AVS2014 Session CS-ThA: Conservation Studies of Heritage Materials 2

The nature of technical studies is necessarily interdisciplinary as they address various questions about works of art and artifacts: What materials did the artist use ; how did he use his materials; and most importantly, what is the significance of this information for a greater understanding of the artist and his work? What is learned in technical studies often both augments art historical research and informs conservation treatment decisions.Collaboration between conservators, scientists, and art scholars is becoming ever more common in the study of works of art. Results of in-depth studies are now featured in exhibitions and scholarly publications, and their importance is increasingly recognized within larger art historical studies of an artist’s oeuvre.

In this light, this presentation will consider two recent technical studies carried out at The Phillips Collection in Washington, D.C. The first, a study of a group of Georges Braque’s mid-career paintings, was conducted in collaboration with conservation scientists from Harvard Art Museums and curators at the Phillips and the Kemper Art Museum in St. Louis. The results of the study were featured in the exhibition Georges Braque and the Cubist Still Life, 1928-1945 and in the related publication. The second study focuses on one painting, The Blue Room (1901), an early Blue Period picture by Pablo Picasso in the collection at the Phillips. Ongoing research of this picture is being conducted in collaboration with independent Picasso scholars and scientists from the Winterthur Museum in Delaware, Cornell University, and the National Gallery of Art.

Both studies began in the conservation studio with the conservator carefully examining each painting in good light and under magnification, considering each artist’s technique and how it may have influenced their material choices. Other examination techniques, such as UV-induced fluorescence, infrared imaging, and x-radiography were also employed.

In both studies, paint samples were taken and analyzed to positively identify the artists' materials and understand them in context. In the Braque study, the increasingly wide-spread use by conservators of portable x-ray fluorescence (pXRF) spectroscopy allowed for comparative pigment analysis of eight paintings from five different collections. For the Picasso, three non-invasive techniques—reflectance imaging spectroscopy, fiber optic reflectance spectroscopy (FORS), and XRF intensity mapping—were used.

The presentation will consider the collaborative nature of both studies and evaluate what made them successful in addition to discussing the process and outcome of each project from the conservator’s perspective.

In recent years, the scientific analysis of painted surfaces has made a giant leap forward. At the same time as innovative standoff macro-scale imaging modalities have been developed to deliver elemental mapping (using macro X-ray fluorescence spectroscopy) and molecular imaging (with Ultra-Violet/ Visible, Near Infrared, Mid-Infrared reflectance, fluorescence, and Raman imaging), our ability to probe local paint chemistries and mechanical properties at the nanoscale has grown exponentially. These recent developments have fundamentally changed the way conservators, curators and conservation scientists approach the study of works of art, leading to cutting-edge research on pigment degradation phenomena and enabling us to retrieve otherwise lost information on altered colors or hidden compositions that make up the original aspect of masterpieces.

This talk will present recent research employing high resolution nanoprobe synchrotron X-ray fluorescence (XRF) mapping of metallic impurities with 30 nm resolution in single grains of zinc oxide pigments used in early 20^th century paints formulated for artists and other commercial uses and widely employed by Pablo Picasso (1881-1973). Such highly detailed chemical characterization of paints at the nanoscale opens the path to a better understanding of their historical fabrication and chemical reactivity.

For the characterization of organic molecules used in paintings Surface-Enhanced Raman Spectroscopy (SERS) has recently been developed into a robust, reliable and highly sensitive technique to detect and unambiguously identify minute quantities of organic colorants. SERS has seen the field of cultural heritage become one of its foremost research and application areas, resulting in improved analytical protocols applicable to several other fields such as pharmaceuticals and forensic analysis. In particular, our efforts to develop methods to identify more than one colorant on a single sample using combined Thin Layer Chromatography / SERS and microfluidics SERS approaches will be discussed. Pushing the envelope of in-situ SERS analysis, first results on the use of Tip-Enhanced Raman spectroscopy (TERS) for the high spatially resolved, highly sensitive and non-invasive investigation of dyes used on paper supports will be discussed and preliminary results presented. This first demonstration of TERS spectral acquisition directly on a paper substrate confirms the analytical potential of TERS to identify organic colorants in artworks with high sensitivity, high spatial resolution, and minimal invasiveness opening the way to future developments for the nano-scale mapping of organic constituents of works of art.

Cadmium carbonate (CdCO₃) has for several years been identified in the altered cadmium yellow (CdS) paints found Impressionist, early modernist, and post-Impressionist works. When it is concentrated at the surface of the painting, CdCO₃ appears to be a result of the photo-alteration of CdS, likely through a CdSO₄-containing phase. However, in other cases CdCO₃ is distributed throughout the paint layer. This is significant because CdCO₃ is highly insoluble (K_sp of 1.0 ×10^-12), and if it were formed solely as a result of photo-alteration it would not be expected to migrate away from the painting’s surface. In cadmium yellow paints in works such as Edvard Munch’s c. 1910 The Scream (The Munch Museum, Oslo), Plahter et al. have recently proposed that CdCO₃ is present because this compound was used in the indirect wet process synthesis of CdS (through, for example, the reaction of CdCO₃ and Na₂S). This would mean that the CdCO₃ is a residual starting reagent rather than a photo-alteration product. Such an interpretation is supported by the identification of CdCO₃ in the unaltered cadmium yellow paints of early modernist works such as Pablo Picasso’s The Blue Room (The Phillips Collection, 1901) and Henri Matisse’s Flower Piece (The Barnes Foundation, 1906).

To address this question of CdCO₃’s role, a flake of apparently nondegraded cadmium yellow paint was removed from Henri Matisse’s Flower Piece so that the distribution of CdCO₃ could be studied, both as a function of depth in the paint layer and in individual pigment particles.

X-Ray microspectroscopy and microdiffraction were respectively carried out at ESRF ID21 and Petra III, supplemented by light microscopy, backscattered electron microscopy with X-ray microanalysis, and Fourier transform infrared spectroscopy. This presentation will focus on results from X-ray microspectroscopy: μ-X-ray fluorescence (XRF scanning) allowed precise mapping of local elemental distribution, and Full Field X-ray Near-Edge Absorption Structure (FF-XANES) for mapping the chemical speciation using the Cd-L₃ and S-K edges.

FF-XANES imaging of a 15 micrometer thin section at CdL3-edge revealed the presence of three Cd-based phases: CdS, CdSO₄, and CdCO₃. The CdSO₄ was concentrated on only one surface of the sample, suggesting its role as a photo-alteration product. Cadmium carbonate was found to comprise the bulk of the individual pigment particles, suggesting that it is a synthesis starting reagent. CdS was found to be concentrated on the surface of these CdCO₃ particles. CdSO₄ could also be observed to surround some of the CdCO₃ particles, suggesting the beginnings of photo-oxidation of the thin CdS coating.

To conserve old masterpieces is, of course, critical but modern fine/contemporary arts also need preservation as future cultural heritage. Therefore, nowadays many modern artistic works have been studied using scientific techniques to preserve their condition. As with easel oil paintings, modern watercolour paintings represent some of our great cultural heritage from artists such as J.M.W. Turner, Paul Klee or Georgia O’Keeffe. For conservation scientists, the characterization of the binder and pigments in modern paintings is especially important yet problematic in terms of conservation treatments and environmental conditions for display or storage.

In investigation of modern paintings it is often critical to identify the origin of organic molecules in the paintings, since modern paintings have commonly used artists’ paints containing synthetic organic pigments due to their greater selection of colours. In addition, in terms of watercolour paints, commonly used binders such as gum arabic, gum tragacanth or honey are not straight forward to identify due to their organic content.

To contribute to a better understanding of modern artists’ paints for conservation, this study presents a scientific investigation into commercially prepared watercolour cakes and binders from the 20^th century. Analysis focuses on the characterisation of commercial watercolour paints (red colour) that may contain quinacridone and/or saccharide materials, and shows different surface chemistries of the paint between powder and cake types. Moreover, we attempt the identification of the synthetic organic pigments and the plant gum binder from the watercolour paint using surface analysis techniques and principal component analysis (PCA). Especially, we feel surface analysis techniques such as x-ray photoelectron spectroscopy (XPS), helium ion microscopy (HIM) and time of flight secondary ion mass spectrometer (ToF-SIMS) are powerful techniques for cultural heritage preservation.