Katrien Keune

Analytical Imaging Studies of Cross-Sections of Paintings Affected by Lead Soap Aggregate Formation

Abstract Lead soap aggregateshave 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-sectionsfrom 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 aregiven of lead soapsforming in a mature paint system or, alternatively, in the early drying stage of the oil; lead soapsforming from various types of lead-containing pigments or driers; lead soapsforming in multiple paint layers; and lead-containing crystallization products inside aggregates. The phenomenon of lead soap aggregatesis multifaceted, and one general scenario describing theformation of lead soap aggregatescannot explain all aspects. However, the integration of the chemical information and its distribution among the paint layers leads to the proposal that reactivefree monocarboxylicfatty acids playa key role in lead soap aggregateformation. The availability and release of thesefatty acids depends on the original paint composition, the build-up of the layers, and the conservation/environmental exposure history of the painting.

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.

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.

Degradation of Emerald green in oil paint and its contribution to the rapid change in colour of the Descente des vaches (1834-1835) painted by Theodore Rousseau

Abstract Descente des vaches (1836) by Théodore Rousseau in the Mesdag Collection in The Hague is barely readable and its paint layers are in poor condition. The surface of the painting is strongly deformed and cracked, the whole painting has darkened and especially the greens have lost all or most of their colour resulting in brown passages. Large passages of the painting that were painted with multiple thick and medium-rich layers have darkened dramatically. This paper proposes that the degradation of Emerald green (Cu(C2H3O2)2·3Cu(AsO2)2, copperacetoarsenite) – the main green pigment used in this painting – is a significant factor in the cause of the darkening. Electron backscatter images reveal that the Emerald green particles are shown different degrees of degradation: from partially to completely disintegrated. Elemental maps show that arsenic is distributed throughout the paint cross section, with relatively higher concentrations around iron- and aluminium-containing particles, and in the varnish layer. Imaging-Fourier-transform infrared microscopy detects copper soaps in the degraded Emerald green-containing layers. Analytical data from four paint cross sections strongly suggest that Emerald green reacts with free fatty acids derived from the binding medium forming copper soaps and mobile arsenic-based species. Chemical laboratory experiments fully support this hypothesis. Emerald green and palmitic acid in chloroform form copper palmitate and arsenic trioxide (arsenolite, cubic) under room temperature and normal light conditions. The degradation of Emerald green particles in Descente des vaches has resulted in a loss of light-reflecting surfaces and in newly formed compounds in the paint, both contribute to the colour change from green to brown.

A study of zinc soap aggregates in a late 19th century painting by R.G. Rivers at the Queensland Art Gallery

Abstract Metal soap formation in paintings has been implicated in a serious deterioration phenomenon. The present study documents zinc soap aggregates observed in a late 19th century painting by R. Godfrey Rivers. Optical microscopy and scanning electron microscopy with energy dispersive X-ray analysis of paint cross-sections are used to describe the appearance and elemental composition of affected paint layers. Fourier Transform Infrared Spectroscopy (FTIR) is used to confirm the presence of zinc carboxylates and static secondary ion mass spectrometry (SIMS) results are given for one sample. Scanning electron microscopy energy dispersive X-ray (SEMEDX) maps and spot analyses are used to examine aggregates in detail. In addition to zinc, carbon and oxygen, magnesium is frequently present. Paint composition and environment are discussed in terms of their potential to influence soap formation.

Analytical imaging studies of the migration of degraded orpiment, realgar, and emerald green pigments in historic paintings and related conservation issues

Yellow orpiment (As2S3) and red–orange realgar (As4S4) photo-degrade and the nineteenth-century pigment emerald green (Cu(C2H3O2)2·3Cu(AsO2)2) degrades into arsenic oxides. Because of their solubility in water, arsenic oxides readily migrate and are found throughout the multi-layered paint system. The widespread arsenic migration has consequences for conservation, and this paper provides better insight into the extent of the problem. Five paint samples containing orpiment, realgar or emerald green pigments deriving from paintings by De Heem (17th C), Van Gogh (19th C), Rousseau (19th C), an unknown 17th C northern European artist and an Austrian painted cupboard (19th C) were investigated using SEM/EDX, imaging ATR-FTIR and arsenic (As) K–edge μ-XANES to obtain the spatial distribution and chemical speciation of arsenic in the paint system. In all of the samples investigated arsenic had migrated throughout the multi-layered paint structure of the art object, from support to varnish. Furthermore, As5+-species were found throughout the entire paint sample. We hypothesize that arsenic trioxide is first formed, dissolves in water, further oxidizes to arsenic pentaoxide, and then reacts with lead, calcium and other ions and is deposited in the paint system as insoluble arsenates. Since the degradation of arsenic pigments such as orpiment, realgar and emerald green occurs through a highly mobile intermediate stage, it not only affects the regions rich in arsenic pigments, but also the entire object, including substrate and top varnish layers. Because of this widespread potential for damage, preventing degradation of arsenic pigments should be prioritized and conservators should minimize exposure of objects containing arsenic pigments to strong light, large fluctuations in relative humidity and water-based cleaning agents.

Its surreal: zinc-oxide degradation and misperceptions in Salvador Dalí’s Couple with Clouds in their Heads, 1936

The painting Couple with Clouds in their Heads by Salvador Dali (1936) from the Museum Boijmans van Beuningen in Rotterdam was seriously disfigured by ring formations on the edges of the painting and a matt, white-looking semi-transparent material on the surface. Paint samples, taken from affected and unaffected paint areas, investigated with light, scanning electron and ATR-FTIR microscopy and X-ray diffraction showed that the zinc white-containing oil paint was converted into zinc soaps, zinc formate dihydrate, zinc acetate dihydrate and zinc sulfide. The conversion of the zinc white started at the surface of the painting. Zinc soaps are formed by a reaction of zinc white and fatty acids from the oil. Formate and acetate, most likely emitted from the wooden frame, reacted with the zinc white pigment. Finally, the zinc sulfide is suggested to derive from zinc white reacting with hydrogen sulfide.

An Investigation into the Viability of Removal of Lead Soap Efflorescence from Contemporary Oil Paintings

Metal soap efflorescent hazes and crusts are one of the most visually disturbing of all metal soap related alterations in paintings. The phenomenon presents particular challenges for conservators as the surface deposits that form are insoluble and intimately bound with the paint film. The superficial deposits accumulate following migration of metal carboxylates, a product formed within the paint film from reaction of free fatty acids with metal ions from pigments and/or driers. Upon deposition at the surface, the metal carboxylates undergo a process of remineralisation, the products of which are determined by the atmospheric conditions that the painting is exposed to. The material can appear optically similar to the whitish haze of organic efflorescence though manifestations also range from crystalline clusters to thick crusts, and in some cases the painting may consequently be rendered partially unreadable. This study aimed to investigate the removal of visually disturbing lead soap efflorescence with Ethylenediaminetetraacetic acid (EDTA), which sequesters the lead ions from the remineralised crust. Through a careful balance of variables such as pH, concentration and application, it is feasible to thin the superficial material significantly. By this means it is possible to saturate the surface of the paint film and render the previously disfiguring efflorescence transparent. Paintings previously treated with this method show no signs of recurrence and thus it appears to be a viable, long-term treatment option.