Particle accelerator reveals hidden van Gogh painting

Using a thin beam of synchrotron X-rays generated by a particle accelerator, European scientists have reconstructed a portrait of a peasant woman painted by Vincent van Gogh that had been concealed beneath another painting for 121 years.

The image, unveiled in a scientific journal to be published today, bears a striking resemblance to a series of somber portraits the artist produced in the Dutch town of Nuenen, where he composed “The Potato Eaters,” regarded as his first major work.

Conventional X-rays had revealed rough outlines of the portrait, which Van Gogh covered 2 1/2 years later with a vibrant landscape of a flowering meadow after he moved to Paris and embraced Impressionism.

But the X-rays weren’t good at distinguishing between the many layers of paint on the single canvas, and pigments made from heavy metals obscured colors derived from other elements.

“We get a very partial, fragmentary, color-blind view,” said Joris Dik, a materials scientist and art historian at the Technical University of Delft in the Netherlands.

So Dik and his colleagues took the painting, titled “Patch of Grass,” to a particle accelerator in Hamburg, Germany. The small but intense X-ray beam excited the atoms on the canvas, causing them to emit X-rays of their own that were captured by a fluorescence detector. It took two days to scan the roughly 7-by-7-inch portion of the meadow that masked the portrait.

Since each element in the painting had its own X-ray signature, scientists were able to identify the distribution of metals in the various layers of paint and construct a three-dimensional model of the work. Then the team peeled off the layers one by one.

The top layer consisted of paints made with zinc, barium, sulfur and other elements. Behind that they found a uniform distribution of lead, which was used as a primer to hide the portrait and prepare the canvas for a new painting. Once that was removed, they combined the distributions of two more elements – mercury and antimony – to produce the outlines of the hidden portrait.

Then, with the help of computer software, the team embarked on an elaborate version of painting by numbers.

“We colorized those two distributions according to the color that the pigment would have had,” Dik said.

Chemical analysis revealed the mercury was an ingredient of vermilion, the red pigment used to color the woman’s lips, cheeks and forehead. Antimony was a component of Naples yellow, which was mixed with zinc white paint to highlight certain areas of the woman’s face, according to the report in the August issue of Analytical Chemistry.