Deterioration
Deterioration of Outdoor BronzesBronze sculptures, when exhibited outdoors, deteriorate with time; the rate of alteration depends on the aggressiveness of the environment. All outdoor environments contain pollutants that arise from specific local and regional sources. The concentrations of those pollutants change with the prevalent directions of wind, rain, and airborne particles, and may vary from season to season and year to year. The green and blue colors seen on many outdoor bronzes in Philadelphia derive mainly from sulfates of copper (the minerals brochantite and antlerite). Unfortunately, these green/blue layers are not protective; corrosion continues to attack the metal underneath. The line drawings illustrate the stages of deterioration that often appear on an outdoor bronze sculpture. The boxes show the corrosion stages in hypothetical, highly magnified cross sections. What is a patina?
Created by the reaction of various chemical solutions on a bronze surface, a patina may be dark or bright and made up of one or many layers. Often applied with heat, a patina usually eats slightly into the metal during application, forming a layer/layers of mineralization. However, artificial patinas are not chemically inert. They change as they age or as they weather from exposure outdoors; they develop into new mineral forms. If no protective coatings are applied over a patina, it may grow slightly in thickness during the initial or induction phase of corrosion from outdoor exposure. The rate at which a patina will alter is determined by a number of factors, including the acidity of local rain, the amount of abrasion from airborne particles, the accumulated material on the surface, the amount of moisture on the surface, and the amount of time that moisture stays on the surface. Stage 1
A patina is applied intentionally to the clean surfaces of a bronze, usually at the foundry where the bronze is cast. Stage 2
The most exposed surfaces of a patinated cast bronze begin to be converted to bright-green sulfate, while airborne particles build up as dark accretions on surfaces that are not well washed by rain or other condensate. Stage 3
Enough soluble corrosion products accumulate on the surface to cause streaking–acid etching–when the sculpture becomes sufficiently wet. In the crevices, where water does not reach, dark accretions continue to spread and thicken; these accretions typically contain copper, sulfur, and oxygen as well as carbon, calcium-, silicon-, or iron-containing materials. Corrosion penetrates further into the metal. Stage 4
Pitting develops in and around the black accretions. Pitting may spread underneath what appears to be a stable green/blue crust, especially in the presence of sufficient chloride ions. Stage 5
Theoretically, the final stage of deterioration would be the complete conversion of the bronze to copper sulfate corrosion products; that is, complete mineralization will be the ultimate fate of the sculpture unless the corrosion processes are arrested by suitable treatment.
For more information, please contact the Rodin Museum at (215) 568-6026.
