Vincent Van Gogh painted the “Flowers in a Blue Vase” in 1887 and in Paris and in the early twentieth century this painting was purchased by the Kröller-Müller Museum. There, it was covered with a varnish that was intended to protect, so much that was used on many other paintings by Van Gogh and other authors, but in the long term in some cases it ended up causing damage.
To paint sunflowers, the flowers he loved so much part of many of his paintings, in “Flowers in a Blue Vase” Vincent Van Gogh used cadmium yellow, made of cadmium sulphide (chemical formula CdS), a pigments that at the time was new. Last year it a problem of oxidation was discovered that could cause a loss in the brightness of the yellow sunflowers turning their color in cadmium sulphate (chemical formula CdSO4) when it’s not protected.
In 2009, instead, it was found that a part of the “Flowers in a Blue Vase” had formed an orange-gray crust on parts of the painting in which cadmium yellow was used. The Kröller-Müller Museum had two samples of the painting a fraction of a millimeter in size analyzed to understand what happened.
The research was conducted by a team at the University of Antwerp, Belgium, in collaboration with the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, and the Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany. These two particle accelerators that allowed them to investigate thoroughly on the molecular structure of the samples using high-intensity X-rays on them.
The result has been surprising. Instead of the oxidation they expected, the researchers found that anglesite had formed, a lead sulfate (chemical formula PbSO4) which is opaque in the varnish. Probably, lead was part of a desiccant added to the varnish.
At the interface between paint and varnish the researchers found cadmium oxalate (chemical formula CdC2O4), formed by the ions of cadmium combined with products of the degradation of the varnish. This compound forms the orange-gray crust that caused the biggest damage to the painting.
Again, the use of sophisticated technological instruments has allowed us to understand what’s happening to some of the Vincent Van Gogh’s paintings. The study will continue to find the best way to preserve them and at the same time keep on exposing them to the public.