Lead soap aggregates have been found in lead-containing oil paint layers in paintings from the thirteenth to the twentieth century. They severely affect the stability of the paint layers and disturb the surface of the paintings. Paint cross-sections from five paintings affected by lead soaps were selected to illustrate and investigate this degradation phenomenon with the analytical imaging techniques of Fourier transform infrared spectroscopy, secondary ion mass spectrometry and scanning electron microscopy combined with X-ray analysis. Examples are given of lead soaps forming in a mature paint system or, alternatively, in the early drying stage of the oil; lead soaps forming from various types of lead-containing pigments or driers; lead soaps forming in multiple paint layers; and lead-containing crystallization products inside aggregates. The phenomenon of lead soap aggregates is multifaceted, and one general scenario describing the formation of lead soap aggregates cannot explain all aspects. However, the integration of the chemical information and its distribution among the paint layers leads to the proposal that reactive free monocarboxylic fatty acids play a key role in lead soap aggregate formation. The availability and release of these fatty acids depends on the original paint composition, the build-up of the layers, and the conservation/environmental exposure history of the painting.

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