Silvia A. Centeno

207Pb and 119Sn Solid-State NMR and Relativistic Density Functional Theory Studies of the Historic Pigment Lead–Tin Yellow Type I and Its Reactivity in Oil Paintings

Lead soaps (lead carboxylates) have been detected in traditional oil paintings in layers containing the pigment lead-tin yellow type I (LTY-I). LTY-I has been used by artists from at least the second quarter of the 15th century until the first half of the 18th century. Soap formation can lead to protrusions in paint layers and increased transparency, causing the paint support to become visible. We have characterized LTY-I by (119)Sn and (207)Pb solid-state NMR (ssNMR) spectroscopy. Using a combination of NMR techniques and DFT molecular cluster calculations, we identify the individual species in LTY-I and determine their (119)Sn and (207)Pb chemical-shift tensors. The presence of starting materials from the synthesis, minium, and tin(IV) oxide was also verified. Knowledge of the chemical-shift tensor components and the impurities in LTY-I is important for examining the chemistry of degradation processes and soap formation. We demonstrate that ssNMR can be used to detect reaction between Pb2SnO4 and added palmitic acid in a model paint sample containing LTY-I.

Nuclear magnetic resonance spectra and (207)Pb chemical-shift tensors of lead carboxylates relevant to soap formation in oil paintings.

Soap formation in traditional oil paintings occurs when heavy-metal-containing pigments, such as lead white, 2PbCO3·Pb(OH)2, and lead tin yellow type I, Pb2SnO4, react with fatty acids in the binding medium. These soaps may form aggregates that can be 100–200 μm in diameter, which swell and protrude through the paint surface, resulting in the degradation of the paint film and damage to the integrity of the artwork. The factors that trigger soap formation and the mechanism(s) of the process are not yet well understood. To elucidate these issues, chemical and structural information is necessary, which can be obtained using solid-state 207Pb and 13C nuclear magnetic resonance (NMR). In this article, we report 207Pb and 13C solid-state NMR spectra and 207Pb chemical-shift tensors of lead carboxylates implicated in soap formation: lead stearate, lead palmitate, and lead azelate, in addition to lead oleate and lead heptanoate for comparison. The 13C cross polarization with magic-angle spinning (MAS) spectra of these lead carboxylates show resonance doubling for the carbons closest to the lead, indicating two different conformations of the fatty acid chains in the asymmetric unit. The 207Pb NMR spectra, from which tensors were determined, were obtained with direct excitation and spin-temperature alternation, with and without MAS, and with the wide band uniform rate smooth truncation Carr-Purcell-Meiboom-Gill pulse sequence. The results of these experiments show that the local coordination environment of lead azelate is different from lead palmitate and lead stearate and could thus be distinguished from these in a paint film displaying soap formation. In addition, comparing the 207Pb NMR chemical-shift tensors of the lead carboxylates studied shows that crystal packing of the acyl chains may be a factor in determining the coordination environment around the lead.

Van Gogh’s Irises and Roses : the contribution of chemical analyses and imaging to the assessment of color changes in the red lake pigments

Vincent van Gogh’s still lifes Irises and Roses were investigated to shed light onto the degree to which the paintings had changed, both individually and in relation to each other since they were painted, particularly in regard to the fading of the red lakes. Non-invasive techniques, including macroscopic X-ray fluorescence mapping, reflectance imaging spectroscopy, and X-radiography, were combined with microanalytical techniques in a select number of samples. The in-depth microchemical analysis was necessary to overcome the complications that arise when evaluating by non-invasive methods alone the compositions of passages with complex layering and mixing of paints. The results obtained by these two approaches were complemented by color measurements performed on paint cross-sections and on protected edges, and with historical information provided by the artist’s own descriptions, early reviews and reproductions, and the data was used to carry out digital color simulations that provided, to a certain extent, a visualization of how the paintings may have originally appeared.Graphical abstractIrises, 1890, Vincent van Gogh. The Metropolitan Museum of Art #58.187. Zn (upper right), Pb (bottom left), and Br (bottom right) distribution maps acquired by XRF imaging.

“Picasso in The Metropolitan Museum of Art”: An Investigation of Materials and Techniques

Abstract The 2010 exhibition “Picasso in The Metropolitan Museum of Art” presented the unparalleled opportunity to examine the working methods and materials of Pablo Picassos paintings in the museums collection, 34 works in total, covering a span of over 60 years. All paintings were comprehensively examined and studied using infrared reflectography and x-ray radiography. The paintings with paint surfaces that visually presented qualities associated with alkyd, enamel, or house paints were analyzed. Four paintings presenting such significant characteristics emerged from the group and are the focus of this study. Pigment and binding media analyses indicated the possible use of non-traditional artists paints but not necessarily house, alkyd, or enamel paints. Picassos infinite creativity was reflected in his experimental nature with materials. Specifically, his manipulation and alteration of the oil medium, which directly impacted the appearance and handling properties of the paint, must be taken into account. Ultimately, the results of this study illustrate the difficulty in identifying house, boat, or other non-traditional artists paints by visual examination alone and the challenges of their analytical characterization. Our analyses showed the presence of traditional artists materials of the time period but did not definitively identify chemical markers of house or other non-traditional artists paints.

Early science commissioning results of the sub-micron resolution X-ray spectroscopy beamline (SRX) in the field of materials science and engineering

Beamline commissioning activities at the Sub-micron Resolution Spectroscopy Beamline, SRX, one of the project beamlines of the National Synchrotron Light Source II, began in December 2014. SRX is a hard x-ray micro-probe beamline. The technical capabilities presented in this paper include scanning micro-fluorescence microscopy (µ-XRF) and x-ray absorption near-edge structure (µ-XANES) spectroscopy. The high flux KBs station with sub-micron resolution in the step-scanning mode has been commissioned with results presented in this paper. Capabilities under commissioning/planning include XRF-XANES stack imaging, a high resolution station (sub-100 nm), x-ray fluorescence tomography, integration of Maia detector, and fly-scan mode. Early science commissioning results from SRX in the materials science field are presented in this paper. Topics being studied include nanoporous materials for energy conversion/storage, thin film materials for electronics, degradation of paint materials for art conservation, and grai...

Elemental and Molecular Segregation in Oil Paintings due to Lead Soap Degradation

The formation of Pb, Zn, and Cu carboxylates (soaps) has caused visible deterioration in hundreds of oil paintings dating from the 15th century to the present. Through transport phenomena not yet understood, free fatty acids in the oil binding medium migrate through the paint and react with pigments containing heavy metals to form soaps. To investigate the complex correlation among the elemental segregation, types of chemical compounds formed, and possible mechanisms of the reactions, a paint sample cross-section from a 15th century oil painting was examined by synchrotron X-ray techniques. X-ray fluorescence (XRF) microscopy, quantified with elemental correlation density distribution, showed Pb and Sn segregation in the soap-affected areas. X-ray absorption near edge structure (XANES) around the Pb-L3 absorption edge showed that Pb pigments and Pb soaps can be distinguished while micro-XANES gave further information on the chemical heterogeneity in the paint film. The advantages and limitations of these synchrotron-based techniques are discussed and compared to those of methods routinely used to analyze paint samples. The results presented set the stage for improving the information extracted from samples removed from works of art and for correlating observations in model paint samples to those in the naturally aged samples, to shed light onto the mechanism of soap formation.

Understanding the gum dichromate process in pictorialist photographs: A literature review and technical study

Abstract In the late 1800s, pictorialist photographers favored a diversity of photographic techniques, including the gum dichromate process. Sometimes superimposed over other photographic images such as platinum and silver prints, the gum dichromate process utilizes a light-sensitive mixture of gum arabic, pigment, and a potassium dichromate solution hand-applied onto a sheet of paper and exposed to light while in direct contact with a negative. The definitive identification of this process has proven to be a challenge due to many variations and intermingling of techniques used by photographers of this period. This research began with a search through the historic literature, followed by the creation of test samples based on historic recipes, and the X-ray fluorescence analysis of these tests. The identification of pigments and the presence of chromium have been associated with the gum dichromate or other dichromated colloid processes in the past. Research results reveal that the presence of chromium may have more complex sources, requiring a more discriminating approach and a modified protocol for the identification of gum dichromate photographs.

Raman Mapping in the Scientific Investigations of Works of Art

Raman mapping in works of art has traditionally been performed using a motorized xy stage that moves a small artwork or a sample taken from the object in a step-by-step manner in two directions under the microscope objective. This configuration allows to obtain important information about the objects in a non-invasive manner and, in the case of samples, on the stratigraphy and the composition of all layers. Three case studies are presented to discuss the capabilities and limitations of this approach. In the first case, the distribution of crystalline and amorphous phases in a sample from a historical glaze were mapped and the resulting composition was related to the production technology of the decoration. In the second case, Raman mapping was performed non-destructively in a ceramic fragment to contribute to elucidate the complex process involved in the lustre decoration formation. And in the third case, the composition of the different paint layers in a sample cross-section removed from a polychromed wooden sculpture was mapped in order to help differentiate original paint layers from those resulting from restoration interventions. Among the limitations of the mapping approach using a motorized xy stage are the impossibility to use it with art objects that do not fit on the stage, under the microscope objective, and the time required for the automatic optimization of the focus distance at each point. Examples of in situ non-invasive mapping experiments in two contemporary paintings using a novel configuration in which a set of mirrors is placed in the horizontal exit of the microscope attached to the spectrometer are discussed. The advantages of this system are, in addition to allowing to map larger objects, its full confocality and the possibility to work with multiple laser excitations. The limitations of this approach are also discussed.

Anselm Kiefer: a study of his artistic materials

The style of the contemporary German artist Anselm Kiefer is highly innovative and unconventional and is characterized by the use of diverse materials that he selects and combines according to the emotions that they stir in him. The conservation and preservation of works by Kiefer are particularly difficult tasks because of the heterogeneity and, in some cases, the incompatibility of the materials used; therefore, a thorough characterization is crucial before any intervention is considered. In this paper, we report the results of an investigation on a fragment from a multimaterial work and on samples from the paintings Bohemia Lies by the Sea and Die Größe Fracht. The large fragment was cut by the artist himself from a work in progress and is considered destitute of any artistic value; therefore, it was possible to sample it extensively. This fragment and the samples from the Die Größe Fracht and Bohemia Lies by the Sea paintings were analyzed by ultraviolet (UV) fluorescence imaging, fiber optic reflectance spectroscopy, Fourier transform infrared spectroscopy, nuclear magnetic resonance, Raman spectroscopy, X-ray diffraction, gas chromatography-mass spectrometry, and pyrolysis gas chromatography-mass spectrometry. This multitechnique approach allowed us to fully characterize and identify pigments, dyes, and organic components that reflect the diversity of the materials typically chosen by the artist. The results are evaluated in the context of the interview that Antonio Rava had with the Anselm Kiefer in 2000.

AN INVESTIGATION INTO JAPINE PLATINUM PHOTOGRAPHS: WILLIAM WILLIS'S PROPRIETARY PAPER

Abstract Analyses of two samples of the commercial platinum photographic paper “Japine” revealed that this trade-name product was sold with a range of surface properties from matte to glossy. An in-depth examination of an unprocessed glossy Japine sample showed a chemically modified surface resulting from parchmentization by treatment with sulfuric acid. This treatment results in a thin film of amorphous cellulose on the printing surface, altering its visual appearance and wet strength. Photographers selected this paper due to its aesthetic qualities. However, such papers have never been characterized and are essentially unknown to modern-day scholars who may mistake them as prints with thin binder layers, such as albumen or gelatin. Indeed, the approach to conservation of these photographs may differ from one designed for a traditional platinum print. The parchmentized Japine paper was criticized for its tendency to crack, one characteristic that may have implications for its handling and conservation treatment today.