Chemical Analysis Of Paint Sample Taken From “Wheat Stack Under A Cloudy Sky” Reveals The Intricacies Of Degradation Of Pigment Made From Lead

To achieve the brilliant reds in his paintings, Vincent van Gogh used paint made with Pb₃O₄, a red lead-based pigment. But this particular pigment appears to be slowly turning white, according to a report from Koen Janssens, Frederik Vanmeert, and Geert Van der Snickt of the University of Antwerp, in Belgium (Angew. Chem. Int. Ed. 2015, DOI: 10.1002/ange.201411691). The discoloration of lead-based pigmented paints is a well-known phenomenon. These paints, which have been used since antiquity, will darken or whiten, depending on the degradation chemistry of the pigment. What the Antwerp team found was a chemical “missing link” in this degradation process. The chemists used X-ray powder diffraction tomography to study a sample taken from van Gogh’s 1889 painting “Wheat Stack under a Cloudy Sky” (shown). In the space between the sample’s reddish-orange Pb₃O₄ core and the light blue PbCO₃ layer that surrounds it, the chemists found plumbonacrite, 3PbCO₃•Pb(OH)₂•PbO. This is the first time this compound has been found in a painting dating before the mid-20th century, the authors note. The discovery, they say, sheds new light on how red lead degrades, and they propose a mechanism that involves carbon dioxide and light.