When Vincent van Gogh moved to the south of France in the late 1880s, he began to paint sunflowers in vibrant chrome yellow. But even before his untimely death, some of his paintings had lost their sheen and started to turn brown. 
 
 The chemistry behind the discolouration has stumped conservationists, but using tiny flakes of paint and an enormous x-ray machine, scientists believe they finally know the cause of the problem. 
 
 Conservationists can slow down the degradation, for example by installing air-conditioning units to keep the paintings cool in the summer. 
 
 One enduring mystery was why some paintings that used chrome yellow turned brown while others were unaffected. The paintings that suffered most used yellow paint that had been lightened with white pigments. 
 
 The researchers found that sunlight kicks off a chemical reaction that ultimately turns yellow paint brown. The sunlight oxidises the oil in the paint, releasing electrons. These are then taken up by the yellow pigment – lead chromate – turning it green. The mix of green paint with oxidised oil produces a chocolate brown colour. 
 
 The team led by Koen Janssens at the University of Antwerp took samples of yellow chrome paint from left-over tubes belonging to 19th century artists. To simulate the effects of sunlight, they exposed them to UV light. After three weeks, paint from one of the artists, the Flemish Fauvist, Rik Wouters, had transformed from bright yellow to deep brown. 
 
 The team analysed flecks of the paint using an intense x-ray beam less than one thousandth of a millimetre wide at the European Synchrotron Radiation Facility in Grenoble. Those tests revealed that particles of lead chromate had been "reduced" – picking up extra electrons. 
 
 Further tests showed that the reaction only took place when white pigments based on sulphates were mixed into the yellow paint. 
 
 "By mixing these white powders in, van Gogh intended to make a lighter yellow paint, but through this effect, nature darkens it. While he wanted to show a light, pale and delicate yellow, it instead becomes a darker, brownish yellow," said Janssens. 
 
 Another series of tests on flakes of paint from two other van Gogh paintings, Bank of the Seine (1887) and View of Arles with Irises (1888) confirmed the same yellow-to-brown reaction had taken place. 
 
 The findings were published on Monday in the journal Analytical Chemistry. 
 
 Vincent van Gogh began painting in his late 20s and shot himself in 1890 at the age of 37 after completing more than 2,000 works of art. 
 
 Janssens said that paintings vulnerable to the discolouration could be preserved by reducing light levels and ensuring they do not get too warm in the summer, as heat accelerates the reaction. ||||| This illustration shows how X-rays were used to study why van Gogh paintings lose their shine. Top: a photo of the painting "Banks of the Seine" on display at the Van Gogh Museum, divided in three and artificially colored to simulate a possible state in 1887 and 2050. Bottom left: microscopic samples from art masterpieces molded in Plexiglas blocks. The tube with yellow chrome paint is from the personal collection of M. Cotte. Bottom right: X-ray microscope set-up at the ESRF with a sample block ready for a scan. Center: an image made using a high-resolution, analytical electron microscope to show affected pigment grains from the van Gogh painting, and how the color at their surface has changed due to reduction of chromium. The scale bar indicates the size of these pigments. 
 
 High-tech analysis is showing why impressionist painter Vincent van Gogh's bright yellows are turning to dull browns. The chemical finding could help restorers preserve the 200-year old paintings. 
 
 "This type of cutting-edge research is crucial to advance our understanding of how paintings age and should be conserved for future generations," said Ella Hendriks of the Van Gogh Museum Amsterdam, where the two Van Gogh paintings studied are on display. 
 
 This is an image, made using an optical microscope, of the sample taken from “Bank of the Seine” studied with synchrotron X-rays. The brown layer on top of the paint is varnish, it appears opaque but in reality it lets light through. The brown pigments are invisible to the optical microscope. They are located at the interface between varnish and paint, in a layer less than three micrometers thick. The scale bar at the bottom indicates the size of the sample. Credit: University of Antwerp, Department of Chemistry. 
 
 The yellow pigment, used by Van Gogh and his contemporaries, has been undergoing a chemical reaction when exposed to ultraviolet light (including sunlight) that turns the outer layers of the painting brown. The yellow pigment is called chrome yellow, and in a thin layer where the dried paint meets the surface varnish, sunlight penetrates the top layer of the paint. This sunlight triggers a chemical reaction that turns the bright yellow into a dirty brown. 
 
 The research team, which included Koen Janssens of Antwerp University in Belgium and Letizia Monico of Perugia University in Italy, discovered that the change was caused when the chromium in the yellow paint was reduced (meaning it gained electrons) from chromium (VI) to chromium (III), changing the color of the pigment. 
 
 Not all of the paintings from this period seem to undergo this change in the same time frame. Some haven't been darkened at all. Different painters used different pigments and switched over time, especially since chromium yellow is toxic, so the researchers had to track down some historical paint samples to test. 
 
 They found three such tubes of yellow paint and artificially aged the paint by exposing it to 500 hours under a UV lamp. Only one of the paint samples turned brown, one belonging to Flemish artist Rik Wouters. The color change was similar to that seen in the Van Gogh painting, and with X-ray analysis the researchers pinpointed the change to the chromium reduction. 
 
 This is a photo of “Banks of the Seine” (1887), one of the two paintings by Vincent van Gogh from which microsamples were taken. Credit: Vincent van Gogh Museum, Amsterdam. 
 
 To check that this is what happened to the actual paintings, the researcher took samples from Van Gogh's "View of Arles with Irises" and "Banks of the Seine" and analyzed the pigments. Though the multicolored samples were difficult to analyze, researchers 
 
 are confident that they likely underwent the same chromium reduction as the artificially aged samples. 
 
 "Our next experiments are already in the pipeline. Obviously, we want to understand which conditions favor the reduction of chromium, and whether there is any hope to revert pigments to the original state in paintings where it is already taking place," Janssens said in a statement.
– Vincent van Gogh's paintings of sunflowers aren't as vivid as they used to be and the sun is to blame, say researchers who have solved a problem that has long stumped art conservationists. A team of chemists experimenting with ultraviolet light and tubes of paint belonging to 19th-century artists found that by mixing in white paint to make his yellows brighter, van Gogh doomed his sunflowers to turn brown over time, the Guardian reports. "While he wanted to show a light, pale and delicate yellow, it instead becomes a darker, brownish yellow" after sunlight kicks off a chemical reaction, the lead researcher explains. To preserve van Gogh's masterpieces and other works from the same era, he says, conservationists will need to keep the paintings cool in summer—and away from sunlight. "This type of cutting-edge research is crucial to advance our understanding of how paintings age and should be conserved for future generations," an official at Amsterdam's van Gogh Museum tells LiveScience.