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Foto: Sara Kerklaan

UvA chemists discover new lead compound in The Night Watch

Sija van den Beukel,
1 februari 2023 - 13:55

The Night Watch contains a lead compound not previously found in historical paintings. This was published by an international team of scientists, including UvA scientists, in the journal Angewandte Chemie. “The finding could tell us something about how Rembrandt made his paint.”

“You think you understand a painting, yet you can be surprised again by new things,” says associate professor of molecular spectroscopy Katrien Keune. In addition to being a professor, she is head of science at the Rijksmuseum. In collaboration with European colleagues, she and her research team succeeded in detecting lead formate, a lead salt of formic acid, in Rembrandt's masterpiece using a special X-ray scanner. The X-ray scanner, developed at the University of Antwerp, measured pigments on the surface of the painting and detected lead formate. The result was confirmed by analyses of paint samples with a specific type of particle accelerator called a synchrotron in Hamburg and Grenoble.


Operation Night Watch

The finding is part of the large-scale research project Operation Night Watch. Since 2019, the Rijksmuseum has been researching the chemical composition of The Night Watch in collaboration with various external experts. Using advanced techniques, the team is trying to determine how Rembrandt painted his masterpiece, its condition, and how best to preserve it for future generations.

Foto: Katrien Keune
Jordi Huisman

What makes the find special? Was lead never found in paintings before?

“Lead is actually very common in paintings. It is found in pigments like lead white and yellows such as lead-tin yellow. It was also added to oil paint in small amounts to make it dry faster. We have been researching lead in paintings for 20 years. We now know that lead does not stay in place because it reacts with fatty acids from the paint to form lead soaps. But the fact that it can also react with weaker acids to make lead formate is new and a big surprise. We had never found this lead compound in paintings before.”


What does this discovery tell us about The Night Watch?

“The finding could tell us something about how Rembrandt made his paint. For the article in Angewandte Chemie, we recreated paint mixtures from the 17th century. This allowed us to see whether lead formate could be created from the linseed oil and dissolved lead oxides that Rembrandt used. And to our great surprise, it turned out that it could. Another hypothesis is that the lead formate appeared later, following restoration. We suspected that during 19th century restorations, varnishes containing lead and oil were applied to the painting.”


How did you now manage to find the connection?

“There could be two reasons: either because the techniques we use have become more sensitive or because lead formate is unique to The Night Watch. We have another meeting with our Belgian colleagues this week to see if we can find more of it in other paintings. From a chemical perspective, I expect we will. Once you discover a substance somewhere, you start to recognize it in other places.”

“Personally, I think the lead formate is more likely to come from restoration”

Can you also determine how old the lead formate is?

“That's a question I have myself. The date of the painting suggests that it could be very old, if it is indeed from Rembrandt's paint. Personally, I think the lead formate is more likely to come from restoration: old layers of varnish. In that case, dating it is very difficult and we can at most make an estimate of around the 18th or 19th century.”


Can you also see the lead formate on the painting?

“No, it cannot be seen with the naked eye. But if you zoom in on The Night Watch on our website, you will see small dots: bulges of lead formate, a chemical change that you can observe very easily.”


How surprising is this discovery in the bigger picture of Operation Night Watch?

“I am surprised every time that there is still so much to discover. You think you know a painting, but then there is always a small piece in that big puzzle that gives us new insight. For instance, there was arsenic pigment in the coat of Lieutenant Ruytenburch, one of the main figures in The Night Watch. In the bigger picture, it adds to the surprise of how much you can still get out of it.”



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