Abstract

Soap formation in traditional oil paintings occurs when heavy-metal-containing pigments, such as lead white, 2PbCO₃·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. 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 using solid-state ²⁰⁷Pb and ¹³C nuclear magnetic resonance (NMR). In this article, we report ²⁰⁷Pb and ¹³C solid-state NMR spectra and ²⁰⁷Pb chemical-shift tensors of lead carboxylates implicated in soap formation: lead stearate, lead palmitate, and lead azelate, in addition to lead oleate and lead heptanoate for comparison. The ¹³C cross polarization with magic-angle spinning (MAS) spectra of these lead carboxylates show resonance doubling for the carbons closest to the lead, indicating two different conformations of the fatty acid chains in the asymmetric unit. The ²⁰⁷Pb NMR spectra, from which tensors were determined, were obtained with direct excitation and spin-temperature alternation, with and without MAS, and with the wide band uniform rate smooth truncation Carr-Purcell-Meiboom-Gill pulse sequence. The results of these experiments show that the local coordination environment of lead azelate is different from lead palmitate and lead stearate and could thus be distinguished from these in a paint film displaying soap formation. In addition, comparing the ²⁰⁷Pb NMR chemical-shift tensors of the lead carboxylates studied shows that crystal packing of the acyl chains may be a factor in determining the coordination environment around the lead.

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