Shining an infrared light on how “metal soaps” threaten priceless oil paintings

by AryanArtnews
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expansion / / NIST researchers have worked with the National Museum and other organizations to study the “metal soaps” found in oil paintings. Soap can deteriorate the paint over time.

National Museum / A. Centrone / NIST

Scientists at the National Institute of Standards and Technology worked with the National Museum and other institutions to study the deterioration of oil paintings. Gypsy woman with mandolin (Around 1870), by the 19th century French landscape and portrait painter Jean-Baptiste Camille Corot. According to a recent paper published in the journal Analytical Chemistry, researchers used three complementary techniques to analyze paint samples under infrared light and form harmful metallic carboxylic acids on the top layer of paint. The composition of salt soap has been decided.

“There were some problems with this painting that the art restorer pointed out,” said Andrea Centerone, co-author and NIST researcher. “There are 13 layers, many of which have been repaired long after painting, and at least the top layer has deteriorated. They restored the painting to its original appearance and the finest layers of the painting. We wanted to know what was happening at the visual level, so we started helping. “

In 2019, I reported that in an oil painting at the Georgia O’Keeffe Museum in Santa Fe, New Mexico, small pin-sized blisters like acne had been occurring for decades. Conservationists and scholars initially assumed that the wound was a grain of sand trapped in paint. However, after that, the protrusions grew, spread, and began to peel off, raising concerns. Some paintings have more pronounced protrusions than others, but even when the conservator-restor repairs the most damaged canvas, wrinkles (or “art acne”) have returned.

Chemists conclude that blister is actually a metal carboxylate soap, the result of a chemical reaction between the metal ions of lead and zinc pigments and the fatty acids of the binding medium used in the paint. I did. The soap begins to aggregate and forms blisters, moving through the paint film. “They form exudates on the surface, which can obscure the painting itself and create insoluble films and transparent effects, so you can see those layers that were not the artist’s intentions. “Mark Walton of Northwestern University told Ars. In 2019.

This “paint disease” is not limited to O’Keefe oeuvre.. Conservator-restors have seen similar deterioration in oily masterpieces over the entire period, including Rembrandt’s work. For example, at the Metropolitan Museum of Art in New York City, there is an ongoing project to identify the causes and mechanisms of metal soap formation in traditional oil paintings. We are working with scientists at Brookhaven National Laboratory to analyze samples using nuclear magnetic resonance spectroscopy and synchrotron-based X-ray methods. (Especially the latter has become almost ubiquitous in the scientific analysis of artistic and archaeological relics.)

<em>Gypsy woman with a mandolin</em>By 19th century painter Jean-Baptiste Camille Koro.  “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 “/><figcaption class=
expansion / / Gypsy woman with mandolinBy 19th century painter Jean-Baptiste Camille Koro.

Heritage Art / Heritage Images by Getty Images

“Oil paints can last for centuries, but they are not inert,” Centrone and her co-author wrote in their dissertation. “A work made of oil paint is made up of several layers, each with specific functions such as adhesion to the base (stratum), painting layer, saturation, and protection from the environment (varnish layer). Art is a difficult task to analyze because paint films are composed of slowly evolving non-uniformities on the micro and nano scales. “

The NIST team used a surgical scalpel to scrape a small sample from a portion of the already degraded painting, and the paint contained a dry oil, cobalt green, and white lead pigment in addition to the metal soap. I found. They scrutinized the soap using an infrared microscope. This is ideal for monitoring changes in paint composition over a wide area, but that method could not identify a particular type of soap in order to obtain a chemical fingerprint.

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