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The red pigments in some of Vincent van Gogh's most valuable paintings are fading, according to new research.

The Dutch impressionist's work is famed for its rustic beauty and bold colours, but some of the pigments are now at risk of disappearing entirely.

Scientists studying the colour blue irises in one of Van Gogh's later paintings have found that the flowers were actually once purple.

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The blue irises in the foreground of Vincent van Gogh's Field with Irises near Arles, painting in 1888, were found to have actually contained a red pigment that has faded over time and were originally a purple colour

The red pigments used by the artist are fading and experts fear that they could deteriorate completely within a few decades.

Dr Ella Hendricks, head of conservation at the Van Gogh Museum in Amsterdam, made the discovery as part of research during restoration work on some of the 200 of Van Gogh's paintings held at the museum.

Lighting levels at the museum have now been reduced by half in an attempt to slow the deterioration of the paint.

Speaking to the Dutch public broadcaster NTR, she said: 'These blue irises were once purple.

'This is the result of the deterioration of red lake pigments by light.'

Dr Hendriks, who is also a lecturer at the University of Amsterdam, was conducting her research in a joint project with scientists at the University of Tilburg, the Van Gogh Museum and the National Office of Cultural Heritage.

They mapped the decay of colour in Van Gogh's Field with irises at Arles, which he completed in 1888. It is thought to be the first of a series of paintings of irises that he completed while at an asylum in Arles.

Another of these, known simply as Irises, is listed among the most expensive paintings ever sold and is thought to be now worth more than £70.6 million.

Their work raises the prospect that many of Van Gogh's other paintings, such as his famous still life of Sunflowers, are also at risk.

Van Gogh struggled to sell his paintings during his life and wrestled with mental illness that led to him cutting off his own ear and later shot himself at the age of 37.

It was only after his death that the importance of his work began to be truely recognised and his works are now among the most expensive paintings ever sold.

In 1993 his painting A Wheatfield with Cypresses sold for £36 million.

Descriptions by Van Gogh of the colour of his lodgings that he painted in The Bedroom show that the floor and walls actually contained more red pigment, giving them more vivid shades than they have today

Experts used computer reconstructions to boost the red in Van Gogh's paintings to see how they would have originally looked before they faded under the bright lights that has caused the red pigment to degrade

The first hints that Van Gogh's paintings were fading came during the restoration of another of his artworks, The Bedroom.

In a series of letters to his brother and sister, Van Gogh described in detail the vivid colours of his room. This allowed scientists to reconstruct how the painting would have originally looked.

They found that the pale brown floor should have been a richer purple and peach colour while the blue walls were originally an orange-red and violet.

Writing about the restoration work conducted on Van Gogh's The Bedroom, Leo Jansen, a curator at the Van Gogh Museum, said: 'The most intriguing issue, of course, is discolouration.

'Now that we know the purplish-peachy colour of the floor and the blue of the walls were originally more like orange-red and violet, respectively, we would love to figure out how the work originally looked.

'Though I doubt we will ever be able to reconstruct the original colour scheme exactly, in the future we will look at other works for which Van Gogh's letters mention one colour, violet, for instance, while we now see another,such as blue.

'To begin with, there are the other versions of The bedroom, but we could also look at Van Gogh's oil copies of black-and-white prints, which he called 'translations in colour'. For instance Snow-covered field with a harrow.'

The researchers hope to examine other works by Van Gogh, such as his famous Sunflower still lifes, that may have contained red lake pigment to see if their colour has changed over time after years on display

Van Gogh's paintings, such as this painting of Vase with Daises, and Poppies, rarely come up for sale but this still life sold for more than £39.3 million when it sold at an auction at Sotheby's in New York last month

Dr Hendricks and her team are now using computer software to build up a picture of how the paintings originally looked.

For now, however, they will not be attempting to restore the original colour to the paintings.

Instead curators at the Van Gogh Museum have lowered the level of ambient light and introduced dark backgrounds to help reduce reflected light to help extend the life of the paintings.

Kees van den Meirackeruit, head of the collection at the Van Gogh Museum, said: Depending on the amount of hours that you exhibit a painting, it may be that you have reached the limit after 20 years, or you can display it less to stretch to 30 years.

'With measures such as ambient, dark backs and bringing the light back to about half of what it was several years ago, we can solve much.'