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When Vincent van Gogh painted Flowers in a Blue Vase in 1887 in Paris, he chose a bright, cheery yellow for the flowers. As years passed, the vivid hue faded to a dull orange-gray and scientists have just found out that a coat of protective varnish is to blame.

After van Gogh’s death in 1890, the varnish was applied to preserve the work, a common practice at the time. However, when the paint mixed with the varnish, a chemical reaction occurred causing the colors to change.

Something seemed awry back in 2009 when Margje Leeuwestein, painting conservator at the Kröller-Müller Museum in the Netherlands, attempted a conversation treatment on the piece. An unusual gray crust containing cadmium yellow paint formed on the surface, signaling that they were dealing with more than just aging varnish which is known to darken over time.

 As the painting had become increasingly brittle, experts at the Kröller-Müller Museum carefully took two microscopic paint samples from the original work and used X-ray beams to determine the chemical composition and structure where the paint and varnish met. A lead-sulfate compound, the result of photo-oxidation that separates cadmium and sulfate ions from that particular paint, was revealed. The researchers deduced that the negatively charged sulfate ions hooked up with the lead ions in the varnish to form anglesite, an opaque lead-sulfate compound that caused the color to transform.

 By keeping the painting in lower light conditions and using more advanced varnish, the deterioration should be halted. The surprising findings will be chronicled in the upcoming issue of the scholarly journal, Analytical Chemistry.

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