The beautiful green color is the result of decades of air and moisture reacting with the copper in the bronze. Although a natural patina can have streaks and spots, it is highly coveted by antique bronze collectors because is it gives a sculpture is character and takes many years to fully develop. However, a natural patina usually develops over a man-made patina that was applied by the artist during the casting process.
To truly understand the process of applying the patina to a sculpture you need to go back to the raw bronze material that was used to cast the sculpture in the first place.
Bronze metal is not a naturally occurring metal in and of itself, but rather an alloy of copper, meaning copper mixed with other metals.
To apply a patina an artist reheats the bronze sculpture using a roofer's torch or to a temperature over degrees Fahrenheit. Chemicals are then applied with a brush or spray bottle.
Once the desired color is achieved the sculpture is rinsed with cold water to stop the chemical reaction. Once the desired color is achieved two coats of wax are applied to the sculpture while it is still warm to seal the patina.
Different chemicals react with the bronze to achieve different colors. One of the most common patinas is Liver of Sulfur potash which results in a golden-brown to almost black color.
Other chemicals include Ferric Nitrate golds, browns and reds and Cupric Nitrate greens and blues. They can also be used in layers to achieve a variety of artistic effects. While most patinas are applied hot, patinas can also be applied cold. Cold patinas are used most frequently to make repairs.
When copper reacts with oxygen, it oxidizes and generates a greenish-blue layer that protects the metal from further corrosion. Any metal that contains a high amount of copper can turn green. But both are types of oxides. The copper has reacted with oxygen, water, and carbon dioxide in the atmosphere. Both of these metals have a really high copper content, which is why they form that same patina when they oxidize.
Patina finishes are actually pretty popular for things like brass door handles, copper roofing, and other kinds of ornamental pieces.
But beyond cosmetics, there are some practical reasons why you might want it. Copper oxide is an interesting substance. The thicker the copper oxide layer gets, the more it will resist further corrosion. The result is that rust can eat away entire structures over time. One example is the Statue of Liberty, built in Over years of being exposed to the elements later, the thickness of the copper oxide layer is only 0. Or you might just prefer the bright look of copper instead of the dull greenish color of the patina.
Since copper is a popular material for things like wires and electrical connections, oxidation is a complete pain in the backside. Ok, so copper and metals with a lot of copper in them will eventually turn green on their own by just being exposed to air and water. But what if you want to speed this process up? First off, clean the metal thoroughly with a degreaser. Any oils will end up protecting the metal from oxidizing and turning green.
Coatings may also reduce the ability of water to react with the copper salts. Perhaps the most effective treatment of all is prevention. Keeping unstable bronze sculptures in low humidity—as in several displays of the exhibition Power and Pathos: Bronze Sculpture of the Hellenistic World —inhibits the chemical reactions that cause damage in the first place. My work as conservator in the Antiquities Conservation Department at the J.
Paul Getty Museum involves a variety of ancient materials. Comments on this post are now closed. Please e-mail web getty. Your explanation of the good and bad effects of patinas with pertinent images was so helpful to me. Your article was easy to access, interesting to read, due to the array of details included, and has the authority of your experience working to conserve ancient objects.
I am teaching an introductory course in Humanities and I appreciate your sharing this! Many thanks!! Bronze Statues and sculptures are well known to be used in temples since chola periods in south India. Bronze is used in massive sculptures as well as statues with minute decorative details in them.
Well known for its durability to retain its structure for long makes bronze a preferred choice for sculptures in open display. TY for your response — Johnny. Thank you for posting this. Font Size. Chemical Finishing Techniques. Displacement Plating Electrochemical Plating. Physical Finishing Techniques.
Support Conservation. Chemical Finishing Techniques : Patination.
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