What is a Patina?
A bronze sculpture’s patina is the color change of the surface of the sculpture due to the copper oxidizing. The study of patinas dates back into antiquity with the Roman historian Pliny describing two types of patinas[1]. Pliny described a patina that was stable and enhanced surface appearance as Aerugo nobilis or “noble patina”. An unstable, destructive patina was called Virus aerugo or “vile patina.”
Noble Patina
Most of the color that occurs naturally on bronze is a result of the copper oxidizing into three copper minerals: Cuprite, Malachite, and Azurite. Initially the bronze develops a film of dark brown copper oxide. This is why bronzes darken with age. Eventually other elements react with the copper to create Cuprite, Malachite, or Azurite.
Cuprite is a reddish to dark red almost black oxide mineral that is often formed with exposure to sulfides. Cuprite is a secondary mineral which is relatively soft and brittle. When bronze is exposed to sulfides over time the copper can begin to react and create cuprite formations. Cuprite is not commonly seen forming on contemporary bronzes but may be found on historic artifact bronzes.
Malachite is a greenish mineral formation of copper carbonate hydroxide. Malachite is another secondary mineral formation that can occur on historic artifact bronzes. Over time with exposure to the carbonate ions and often when buried deep enough the formation may occur in the patina of the bronze.
Azurite is close in composition to Malachite but has a more blueish tint ranging from an Azure, light, or dark blue color. Azurite is unstable in open air and unless stored properly may begin changing into malachite as some of the carbon dioxide units are lost due to the low pressure of carbon dioxide in air.
These color changes are an actual change to the surface of the bronze. Removing surface oxidation layers removes part of the actual surface of the bronze sculpture. For modern bronzes many artists and foundries will apply a clear seal coat to prevent the surface from further oxygen exposure and oxidation. Regular maintenance of a modern bronze sculpture can preserve the artistic appearance of a piece for many years.
Vile Patina
A vile patina is often the result of exposure to additional sulfides and chlorides that react corrosively with copper when exposed to moisture. Copper sulfate often occurs on surfaces with severe exposure such as horizontal surfaces. Oxygen deprivation, particulate deposits, and moisture create a catalytic situation which is much like the fuzzy green corrosion seen on the copper wires on a car battery. Vile or unstable patinas are ultimately damaging to the bronze sculpture and have a negative impact on the appearance, integrity, and value of a bronze sculpture.
Modern Patination
The development of patination as it is performed today probably began during the Renaissance as a means of giving a sculpture the appearance of antiquity. While traditional patinas vary in shades of brown and green in imitation of naturally occurring patinas, modern formulations result in a broader range of coloration. In modern bronze sculpture, the patina is an integral part of the artistic intention.
The modern bronze artist and foundry tradition involves the artificial creation of a patina. The application of a patina on a modern bronze piece involves consideration of a variety of factors. The artistic vision of the sculptor is the primary factor. However, additional factors include such things as the composition of the metal (alloy type, metal thickness, quality of metalworking), environmental factors during patination (temperature of the metal, air quality, ambient humidity), and even the planned placement of the sculpture should be considered in the patina process.
The patineur or patina artist is a mixture of artist, chemist, and metalworker. The patineur is the modern alchemist transmuting the unfinished bronze sculpture into an artistic work of beauty conveying the vision of the artist with an appearance of age and permanence.
The process of patination requires multiple steps. First, the metal must be prepared for the patina. Oils and residue from handling the sculpture can interfere with the application of the patina. The metal needs a surface suited for the type of patina to be applied. There are several methods of prepping the metal surface for patination. However, once the surface has been prepped the process truly begins.
The patineur starts the process by heating up the bronze. The goal is to have a consistent even temperature across the surface of the metal. The exact temperature varies depending on the color desired and the chemicals to be used. Vastly uneven heating or inconsistent metal thickness can result in a patchy looking patina. The heating also aids in the durability of the patina. However, there are a select few chemicals that should be applied to a cold bronze surface. The expertise of the patineur is crucial in understanding the temperatures and chemicals to use in the patination process.
Spray Application
The method of applying chemicals also may vary depending on the desired effect, chemical composition, and the amount of control the patineur needs in applying the patina.
Spray application is typically a faster process and can help achieve an even base coat. Spray techniques are typically more often used on contemporary style patinas rather than traditional.
Brush application of patinas is the more traditional method. The range of brush types also allows for a wide variety of results in the process.
Brushed on patinas are able to be more precise and more organic in application appearance. Brush application can achieve effects that mimic the look of marble. By layered application of heat and chemicals it can create a look of depth as well as imitate the look of natural patination.
[1] https://www.getty.edu/news/bronze-patinas-noble-and-vile/