Duke University: Laser imaging holds promise for early detection of risky artworks

Compared to Impressionist paintings taken 50 years ago, upon closer inspection of Impressionist paintings in museums, you may notice some strange things: some are losing their bright yellow hue.
Taking the dramatic sunset in Edward Munch's masterpiece "The Scream" as an example. The once bright orange yellow parts of the sky have faded to off white.

Similarly, in his painting "The Joy of Life", Henri Matisse brushed some sunny yellow between the reclining nudes, now appearing more like a monotonous beige.
Several other paintings during this period also faced similar problems. The bright yellow paint used by these artists is made from the chemical compound cadmium sulfide. This pigment was highly favored by many European artists in the late 19th and early 20th centuries. Claude Monet, Vincent van Gogh, and Pablo Picasso all used it to rally votes.

Zhou, who obtained his doctoral degree from Duke University's chemistry professor Warren Warren's laboratory, said, "Many painters really like this pigment.".

But as decades have passed, many artists and art conservationists have realized that they have a problem: their cadmium yellow strokes don't look as vibrant as before.

Over time, artworks are exposed to light, moisture, dust, and other natural elements, which can easily cause pigments to fade.
In a new study, researchers from Duke University have shown that their developed laser microscopy technology can provide an early detection method that can be identified even before the eyes can see the initial small signs of color change.

There are several techniques to study which pigments are used in painting and how much they decompose. But they usually require a surgical knife to scrape off a small piece of paint to analyze its composition. Zhou said that this method may damage the workpiece and limit the scope of the study.
"It's a bit like surgery," she added.

Enter the pump probe microscope. It can peek at the paint layer and detect the chemical changes that mark the beginning of pigment decay, without the need to capture a cross-section of the original artwork.

This technology uses ultra fast pulses of harmless visible or near-infrared light, with a duration of less than trillions of a second, and measures how they interact with pigments in paint. The obtained signal can be used as a chemical fingerprint to identify which compounds are present.
By focusing the laser beam at different positions and depths within the sample, researchers can create 3D maps of certain pigments and monitor what happens within a range as small as one hundredth of a millimeter.

In this new study published on April 26th in the Journal of Physics: Photonics, researchers analyzed cadmium yellow paint samples that had undergone artificial aging using a pump probe microscope.

In a laboratory at Duke University West Campus, Zhou stirred samples of this famous color. She took a bottle of powdered cadmium sulfide pigment from the shelf, mixed it with flaxseed oil, and then brushed it dry on a microscope slide.

Some samples were placed in a dark and dry environment, protected from moisture and light damage. But the rest are placed in a special room, exposed to light and high humidity, which can cause serious damage to unstable colors.

Then, the researchers used a pump probe microscope to image the paint sample and track the degradation process at the microscopic scale.
Compared with the control sample, the aged sample appears to have more severe wear. After being placed in the aging room for four weeks, they have faded to a lighter yellow color.

But Zhou said that even before these changes became apparent, there were clear signs of decay in the data of the pump probe.
The cadmium sulfide signal began to weaken as early as the first week and eventually decreased by more than 80% in the fourth week.
Zhou said that signal loss is the result of pigment chemical changes. Moisture can promote the conversion of yellow cadmium sulfide into white cadmium sulfate, resulting in white or dull castings.

Senior co authors Warren and Martin Fischer initially developed a technique for analyzing pigments in human tissue, rather than art, to detect signs of cancer in skin moles. But later they realized that the same method could also be used for art preservation.

It should be noted that although this technology discovered early changes in a non-destructive manner, protectors were unable to easily reconstruct massive laser devices in their own museums.
The team suggests that in the future, a cheaper and more portable version may be developed for studying paintings that are too fragile or too large to be transported and analyzed off-site.

Of course, any color loss that has already occurred is irreversible. But one day, art protectors may have a new tool to detect these changes earlier and take measures to slow down or stop the process at the beginning.

This study has potential applications beyond artist pigments. Warren said that observing the degradation of cadmium yellow in century old paintings can help researchers better understand that modern materials are also susceptible to the influence of these elements, such as cadmium sulfide used in solar cells.

This study was funded by the National Science Foundation and the Chen Zuckerberg Initiative.

Source: Laser Net