The yellows in one of Van Gogh's paintings, "Flowers in a Blue Vase", have been turning orange-grey. This is not what a museum curator or collector wants! So laboratories in France and Germany ran an X-ray analysis on the painting, and they discovered the problem. It seems that sometime after Van Gogh's death there was a layer of varnish applied to the painting, as a protection. The cadmium and varnish do not get along, and so the yellows are darkening. Normally, cadmium yellow becomes paler and less vibrant with age, but not in this case. The specific chemical cause of the degredation is called cadmium oxalate. Luckily, cadmium yellow was hard to find during Van Gogh's life. It was expensive and only available in the cities. That meant Van Gogh did not use it in many of his paintings.
"Flowers in a Blue Vase" was painted in 1870, in Paris. It has been in the Kröller-‐Müller Museum in the Netherlands since the early part of the 20th century.
To read more about research on the painting, click here for details: http://pubs.acs.org/doi/abs/10.1021%2Fac3015627