Feasibility study of the application of micro-Raman imaging as complement to micro-XRF imaging

Abstract

X-ray fluorescence (XRF) spectroscopy and Raman spectroscopy are preferential analytical techniques to study cultural heritage objects, since both techniques may provide complementary information in a non-destructive manner. Moreover, the application of microscopic beams allows the investigation of heterogeneous samples on the microscopic level and the study of the heterogeneity of particular samples. The micro-XRF method became already a routine analytical imaging method also because of the well-established spectrum evaluation methodology enabling specific data handling procedures. These include multivariate statistical analysis procedures such as principal components analysis (PCA) in order to explore and describe the acquired data, and clustering techniques in order to find similar pixels (or areas) in the obtained images. In the case of the micro-Raman technique, however, the usual approach is to perform a single spot analysis of only a few selected positions in order to ultimately identify the material on the basis of the comparison with Raman spectra obtained from reference materials. However, when samples are heterogeneous, imaging is still to be preferred in order to deal with the problem of sampling. With the arrival of a new micro-Raman spectrometer at the UGent laboratories, there was the need to explore the possibilities of Raman imaging. One of the most important aspects of imaging is the time needed for the analysis. Therefore, the influence of different instrumental parameters, such as resolution (low or high) and measuring time per pixel, on the quality of Raman spectra and images was investigated in order to evaluate the possibility of performing fast Raman mappings because of the need to identify the regions of interest on the art object in a more systematic manner

Discovering the material palette of the artist: a p‐XRF stratigraphic study of the Giotto panel ‘God the Father with Angels’

“…Beyond its own history, a work of art as a painting or a fresco can reveal details regarding its historical age such as the habits or the tastes of people, and, more important for scientists and art historians, information regarding the technique, and the creative process used by the painter. Discovering how painted works of art were realized or, eventually modified, can be done by gathering data about its structure and composition and, because a great deal of information can lie tens or hundreds of microns below the surface, the possibility of study artworks layer by layer is valuable for art research In this scenario, X‐ray fluorescence (XRF) results as the most diffused and powerful technique; it gives extremely useful results, also as a consequence of the development of new devices, more and more sensitive and reliable, mainly when coupled with other spectroscopic techniques . Thanks to the fact that XRF allows a non‐destructive and non‐invasive elemental analysis also in air, XRF makes the study of the stratigraphic distribution of elements feasible also for cultural heritage objects: samples that require damage avoiding as a key feature to perform experimental studies.…”

Raman Spectroscopy of cultural heritage Materials: Overview of Applications and New Frontiers in Instrumentation, Sampling Modalities, and Data Processing

“…Raman mapping using a conventional motorised stage is used in the cultural heritage field on micro-samples and cross-sections of samples such as paint layers (in favourable cases where fluorescence is not a complete impediment to analysis) [94,153,287], calcium oxalates layers deposited on the surfaces of monuments as deterioration products or as intentional mineral consolidation treatments [92,93], iron corrosion layers [210], and other corrosion products of archaeological metal objects [18] or porcelain cards [116]. When it comes to in situ analyses, unless the object fits under the microscope and can be securely held on the motorised stage, which is not often the case, different approaches must be considered.…”

Mobile depth profiling and sub-surface imaging techniques for historical paintings—A review

“…So we will, especially in case of X-ray methods, include techniques that are not (yet) available as mobile instruments or studies that could have been performed insitu with comparable out-come. Further, we omit techniques that are only sensitive to the surface of paintings, such as multi-spectral imaging in the visible range [7] and Raman imaging [8]. Nonlinear imaging [9] is omitted, as currently too few case studies are available that allow to estimate the practical value of this technique for the investigation of hidden features in historical paintings.…”