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Scientists identify rare lead compounds in Rembrandt’s The Night Watch


Enlarge / Scientists and conservators are working together to preserve The Night Watch, by Rembrandt van Rijn (1642), for future generations.
Rijskmuseum Amsterdam

One of the most famous paintings from the Dutch Golden Age is Rembrandt van Rijn‘s 1642 masterpiece The Night Watch.  An interdisciplinary team of researchers has conducted a fresh, in-depth analysis and found rare traces of a compound called lead formate in the painting, according to a recent paper published in the journal Angewandte Chemie. The work was part of the Rijksmuseum’s Operation Night Watch, the largest multidisciplinary research and conservation project yet undertaken for Rembrandt’s famous painting, devoted to its long-term preservation.

“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,” said Katrien Keune, head of science at Rijksmuseum and professor at the University of Amsterdam (the Netherlands). “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.”

Science has become a valuable tool for art conservationists, especially various X-ray imaging methods. For instance, back in 2019, we reported on how many of the oil paintings at the Georgia O’Keeffe Museum in Santa Fe, New Mexico, had been developing tiny, pin-sized blisters, almost like acne, for decades. Conservationists and scholars initially assumed the blemishes were grains of sand trapped in the paint. Chemists concluded that the blisters are actually metal carboxylate soaps, the result of a chemical reaction between metal ions in the lead and zinc pigments and fatty acids in the binding medium used in the paint. The soaps start to clump together to form the blisters and migrate through the paint film.

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Conservators have found similar deterioration in oil-based masterpieces across all time periods, including in works by Rembrandt. For instance, the Metropolitan Museum of Art in New York City has an ongoing project to determine the causes and mechanisms of metal soap formations on traditional oil paintings; it is collaborating with scientists at Brookhaven National Laboratory to analyze samples using nuclear magnetic resonance spectroscopy and synchrotron-based X-ray methods.

In 2020, scientists analyzed Edvard Munch’s The Scream (which was showing alarming signs of degradation) and concluded the damage was not the result of exposure to light, but humidity—specifically, from the breath of museum visitors, perhaps as they lean in to take a closer look at the master’s brushstrokes. In March 2022, scientists studied the deterioration of Jean-Baptiste-Camille Corot’s Gypsy Woman with Mandolin (circa 1870). They used three complementary techniques to analyze paint samples under infrared light to determine the composition of the damaging metal carboxylate soaps that had formed on the top layer of paint.

Enlarge / Co-author Ida Fazlić at the ESRF analyzing tiny paint samples with synchrotron radiation.
ESRF / Stef Candé

Also in 2022, Dutch and Belgian researchers used macroscopic X-ray fluorescence imaging (MA-XRF), macroscopic X-ray powder diffraction imaging, and reflectance imaging spectroscopy to map the distribution of elements present in the degraded painted yellow flower in Abraham Mignon’s Still Life with Flowers, as well as 3D microscopy of the rose’s paint surface. That analysis revealed the details of the original brushwork used to create the flower’s 3D illusion, which is no longer visible to the naked eye.

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Past analyses of Rembrandt’s paintings identified many pigments the Dutch master used in his work, including lead white, multiple ochres, bone black, vermilion, madder lake, azurite, ultramarine, yellow lake, and lead-tin-yellow, among others. The artist rarely used pure blue or green pigments, with Belshazzar’s Feast being a notable exception. (The Rembrandt Database is the best resource for a comprehensive chronicling of the many different investigative reports.)

For their new analysis of The Night Watch, Keune and her collaborators employed complementary techniques covering multiple length scales to determine the composition and the distribution of the materials (pigments, drying agents, and so forth) used to create the painting, focusing particularly on lead compounds. First, they scanned about half a square meter of the painting’s surface with X-ray powder diffraction mapping and analyzed tiny fragments from the painting with synchrotron micro X-ray probes. This revealed the presence of the lead formates.

The Night Watch.”>Crystalline phases distribution obtained via structural imaging on an area of <em>The Night Watch</em>.” src=”https://cdn.arstechnica.net/wp-content/uploads/2023/01/rembrandt2-640×197.jpg” width=”640″ height=”197″ srcset=”https://cdn.arstechnica.net/wp-content/uploads/2023/01/rembrandt2.jpg 2x”></a><figcaption class=
Enlarge / Crystalline phases distribution obtained via structural imaging on an area of The Night Watch.
University of Antwerp

“In paintings, lead formates have only been reported once in 2020, but in model paintings (mock-up, fresh paints),” said co-author Victor Gonzalez of the Supramolecular and Macromolecular Photophysics and Photochemistry (PPSM) laboratory (CNRS/ENS Paris-Saclay). “And there, surprise: not only do we discover lead formates, but we identify them in areas where there is no lead pigment, white, yellow. We think that probably they disappear fast, this is why they were not detected in old master paintings until now.”

So why didn’t the lead formate disappear in The Night Watch? And where did it come from in the first place? Hoping to answer these questions, the team whipped up model “cooked oils” from a 17th-century recipe, which called for mixing and heating linseed oil and lead oxide, then adding hot water to the reacting mixture. They analyzed those model oils with synchrotron radiation. The results supported their hypothesis that the oil used for light parts of the painting was treated with an alkaline lead drier. The fact that The Night Watch was revarnished with an oil-based varnish in the 18th century complicates matters, as this may have provided a fresh source of formic acid, such that different regions of the painting rich in lead formates may have formed at different times in the painting’s history.

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That will be the focus of future research. “Until now, metal formates have been mainly associated with corrosion and degradation products,” the authors concluded. “It will be interesting to see if lead formate plays any role (positive or negative) in the stability and optical properties of oil paintings.”

DOI: Angewandte Chemie, 2022. 10.1002/anie.202216478  (About DOIs).

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