Joen J. Hermans

An infrared spectroscopic study of the nature of zinc carboxylates in oil paintings

The formation of metal soaps is a major problem for oil paintings conservators. The complexes of either lead or zinc and fatty acids are the product of reactions between common pigments and the oil binder, and they are associated with many types of degradation that affect the appearance and stability of oil paint layers. Fourier transform infrared spectroscopy (FTIR) reveals that a paint sample from The Woodcutter (after Millet) by Vincent van Gogh contains two distinct zinc carboxylate species, one similar to crystalline zinc palmitate and one that is characterized by a broadened asymmetric stretch COO− band shifted to 1570–1590 cm−1. This observation has been made in many paintings. Although several hypotheses exist to explain the shifted broad carboxylate band, these were not supported by experimental evidence. In this paper, experiments were carried out to characterize the second zinc carboxylate type. It is shown that neither variations in the composition of zinc soaps (i.e. zinc soaps containing mixtures of fatty acids or metals) nor fatty acids adsorbed on pigment surfaces are responsible for the second zinc carboxylate species. X-Ray diffraction (XRD) and FTIR analysis indicate that the broad COO− band represents amorphous zinc carboxylates. These species can be interpreted as either non-crystalline zinc soaps or zinc ions bound to carboxylate moieties on the polymerized oil network, a system similar to ionomers. These findings uncover an intermediate stage of metal soap-related degradation of oil paintings, and lead the way to improved methods for the prevention and treatment of oil paint degradation.

Ionomer-like structure in mature oil paint binding media

Infrared spectra of samples from oil paintings often show metal carboxylate bands that are broader and shifted compared to those of crystalline metal soap standards (metal complexes of long-chain saturated fatty acids). Using quantitative attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), it is demonstrated that the broad metal carboxylate band is typically too intense to be explained by carboxylates adsorbed on the surface of pigment particles or disordered metal complexes of saturated fatty acids. The metal carboxylate species associated with the broad bands must therefore be an integral part of the polymerized binding medium. Small-angle X-ray scattering (SAXS) measurements on model ionomer systems based on linseed oil revealed that the medium contains ionic clusters similar to more classical ionomers. These structural similarities are very helpful in understanding the chemistry of mature oil paint binding media and the potential degradation mechanisms that affect oil paintings.

Random Graph Approach to Multifunctional Molecular Networks

Formation of a molecular network from multifunctional precursors is modelled with a random graph process. The random graph model favours reactivity for monomers that are positioned close in the network topology, and disfavours reactivity for those that are obscured by the surrounding. The phenomena of conversion-dependant reaction rates, gelation, and micro-gelation are thus naturally predicted by the model and do not have to be imposed. Resulting non-homogeneous network topologies are analysed to extract such descriptors as: size distribution, crosslink distances, and gel-point conversion. Furthermore, new to the molecular simulation community descriptors are invented. These descriptors are especially useful for understanding evolution of pure gel, amongst them: cluster coefficient, network modularity, cluster size distribution.