Unusual Lead Compound Found in a Rembrandt Painting
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Rembrandt van Rijn was one of the most important 17th century Dutch painters. His most famous painting The Night Watch (1642) hangs in Amsterdam's Rijksmuseum. An international team has now identified lead formate—a compound very unusual for paintings—in various areas of The Night Watch. The results presented in the journal Angewandte Chemie provide clues about the pictorial practices of Rembrandt and the reactivity of lead driers in the oil matrices of historical paintings.
“Operation Night Watch” is a comprehensive research and conservation project in which conservators, art historians, and other scientists across various disciplines are collaborating closely on Rembrandt's Night Watch. As part of this project, the composition and distribution of materials were examined by macro-X-ray powder diffraction mapping. Synchrotron-based micro-X-ray powder diffraction and infrared microscopy studies were also carried out in parallel on tiny samples. This made it possible for the team from the Rijksmuseum, the University of Amsterdam (the Netherlands), the CNRS, and the European Synchrotron Radiation Facility (France), and the University of Antwerp (Belgium), among others, to identify and map various lead compounds present in Rembrandt's paint layers.
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Lead pigments were widely used by Rembrandt. The most common was white lead, a mixture of lead carbonates hydrocerussite Pb3(CO3)2(OH)2 and cerussite PbCO3. Lead is also present in other pigments, and their associated alteration products. However, one discovery made by the team headed by Victor Gonzalez, Ida Fazlic, and Marine Cotte was more unusual: lead(II) formate Pb(HCOO)2—a compound that has never before been found in historical oil paintings. Lead formate, the lead salt of formic acid, was found in several areas of The Night Watch—sometimes together with plumbonacrite Pb5(CO3)3O(OH)2, another rare lead compound.
To investigate the chemical origin of lead formate, the team produced model paint layers according to old recipes. For example, siccative oil was prepared by heating linseed oil, the most common binding agent for paints at the time, with lead oxide PbO. Lead oxide is a metallic dryer, causing paints to harden more quickly.
The study showed that PbO in oil paint can react to form lead formate. Even though no crystalline PbO was detected in The Night Watch, the results support the hypothesis that an oil containing such a lead dryer was used. But other hypotheses have to be considered. Past conservation works on The Night Watch, notably the possible addition of an oil-based varnish in the 18th century could have favored the formation of lead formate on the painting. The team is now investigating the kinetics of lead formate formation, and their stability in oil paint.
Reference: Gonzalez V, Fazlic I, Cotte M, et al. Lead(II) formate in Rembrandt’s Night Watch: Detection and distribution from the macro- to the micro-scale. Angew Chem Int Ed. 2023:e202216478. doi: 10.1002/anie.202216478
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