Conservators at the Art Institute of Chicago wanted to solve the mystery of the missing pigment.

They were looking at “Madame Léon Clapisson,” an 1883 portrait by the French Impressionist Pierre-Auguste Renoir, which had just been removed from its frame for detailed study. The part that had been hidden behind the frame was markedly more vivid.

“It’s been protected from light, so we were able to really appreciate the original color in this unfaded area,” said Francesca Casadio, the museum’s senior conservation scientist. “It’s really quite striking. It’s a very scarlet-purple kind of paint.”

Although the bright edges were observed about 15 years ago, the museum did not have the scientific facilities then to investigate. But four years ago, a project to create a digital catalog of the museum’s Renoir holdings provided an opportunity to run a battery of scientific tests.

Kelly Keegan, the conservator working on the project, said a look under the microscope quickly revealed that the color shift was not a revision by Renoir himself or caused by 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.”