A technique called In-Air-Plasma-Induced Luminescence (In-Air-PIL) could be just as good as classical chemical and crystallographic analysis methods to study how exposure to sunlight darkens some commonly employed yellow pigments in paintings.
Van Gogh used chrome yellow (which is a bright yellow mainly composed of lead chromate and sulphide) in many of his paintings, but this pigment darkens over time. At the base of his “Les Aliscampes,” for example, large yellow areas have completely lost their brightness.
Researchers believe that the yellow deteriorates because of the formation of a Cr(III)-species and to the loss of orthorhombic crystallinity. Additional studies on the Matisse’s paintings have revealed that cadmium yellow degrades because of the formation of cadmium carbonates and sulphates present in the altered paint layers.
Little is known about the darkening mechanisms caused by light ageing of other yellow pigments that were widely used in paintings by artists ranging from the Babylonian era up to the 19th century. These include: Naples yellow (O7Pb2Sb2), cadmium yellow (CdS), chrome yellow (PbCrO4), litharge (PbO), and orpiment (As2O3). Each artist had his own favourite – Naples yellow was mainly used by “The Old Masters” painters, for example (one good example is “The Sleeping Venus” by Giorgione).
Developed by Patrizio Antici of the INRS in Canada and colleagues, In-Air-PIL makes use of a small particle accelerator and a laser-generated photon source. In the technique, the laser interacts with air to produce a plasma that generates electrons. When these electrons reach the sample surface, they cause luminescence emission and plasmonic resonance that can be used to characterize the chemical and optical properties of the pigment.
The researchers used In-Air-PIL to analyse how the five yellow pigments mentioned above darkened on exposure to a light from a white lamp for several hours. They compared the results they obtained using this technique with classical chemical analysis approaches, such as Energy Dispersive X-Ray Spectroscopy.
Both techniques revealed that the chemical composition of all samples was unaffected by light ageing, but the In-Air-PIL results also confirmed a strong shift in the crystal lattice structure of the pigments. This altered structure increases the amount of light absorbed by the pigments, causing them to darken.
The In-Air-PIL technique only takes five minutes to produce a reading of chemical composition and crystal characteristics and works on larger surfaces than the leading technologies currently used. It is also more compact and easier to set up and might thus be a promising alternative to the PIXE (Particle-Induced X-ray Emission) diagnostic, considered to be the gold standard in the field. PIXE produces extremely precise analyses but it is also expensive and bulky. “In many cases, that kind of extreme precision isn’t essential making in-Air PIL a highly viable alternative,” says Prof. Antici.
The team at INRS worked with scientists from Italy, Greece and France. In France, the Bordeaux Nuclear Research Centre (AIFIRA-CENBG) is well known for its specialized particle accelerator used in artwork analysis and its work with the Musée du Louvre.
The research is detailed in Science Advances 10.1126/sciadv.aar6228.
Read the research paper: Pigment darkening as case study of In-Air Plasma-Induced Luminescence, M. Barberio et al., Science Advances 10.1126/sciadv.aar6228