Across the world, thousands of bronze sculptures stand outside and in public areas. Depicting historical figures or even abstract art, these monuments may be part of cultural heritage and give distinction to an area. Undoubtedly, there is also an impact that the outside setting grants to the aesthetic value of these structures. Many examples come to mind: Copenhagen’s The Little Mermaid is perched atop a boulder at the waterside and gazes longingly at pedestrians strolling on the promenade; Manhattan’s Charging Bull shows assertion in a tempestuous Wall Street as it aggressively leans back to dash ahead; near Arlington National Cemetery, the Iwo Jima Memorial honors Marines and their comrades by commemorating the iconic photo of six Marines raising the American flag on the island of Iwo Jima. Of course, while moving these monuments indoors would protect them from the elements, it would also detract from their impact.

Environmental Risks Faced by Bronze Sculptures and Statues

Lamentably, the consequences of a placing a bronze statue outdoors can be severe but not unexpected. Exposed to the elements and the public, the damage a bronze structure may undergo includes corrosion, bird droppings, plant debris and even vandalism, all of which can obscure the appearance and message of the structure and necessitate special cleaning procedures. Corrosion is among the most detrimental damage to befall a bronze statue. The severity of bronze corrosion depends on the metal's composition, the climate, pollutants in the atmosphere and previous protective measures. While a dark brown copper oxide or light green copper sulfate patina may sometimes be a desirable appearance, severe forms of chloride corrosion or bronze disease require complex treatments to save the piece from destruction. (Learn more in the article The 5 Factors of Atmospheric Corrosion.)

Five Stages of Bronze Corrosion

The U.S. General Service Administration’s report on bronze protection lists five stages that a corroding outdoor bronze undergoes. Initially, a copper oxide film forms. The composition and rate that this initial film forms factor into how well this layer acts as a protective barrier. Next, the metallic surface converts to copper sulfate over areas with the harshest exposure. Over time, the next stage occurs where copper sulfate and sulfide products pool and streak. The appearance of black “scabs” and discoloration caused by the pooling and streaking of these compounds can make a piece unrecognizable. Depending on the presence of chloride, the pitting stage spreads around the scabs and can spread below the surface and cause unseen damage. Finally, the exposed surface completely transforms to a green copper sulfate.

Bronze Corrosion Treatment and Cleaning Options

The treatment of a deteriorating bronze sculpture, especially an older, iconic one, can be a tricky process that depends on the status of the bronze and the opinion of the art conservationist. Often, a copper sulfate patina is deemed to contribute to the piece’s aesthetics and is left alone. It is when the black streaking occurs and there is significant detritus or damage on the structure that removal and repair is advised. The conservationist deems that repair measures only occur if the message or character of the piece will be obscured. On the other hand, the corrosion/coating scientist insists the best possible corrosion-prevention and durable system should be used. The compromise is that the pretreatments and coatings used must be completely removable and not result in a change to the structure’s appearance. However, this means that treatments do not last long and the structure must be monitored and re-coated regularly.

The actual cleaning process is even more delicate. It usually is laid out by first inspecting the statue, washing away debris, removing corrosion products, treating the statue with a corrosion inhibitor, and finally applying coatings of waxes and lacquers. Dennis R. Montagna of the U.S. National Park Service provided a detailed account of this process in 1987 with the repair of the severely deteriorated Thaddeus Kosciuszko Monument in Washington, D.C. With the use of a 62-foot (19-meter) hydraulic lift, the team inspected the statues for any lack of stability or parts needing replacement. Next, a pressure washer spraying water and detergent removed droppings, bird nests and other plant debris. To remove corrosion products, ground walnut shells meticulously abraded the structures at a controlled pressure and distance, 6-7 in2 (15-18 cm2) at a time. (Discover the advantages of this technique in Understanding the Industry Shift to Wet Abrasive Blasting.) After another wash, a corrosion-inhibiting benzotriazole solution was pumped over the figures and dried. One more wash removed benzotriazole crystal deposits before the statues were heated with a blowtorch and protective wax coatings applied. The lifespan performance of the coating was deemed to be between one and three years and annual inspection and reapplication was imperative. Total cost of labor and materials was $17,500 (adjusted to 2017 dollars). The cost of annual touch ups, cleanings and inspections is $1,300.

The future of outdoor bronze protection will likely involve more efficient and durable anti-corrosion pretreatments and coatings that do not change the piece's appearance. The conservationist can then be pleased with the retention of character and the scientist can be confident that the figure is better protected from corrosion.