Shining Infrared Light On How ‘Metal Soaps’ Threaten Priceless Oil Paintings

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Enlarge / NIST researchers collaborated with the National Gallery of Art and other organizations to study “metallic soaps” found in oil paints. Soaps can degrade paint over time.

National Gallery of Art/A. Centrone/NIST

Scientists from the National Institute of Standards and Technology collaborated with the National Gallery of Art and other institutions to study the deterioration of an oil painting titled Mandolin Gypsy (circa 1870), by the 19th-century French landscape and portrait painter Jean-Baptiste-Camille Corot. The researchers used three complementary techniques to analyze paint samples under infrared light to determine the composition of harmful metal carboxylate soaps that had formed on the top layer of paint, according to a recent article published in the journal Analytical Chemistry.

“The painting had some issues which the art restorers pointed out,” said co-author and NIST researcher Andrea Centrone. “It has 13 layers, many of which are due to restorations that occurred long after the painting was done, and at the very least the top layer was degrading. They wanted to restore the painting to its original appearance and find out what was going on. was happening at the microscopic level on the top layer of the paint, and that’s where we started to help.”

In 2019, we reported how many of the oil paintings in the Georgia O’Keeffe Museum in Santa Fe, New Mexico had grown tiny, pin-sized blisters, almost like acne, for decades. Conservationists and academics first assumed that the imperfections were grains of sand trapped in the paint. But then the protrusions grew, spread and started flaking off, causing growing concern. Some paintings have more pronounced protrusions than others, but even when restorers restored the most damaged canvases, the pimples (or “art acne”) returned.

The chemists concluded that the blisters are actually metal carboxylate soaps, the result of a chemical reaction between metal ions from lead and zinc pigments and fatty acids from the binder used in the paint. The soaps begin to clump together to form blisters and migrate through the paint film. “They can form exudates on the surface, which obscure the paint itself, creating an insoluble film or see-through effect, so you can look through those layers, which was not the artist’s intention. “said Marc Walton of Northwestern University in Ars. in 2019.

This “painting disease” is not limited to O’Keeffe artwork. Restorers have seen similar deterioration in oil-based masterpieces from all eras, including works by Rembrandt. For example, the Metropolitan Museum of Art in New York has an ongoing project determine the causes and mechanisms of metallic soap formations on traditional oil paints; he collaborates with scientists at Brookhaven National Laboratory to analyze samples using nuclear magnetic resonance spectroscopy and synchrotron-based X-ray methods. (The latter in particular have become almost ubiquitous in the scientific analysis of art and archaeological artifacts.)

<em>Mandolin Gypsy</em>, by the 19th century painter Jean-Baptiste-Camille Corot.  ” src=”https://cdn.arstechnica.net/wp-content/uploads/2022/03/corot-640×733.jpg” width=”640″ height=”733″ srcset=”https://cdn.arstechnica .net/wp-content/uploads/2022/03/corot.jpg 2x”/></a><figcaption class=
Enlarge / Mandolin Gypsyby the 19th century painter Jean-Baptiste-Camille Corot.

Heritage Art/Heritage Images via Getty Images

“Oil paint may last for centuries, but it is not inert,” Centrone and co-authors wrote in their paper. “Works of art made with oil paints are made up of several layers, each with specific functions, such as adhesion to the substrate (base coat), paint layers, color saturation and protection environment (varnish layer). Understanding the detailed composition of works of art is a difficult analytical challenge because paint films are made up of slowly evolving heterogeneities at micro and nano scales.”

The NIST team used a surgical scalpel to scrape off a small sample of some of the paint that had already degraded and found the paint contained dried oil, cobalt green and white lead pigments. , in addition to metallic soaps. They took a closer look at the soaps using infrared microscopy – ideal for monitoring changes in paint composition over large areas – to get a chemical fingerprint, but they were unable to identify specific types of soaps with this method.

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