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It is well known that the rise of Impressionism was due in part to the new developments in paint chemistry which created brighter, more stable colors previously unavailable for oil painting.
|View of Arles with Irises by Vincent van Gogh, oil painting.|
One of the artists who embraced and experimented with the new color technology was Vincent van Gogh. The brilliance of some of van Goghs most famous fine art oil paintings are due to his use of these newly available vivid industrial pigments. These pigments begin to show up in his work after his move from Holland to France. Unfortunately, the chemists of the time did not have the tools to perform accelerated aging tests to determine the lightfastness over time of their new creations.
One of the colors, Chrome Yellow, is not only toxic, but is now also known to darken under exposure to the ultraviolet rays in sunlight. As less toxic alternatives to Chrome Yellow were finally developed in the mid-1900s, artists tended to switch to them. But, for a critical period of time, the brilliant color that Chrome Yellow provided was an important tool in van Goghs and other artists palettes. It was perhaps most famously used in some of van Goghs Sunflower paintings. Researchers have found that some, but not all, paintings containing chrome yellow suffer from the severe browning that exposure to sunlight can cause.
To understand how paintings age and how best to preserve them for the future, researchers conducted an in-depth study of the browning seen in some of Van Goghs work painted with Chrome Yellow. Scientists from four countries performed the study. They began by collecting samples from three historic chrome yellow paint tubes and artificially aging the paint for 500 hours using an ultraviolet lamp. They also took samples from two van Gogh paintings, View of Arles with Irises and Bank of the Seine. The paint from the tubes darkened to a chocolate brown after exposure to the UV light.
Using an X-ray beam that is one hundred times thinner than a human hair, the scientists were able to analyze the darkened chrome yellow at the very surface of the two paintings as well, just below the varnish. Their analyses discovered that the chromium in the pigment gained electrons from the UV light, effectively reducing Chromium (VI) to Chromium (III), turning bright yellow to brown.
Of great interest was the finding from the microscopic X-ray beam that the darkening was most prominent where chemical compounds containing barium and sulphur were. This may prove why some of Van Goghs paintings seem to be especially susceptible to the darkening, as it is speculated that he sometimes blended white (containing barium and sulphur) with his yellow paint. The next phase of the research will be the most important of all in trying to understand if there is any hope to revert pigments to the original state in paintings where the darkening is already taking place.
In yet another blending of science and art, we are able to look backwards to understand more about the lives and the works of the great masters. I am not aware of a similarly big effort ever having been made for the chemistry of an oil painting. – Joris Dik, Professor at Delft Technical University. (More information is available from the Journal of Analytical Chemistry.)
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–John and Ann