Vincent van Gogh's paintings aren't quite what they used to be.

The paintings are fading. Studies find the yellows are turning brown. The color red also poses problems.

Researchers are now moving closer to finding out why the paint in van Gogh's masterpieces is fading. It's because van Gogh preferred to use a pigment known as red lead.

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Red lead is most familiar to us as orange-red rustproof paint. Artists have treasured the brilliant color of this pigment for their paintings since ancient times. Various ageing processes cause discoloration of the saturated hue over time.

A team headed by Koen Janssens at the University of Antwerp has been able to further clarify the degradation process of red lead that causes this bleaching of the color. The researchers examined a microscopically small sample of van Gogh's "Wheat Stack under a Cloudy Sky," an oil on canvas, using the micro/nanofocus beamline P06 of PETRA III.

Located in Germany, PETRA III is the most brilliant storage-ring-based X-ray radiation source in the world.

Researchers made use of X-ray powder diffraction mapping and tomography techniques to determine the distribution of different crystalline compounds within the sample with very high spatial resolution and specificity. Their methods resulted in a depth profile of the sample's composition without cutting it open in contrast to conventional X-ray crystallographic methods.

They found the red lead is covered in a "degradation product." One chemical may be the culprit: plumbonacrite.

It's causing a chemical buildup when light is shone upon it. The researchers say plumbonacrite has never been reported "in a painting dating from before the mid 20th century."

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