aggressive and if left untreated can cause fast and often complete destruction of the object.

Patinas can also be artificially created through a series of more less complex chemical reactions. Some of these processes were already known by the ancient Greeks and Romans who used simple and common substances such as vinegar or urine. However, it was not until the nineteenth century that artificial patination of bronzes by chemical means was generally and widely practiced on a large scale. It is also interesting to note that methods of artificial patination have been used form the Renaissance not only to alter the visual qualities of the object but, in somewhat dubious practices, to ‘verify’ authenticity, age, and value of objects of otherwise questionable provenance.

Protection of outdoor bronzes
The dangerous character of some of the patinas has been known since antiquity. Frequently discussed recipes for keeping the sculptures clean include the use of coating layers to exclude moisture and preserve the bright polish of the metal. This treatment, together with regular washing and cleaning, remains until this day the most effective way of preserving the natural or artificial patina as intended by the maker of the object. Different coating systems can be chosen depending on the bronze surface, environmental, and aesthetic considerations.

Many outdoor sculptures in big, industrial cities display the results of our modern destructive environment. Streaked, uneven, or thick patinas are just small signs of destruction of our cultural heritage.

The National Gallery’s outdoor bronzes have been installed in the Sculpture Garden relatively recently (some 15 years ago) and have been lucky to be placed in such a clean, and virtually pollution-free environment. The level of man-made pollutants in Canberra is very low, on average too low to monitor. Therefore, levels of corrosive sulphur compounds are so negligible that the bronzes can be still admired in their original condition, with a beautiful dark brown patina.

All outdoor bronzes at the National Gallery of Australia are routinely cleaned and washed to remove dirt and dust, vegetation is kept well clear, and they undergo regular annual waxing with a special wax formulated for outdoor conditions. These procedures ensure that all the sculptures are protected from the natural elements and can remain trouble-free for many years to come.