Unusual chemical found in Rembrandt masterpiece could shed light on ancient art techniques

In the process of conserving a 17th-century painting by Dutch master Rembrandt, scientists have isolated an unexpected chemical within the paint, shedding light on some of the techniques used to create ancient masterpieces.

The Night Watch, arguably Rembrandt’s most famous work of art, was painted I 1642 and now lives in the Rijksmuseum of Amsterdam in the Netherlands.

As part of a research and conservation project for the painting called Operation Night Watch, first embarked upon in 2019, scientists took a microscopic look at the compounds in the painting, leading to an unusual discovery.

In several areas of the painting, they found the presence of lead (II) formate—something that has never been reported in historical oil paintings before.

The discovery was described in a paper published in the peer-reviewed journal Angewandte Chemie, the journal of the German Chemical Society.

Lead-based pigments were commonly used in historical paintings and by Rembrandt, but this specific lead formate was described as “very unusual,” in the paper.

Victor Gonzalez, a researcher at the Supramolecular and Macromolecular Photophysics and Photochemistry lab and first author of the paper, explained in a press release that lead formates have only been reported in fresher model paintings before. They didn’t expect to find any in The Night Watch.

“And there, surprise: not only do we discover lead formates, but we identify them in areas where there is no lead pigment, white, yellow,” he said.

“We think that probably they disappear fast, this is why they were not detected in old master paintings until now.”

But if this is the case, why didn’t it vanish from Rembrandt’s work?

The answer not only would allow us to learn more about Rembrandt’s techniques, but could potentially provide a pathway to better preservation techniques for modern scientists looking to extend the life of old paintings, the paper authors say.

“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,” Katrien Keune, head of science at Rijksmuseum and a professor at the University of Amsterdam, said in the release. “The lead formate gives us valuable new clues about the possible use of lead-based oil paint by Rembrandt and the potential impact of oil-based varnishes from past conservation treatments, and the complex chemistry of historic oil paintings.”

The Night Watch, which is nearly four metres tall and 4.5 metres wide, was painted for one of the headquarters of Amsterdam’s civic guard, civilian soldiers which defended the city.

In the centre of the painting, a captain dressed in black is giving the order to march out to the guardsmen around him. Like many of Rembrandt’s works, it is known for its striking use of light and darkness, with the bright figure of a lieutenant at the captain’s side balanced out by a young girl in a yellow dress.

Some of the lead formate was found in the collar of the brightest figure, the lieutenant positioned in the centre of the canvas.

Researchers compared tiny fragments of the painting to model paint samples they’d mixed according to a recipe from 1633, utilizing linseed oil, the most common organic binder used in the 17th century to make pigments into paint.

They varied their recipe in order to see which mode of preparation was closest to Rembrandt’s paint, working with the assumption that Rembrandt had utilized dissolved lead oxide in the paint. Each batch of paint was then spread onto a sheet of glass and left to dry for three years.

With the use of the European Synchrotron Radiation Facility (ESRF), which produces X-rays 100 billion times brighter than those used in hospitals, researchers were able to track the “the presence of formates at a micrometric scale, and follow their formation over time,” Marine Cotte, a scientist at the ESRF, said in the release.

Researchers say this work has put them closer to understanding how Rembrandt mixed and sealed his paints, but that there’s more work they’re hoping to do in their quest to discover how to maximize preservation.

“In addition to providing information on Rembrandt's pictorial techniques, this research opens up new avenues on the reactivity of historical pigments, and therefore on the preservation of heritage,” Koen Janssens, professor at the University of Antwerp, said in the release.

The painting continues to hang in the Rijksmuseum Amsterdam museum—the display has been set up so that spectators are able to come and view both the painting and scientists conducting research in a glass room set up behind it.

Operation Night Watch has made numerous discoveries so far in its deep dive into the painting. According to the Rijksmuseum Amsterdam museum’s website, researchers have imaged the original sketch lying underneath the painting, found the use of a paint Rembrandt rarely used within the lieutenant’s yellow coat, and discovered that the degradation of a dog on the right side of the painting was due to physical abrasion of the paint and not due to discolouration.