A scientific analysis of historical artworks can tell us a great deal about historical manufacturing practices and artists' techniques. This, in turn, can help with authentication, conservation and restoration. Non-invasive analytical techniques such as hyperspectral imaging, X-ray fluorescence (XRF) spectroscopy, Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy are common and can be directly applied to the artworks without the need of invasive sampling. This is not the case for X-ray Diffraction (XRD), a powerful analysis method giving direct information of the crystalline structure of essential components of heritage materials. Conventional XRD laboratory instruments are not designed to accommodate the different shapes and sizes of artefacts, and XRD analysis requires specific preparation of materials removed from the artwork. Researchers from the University of Leicester are developing an Energy-dispersive X-ray Diffraction (EDXRD) technique that's insensitive to the shape of the sample and very suited to cultural heritage analysis. It will potentially allow the analysis of unusually large items, as often found in heritage collections. In work recently published in the Journal of Cultural Heritage, the Leicester team worked with Nicholas Eastaugh of the Pigmentum Project, using their technique to survey a test panel of forty oil-based artists' paints. Their results show that the back-reflection EDXRD technique can extract a range of information about paint samples non-invasively and at high resolution.
Dr Graeme Hansford and his team at the University of Leicester have been exploring the use of energy-dispersive X-ray diffraction (EDXRD) implemented in a back-reflection geometry to analyse cultural artefacts. This non-invasive technique is highly insensitive to the artefact shape and size, which means no surface preparation or sampling are needed. In order to understand the potential and limitations of the method in the scientific assessment of paintings, the team used it to examine a test panel of forty oil-based paints used by artists in the 20th century and containing a variety of inorganic and organic pigments.
The EDXRD method relies on the availability of wide-band, tunable X-ray sources and high resolution, energy-dispersive detectors. In addition, the use of a back-reflection geometry is key to obtain accurate measurements of the diffraction patterns not affected by irregularities of the sample surface.
Dr Hansford says,
For these experiments we needed a non-standard configuration, and the B18 beamline is perfect because it is very flexible in terms of setup. The beamline staff were extremely knowledgeable and helped us to install our own equipment on the beamline.
Application of this technique to oil paintings has demonstrated that it is possible to obtain high quality crystallographic data. The investigation allowed the team to accurately identify the sample crystalline phases in 86% of the inorganic pigments. They could also detect where they were present as solid solutions, to measure accurate unit cell parameters, crystallite strain and size, and preferred orientation of the crystallites. This information provided a detailed picture of the inorganic components of the paints as deposited on the artefacts. At the same time, the investigation highlighted that back-reflection EDXRD is not effective in identifying organic pigments. This is expected, as tinting components are present in small amounts in commercial paints. They have a low average atomic number, so they are only weakly X-ray scattering, therefore their signature signals are hidden beneath the background given by inorganic fillers and extenders commercially used. At the same time, this is not a significant concern, as those components can be reliably identified with other techniques (typically, Raman). This indicates that the ideal application domain of EDXRD is on the analysis of paintings coming from periods before the introduction of modern organic colours.
Further work is needed to test how effective back-reflection EDXRD is for analysing an actual artwork with multiple paint layers, paint mixtures and varnishes. Therefore, the team is planning systematic studies with prepared materials to test the potential for depth analysis and quantify the effect of varnish.
Dr Hansford explained:
At the moment, this kind of analysis requires access to a synchrotron, but that's not ideal for large items, and institutions are understandably reluctant for priceless artefacts to travel.
Ultimately, we're hoping that back-reflection EDXRD can be implemented using transportable microcalorimeters that offer high resolution, so that it can be used to analyse artefacts at their home locations. Authentication is always going to be a complex process, and we hope that this tool can unearth more pieces of the puzzle.
To find out more about the B18 beamline or discuss potential applications, please contact Principal Beamline Scientist Diego Gianolio: [email protected].
Hiley CI et al. High-resolution non-invasive X-ray diffraction analysis of artists' paints. Journal of Cultural Heritage 53: 1-13 (2022). DOI:10.1016/j.culher.2021.10.008.
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