Why red pigments fade in historical art works
Most artists would surely like to leave behind a rich legacy of their work. Yet artworks are prone to fading – sometimes even within an artist’s lifetime. This unfortunate phenomenon can be seen in many 19th-century European impressionist paintings, especially when artists have used organic red pigments (ORPs). “These spontaneous and unwanted reactions start taking place from the moment the pigment is exposed to light, even before the artist finishes the painting,” explains Alba Alvarez Martin(opens in new window), research scientist at the Rijksmuseum(opens in new window), former postdoctoral researcher at the University of Antwerp(opens in new window) and principal investigator of the RED-OPEN project. This fading presents a serious challenge to conservation scientists dealing with historical artworks. Low concentrations of the original pigments and the complex nature of their degradation make ORP identification difficult or impossible. In the EU-funded RED-OPEN project, undertaken with the support of the Marie Skłodowska-Curie Actions(opens in new window) (MSCA) programme, Alvarez Martin led a team of researchers that used a multi-analytical method to understand the fading mechanism on a deeper level. The project results will contribute to comprehending the chemistry behind the discoloration of paintings.
The chemical weakness of organic red pigments
Organic pigments are thought to fade more than inorganics due to their chemical composition and structure: they often contain a system of conjugated double bonds known as a chromophore, in which chemical bonds alternate between single and double bonds. This allows the molecule to absorb light in the visible range, leading to the perception of colour. However, the double bonds can be very sensitive to light, heat and oxygen, leading to chemical reactions that cause the molecule to break down and a total discoloration of the organic pigment. “One of the pigments included in the project, geranium lake – a pigment used by Van Gogh – can completely fade within a few months when exposed to natural light,” says Alvarez Martin.
Employing mass spectrometry with organic pigments
To investigate the fading mechanism further, the team used an imaging mass spectrometry technique MALDI-MSI(opens in new window), never used before to map organic pigments in cultural heritage-related samples. “We have investigated the efficacy of this method in oil paint samples containing a mixture of two historic organic pigments, geranium lake and lead white, a mixture often employed in Van Gogh’s oeuvre,” adds Alvarez Martin. This allowed the researchers to visualise the molecules responsible for the pink hue of the paint, and also detect the chemical changes behind the discoloration of paintings with this composition. “The results demonstrate that this approach provides valuable molecular compositional information on the degradation pathways of pigments in specific paint layers,” says Alvarez Martin. The results were recently published in the peer-reviewed journal ‘Analytical Chemistry’(opens in new window). “For now, we have applied this method to oil paint reconstructions prepared in the lab following historical recipes,” Alvarez Martin notes. “We expect to start applying this method to historical samples in the coming months.”
Supporting the preservation of invaluable art
Through the project, the team established new collaborations with other European and North American research groups, in academia and in major museums. After completing the MSCA programme, Alvarez-Martin started working as a research scientist at the Rijksmuseum while continuing to collaborate with the group of Koen Janssens, professor in the chemistry department at the University of Antwerp.
