Analysis of Lead Carboxylates and Lead-Containing Pigments in Oil Paintings by Solid- State Nuclear Magnetic Resonance
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Analysis of Lead Carboxylates and Lead-Containing Pigments in Oil Paintings by SolidState Nuclear Magnetic Resonance Jaclyn Catalano1,2, Yao Yao2, Anna Murphy2, Nicholas Zumbulyadis3, Silvia A. Centeno1, Cecil Dybowski2 1
Department of Scientific Research, The Metropolitan Museum of Art, New York, NY 10028, U.S.A. 2 Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716, U.S.A. 3 Independent Researcher
ABSTRACT Soap formation in traditional oil paintings occurs when heavy-metal-containing pigments, such as lead white, 2Pb(CO3)2·Pb(OH)2, and lead-tin yellow type I, Pb2SnO4, react with fatty acids in the binding medium. These soaps may form aggregates that can be 100-200 µm in diameter, which swell and protrude through the paint surface, resulting in the degradation of the paint film and damage to the integrity of the artwork. In addition, soap formation has been reported to play a role in the increased transparency of paint films that allows the painting support, the preparatory drawing, and the artists’ alterations to become visible to the naked eye. The factors that trigger soap formation and the mechanism(s) of the process are not yet well understood. To elucidate these issues, chemical and structural information is necessary which can be obtained by solid-state 207Pb, 119Sn, and 13C nuclear magnetic resonance (NMR). In the present study, a combination of 207Pb NMR pulse sequences was used to determine accurately the NMR parameters of lead-containing pigments and lead carboxylates known to be involved in soap formation, such as lead palmitate, lead stearate, and lead azelate. These results show that the local coordination environment of lead azelate is different from lead palmitate or lead stearate and therefore it is unlikely that lead azelate would be incorporated into an ordered structure containing lead palmitate and lead stearate. In addition, the chemical shifts of the pigments obtained are different from those of the soaps, demonstrating that 207Pb NMR is useful in characterizing the components when present in a mixture, such as a paint film. The NMR methods discussed can also be applied to other Pb-containing cultural heritage materials, electronic and optoelectronic materials, superconducting materials, and environmentally contaminated materials.
INTRODUCTION The formation of lead and other heavy-metal carboxylates, also called heavy-metal soaps, has been reported to be the cause of deterioration of hundreds of oil paintings dating from the fifteenth to the twentieth centuries [1-6]. Soaps form when heavy-metal-containing pigments, such as the commonly used lead white, 2PbCO3·Pb(OH)2, and lead-tin yellow type I, Pb2SnO4, react with fatty acids that result from the hydrolysis of glycerides in the oil binding medium, or from protective coatings [1, 2, 4, 7, 8]. Soaps have been characterized and identified in samples from works of art by microanalytical techniques, such as FTIR, SIMS, GC-MS, DTMS, SEM-
EDX, micro-XRF, and Raman spectroscopy, inside aggregates or inclusions th
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