A new study has provided a deeper insight into why Vincent van Gogh's most prized paintings are turning white.
Researchers at the University of Antwerp, by inspecting the speck under a microscope and using x-ray lasers, were able to determine what minerals it contained.
It was discovered the speck was originally red and contained a lead known as plumbonacrite.
The scientists explained plumbonacrite was one of the first synthetically-made paints known to man and was popular with Van Gogh.
However, unknown to him the time, plumbonacrite degrades colours when exposed to light.
Based on their new insights, the scientists have proposed a possible reaction pathway by which red lead loses its red color under the influence of light and carbon dioxide: Irradiation with light causes electrons to move from the valence band to the conducting band in the red lead, which is a semiconductor.
This initiates reduction of the red lead to PbO. Subsequently, CO2 is absorbed gradually from the air and/or from degradation products of the binding medium from the oil paint. This forms plumbonacrite as an intermediate that is converted to hydrocerussite and then to cerussite (lead carbonate) upon further absorption of CO2. These degradation products are white.
The study is published in the journal Angewandte Chemie.
