Laser Technique Spots Early Signs of Fading in Artworks

A bright yellow pigment favored by Impressionists such as Matisse and Van Gogh a century ago is losing its luster. One day, art conservators could be able to detect the first tiny signs of the pigment’s decay before they are visible and take earlier steps to make the color last, thanks to imaging techniques developed by Duke researchers. Video Credit: Duke University

Comparing the close-ups of Impressionist paintings in museums to images of them from half a century ago, one may notice that some of them are becoming less vibrant yellow.

Consider the striking sunset in “The Scream,” a well-known painting by Edward Munch. A portion of the sky that was initially a bright orange-yellow has turned off-white. Henri Matisse’s painting “The Joy of Life” also originally featured sunlit yellow, which now looks more like a dull beige. Similar problems are present in several other paintings from this era.

Cadmium sulfide is the chemical compound these artists used to make their vibrant yellow paint. Many late 19th and early 20th century European artists were passionate about the pigment. Claude Monet, Vincent van Gogh, and Pablo Picasso all brushed their canvasses with it.

Yue Zhou, who earned her Ph.D. in the lab of Duke Chemistry Professor Warren Warren, said, “So many painters loved this pigment.”

However, as the years went by, many artists and art conservators became aware of a problem: their once-vibrant cadmium-yellow brushstrokes were becoming less vibrant.

Artwork is exposed to light, moisture, dust, and other natural elements over time, which can cause pigments to fade and become discolored.

The study reveals the potential of a laser microscopy technique pioneered by the researchers, which could detect initial signs of color alteration before they become perceptible to the human eye.

Numerous methods exist for determining which pigments were used in a painting and how much they have degraded. However, to examine its composition, they usually require using a scalpel to remove a small chip of paint. Zhou claimed that this approach restricts the study area and has the potential to harm the piece.

She continued, “It is a little like surgery.”

Pump-probe microscopy can look through paint layers and identify chemical alterations that signal the beginning of a pigment’s deterioration without requiring cross-sections of the original artwork.

The method measures the interaction between the paint pigments and ultra-fast, non-harmful visible or near-infrared light pulses that last less than a trillionth of a second. The resulting signals can be utilized as chemical fingerprints to determine which compounds are present.

By focusing the laser beam at various points and depths within the sample, the researchers can make 3D maps of specific pigments and track phenomena at scales as small as a hundredth of a millimeter.

The researchers used pump-probe microscopy to examine samples of cadmium yellow paint that had been artificially aged.

Zhou mixed up samples of the well-known color in a lab on Duke’s west campus. She removed a bottle containing powdered cadmium sulfide pigment from a shelf, blended it with linseed oil, and brushed the mixture onto microscope slides to dry.

A portion of the samples was kept dry and dark to prevent deterioration from light and moisture. The remaining ones were housed in a separate room with high humidity and light, two elements known to severely damage unstable colors.

The researchers then monitored the degradation process on a microscopic level by imaging the paint samples with pump-probe microscopy.

The samples that received the aging treatment showed the most signs of wear and tear compared to the control samples. After four weeks in the aging chamber, they had changed to paler yellow tones.

However, Zhou noted that the pump-probe data showed apparent signs of decay even before these changes became noticeable.

As early as week one, the cadmium sulfide signal began to decline, and by week four, it had decreased by over 80 %.

According to Zhou, chemical alterations in the pigments are the cause of the signal loss. A whitish or dull cast results from the conversion of yellow cadmium sulfide into white cadmium sulfate, which is triggered by moisture.

Senior co-authors Warren and Martin Fischer initially developed the method to examine pigments in human tissue rather than artwork to check skin moles for cancerous signs. However, they later discovered that art conservation could benefit from the same strategy.

Although the method detects early alterations without causing damage, conservators will find it difficult to replicate the heavy laser setup in their museums.

The researchers speculate that a less expensive, more portable version may someday be created and utilized to examine paintings that are too large or delicate to be transported and examined off-site.

Naturally, color loss that has already occurred cannot be undone. However, art conservators may eventually acquire a new tool that will enable them to recognize these alterations sooner and take action to halt or slow the process at its start.

Beyond artist’s pigments, the research may find uses. Researchers may gain a better understanding of contemporary materials susceptible to environmental changes, like cadmium sulfide utilized in solar cells, by examining the degradation of cadmium yellow in paintings dating back hundreds of years, according to Warren.

The National Science Foundation and the Chan Zuckerberg Initiative provided grants for this research.  

Journal Reference:

Zhou, Y., et al. (2024) Non-destructive three-dimensional Imaging of artificially degraded CdS paints by pump-probe microscopy. Journal of Physics Photonics. doi.org/10.1088/2515-7647/ad3e65

Source: https://duke.edu/