Scientists just solved a 100-year-old art mystery — with a particle accelerator

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Edgar Degas’ “Portrait of a Woman” arrived at the National Gallery of Victoria to great fanfare in 1937. It was the Australian museum's first work by the French impressionist master, and it had been a bargain. "It is by a very great artist and has sufficient of him in it to make it desirable," wrote the museum's director, J.S. MacDonald.

Then he saw the painting in person.

The entire right half of the subject's face was dark and discolored, as though she'd been badly bruised. There was something on the canvas underneath Degas’ portrait — a blue and black wash? a mistake? another image? — that was slowly emerging as time turned Degas’ original oil paints translucent.

Art critics sniffed at the spoiled image: "Thoroughly unrealistic," one wrote. "Disappointing," said another. A 21-year-old member of the public wrote an indignant letter to the editor of Melbourne's morning newspaper, The Argus, calling the portrait "the latest addition to our list of costly mediocrities."

Within about eight decades, time and a few rough X-rays revealed that the discoloration was actually another face, obscured when Degas painted the "Portrait of a Woman" on top of it. But it would take some serious science to figure out who was hiding beneath all those layers of paint.

Luckily, serious science is exactly what Daryl Howard does.

Working at the Australian Synchrotron (a particle accelerator whose other uses involve searching for a cure for cancer and probing the fundamental nature of physics), Howard and his colleagues analyzed the chemical composition of the underlying image to construct a rough portrait of the person it once showed.

The synchrotron produces a beam of light a million times brighter than the sun and smaller in diameter than a hair, which scientists can use to scan tiny cells or a framed painting. When the light beam hits its target — in this case, paint molecules — the atoms respond by emitting a secondary set of rays called "fluorescence." The wavelength of the rays will vary depending on the elements being targeted, and the researchers can use that information to determine the composition of the underlying paint. That in turn helps them figure out the colors that Degas used — for example, areas with a lot of zinc were likely covered in white paint.

As the synchrotron beam grazed across the painting, dots of data coalesced into a portrait. One of Howard's colleagues developed software that would translate the data into a color image.

"It became apparent as her face started coming into view that we were really onto something," said Howard. "We were just impatiently waiting for that image to emerge to see what we were getting."

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