London: After analysing one of the greatest works by famous Dutch painter Vincent van Gogh, Belgian scientists have identified a very rare mineral that is turning his paintings white.
The team, headed by Koen Janssens from the University of Antwerp in Belgium, identified plumbonacrite in one of his paintings, clarifying the degradation process of red lead that causes this bleaching of the colour, turning it white.
"This is the first time that this substance has been found in a painting from before the mid-20th century," reported Frederik Vanmeert, first author of the paper.
"Our discovery sheds new light on the bleaching process of red lead," he added. Red lead is a lead oxide whose colour varies over time.
Artists have treasured the brilliant colour of red lead for their paintings since ancient times. However, various ageing processes cause discolouration of the saturated hue over time.
For the study, the team examined the painting titled "Wheat Stack under a Cloudy Sky" by van Gogh (1889).
Thanks to a combination of X-ray diffraction mapping and tomography experiments, scientists explained an additional step in the light-induced degradation of red lead.
Based on their new insights, the scientists have proposed a possible reaction pathway by which red lead loses its red colour 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," the authors wrote.
This initiates reduction of the red lead to plumbonacrite. Subsequently, CO2 is absorbed gradually from the air and/or from degradation products of the binding medium from the oil paint, turning the colour into white.
The discovery was reported in the journal Angewandte Chemie.
Updated Date: Mar 11, 2015 17:39:19 IST