The reds in Van Gogh's Wheat stack under a cloudy are slowly turning white © Peter Horree/Alamy 
 
 The vibrant red leaves on the pond in Vincent van Gogh ’s Wheat stack under a cloudy sky are slowly transforming into the colour of the clouds above. Now, scientists in Belgium may have found the missing chemical link to explain why these reds are turning white 
 
 During his most prolific period, Van Gogh produced 2000 paintings over a decade and was known for his use of intense colours, which, some say, reflected his complex personality. However, many of the painter’s most celebrated works have been degrading due to exposure to light. 
 
 ‘Normally, the idea is these paintings are there for a hundred years, or five hundred years, and they’re static – nothing really changes,’ says Koen Janssens from the University of Antwerp, Belgium. ‘But the opposite is actually true when you look in detail.’ 
 
 Red lead or minium (Pb 3 O 4 ), thought to be one of the earliest synthetically produced pigments in antiquity, was used extensively by Van Gogh in his paintings. Scientists have known that minium ‘whitens’ under light but have not been able to determine why. 
 
 The white degradation product that forms around the red paint is an exotic lead mineral © Wiley-VCH 
 
 To understand this process, Janssens and his colleagues examined a minute white globule taken from the surface of the pond in Van Gogh’s 1889 piece a Wheat stack under a cloudy sky using x-ray powder diffraction tomography. By firing a focused beam of x-rays at the sample from different positions, they were able to gather information on the chemical species present. 
 
 ‘[It is] an enclosed little world,’ explains Janssens. ‘The minium is changing and the products [are] captured below the surface of the paint.’ He goes on to add that the red lead is at the grain’s centre and, over the years, it has become coated with degradation products. 
 
 ‘Starting from minium, we found plumbonacrite (3PbCO 3 .Pb(OH) 2 .PbO), which is a very exotic mineral of lead,’ says Janssens. ‘More towards the outer parts of this sample we found the more commonly known carbonates, hydrocerussites (2PbCO 3 .Pb(OH) 2 ) and cerussites (PbCO 3 ).’ 
 
 The cerussites are a family of compounds that form the white outer shell around the minium but plumbonacrite has never been detected before in a pre mid-20th century painting. Janssens calls this rare mineral the ‘missing link’ and the team are now able to propose a chemical pathway for the photochemical degradation of minium. The researchers think that when red lead is exposed to light it is converted into plumbonacrite, which reacts with carbon dioxide to form hydrocerussite and cerussite. 
 
 ‘It’s a very important [piece of research],’ says Philippe Walter, director of the Laboratory of Molecular and Structural Archaeology in France. ‘For the conservation of works of art, it is important to look at this phenomenon to be sure to take care about the quantity of light [in museums].’ ||||| 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. ||||| 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.
– Look at a Van Gogh painting today, and you're not quite getting the full effect. Reports in 2013, as cited in the Guardian, revealed that the paintings were fading; studies have found yellows are turning brown. The color red is also posing a problem in works like Wheat Stack Under a Cloudy Sky, and researchers are now moving closer to finding out why, Chemistry World reports. Van Gogh often used a pigment known as red lead. The stuff, also called minium, turns whiter when it's exposed to light. Seeking to understand the process, researchers analyzed a tiny dot of paint from the work. The process, which involved shooting X-rays at the paint, showed that the red lead is now covered with what Chemistry World calls "degradation products" in the form of other chemicals. The researchers found a substance they call the "missing link" in the degradation process: It's a mineral lead known as plumbonacrite, Hyperallergic reports. Light shining on the plumbonacrite has apparently caused the buildup of other chemicals. Interestingly, as Hyperallergic notes, the researchers say plumbonacrite has never before been reported "in a painting dating from before the mid 20th century." Of course, the color issues actually make Van Gogh's work even more impressive, Jonathan Jones wrote in the Guardian: "In the end, the fact that some of Van Gogh's colors have changed over the years is simply a homage to his genius. No one, after all, has any complaint about his colors." Last year, Van Gogh—or rather, his "regrown" ear—made some unusual headlines.