The Patina Process at Carolina Bronze Sculpture
Bronze sculpture has long been valued for its long-lasting beauty. While other materials crumble with the ravages of time, bronze endures. Often its beauty is considered to increase with time as its colors grow richer and develop a deeper layered look. Those colors are due to the patina that develops. Here we will look at what a patina is, how it develops, and why it is so crucial to the endurance and beauty of bronze sculpture.
What is a Patina?
A patina is a thin surface layer of a chemical film that forms on bronze. This film is formed as a chemical reaction when bronze is exposed to oxygen and moisture. This process is commonly known as oxidation and is most recognizable when it comes to iron as rust. Where bronze differs from iron is in the way bronze oxidation can reach a point in which it stabilizes. This stable patina layer then protects the underlying metal from further corrosion and deterioration.
The patina layer can occur either naturally over time or artificially through controlled chemical application. While some sculptures are allowed to develop a patina over time, this is not the most common method of modern sculpture creation. (One of the most notable sculptures with a naturally occurring patina is the Statue of Liberty. When it was first installed it was the color of a shiny penny since it was made of copper sheets over an iron framework.) Most modern artists, however, choose an artificially applied patina as it enables them to have some control over the artistic look of the finished sculpture.
A natural patina can develop inconsistent levels of color. Sometimes this results in streaks as the oxidation occurs faster along the pathways that rain flows down a sculpture.
Why Do Patinas Matter in Bronze Sculpture?
Paint Versus Patina
While some foundries and artists choose to apply paint to their bronzes to achieve their artistic goals there are some advantages in a chemical patina over a paint-based patina. Part of that is in the nature of the color. Paint is a pigment applied using a binder and solvent. It sits on the surface of a sculpture and over time especially with outdoor exposure will begin to peel away. Painting bronze also requires the use of primers and generally is not as long lasting or durable as a true chemical-based patina.
Chemical based patinas while not capable of as many color variations as paint do have several positive aspects. First because it is a chemical change to the actual surface of the metal itself and not a separate layer on top of the metal it will not peel or flake off.
Durability
A properly sealed stable patina can help to preserve the overall state of a sculpture for potentially thousands of years. It also has the added benefit of outlasting paint. Typically, there are a few things that can damage a patina.
Friction
Friction is one of the most common. Constant handling of a sculpture will wear away protective coatings and eventually wear away the layers of metal containing the patina layer. This is often the cause of shiny golden looking areas on a bronze sculpture. If handled long enough without protective coatings of patina, lacquer, and wax the combination of friction and skin oils can result in wearing away the actual metal.
Chemicals
Many times, chemicals ranging from household cleaners, pesticides, fertilizers, snowmelt salt, and chlorine can have a damaging effect on a patina. The nature and severity of the damage depends on the chemical and the amount of time the sculpture is exposed to that chemical. Chlorides can react with the copper in the bronze to create hydrochloric acids that will attach the copper forming copper chloride in a repeating cycle that ultimately damages the bronze. This is commonly referred to by conservators as “bronze disease.”
Artistic Intent
Artists often have a specific vision in mind for their sculpture. With the application of a patina the artist can create a protective artistic layer of color that can convey age, enhance the texture and portray the subject matter in a visually fitting way.
What is the Patina Process?
Step One: Surface Preparation
Prior to patination the bronze must be cleaned of any oils from tools or human skin contact. This is done by sandblasting the metal clean. This also provides a uniform surface that will receive the chemicals.
Step Two: Heating the Bronze
Heating the bronze helps the chemicals stick to the metal and speeds up the oxidation reaction. This allows a process that would take months or years to occur naturally to become fully formed in a matter of hours.
Step Three: Applying Chemical Solutions
The chemicals used react with the bronze creating the colored patina. They can be applied in nearly as many ways as paint. Sprays, brushes, cloth, and sponges can be used to create layered depth and visual patterns replicating many artistic designs. Sometimes patina chemicals can even be applied by immersing the sculpture in a chemical bath.
Step Four: Cooling and Rinsing
Rinsing the sculpture with water serves a dual purpose by washing off residual chemicals and cooling the metal to prevent the patina from progressing further. Some of the patina chemicals can include acidic compounds that can etch the metal if left on too long. Rinsing also protects the seal coating from reacting with the patina chemicals.
Step Five: Sealing the Patina
After the desired patina is developed the patina should be sealed. For an outdoor patina a clear lacquer coating will help prevent moisture from making the patina develop beyond the desired level. It also provides a protective layer reducing damage from people touching the sculpture over time. As an added layer of protection, a clear waste coating of wax is also a means of protecting a sculpture patina. The wax coating is called a waste coating because if the wax is defaced by vandalism or environmental damage it can easily be replaced and cleaned to preserve the desired patina. Without a waste coating, graffiti often requires the patina be reapplied to repair. This becomes a costly and lengthy process while a waste coating of wax makes the process much easier.
How Do You Preserve a Patina Long Term?
Even the best applied seal coat and waste coat can be degraded by time and things such as wear and tear due to sunlight, animals, human handling, and pollution. In most outdoor settings waste coatings of wax rarely last longer than a year. During rough weather, or hot summers that wax coating can begin to wear away. This is part of its purpose as a "waste" coating. It is a removable outer coating that can be easily removed during cleaning and replaced relatively easily as well. Sculptures with exposure to salt water or chlorinated water are at even greater risk of wax coating loss due to the caustic nature of salt and chlorine.
Annual maintenance of a sculpture is recommended for outdoor pieces. Coastal or fountain installs may require maintenance twice a year. Maintenance of an outdoor sculpture includes cleaning a sculpture, removing old wax layers, dirt, and chemical residue. After a sculpture has been rinsed clean and dried a new waste coating of wax is heated onto the sculpture. Then the sculpture is buffed lightly after the wax has dried.