Raman spectroscopy illuminates Renoir's intentions

Museum conservators and scientists have teamed up to determine the intentions of 19th century Impressionist painter Pierre-Auguste Renoir in his 1883 portrait Madame Léon Clapisson. The portrait shows Madame Clapisson in an evening dress set against an abstract background.

That background became an issue when the painting was taken into the conservation lab at the Art Institute Chicago. According to the Art Institute, “Removal of the frame led to an unexpected discovery: around the perimeter at the top and left was a sliver of intense violet-red, much more vibrant than the adjacent colors of the background that had not been protected from light by the frame. This deeply hued clue tipped off researchers to the fact that the cool and restrained mood of the current image probably does not match the artist’s original intent.”

Kenneth Chang writing in the New York Times quotes Kelly Keegan, a conservator working on the project, as saying that a look under the microscope revealed that the change in background color was not due to a revision by Renoir himself or subsequent imprudent cleaning. “It’s very clear it’s original paint,” she said. “You can see there are particles on the surface that are sort of translucent now.”

Chang notes that conservators regularly employ infrared, ultraviolet, and X-ray inspection to study how paintings may have changed over time, but such approaches are not effective for studying the organic pigments popular with the Impressionists.

Instead, he writes, the museum’s senior conservation scientist Francesca Casadio and conservation scientist Federica Pozzi, both of whom trained as chemists, employed surface-enhanced Raman spectroscopy (SERS), in which a sample of red pigment from the painting's edge was placed on silver nanoparticles. The scientists were able identify the pigment as carmine lake, made from crushed and ground cochineal insects.

Even in Renoir's era, Chang notes, paint catalogs indicated that the pigment was not stable when exposed to light. He quotes Casadio as saying, “I think the artists didn’t realize the colors could fade away entirely.”

The Art Institute does not plan to restore the original painting based on the Raman spectroscopy results, but using image processing software, the conservators have been able to reconstruct the work’s original colors in a full-scale digital reproduction. The original and reproduction will be on display side-by-side at the Art Institute Chicago through Sunday.