Art conservationists and researchers have long been puzzled by the fading and color changes observed in many Renaissance paintings, even when stored in dark rooms. While light exposure was previously considered the primary factor behind color degradation, a recent study has identified humidity as a new culprit.
Scientists from the Stanford Synchrotron Radiation Lightsource (SSRL) at the Department of Energy's SLAC National Accelerator Laboratory, along with researchers from the University of Amsterdam, Rijksmuseum, and other institutions, conducted a peer-reviewed study to explore this phenomenon. These discoveries were published in the Journal of the American Chemical Society.
The research team sought to understand the presence of a form of arsenic known as As(V) alongside a yellow arsenic sulfide pigment called orpiment.
Previous studies had shown that the conversion of these pigments to As(V) could lead to color degradation, but the underlying causes remained unclear.
To investigate the As(V) species, the researchers selected a sample from the 17th-century painting "Still Life with Flowers in a Glass Vase" by Jan Davidson de Heem. The yellow eglantine rose at the painting's center had been gradually fading to white.
How did the scientists conduct their study?
To examine the color change in greater detail, the scientists subjected the sample to intense X-rays generated at one of SSRL's beam lines.
These X-rays provided 2D images of the sample's cross-section, enabling the researchers to pinpoint the locations of the arsenic species.
SSRL scientists Johanna Nelson Weker explained, "SSRL's X-rays allowed us to penetrate below the surface layers of the paint, which have been exposed to light and study the layers that haven't yet degraded due to light exposure."
After capturing X-ray images, the researchers used an X-ray microscope to study the chemistry of the arsenic species in higher resolution and 3D.
By combining these X-ray techniques, the team successfully mapped the degradation pathways.
The findings revealed that humidity plays a significant role in color degradation by providing a pathway for the migration of arsenic species, according to Weker.
Futhermore, the researchers observed that the arsenic species does not remain confined to the areas where the orpiment pigment was used, as conservators previously believed.
Webb noted, "This means that if you are a conservator, you have to not only look at where the arsenic pigment is but also in a large halo around the pigment."
Uncovering the role of humidity in color degradation required extensive experimentation, including the recreation of an arsenic pigment sample in the lab.
The researchers added egg yolk, a common pigment binder used during the time the painting was created, to the sample.
They then subjected the egg-yolk sample to various humidity conditions and compared the results with observations made within the sample from de Heem's yellow eglantine rose.
The study revealed that the arsenic species can spread after reacting with water, both in lab-grown samples and paintings that are hundreds of years old.
Moving forward, the team aims to gain a more comprehensive understanding of the interactions between light, including experimental X-rays, and the pigments. This knowledge will help elucidate the complex reactions between pigments and binders, contributing to ongoing conservation efforts.
Additionally, the researchers aim to determine the precise range of humidity levels that facilitate the growth of arsenic species within pigments.