This Rembrandt has been hiding a surprising secret for centuries: ScienceAlert

At 3.63 meters (12 feet) long and 4.37 meters wide, the painting commonly known as The Night Watch is Rembrandt’s largest painting. Centuries after its creation, we are still discovering the tiniest details in the pigments, thanks to advances in modern technology.

A team of researchers from all over Europe has found an unexpected molecule while studying the composition of paint that makes up the famous Dutch Golden Age artwork.

Thanks to the layers of pigments and other media such as primers and varnishes, paintings can vary in chemical composition in three dimensions. So in addition to scanning the surface of the masterpiece, experts working on the Operation Night Watch conservation project took minute samples through the layers of the famous painting.

In 2021, two years after the start of the project, macro-XRF mapping revealed Rembrandt’s original sketch under the final version of The Night Watch. By beaming X-rays onto the painting, the technology forces pigments to absorb and then emit high-energy light in distinct ways, identifying the distributions of different elements within individual layers.

This mapping revealed many small details that had changed from the original sketch to what we can see today.

By mapping the high calcium content in the chalky paint that Rembrandt used for his preparatory sketches, researchers revealed the underlying sketch. (Rijksmuseum)

However, it was not only the images that caught the researcher’s attention. A technique called X-ray powder diffraction revealed the presence of lead formates, including the rare lead(II) formate, Pb(HCOO)2.

Commonly used in the making of white and yellow pigments at the time, it is far from surprising to find the element lead in a Rembrandt artwork. However, this particular class of lead compounds has only been discovered in studies modeling the aging of old paints – not in old paintings themselves.

“We not only discover lead formates, but we also identify them in areas where there is no lead pigment, white, yellow,” explains Victor Gonzalez, a photochemist at the French National Center for Scientific Research.

“We think they’re probably disappearing soon, which is why they weren’t found in old master paintings until now.”

The distribution of the lead formats suggests that it was more concentrated in some layers than others and associated with light color paints.

Gonzalez and colleagues modeled scenarios using several well-known paint recipes of the era.

Heatmaps with distributions of lead compounds in the painting.
Distributions of various lead compounds in the painting, the first a common pigment for white paint. (Gonzalez et al., Angewandte Chemie, 2023)

“The unique analytical performance of the ESRF, the world’s brightest synchrotron light source, allowed us to map the presence of micrometric-scale formats and track their formation over time,” explains Beamline scientist Marine Cotte of the European Synchrotron Radiation Facility (ESRF).

Their results suggest that Rembrandt mixed lead(II) oxide into his linseed oil medium for the lighter paint colors to help them dry. Interactions with other molecules over time turned this into the leading formats. It seems that even great painting masters like Rembrandt can become impatient with this slowly drying medium.

However, it’s also possible that these formats evolved from previous restoration attempts, the team explains.

Past restoration efforts have resulted in massive changes to the painting, which is not really a night scene at all. Many layers of varnish, mixed with dirt, darkened the scene to give that impression, eventually leading to its common name towards the end of the eighteenth century.

The official name of the painting is The militiamen of Wijk II led by captain Frans Banninck Cocqand Operation Night Watch is now investigating how to remove these layers of varnish without disturbing the original painting underneath.

“In addition to providing information about Rembrandt’s painting techniques, this research opens new avenues to the reactivity of historical pigments, and thus to the preservation of heritage,” says analytical chemist Koen Janssens of the University of Antwerp.

In 2021, a missing strip that had been cut out of the artwork was also meticulously reconstructed using AI.

“It really gives the painting a different dynamic,” museum director Taco Dibbits told the Associated Press at the time. “And what it taught us is that Rembrandt never does what you expect.”

Rembrandt is known for his exploration of painting materials, techniques and compositions and the discoveries of researchers of Operation Night Watch have proven this time and time again.

This study was published in International edition applied chemistry.

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