New insights into how a yellow pigment widely used in historical artwork fades over time have been gained by researchers in the US. Using an imaging technique that they had developed to detect skin cancer, Martin Fischer and colleagues at Duke University showed how signs of paint degradation can appear on a microscopic scale, before it is visible to the eye.
As they painted their masterpieces, artists throughout history knew that the colours they used would fade over time. Recently, however, analytical techniques are providing insights into the properties of microscopic grains of pigment and why they fade. This allows us to imagine artworks as they looked when they were first painted, and how to conserve and restore paintings.
“Understanding the reason for pigment degradation is extremely important to halt damage that has occurred, prevent damage that has not yet happened, and to get an idea how a degraded masterpiece might have looked originally,” Fischer explains.
One of the most challenging aspects of this task is the deep complexity hidden beneath the surface of a painting. In many paintings, numerous pigments have been mixed together and layered on top of each other, making them difficult to analyse without damaging the artwork.
In their study, Fischer and colleagues overcame this challenge using a method called pump-probe microscopy, which uses pairs of synchronized femtosecond laser pulses. The two different pulsed laser beams are superimposed and then focused onto the sample being imaged, with a controlled delay between the arrival of the pump and pulse. The pump pulse comes first and creates excitations within the sample. Then the probe pulse interacts with the sample such that the reflected light contains information about specific excitations and therefore the chemical composition of the sample.
Powerful technique
Pump–prove microscopy has become a powerful technique for generating high-contrast images of non-fluorescent images, especially in living tissues. Indeed, Fischer’s team have already adapted it to examine moles for signs of skin cancer. Now, the group has used technique to examine the degradation of pigments hidden within complex layers of paint. They focused on pigments containing cadmium sulphide, which are bright yellow and have played an important role in the history of art.
“CdS was popular with artists like Edvard Munch, Henri Matisse, and Pablo Picasso, but is also very prone to degradation,” Fischer explains. “Despite the importance of CdS, the influence of the environmental conditions and manufacturing methods on the degradation process is not very well understood.”
To investigate the effect, the team started by synthesizing a CdS pigment, using a historical method often used by artists of the past. They then accelerated the aging process by exposing their pigment to high levels of light and humidity. This quickly degraded the CdS grains into hydrated cadmium sulphate, causing the yellow colour to fade.
Variable breakdown
During the degradation process, the researchers used pump–probe microscopy to monitor changes to individual CdS grains. Their experiment revealed that the breakdown process can vary widely, depending on the size and shape of the grains.
“We discovered that degradation tends to happen more strongly for small, rough CdS crystals that are closer to the surface,” Fischer explains. In contrast, “degradation in larger crystals tends to start from the outside in, and from top to bottom.”
The experiment also showed that signs of degradation can start to appear well before they are visible even to the sharp eyes of art conservators. “Having such an early warning signal could be very helpful to adjust storage or display conditions for the artwork or to indicate the need for early intervention,” Fischer adds.
Based on their success, Fischer and colleagues now hope that pump–probe microscopy will help them to gain a better understanding of the degradation processes that cause fading in other types of pigment. In turn, their work could enable conservators to develop new and improved techniques, helping them to protect priceless historical artwork for years to come.
The research is described in JPhys Photonics.
