Abstract
The formation of Pb, Zn, and Cu carboxylates (soaps) has caused visible deterioration in hundreds of oil paintings dating from the 15th century to the present. Through transport phenomena not yet understood, free fatty acids in the oil binding medium migrate through the paint and react with pigments containing heavy metals to form soaps. To investigate the complex correlation among the elemental segregation, types of chemical compounds formed, and possible mechanisms of the reactions, a paint sample cross-section from a 15th century oil painting was examined by synchrotron X-ray techniques. X-ray fluorescence (XRF) microscopy, quantified with elemental correlation density distribution, showed Pb and Sn segregation in the soap-affected areas. X-ray absorption near edge structure (XANES) around the Pb-L3 absorption edge showed that Pb pigments and Pb soaps can be distinguished while micro-XANES gave further information on the chemical heterogeneity in the paint film. The advantages and limitations of these synchrotron-based techniques are discussed and compared to those of methods routinely used to analyze paint samples. The results presented set the stage for improving the information extracted from samples removed from works of art and for correlating observations in model paint samples to those in the naturally aged samples, to shed light onto the mechanism of soap formation.
Highlights
Soap formation has been identified in hundreds of works of art dating from the 15th century to the present, and in numerous cases it has been linked to environmental factors such as light exposure, changes in relative humidity, and/or elevated temperatures[10]
Results of the study of the soaps formed in a 15th century oil painting, The Crucifixion by Jan Van Eyck[27,28], show that the combination of synchrotron X-ray fluorescence (XRF) microscopy and μ-X-ray absorption near edge structure (XANES), both with a spatial resolution in the order of a micron, permits one to determine the elemental and molecular distributions providing complementary information to other techniques routinely used in conservation science laboratories
It has been shown that Pb3O4 may be present as an impurity in PbSn yellow type I commercial pigment samples[24], but it is known that this compound forms inside Pb soap protrusions[32]
Summary
Soap formation has been identified in hundreds of works of art dating from the 15th century to the present, and in numerous cases it has been linked to environmental factors such as light exposure, changes in relative humidity, and/or elevated temperatures[10]. A wide range of methods have been used to characterize soap deterioration in paintings and to study the reactions in model paint samples Resolved techniques such as Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy - energy dispersive X-Ray spectroscopy (SEM-EDS), secondary ion mass spectrometry (SIMS), and XRF have been employed to analyze the soaps www.nature.com/scientificreports/. Results of the study of the soaps formed in a 15th century oil painting, The Crucifixion by Jan Van Eyck[27,28], show that the combination of synchrotron XRF microscopy and μ-XANES, both with a spatial resolution in the order of a micron, permits one to determine the elemental and molecular distributions providing complementary information to other techniques routinely used in conservation science laboratories. One should pay attention to potential self-absorption effect if X-ray fluorescence measurements are conducted; the limitations of the techniques regarding this and other aspects are discussed and summarized in the conclusion
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