Watching works of art under the synchrotron lights to reveal their secrets
Authors: Cotte Marine 
Published: December 2016

As early as he discovered X‐rays in 1895, Roentgen envisaged their possible application for the study of works of art, and of paintings in particular. More than one century latter, X‐rays are routinely used for the analysis of artworks. Beyond X‐ray radiography (which application to paintings was patented already in 1914 (Bridgman, 1964)), many other X‐ray based techniques can be used, such as X‐ray fluorescence and X‐ray diffraction, giving access to the chemical composition of ancient and artistic objects. Efforts are made to extend the use of such laboratory techniques in two main directions: on the one hand with the development of portable instruments, allowing on‐site (museums, archaeological sites, historical buildings) analyses; on the other hand, exploiting the additional capabilities offered by synchrotron sources.

The brightness, collimation, polarization, emission spectrum and partial coherence of synchrotron beams offers: i) the possibility to focus the beam to few microns, down to few tens of nanometers, ii) improved detection limits, iii) access to the element speciation thanks to spectroscopy techniques, iv) reduced acquisition times, allowing the acquisition of many points, and in particular 2D and 3D scanning maps, v) additional contrast modes such as phase contrast imaging. Considering the technical constraints associated to the intrinsic properties of artistic materials (precious objects and samples of very limited size when available; highly heterogeneous at multi‐scales; composed of a variety of materials (in/organic, un/crystallized, low/high Z elements)), these combined analytical strengths are highly beneficial (Bertrand, et al., 2012).

Here, we will present a set of recent examples of analysis of artistic and ancient materials, performed at the ESRF. Experiments usually aim at understanding how works of art were created (choice and synthesis of ingredients, in particular of pigments, firing temperature in the case of ceramics, etc). As an example, Figure1 shows a combined micro X‐ray fluorescence, micro X‐ray diffraction, micro X‐ray absorption spectroscopy analysis of blue pigments in Chinese Qinghua porcelains. Fragments were sampled from sherds and prepared as thin sections (Top left). µXRF maps show particular concentration of Co and Ca in the pigment regions, and of iron on the surface (top right). Full‐field XANES at the Co K‐edge reveals the presence of two main Co species, in the pigment and in the glaze (bottom left: (a) transmission image recorded at 7670 eV, (b) the edge jump map, (c) cluster maps and average XANES spectra obtained by PCA, (d) Speciation maps obtained from the least squares linear combination fitting (standards CoAl2O4and Co in glaze)). Micro X‐ray diffraction offers further determination of Co pigment lattice parameters (bottom right) (Wang, et al.).

Other experiments focus on degradation issues, and aim at identifying degradation products, and internal and external factors responsible for such instabilities. Figure 2 shows a typical example with the study of cinnabar darkening in Madonna with Child, St. Sebastian, St. John the Baptist and two donors, Boltraffio, Louvre (© C2RMF) (Cotte & Susini, 2009).