Looking at the paintings in a museum, one may observe that the colors of Renaissance masterpieces are obviously less bright than they were in their original form. For many years, art conservationists and researchers have been baffled by the tendency of these art pieces to fade and change in color.
These changes have been previously attributed to light exposure, but the same things happen even if the historical paintings are stored in dark rooms. A new study on classic paintings has revealed humidity as the real culprit.
Investigating the Renaissance-Type Pigments
Color degradation among historical paintings was explored by a group of scientists from the Stanford Synchroton Radiation Lightsource (SSRL) at the Department of Energy's SLAC National Accelerator Laboratory in collaboration with the researchers from the University of Amsterdam, Rijksmuseum, and other institutions. Their peer-reviewed study aimed to investigate the presence of the arsenic species As(V) together with arsenic sulfide dye called orpiment.
There have been previous studies which reveal the role of As(V) in color degradation, but the underlying causes remain a mystery. To understand the conversion of pigments to As(V), the research team used the 17th-century painting "Still Life with Flowers in a Glass Vase" made by Jan Davidson de Heem. In this masterpiece, the color of the yellow eglantine rose located at the middle of the painting had been slowly turning white.
The sample was subject to intense X-rays generated at SSRL's beam lines in order to extensively analyze the details of the color change. As the X-ray scan provides 2D images of the painting's cross-section, the researchers can pinpoint the exact location of the arsenic species.
After the images are captured, an X-ray microscope was then used in studying the chemistry of the arsenic species using higher resolution and 3D technology. The combined X-ray techniques allowed the team to map the degradation pathways successfully.
The result of the study suggests that the color degradation is attributed to humidity as it provides a pathway for the passage of arsenic species, as confirmed by SSRL scientist Johanna Nelson Weker. It was also observed that contrary to the previous belief of conservators, the arsenic species were not restricted to the areas where the orpiment pigment was applied.
In the future, the researchers plan to gain new insights about the interactions between light and the pigments. Aside from this, they also aim to identify the specific range of humidity levels which trigger the growth of arsenic species within pigments.
Arsenic-Based Pigments
Arsenic is a naturally occurring element in the Earth's crust. This metalloid is widely distributed throughout the environment with small quantities present in rock, air, water, and soil. It usually occurs as a form of mineral in combination with sulfur and other metals, although it can also exist as a pure elemental crystal. In its inorganic form, arsenic could be highly toxic.
Arsenic-bearing minerals include sulfides, arsenates, oxides, and other forms. Minerals that contain arsenic can provide pigments with various colors when associated with other chromophores.
Sixteenth-century painters, especially those from Venice, are considered as "colorists" during the Renaissance period. The particular color in a Venetian palette is rich deep orange which was used generously from about 1490. This color was produced using the arsenic sulfide yellow orpiment and orange realgar.
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