Portrait of a woman
Degas painted "Portrait of a Woman" between 1876 and 1880, and it was purchased by the National Gallery of Victoria in Australia in 1922.
Since the painting arrived in Australia, a discoloration across the face in the portrait, caused by pigments from the earlier painting underneath, has slowly become more visible.
But these techniques were unable to reveal more details in the hidden painting, or to provide any clues about the identity of the mysterious woman in the portrait.
The research required placing the valuable painting in a moving cradle between a stationary X-ray beam and specialized X-ray detector at the Australian Synchrotron in Melbourne.
What lies beneath
It took 33 hours to precisely scan the painting by moving it slowly across the X-ray beam, while measuring the X-ray response from each pixel-sized area of the canvas.
Mapping the painting
The specialised X-ray detector allowed the researchers to monitor the data in real time as the experiment progressed. This photograph shows the first images of the face in the hidden portrait, based on a map of zinc atoms in the pigments of the painting.
The final result is detailed enough to allow the researchers to make a positive identification of the woman in the hidden portrait as Emma Dobigny, an artist's model who sat for Degas and other painters in Paris in the 1860s and 1870s.
The lead author of the latest research, conservator David Thurrowgood, said the hidden image appears to be almost identical to another rarely seen portrait of Dobigny by Degas.