Toward a Complete Molecular Model for the Formation of Metal Soaps in Oil Paints
An overview is presented of the current state of understanding of the chemical pathways that lead to the formation of crystalline metal soap phases in oil paints, based on recent experimental work by the authors and supported by relevant literature. Improved (quantitative) interpretation of Fourier-transform infrared (FTIR) spectra has revealed that metal ions are bound to carboxylate functionalities of the oil polymer during oil paint aging, a state similar to ionomeric polymers. Tailored ionomer-like systems based on linseed oil were synthesized to study the structure of the mature oil paint binding medium, and such systems were used as the starting point for studies on metal soap crystallization. Additionally, series of differential scanning calorimetry (DSC) studies shed light on the driving forces and kinetics of metal soap crystallization, and electron microscopy studies have been used to image the initial stages of metal soap crystallization. The results have been used to construct a model of the chemical reactions leading to metal soaps from a mixture of pigment and oil. Additionally, the model provides insight into diffusion mechanisms for metal ions and fatty acids and potential physical transitions in the structure of metal soaps. The mechanisms described are helpful in explaining the different morphologies of lead and zinc soaps observed in actual samples from historic paintings and the locations within paint films where these are typically found.
The authors are indebted to Dr. John Drennan and his co-workers at the Australian Microscopy and Microanalysis Research Facility (AMMRF) for the collaborative effort to visualize the early stages of zinc soap crystallization. Lambert Baij, Robert Corkery, Ties Korstanje, and Silvia Centeno are thanked for sharing their knowledge and inspiring discussions.
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