Joint Research Discovers Unusual Lead Compound In Rembrandt’s Night Watch

An international team of scientists from the Rijksmuseum, the universities of Amsterdam and Antwerp, CNRS/ENS Paris-Saclay and the European Synchrotron ESRF, have discovered lead formate in Rembrandt’s masterpiece The Night Watch. It is the first discovery of this compound in the history of the scientific study of paintings and provides new insight into 17th-century painting technique and the conservation history of the masterpiece. The study was recently published in Angewandte Chemie – International edition.

The Night Watch, painted in 1642 and displayed today in the Rijksmuseum Amsterdam (The Netherlands), is one of Rembrandt’s most important masterpieces and his largest work of art. In the framework of the 2019 Operation Night Watch, the largest research and conservation project ever undertaken for Rembrandt’s masterpiece, an international research team joined forces to study how the painting materials react chemically and with time. The research is led by Prof. Katrien Keune, head of science at the Rijksmuseum and professor at the Van ‘t Hoff Institute for Molecular Sciences of the University of Amsterdam.

Never before detected
The team of scientists combined multi-scale imaging methods in order to chemically study the materials used by Rembrandt in The Night Watch. An X-ray scanning instrument developed at the University of Antwerp (Belgium) was applied directly to the painting, while tiny fragments taken from the painting were studied elsewhere using synchrotron micro X-ray probes at the ESRF, the European Synchrotron (France), and the PETRA-III facility (Germany).

This revealed the presence of an unexpected organo-metallic compound in the painting called lead formate. It had never before been detected in historic paintings. For Keune, this finding is key to understanding Rembrandt better: “In Operation Night Watch we focus on Rembrandt’s painting technique, the condition of the painting and how we can best preserve it for future generations. The lead formate gives us valuable new clues about the possible use of lead-based oil paint by Rembrandt, the complex chemistry of historic oil paintings, and the potential impact of oil-based varnishes from past conservation treatments.”

Study of model samples
The researchers suspected the lead compound originates from the linseed oil used by Rembrandt. They hypothesized that it contained a dissolved lead oxide (litharge, PbO) meant to enhance the drying qualities of the oil. They prepared model samples in the laboratory, simulating the original historic formulations, and studied these with the synchrotron micro X-ray probes at the ESRF in Grenoble. This enabled mapping the formation and presence of the lead formates at a micrometric scale, and following their formation over time. The research thus confirmed the formation of lead formate in a paint system containing lead-based linseed oil. Keune points out the importance of the spatial results that show the micro-scale formation of formate in relation to the presence of the lead oxide. This finding has already led to new hypotheses on possible formate chemistry in old paint layers. The next step for the team is to further study the origin of the formate, and to check if it might also have originated from past restoration treatments of The Night Watch.