UV–Vis-NIR and microRaman spectroscopies for investigating the composition of ternary CdS 1−x Se x solid solutions employed as artists' pigments

The “Historical Materials BAG”: A New Facilitated Access to Synchrotron X-ray Diffraction Analyses for Cultural Heritage Materials at the European Synchrotron Radiation Facility

The European Synchrotron Radiation Facility (ESRF) has recently commissioned the new Extremely Brilliant Source (EBS). The gain in brightness as well as the continuous development of beamline instruments boosts the beamline performances, in particular in terms of accelerated data acquisition. This has motivated the development of new access modes as an alternative to standard proposals for access to beamtime, in particular via the “block allocation group” (BAG) mode. Here, we present the recently implemented “historical materials BAG”: a community proposal giving to 10 European institutes the opportunity for guaranteed beamtime at two X-ray powder diffraction (XRPD) beamlines—ID13, for 2D high lateral resolution XRPD mapping, and ID22 for high angular resolution XRPD bulk analyses—with a particular focus on applications to cultural heritage. The capabilities offered by these instruments, the specific hardware and software developments to facilitate and speed-up data acquisition and data processing are detailed, and the first results from this new access are illustrated with recent applications to pigments, paintings, ceramics and wood.

PLS-DA and data fusion of visible Reflectance, XRF and FTIR spectroscopy in the classification of mixed historical pigments

In this work samples of historical pigments of green hue were brushed on a canvas and studied by Visible Reflectance, X-Ray Fluorescence and Fourier Transform Infrared Spectroscopy. One hundred samples were investigated, all with green hue, these prepared from pigments themselves green, such as chromium oxide (Cr₂O₃) or from a mixture of pigments that result in green, for example, chrome yellow (PbCrO₄) and Prussian blue (Fe₄[Fe(CN)₆]₃). Because every sample investigated through the spectroscopic techniques were of green hue, the characterization of the pigments present in the mixtures through the visual inspection of spectra has become a complex task in some cases, also, due the large number of recorded spectra. In this work, classification models were developed using the multivariate statistical method Partial Least Squares Discriminant Analysis (PLS-DA) to automate the characterization of the pigments present in the mixtures. The models were developed to classify chromium oxide (Cr₂O₃), chrome yellow (PbCrO₄), cerulean blue (CoO.nSnO₂) and yellow ochre (Fe₂O₃·H₂O + clay + silica). The models were developed from the fusion of data from the three spectroscopic techniques. However, before data fusion, pre-treatments of the spectral data were tested for their influence on the PLS-DA models. The models developed with data from the three techniques made it possible to classify the pigments of interest in the samples with up to 100% effectiveness. The results also indicate that fusion of the data from the three techniques allows to obtain fingerprints of the pigments of interest, which is not always possible using data from only one or two of the techniques applied in this work.

Photoluminescence excited at variable fluences: a novel approach for studying the emission from crystalline pigments in paints

Crystalline solids can exhibit photoluminescence when properly excited by sufficiently energetic light radiation. Following excitation, different radiative and non-radiative recombination pathways can occur that are informative of the energetic structure of the material as well as of the presence of crystal defects and impurities. Usually, the characterization of the optical emission of crystalline materials is achieved through the study of emission spectra as a function of the excitation wavelength. A different approach employs variable excitation fluence to populate the energetic levels until saturation, which promotes the emission from other radiative and non-radiative pathways. The method is particularly effective for understanding conduction phenomena and studying charge recombination channels in semiconductor materials. In this work, we propose its application for characterizing radiative recombination paths in crystalline pigments. The approach has been tested in spectroscopy mode for the identification of paints in a model painting and in micro-imaging modality for the study of paint stratigraphies. We demonstrate that the method is highly informative of the nature of different recombination paths in crystalline pigments and allows a deeper characterization of the emission from luminescent paints with respect to the conventional steady-state photoluminescence approach.

In-Depth Analysis of Egg-Tempera Paint Layers by Multiphoton Excitation Fluorescence Microscopy

The non-invasive depth-resolved imaging of pictorial layers in paintings by means of linear optical techniques represents a challenge in the field of Cultural Heritage (CH). The presence of opaque and/or highly-scattering materials may obstruct the penetration of the radiation probe, thus impeding the visualization of the stratigraphy of paintings. Nonlinear Optical Microscopy (NLOM), which makes use of tightly-focused femtosecond pulsed lasers as illumination sources, is an emerging technique for the analysis of painted objects enabling micrometric three-dimensional (3D) resolution with good penetration capability in semi-transparent materials. In this work, we evaluated the potential of NLOM, specifically in the modality of Multi-Photon Excitation Fluorescence (MPEF), to probe the stratigraphy of egg-tempera mock-up paintings. A multi-analytical non-invasive approach, involving ultraviolet-visible-near infrared (UV-Vis-NIR) Fiber Optics Reflectance Spectroscopy, Vis-NIR photoluminescence, and Laser Induced Fluorescence, yielded key-information for the characterization of the constituting materials and for the interpretation of the nonlinear results. Furthermore, the use of three nonlinear optical systems allowed evaluation of the response of the analyzed paints to different excitation wavelengths and photon doses, which proved useful for the definition of the most suitable measurement conditions. The micrometric thickness of the paint layers, which was not measurable by means of Optical Coherence Tomography (OCT), was instead assessed by MPEF, thus demonstrating the effectiveness of this nonlinear modality in probing highly-scattering media, while ensuring the minimal photochemical disturbance to the examined materials.

Time-Resolved Photoluminescence Microscopy for the Analysis of Semiconductor-Based Paint Layers

In conservation, science semiconductors occur as the constituent matter of the so-called semiconductor pigments, produced following the Industrial Revolution and extensively used by modern painters. With recent research highlighting the occurrence of various degradation phenomena in semiconductor paints, it is clear that their detection by conventional optical fluorescence imaging and microscopy is limited by the complexity of historical painting materials. Here, we illustrate and prove the capabilities of time-resolved photoluminescence (TRPL) microscopy, equipped with both spectral and lifetime sensitivity at timescales ranging from nanoseconds to hundreds of microseconds, for the analysis of cross-sections of paint layers made of luminescent semiconductor pigments. The method is sensitive to heterogeneities within micro-samples and provides valuable information for the interpretation of the nature of the emissions in samples. A case study is presented on micro samples from a painting by Henri Matisse and serves to demonstrate how TRPL can be used to identify the semiconductor pigments zinc white and cadmium yellow, and to inform future investigations of the degradation of a cadmium yellow paint.

Non-Invasive and Non-Destructive Examination of Artistic Pigments, Paints, and Paintings by Means of X-Ray Methods

Recent studies are concisely reviewed, in which X-ray beams of (sub)micrometre to millimetre dimensions have been used for non-destructive analysis and characterization of pigments, minute paint samples, and/or entire paintings from the seventeenth to the early twentieth century painters. The overview presented encompasses the use of laboratory and synchrotron radiation-based instrumentation and deals with the use of several variants of X-ray fluorescence (XRF) as a method of elemental analysis and imaging, as well as with the combined use of X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). Microscopic XRF is a variant of the method that is well suited to visualize the elemental distribution of key elements, mostly metals, present in paint multi-layers, on the length scale from 1 to 100 μm inside micro-samples taken from paintings. In the context of the characterization of artists' pigments subjected to natural degradation, the use of methods limited to elemental analysis or imaging usually is not sufficient to elucidate the chemical transformations that have taken place. However, at synchrotron facilities, combinations of μ-XRF with related methods such as μ-XAS and μ-XRD have proven themselves to be very suitable for such studies. Their use is often combined with microscopic Fourier transform infra-red spectroscopy and/or Raman microscopy since these methods deliver complementary information of high molecular specificity at more or less the same length scale as the X-ray microprobe techniques. Since microscopic investigation of a relatively limited number of minute paint samples, taken from a given work of art, may not yield representative information about the entire artefact, several methods for macroscopic, non-invasive imaging have recently been developed. Those based on XRF scanning and full-field hyperspectral imaging appear very promising; some recent published results are discussed.