Monitoring Bodhisattva Maitreya

Bodhisattva Maitreya, Thailand: Buriram Province, Prakhon Chai, 8th century

Bodhisattva Maitreya, Thailand: Buriram Province, Prakhon Chai, 8th century

John Griswold, Conservator
July 16, 2020

“Is it contagious? Can other sculptures catch it?” This was a question a student once asked me about “bronze disease,” a particularly insidious form of corrosion that attacks copper alloys. This was a fair question, given the rather unfortunate colloquial name for the particular cycle where chloride ions react with copper alloys to form bright green, powdery or waxy eruptions and pitting. These unstable chloride-containing compounds, in contact with moisture, in turn form concentrated hydrochloric acid, further eating into the bronze, and the cycle continues at a potentially rapid rate if left unchecked. But no, other sculptures nearby cannot “catch it.” The object must have had prolonged contact with chloride-containing solutions, such as seawater, salt-laden soils, etc. It can also be found on bronze sculptures where chlorine has been used to treat fountain water, or where winter roads are salted to reduce ice formation.

I’ve been monitoring this tiny patch of “bronze disease” on the broken end of the left arm of this Thai bronze figure since I came on staff in 2007. It might have originated when the surface was cleaned long ago with acid, a formerly common practice for removing mineral accretions that obscured surface detail. Some advancement of the bright green patch, seen in the turquoise rectangle on the photograph, had occurred. I excavated the corrosion spot with a sharp dental probe, and then applied a stabilizing chemical solution. Next, I packed in a dry, black powder containing silver, which would react with any new acid to form a silver salt, instead of attacking the copper alloy. I then sealed it all in place with a synthetic lacquer, pigmented to mask the black color of the silver oxide powder. I’ll still have to monitor this area for a new outbreak, but It should remain stable for years to come.

John Griswold is the Conservator of the Norton Simon Museum, where he leads efforts to preserve and care for the Simon collections. This article is from the series "Dispatches from the Conservation Studio."