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Research Looks at Aging, Deterioration of Oil Paints

Friday, September 6, 2019

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A team of researchers is getting together to watch paint dry. (Or, more accurately, to watch paint age.)

An international team led by researchers at the National Gallery of Art and the National Institute of Standards and Technology recently conducted studies that included 3D X-ray imaging of a paint sample at the Advanced Light Source, a synchrotron at the Department of Energy’s Lawrence Berkeley National Laboratory.

The tests were done to learn more about the chemical processes involved in aging oil paints.

What’s the Problem?

In artwork composed of oil paints, the formation of what are called metal soaps—also known as “art acne”—occurs, pimpling and further deteriorating the work.

Berkeley Lab

An international team led by researchers at the National Gallery of Art and the National Institute of Standards and Technology recently conducted studies that included 3D X-ray imaging of a paint sample at the Advanced Light Source, a synchrotron at the Department of Energy’s Lawrence Berkeley National Laboratory.

“An estimated 70% of oil paintings might already have or will have these metal-soap problems,” said Xiao Ma, Charles E. Culpeper Fellow at the National Gallery of Art who was the lead author of the team’s study, published in the journal Angewandte Chemie International Edition.

“In our collections we see soaps in the paintings—I would say it’s not uncommon,” he noted. “They might not already show at the surface, but exist at the ‘ground,’ or priming layers.”

The same damaging chemistry—which previous studies have traced to the mixing of fatty acids with metal ions present in paint pigments including lead, zinc, copper, cadmium and manganese—has been found to also occur in some organic coatings, too, such as those used for bronze sculptures.

The Studies

The latest study focused on one paint called “Soft Titanium White” that was painted on a canvas in 1995, according to Berkeley. In addition to titanium dioxide, it contains zinc oxide, which is known to form soaps. Paints like that have been in use since around 1930, Ma said.

The aged sample hasn’t been treated in any way and has remained in a controlled environment. The study found that clusters of a compound called aluminum stearate are distributed randomly in the paint, and that zinc carboxylates—the soaps— are mixed in. The high spatial resolution analysis showed that one sort of zinc soap, zinc stearate, aggregates in proximity to these clusters.

While that paint sample didn’t yet show any deterioration, researchers found signs that paint fragmentation and chipping could occur of the soaps become more concentration and localized over time.

“We’re trying to get a handle on the very beginning processes to understand where the soaps might be forming and where they might be moving—if they’re moving,” said Barbara Berrie, who leads the Scientific Research Department at the National Gallery of Art and served as a co-leader of the study.

“We want to make sure we understand what’s going on in more contemporary paintings so that these works are here for the future.”

The X-rays used in the study look at the size, shape and distribution of those bubbles of paint, measured just millimeters across. And besides the X-ray exploration, the team also used photothermal induced resonance (PTIR) that exceeded the magnification limits of conventional light-based microscopes. (PTIR couples infrared lasers with an atomic force microscope to provide a nanoscale window into the paint’s chemistry at a scale much smaller than is achievable with conventional IR microscopes.)

And yet another technique, called Fourier transform infrared micro spectroscopy, provided a broader view of the chemical composition across varying layers of paint samples.

“We are able to capture very small details down to 10 or 20 nanometers,” or billionths of a meter, said Andrea Centrone, a project leader for the Nanoscale Spectroscopy Group at NIST who co-led the study with Berrie. “We were able to detect which kind of metal soap had formed in the paint samples.”

The study could have broader implications for methods of conservation, according to researchers, who also note that the same techniques could be applied more broadly in other fields where paint samples are challenging because their chemistry isn’t uniform, and detailed knowledge of chemistry over different scales is required.

   

Tagged categories: Asia Pacific; Coating Materials; Deterioration; EMEA (Europe, Middle East and Africa); Latin America; Lawrence Berkeley National Laboratory; North America; Research; Research and development; Z-Continents

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