Understanding the Dynamics and Structure of Lead Soaps in Oil Paintings Using Multinuclear NMR

Abstract

To understand the mechanisms and factors that trigger soap formation and the dynamics of the reactive compounds in paints, advanced nuclear magnetic resonance (NMR) and X-ray analyses, complemented by analysis with FTIR spectroscopy, were performed on a series of lead carboxylates and model paint samples. Similar spectroscopy and lead coordination were observed for lead carboxylates of carbon chain length of 9 to 11 and 16 (palmitic acid) and 18 (stearic acid). Experiments as a function of temperature and humidity provided insight into the factors that increase soap formation. The local dynamics of palmitic acid and lead palmitate, in a linseed oil matrix at different temperatures (T) were measured by 2H NMR spectroscopy. The results show the extent of mobility of palmitic acid and lead palmitate in the paint matrix, how they differ, and how they depend on T. The kinetics of soap formation in model paint films subjected to different relative humidities was monitored by 13C NMR spectroscopy; the rate of soap formation increases with relative humidity. The results are discussed in the context of their implications for the conservation and preservation of works of art affected by lead soap formation.