Painters throughout the ages have favored the vibrant color of red lead. Today, we know red lead in the form of rustproof paint. Seems like brilliant, durable stuff, but it turns out, red lead has its limits.
X-ray diffraction mapping and tomography experiments at the DESY synchrotron light source PETRA III showed the degradation of lead red caused by light. The big breakthough about the breakdown was the identification of a rare lead carbonate mineral called plumbonacrite used by Vincent van Gogh in his work, according to Science 2.0.
Red lead (Pb3O4) sometimes darkens as the pigment is turned to plattnerite (beta-lead dioxide) or galena (lead sulfide), but sometimes it goes in a different direction - the red lightens as the red lead turns into lead sulfate or lead carbonate.
Koen Janssens and his colleagues at the University of Antwerp examined a microscopic sample of "Wheat Stack under a Cloudy Sky" by Vincent van Gogh (1889, oil on canvas, Kröller Müller Museum, Netherlands) at the micro/nanofocus beamline P06 of PETRA III, according to Science 2.0.
Various X-ray powder diffraction mapping and tomography techniques were used to identify the crystalline compounds. The researchers were able to obtain a depth profile without cutting.
What they found was the very rare lead carbonate mineral called plumbonacrite (3 PbCO3·Pb(OH)2·PbO). "This is the first time that this substance has been found in a painting from before the mid twentieth century," first author Frederik Vanmeert told Science 2.0. "Our discovery sheds new light on the bleaching process of red lead."
Irradiation causes electons to to move to the conducting band in the red lead (a semiconductor) from the valance band. Red lead is then reduced to PbO. As CO2 is absorbed, plumbonacrite converts to hydrocerussite and then to cerussite (lead carbonate) as more CO2 is absorbed. The products of degredation are white.
The results of the study can be found in 'Plumbonacrite Identified by X-ray Powder Diffraction Tomography as a Missing Link during Degradation of Red Lead in a Van Gogh Painting,' Angewandte Chemie DOI: 10.1002/ange.201411691 by authors Frederik Vanmeert, Geert Van der Snickt and Koen Janssens.
