A Vincent van Gogh portrait of a peasant woman that was painted over by the artist has been revealed in extraordinary detail through use of an x-ray technique that has never before been applied to a painting.

Research had previously disclosed the vague outline of a head behind the painting, entitled Patch of Grass, but the face of the woman emerged from the centre of the work only after the picture was subjected to x-ray fluorescence spectroscopy.

Often Van Gogh painted over his earlier works; experts estimate that about a third of his early pictures conceal other compositions. Patch of Grass was completed in Paris in 1887 and is owned by the Kroller-Muller museum in the Netherlands.

Scientists scanned the picture over two days with a pencil-thin beam of "energetic" x-rays generated by a synchrotron, a machine that accelerates sub-atomic particles. The atoms in the layers of paint released "fluorescent" x-rays, which were used to map the picture's chemicals. Elements from specific paint pigments - deriving, for instance, from mercury and antimony - allowed a "colour photo" of the concealed work to be produced.

The team, led by Joris Dik, from Delft University of Technology, in the Netherlands, and Koen Janssens, from the University of Antwerp, in Belgium, then reconstructed the hidden picture.

The portrait may have been one of a series of heads painted by Van Gogh, between 1884-85, while he stayed in the Dutch village of Nuenen.

Press Association