A team of European scientists unveiled on Wednesday a new method for extracting images hidden under old masters' paintings, recreating a color portrait of a woman's face unseen since Vincent van Gogh painted over it in 1887.

Undated handout picture shows a painting underneath Van Gogh's famous work 'Patch of grass' in Hamburg. Scientists have made a coloured view of an early rejected painting underneath Vincent van Gogh's 'Patch of Grass' painting, using advanced X-ray techniques, a Dutch university said on Wednesday. [AP]

For years, art historians have been using x-rays to probe artworks hidden underneath other paintings, a technique resulting in a fuzzy, black-and-white image. But Joris Dik, a materials scientist from Delft University, and Koen Janssens, a chemist from the University of Antwerp in Belgium, combined science and art to engineer a new method of visualizing hidden paintings, using high-intensity x-rays and an intimate knowledge of old pigments.

The pair used the new approach on "Patch of Grass," a small oil study of a field that Van Gogh painted in Paris while living with his brother Theo, who supported him.

While not exact in every detail, the image produced is a woman's head that may be the same model Van Gogh painted in a series of portraits leading up to the 1885 masterpiece "The Potato Eaters."

The new method will allow art historians to obtain higher quality and more detailed images underlying old masterpieces. In Van Gogh's case, it could reveal details of works that were painted over. For other works, it could provide new insights into the studies that the artist built a painting on.

Dik and Janssens used high-intensity x-rays from a particle accelerator in Hamburg, Germany to compile a two-dimensional map of the metallic atoms on the painting beneath "Patch of Grass," which is part of the large Van Gogh collection in the Kroller-Muller Museum in the Netherlands.

Knowing that mercury atoms were part of a red pigment and the antimony atoms were part of a yellow pigment, they were able to chart those colors in the underlying image.

"We visualized -- in great detail -- the nose, the eyes, according to the chemical composition." Dik said. Scanning a roughly 7-inch square of the larger portrait took two full days.

Though his paintings are now worth millions, Van Gogh was virtually unknown during his lifetime and struggled financially before committing suicide in 1890. He often reused canvas to save money, either painting on the back or over the top of existing paintings, and experts believe roughly a third of his works hide a second painting underneath.

The painting under "Patch of Grass" adds weight to the theory that Van Gogh mailed paintings from the Netherlands to his brother Theo, and, after moving to Paris to join him, found the old works and painted over them.

Teio Meedendorp, an independent Van Gogh expert in Amsterdam, said the underlying woman was probably painted between November 1884 and March 1885, while Van Gogh was living in the Dutch village of Nuenen. In that period he painted a series of heads in what Meedendorp called "oil lamps and candlelight," followed by the famous "Potato Eaters" of April 1885.

Both Dik and Meedendorp were excited about the prospect of using the technique to probe paintings by Van Gogh and other famous artists such as Rembrandt and Picasso.

"I was really surprised by the quality of the image, which is really promising for the future of research," Meedendorp said.

However, scanning other paintings may be difficult since the technique requires a particle accelerator, and few exist in the world and none in the Netherlands.

Dik and Janssens' scientific paper was published online Wednesday in the Journal of Analytical Chemistry.