Myriam Eveno

Depth profiling reveals multiple paint layers of Louvre Renaissance paintings using non-invasive compact confocal micro-X-ray fluorescence

The High Renaissance marks one of the most significant and prolific periods of artistic production in painting. Artists of this epoch advanced many aspects of pictorial art that continue to be debated and researched today. The typical painting technique during this time period consisted of superimposing multiple layers of thinly applied paint to achieve realistic optical impressions and desired visual effects. The presence of multiple thin layers in Renaissance paintings presents a challenging case study. In the work presented here, a laboratory-based confocal micro-X-ray fluorescence analysis (CXRF) spectrometer called LouX3D is applied to the non-destructive study of the Renaissance paintings series Famous Men, from the Louvre collection. Elemental depth profiles measured directly on the paintings are compared to lateral scans and results of conventional analyses on cross-sections removed from the paintings in the same locations. This comparison enables a better evaluation of the feasibilities of CXRF depth profiling using the LouX3D set-up for the investigation of paint layers. The limit of depth profiling is reached due to absorption effects, which depend on the chemical composition of the paint layers, on their order and on the performance of the set-up. Care must be taken if the same element is present in adjacent layers, if the layers are thin (<10 μm), or if the composition within a layer is highly heterogeneous. Paint sequences on the original paintings, including later retouchings, were successfully detected using CXRF measurements. This study highlights the great potential of this laboratory-based method when used in combination with other techniques for completely non-invasive painting analyses.

Portable apparatus for in situ x-ray diffraction and fluorescence analyses of artworks.

A portable X-ray fluorescence/X-ray diffraction (XRF/XRD) system for artwork studies has been designed constructed and tested. It is based on Debye Scherrer XRD in reflection that takes advantage of many recent improvements in the handling of X-rays (polycapillary optics; advanced two-dimensional detection). The apparatus is based on a copper anode air cooled X-ray source, and the XRD analysis is performed on a 5-20 μm thick layer from the object surface. Energy dispersive XRF elemental analysis can be performed at the same point as XRD, giving elemental compositions that support the interpretation of XRD diagrams. XRF and XRD analyses were tested to explore the quality and the limits of the analytical technique. The XRD diagrams are comparable in quality with diagrams obtained with conventional laboratory equipment. The mineral identification of materials in artwork is routinely performed with the portable XRF-XRD system. Examples are given for ceramic glazes containing crystals and for paintings where the determination of pigments is still a challenge for nondestructive analysis. For instance, lead compounds that provide a variety of color pigments can be easily identified as well as a pigment such as lapis lazuli that is difficult to identify by XRF alone. More than 70 works of art have been studied in situ in museums, monuments, etc. In addition to ceramics and paintings, these works include bronzes, manuscripts, etc., which permit improvement in the comprehension of ancient artistic techniques.

Confocal XRF depth profiling non-destructively reveals the original blue pigments in a Renaissance painting by Caroto

Recent restoration of The Rest on the Flight into Egypt by Giovanni Francesco Caroto, from the Louvre collections, included a technical examination of the Virgins blue robe that aimed to characterize the original materials and later additions. The painting was examined using light microscopy and ultraviolet and infrared imaging, and one cross section was taken. None of these methods showed a clear distinction between the layers that were artist applied or modifications made by another hand. To gain information about the paint stratigraphy without taking additional samples, non-invasive confocal X-ray fluorescence (CXRF) analysis was carried out to examine several areas of the Virgins robe. Using this technique, it was possible to identify the pigments and order of application of paint by the artist. All the pigments identified were considered to be artist applied, and on this basis no further cleaning was carried out and conservation proceeded with retouching of minor areas of loss or damage. This study highlights the advantages of CXRF analysis for the technical study of paintings, used in combination with classical methods for making informed conservation treatment decisions.

Synchrotron-based high angle resolution and high lateral resolution X-ray diffraction reveals lead white pigment qualities in Old Masters paintings

Microsamples collected on 27 major paintings by Old European Masters dating from the 14th to the late 19th centuries were analyzed using synchrotron-based X-ray diffraction. Two complementary analytical configurations were used at beamlines ID22 (high angle resolution) and ID21 (high lateral resolution), in order to highlight markers of the different grades of the lead white pigments (mixture of cerussite PbCO3 and hydrocerussite Pb3(CO3)2(OH)2). Rietveld analysis and crystalline phases mapping at the microscale revealed the composition and microstructure of the pigments, shedding light on the preparation recipes and pigment choices of the artists through History.

Determination of the complex optical index of red pigments, vermillon

The non-destructive analysis of works of art and more specifically the paintings with the aim of a non-ambiguous identification of their components and the understanding of the techniques of the artists still remains a challenge. The aim of our research is to elaborate a purely optical way for this identification, based on the exclusive use of the intrinsic characteristic optical parameters of the components, instead the derived parameters presently commonly used, depending on several other parameters (morphology, environment...). The approach we propose is based on the resolution of the RTE using the 4-Flux approximation, combined with the Mie theory, allowing the identification of the pigments via the spectrum of their complex optical index entered into the model via a database. The key point of this approach is the index data bank. We report in this communication one the methods crucial steps: the determination of the intrinsic optical index of pigments under the form of grains of micrometric size. This step is far from trivial and presents many difficulties that are not completely solved. This is one of the reasons why a more rigorous analysis of the paintings has not been up to now developed. We illustrate this problem with a red pigment: vermillion randomly dispersed at low concentration in a transparent polymer. The morphology of the sample is well characterized (thickness, concentration, size and dispersion of the pigments, surface roughness) as well as the index of the matrix. We use the same approach and model as presented above, applied this time to the calculation of the complex index of the pigments. The model is supposed to account for the diffuse flux and the specular flux, both measured on our samples, by spectrophotometry with an integrating sphere in the visible spectral range 400-800 nm. This resolution allows determining independently the coefficients of scattering and absorption of the pigment, which are finally related to the complex index of refraction through Mies Theory.