Dead red: Discoloured pigment X-rayed

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  • Published: Mar 15, 2015
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Dead red: Discoloured pigment X-rayed

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Red lead is familiar as a component of the orange-red rustproof paint used on boats and other areas exposed to sea and weather.

"Red lead" is familiar as a component of the orange-red rustproof paint used on boats and other areas exposed to the sea and the weather. It has also been used by countless artists through the centuries. Unfortunately, the pigment undergoes discoloration over time. Now, Belgian researchers at DESY have used X-ray diffraction mapping and tomography with synchrotron light source PETRA III to help them explain a new step in the discolouration process - formation of the rare lead carbonate mineral plumbonacrite - in the light-induced degradation of red lead in a Van Gogh painting.

Red lead (also known as the mineral minium or chemically as lead(II,IV) oxide, Pb2+2Pb4+O4) is vivid, but its hue can change with time and exposure to light, humidity, air and sulfur-containing chemicals. It can darken as the red lead pigment is converted to plattnerite (beta-lead dioxide) or galena (lead sulfide). But, it can also become "bleached" turning almost white as lead sulfate or lead carbonate form.

Van Gogh revelations

Now, Koen Janssens and his colleagues at the University of Antwerp have revealed a new clue regarding this latter, bleaching, process, which could have implications for the conservation of classic paintings by Vincent Van Gogh and other artists. The team examined a microscopic sample taken from Van Gogh's “Wheat Stack under a Cloudy Sky” (an 1889, oil on canvas, currently held in Kröller Müller Museum, Netherlands), they used the facilities available to them on the micro/nanofocus beamline P06 of PETRA III. X-ray powder diffraction mapping and tomography allowed them to determine the distribution of the different crystalline compounds within this tiny sample but with very high spatial resolution and specificity; something that is not possible with conventional X-ray crystallography.

Mineral surprise

The team was surprised to discover the very rare lead carbonate mineral plumbonacrite (Pb10CO36O(OH)6). “This is the first time that this substance has been found in a painting from before the middle of the twentieth century,” explains team member Frederik Vanmeert. “Our discovery sheds new light on the bleaching process of red lead.” Based on these findings, the scientists have suggested a possible reaction pathway by which the pigment loses its red colour under the influence of light and exposure to carbon dioxide. Light causes electrons to move from the valence band to the conducting band in the semiconducting red lead, this leads to reduction to lead oxide. Gradual absorption of carbon dioxide from the atmosphere eventually forming the plumbonacrite, this mineral then degrades further to hydrocerussite and then to white cerussite (lead carbonate) with continued carbon dioxide absorption.

"We are currently investigating in a more quantitative manner what factors stimulate the degradation such as carbon dioxide concentration, light wavelength, residual PbO in the minium, way of preparing the minium," Janssens told SpectroscopyNOW. "Our ultimate goal is to gain a complete understanding of the transformation process and use this insight to slow down any ongoing degradation phenomena."

Related Links

Angew Chem Int Edn 2015, online: "Plumbonacrite Identified by X-ray Powder Diffraction Tomography as a Missing Link during Degradation of Red Lead in a Van Gogh Painting"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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