The characterisation of lead fatty acid soaps in 'protrusions' in aged traditional oil paint

Lead(II) carboxylate soaps of two fatty acids, palmitic (C15H31COOH) and stearic acids (C17H35COOH), and a dicarboxylic acid, azelaic acid (HOOCC7H14COOH), have been synthesised and characterised by FTIR spectroscopy. These acids are all encountered in aged traditional oil paint, the azelaic acid resulting from the oxidative degradation of unsaturated fatty acids in the oil. Lead(II) azelate synthesised by hydrothermal methods was characterised by single crystal structure determination. This has a 3D polymeric structure with lead(II) ions linked by carboxylate bridges to form an infinite stack of (PbO4)(n) units. These layers are connected to adjacent layers by an infinite number of parallel C(CH2)(7)C chains arranged perpendicularly to the stacks. The lead(II) ions display an unusual 7-fold coordination. The first direct evidence that the 'protrusions' encountered in aged traditional lead-containing oil paints contain lead soaps is reported. Their mechanism of formation is discussed. (C) 2003 Elsevier Ltd. All rights reserved.