By Emily Farek

If you’ve ever had a sunburn, you know your skin’s appearance can change depending on what exposure it’s experienced. Metal sculptures have a skin, too — it’s called the patina. Patina is the outermost layer of the sculpture that has reacted with its environment in natural aging, or has been purposefully treated with certain chemicals to give it an particular appearance.

In the Rodin Sculpture Garden, all of the sculptures have a dark brown patina that was chemically applied by the foundry where the sculptures were cast. This dark brown patina was approved by Rodin, and is a key part of the appearance that Rodin wanted for his artwork.

Rodin’s signature on the base of The Three Shades. Photo by Emily Farek.

This particular color was applied in a process called chemical patination. Different colors can be obtained by using sulfides, nitrates, oxides, acetates, and chloride solutions. Typically, brown patinas are created using ammonium sulfide, iron oxide, and/or ferric nitrate solutions.  The original patina on the Rodin bronzes required an extra element to form — heat. Hot chemical patination uses an open flame or a blow torch to heat the metal, which is brushed with liquid salt solutions to get the desired color and tint. There are many variables involved in the process of making a certain patina. The amount of heat applied, the amount of salt solution brushed on, the strength of the solution, the size of the brush, and the timing of everything all influence the resulting color. This makes a specific patina hard to replicate.

Brushing salt solution onto a sculpture during hot chemical patination. Photo courtesy Tom Schrey, Artworks Foundry.

Every day in the sculpture garden, visitors interact with and enjoy the sculptures. Unfortunately, sometimes the sculptures are touched and climbed on, which damages the surfaces over time. When we touch metals with our bare hands, the oils and whatever else is on our hands deposit onto the metal surface. These oils and materials react with the surface, and can abrade, or wear down, the patina. When the sculptures are climbed on, rocks and dirt are ground into the sculpture’s surface by the climber’s shoes, potentially scratching the patina and causing a lot of damage. When the patina is altered or damaged, the sculptor’s vision for the artwork is also being altered or damaged.

A hand battery at the Exploratorium in San Francisco, CA. You can see how the copper plates have discolored over time due to touching. Photo by Emily Farek.

Orpheus and The Three Shades are two bronzes in the Rodin sculpture garden, which have some patina loss on their bases. In these places, the bare bronze is exposed to humidity and air in the outdoor environment — the way a skinned knee exposes underlying tissue. When the metal ions in bronze react with the environment, they form green cupric carbonate corrosion products. This color is much different from the patina Rodin purposefully chose for his sculptures and alters the sculptures’ appearance, much like a scar does on human skin. If the patina loss continues, the corrosion product formation will only grow.

To prevent this from occurring, Sam and I repatinated the bare areas on Orpheus using a cold chemical patination technique followed by rinsing and waxing. By swabbing the bare areas with Sur-Fin Oil Rub Bronze EZ Brown-70 using cotton applicators, we were able to react the outer layer of metal so that the bronze would no longer be bare. The EZ Brown-70 chemical was chosen because of its similarity in color to Orpheus’ original patina.

Our treatment on Orpheus was successful in that the bare bronze reacted with the chemical patina and blended aesthetically with the rest of the sculpture. The treatment also inhibited the formation of corrosion product. Unfortunately, the repatinated areas wore off within a few weeks, once again exposing the bronze. Perhaps this was due to the chemical composition of the solution we used to repatinate, or maybe human interaction with the base caused the outer layer to rub off once again. More research on chemical patination solutions will be necessary to make an informed decision for future treatments. In the meantime, we’ll monitor the sculptures and maintain a thin layer of wax over the surfaces. We want to be sure that these sculptures can be enjoyed the way they were intended for many years to come!

Orpheus  in the Rodin Sculpture Garden, Cantor Arts Center at Stanford University. Photo by Emily Farek.

References:

Fucito, Luisa. “Chapter 10: Methods and Materials Used for Patination at the Fonderia Chiurazzi.” The Restoration of Ancient Bronzes: Naples and Beyond. Ed. Erik Risser and David Saunders. Los Angeles: Getty Publications, 2013. 137-47. The Restoration of Ancient Bronzes: Naples and Beyond. J. Paul Getty Museum, Los Angeles. Web. 12 Sept. 2015. <http://www.getty.edu/museum/symposia/restoring_bronzes/&gt;.

Gettens, R.J. 1969. The Freer Chinese Bronzes, vol. II Technical Studies. Washington, D.C.: Smithsonian Institution, Freer Gallery of Art, Oriental Studies, No. 7. 171.

Hamilton, Donny L. “Cupreous Metal (Copper, Bronze, Brass) Conservation.” Methods of Conserving Archaeological Material from Underwater Sites. Nautical Archaeology Program, Texas A&M University, n.d. Web. 17 Sept. 2015. <http://nautarch.tamu.edu/CRL/conservationmanual/File12.htm&gt;.