Annelies van Loon

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.

Tracking the transformation and transport of arsenic sulfide pigments in paints: synchrotron-based X-ray micro-analyses

Realgar and orpiment, arsenic sulfide pigments used in historic paints, degrade under the influence of light, resulting in transparent, whitish, friable and/or crumbling paints. So far, para-realgar and arsenic trioxide have been identified as the main oxidation products of arsenic sulfide pigments. This paper shows that after photo-degradation, various oxidation and migration processes take place. Synchrotron radiation (SR) micro-X-ray fluorescence (μ-XRF) reveals arsenic to be distributed throughout the whole multi-layered paint system. Arsenic (As) K-edge micro-X-ray absorption near edge structure (μ-XANES) analyses indicate the presence of an intact AsxSy pigment, arsenite compounds (As3+; As2O3), and arsenate compounds (As5+); the latter are certainly present as calcium, lead, aluminium and iron arsenates. Sulfur (S) K-edge μ-XANES points to the conversion of the sulfide (S2−) group to a sulfate (SO42−) group, probably via an elemental sulfur (S0) or sulfoxide (S2+) compound. Principal Component Analysis (PCA) and subsequent k-means clustering of multi-energy SR μ-XRF maps and μ-XANES were performed to identify the various arsenic species and visualize their distribution. The arsenates (As5+) are spread throughout the entire paint system and dominate the photo-degraded paint and ground layers, while the arsenite compounds (As3+) are located close to the intact arsenic sulfide pigment. The oxidation of arsenic trioxide into arsenates likely takes place in aqueous solutions. The presence of As5+ compounds in the paint systems indicates that the arsenic trioxide is dissolved by ambient water present in the paint. Arsenite and arsenate compounds are water soluble and are transported by water throughout the paint system. This knowledge is crucial for the conservation field, as this is the first time that (indirect) evidence of water transport within paintings has been given.

Photochemistry of the metal-metal-bonded complexes [(CO)5Mn-Ru(Me)(CO)2(alpha-diimine)]. Crystal structure of the photoproduct [(CO)4MN-mu-(sigma(N), sigma(N'), etha2(CN)-i-Pr-PyCa)Ru(Me)(CO)2]

Abstract Photochemical reactions are reported for the metal-metal-bonded complexes [(CO)5MnRu(Me)(CO)2(α-diimine)] [α-diimine = pyridine -2- carbaldehyde -N- isopropylimine ( i PrPyCa) , N,N′-diisopropyl-1,4-diaza-1,3-butadiene (iPr0DAB)] upon irradiation into their lowest-energy absorption band. The radicals [Mn(CO)5] and [Ru(Me)(CO)2(α-diimine)] are formed by homolytic splitting of the MnRu bond; the latter radicals have been characterized by ESR spectroscopy. In THF, the [Ru(Me)(CO)2(iPr-DAB)] radicals dimerize to give [Ru(Me)(CO)2(iPr-DAB)]2, and the corresponding radicals from the iPr-PyCa complex gave an unidentified product. Irradiation of a solution of the complexes in CH2Cl2 or CHCl3 afforded [Mn(Cl)(CO)5] and [Ru(Cl)(Me)(CO)2(α-diimine)] but in apolar solvents such as hexane, completely different photoproducts, viz. [(CO)3Mn-μ-(σ(N),σ(N′),ν2(CN),ν2(C′N′)-iPr-DAB)Ru(Me)(CO)2] and [(CO)4Mn-μ-(σ(N),σ(N′),ν2(CN)-iPr-PyCa)Ru(Me)(CO)2] were obtained. The crystal structure of the last complex was determined. The photochemical quantum yield for the disappearance of the iPr-DAB complex was ca. 0.30, but only 0.05 for the iPr-PyCa complex. Low-temperature measurements and flash photolysis data showed that this difference in behaviour is due to the thermal instability of the dimer [Ru(Me)(CO)2(iPr-PyCa)]2, which leads to partial regeneration of the parent complex at room temperature. At temperatures below 163 K, the photodecomposition into radicals was followed by electron transfer, leading to formation of the ions [(CO)5Mn]− and [Ru(Me)(S)(CO)2(α-diimine)]+ in 2-MeTHF and the contact ion-pair [(CO)5Mn−/3. Ru(Me)(CO)2(α-diimine)+] in 2-chlorobutane. Similar photodisproportion products were formed upon irradiation of the complexes at room temperature in the presence of N- or P-donor ligands.

SEM Backscattered-Electron Images of Paint Cross Sections as Information Source for the Presence of the Lead White Pigment and Lead-Related Degradation and Migration Phenomena in Oil Paintings

Scanning electron microscopy backscattered-electron images of paint cross sections show the compositional contrast within the paint system. They not only give valuable information about the pigment composition and layer structure but also about the aging processes in the paint. This article focuses on the reading of backscatter images of lead white-containing samples from traditional oil paintings (17th-19th centuries). In contrast to modern lead white, traditional stack process lead white is characterized by a wide particle size distribution. Changes in particle morphology and distribution are indications of chemical/physical reactivity in the paint. Lead white can be affected by free fatty acids to form lead soaps. The dissolution of lead white can be recognized in the backscatter image by gray (less scattering) peripheries around particles and gray amorphous areas as opposed to the well-defined, highly scattering intact lead white particles. The small particles react away first, while the larger particles/lumps can still be visible. Formed lead soaps appear to migrate or diffuse through the semipermeable paint system. Lead-rich bands around particles, at layer interfaces and in the paint medium, are indications of transport. The presence of lead-containing crystals at the paint surface or inside aggregates furthermore point to the migration and mineralization of lead soaps.

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.

Synthesis, structure and spectroscopic properties of novel metal-metal bonded manganese-ruthenium complexes with α-diimine ligands. X-ray structure of [(CO)5MnRu(Me)(CO)2(σ(N′)-iPr-PyCa)] (iPr-PyCa = pyridine-2-carbaldehyde-N-isopropylimine)

The synthesis and spectroscopy of the complexes [(CO)5MnRu(Me)(CO)2(α-diimine)] (α-diimine = N, N′-diisopropyl-1,4-diaza-1,3-butadiene (iPr-DAB), pyridine-2-carbaldehyde-N-isopropylimine (iPr-PyCa) are reported. The metal fragments are bonded to each other by an uncommon manganese-ruthenium bond. The single crystal X-ray structure has been determined. The purple crystals of [(CO)5MnRu(Me)(CO)2(iPr-PyCa) are monoclinic, space group P21n, Z=4, with unit cell dimensions a=9.6297(7), b=20.923(1), c=10.166(1) A and β=92.190(8)°. The structure refinement converged to R=0.035 for 3277 observed reflections. Both complexes show a strong, solvatochronic absorption band in the visible region, which is assigned to Ru (dπ)→ α-diimine (π∗) transitions. In aggreement with this assignment, the Raman spectra show resonance enhancement of Raman intensity for νs(CO) and νs(CN) of the Ru(Me)(CO)2(α-diimine) fragment.

THE RELATIONSHIP BETWEEN PRESERVATION AND TECHNIQUE IN PAINTINGS IN THE ORANJEZAAL

Abstract This paper describes natural aging processes in the seventeenth-century oil paintings from the Oranjezaal ensemble in the Huis ten Bosch Palace (The Hague). Degradation of a wide range of pigments – lead white, lead-tin yellow, minium, smalt, azurite, ultramarine, vivianite, orpiment, vermilion, bone black, organic lakes, indigo and Kassel earth – in oil paints is discussed. In this unique case of an ensemble kept under known conditions and with similar restoration histories, it was possible to relate the degree of aging to differences of quality of pigment and binding medium and the ways the artists applied their materials.