Elemental and Molecular Segregation in Oil Paintings due to Lead Soap Degradation

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.

On the photoluminescence changes induced by ageing processes on zinc white paints

“…One significant consequence of the interaction is the formation of metal carboxylates, which can be detected by Fourier Transform Infrared (FTIR) spectroscopy [8] [10]. It has been shown that in the case of oil and metal-based pigments, the unbound fatty acids present inside the oleic binder bond to metal ions and, via a hopping mechanism, cause the migration of metal ions from the metal oxide to the binding medium where the metal ions attach to the polymer network and form metal carboxylates [11] [12]. Another mechanism suggested to occur in paint material is the adsorption of the organic acids on the pigment particle surface [10] [13].…”

Review of the use of NMR spectroscopy to investigate structure, reactivity, and dynamics of lead soap formation in paintings

“…The formation of metal carboxylates (soaps) has been reported in hundreds of oil paintings dating from the 15th to the 20th century. [1][2][3][4][5][6][7][8][9][10] Soaps are the result of reactions between metals in pigments or in other compounds present in the paints, such as driers or other modifiers, and long chain fatty acids originating in the binding medium or coatings. The triggering factors and underlying mechanisms behind the processes are not yet completely understood.…”

“…The use of a micron-sized beam in XRF mapping, with scanning of the X-ray photon energy, allows micro-X-ray absorption near-edge structure (micro-XANES) spectroscopy across the Pb-M 4 and M 5 edges that permits differentiation of Pb pigments and Pb soaps. 9,34,35 Scanning electron microscopy (SEM), in combination with optical microscopy, has been applied to image the characteristic morphology of soap protrusions localized in the stratigraphy of paint crosssections. 36 Chromatographic and/or mass spectrometric techniques, such as gas chromatography-mass spectrometry (GC-MS), pyrolysis-GC-MS, secondary ion mass spectrometry (SIMS), high-performance liquid chromatography (HPLC), direct temperature-resolved mass spectrometry (DTMS), and related methods, have also been utilized in samples removed from works of art and in model paints to identify fatty acids involved in soap formation.…”

Nano- to microscale three-dimensional morphology relevant to transport properties in reactive porous composite paint films

“…A wide range of methods have been used to characterize the reactive species, free fatty acids and pigments, and products of soap deterioration in both microsamples removed from artistic paintings and model paint samples (See, for example 1 , 3 , 5 , 6 , 16 – 22 ). Boon et al utilized neutron radiography on a mesoscopic level to study water uptake by model samples of painting canvas supports and ground preparations.…”