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

Scientists at 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, entitled Gypsy Woman with Mandolin (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 the damaging metal carboxylate soaps that had formed on the top layer of paint, according to a recent paper published in the journal Analytical Chemistry. “The painting had some problems that art conservators pointed out,” said co-author and NIST researcher Andrea Centrone. “It has 13 layers, many due to restorations that occurred long after the painting was made, and at the very least, the top layer was degrading. They wanted to restore the painting to its original state of appearance and find out what was happening on a microscopic level on the top layer of the painting, and that’s where we started to help.” 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. But then the protrusions grew, spread, and started flaking off, leading to mounting concern. Some paintings have more pronounced protrusions than others, but even when the conservators restored the most damaged canvases, the pimpling (or "art acne") returned. 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. "They can form exudates on the surface, which obscure the painting itself, creating an insoluble film or an effect of transparency, so you can look through those layers, which was not the intention of the artist," Marc Walton of Northwestern University told Ars in 2019. This "paint disease" isn't limited to O'Keeffe's oeuvre. 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. (The latter in particular have become almost ubiquitous in the scientific analysis of art and archaeological artifacts.)

"Oil paint can last for centuries, but it is not inert," Centrone and her co-authors wrote in their paper. "Artworks made with oil paints consist of several layers, each with specific functions, such as adhesion to the substrate (ground layer), pictorial layers, color saturation, and protection from the environment (varnish layer). Understanding the detailed composition of works of art is a tough analytical challenge since paint films consist of slowly evolving heterogeneities at micro- and nanoscales." The NIST team used a surgical scalpel to scrape off a small sample from a part of the painting that had already degraded and found the paint contained dried oil, cobalt green, and lead white pigments, in addition to metal 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 weren't able to identify specific types of soaps with that method. “A fundamental limit of optical microscopy is that light cannot be focused to a spot smaller than half its wavelength,” said Centrone. “Infrared light has wavelengths between 2 and 20 micrometers, and although it sounds small, it is too big for measuring details with nanometer-scale spatial resolution.” That's the scale needed to truly delve into how the paint components and the byproducts from all the alterations over the ensuing decades intermixed. So Centrone and her colleagues turned to two other complementary methods to learn more. Optical photothermal infrared spectroscopy (O-PTIR)—ideal for rapidly identifying chemical compounds at resolutions of around 500 nanometers—uses a green laser light in combination with pulses of infrared light to heat a sample. Scientists can then measure how much of the green laser light is reflected to get a chemical fingerprint of the samples.

O-PTIR's much higher resolution is due to the fact that the green laser light's wavelength is so much smaller than the infrared wavelength. Notably, Centrone et al. found that the sample contained zinc soaps but not lead soaps. They were even able to distinguish between denser, more ordered zinc stearate and zinc oleate soaps and more disordered species of zinc soap. The authors hypothesized that the latter's disordered structure is what enables the diffusion of various chemical compounds (water, ions, etc.) in the paint, and those compounds can then chemically react. To measure the distribution of the zinc soaps in the sample, Centrone et al. turned to photothermal induced resonance (PTIR), which combines pulses of infrared light with atomic force microscopy. It takes a bit more time to analyze a sample using this technique, but it can produce a map of the sample surface at high resolutions of between 10 and 20 nanometers. This is just an initial step; the authors hope to further improve the sample preparation to achieve thinner and smoother cross-sections. The research will help art conservationists identify which oil pigments are most prone to developing metal soaps as they degrade. "This work highlights the potential of using IR spectroscopy techniques operating at different length scales to probe the evolution of alteration products in chemically complex systems, which is necessary to understand the chemistry of paints," the researchers concluded. "We believe this work will stimulate robust work to determine the nanoscale composition of oil paints and foster the development of art preservation and conservation practices." DOI: Analytical Chemistry, 2022. 10.1021/acs.analchem.1c04182  (About DOIs).