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Abstract: Back-reflection energy-dispersive XRD (BR-EDXRD) is a non-invasive technique that offers extremely high-resolution and insensitivity to surface morphology. It is notably effective in the analysis of cultural heritage objects, where non-invasive analytical techniques are clearly advantageous. In particular, the technique is ideally suited to the analysis of paints, where samples are typically thin, have components that are finely-powdered and can consist of complex phase mixtures. Whole-pattern fitting (i.e. Rietveld or Pawley fitting) to diffraction patterns offers a wealth of information such as chemical composition, polymorphism, phase ratios and microstructural details which can help in authentication, attribution and conservation, as well as yielding insight into historic pigment manufacture, trade and artists’ preferences. Rietveld fitting is challenging for energy-dispersive XRD because incident and diffracted intensities are non-uniform as a function of energy, and pigments and fillers in paints are typically contained in an amorphous matrix with an unknown absorption coefficient. For BR-EDXRD this is exacerbated; the experimental configuration employed utilises an air-gap between sample and detector window which is poorly constrained from sample to sample, making it challenging to model. Herein we present results from a selection of historically relevant paints examined by BR-EDXRD3 (Figure 1a) to establish the limitations and capabilities of the technique, including extraction of accurate lattice parameters, microstructural analysis and phase quantification (Figure 1b). This was achieved by introducing an empirical intensity scaling correction function to allow Rietveld fitting to the BR-EDXRD data.

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Abstract: Chinese porcelains have been in production for hundreds of years and are important evidence for informing our understanding of Chinese history, culture, technology and trade. With this in mind, it is clear that accurate determination of the provenance and authenticity of an object is crucial. However, scientific investigation is limited by the high value of the objects which hinders or precludes destructive or invasive analyses. Famille Rose porcelain was first produced in China in the early 18th century CE. It is characterised by its pink-coloured overglaze enamel arising from the inclusion of colloidal gold (‘Purple of Cassius’) in a glassy matrix, a technique that may have been brought to China by Jesuit missionaries. In conjunction with colloidal gold, opaque enamels such as lead stannate (PbSbO3) and lead arsenates were commonly used. Back-reflection energy-dispersive XRD (BR-EDXRD) offers high-resolution and insensitivity to surface morphology which negates the need for sample preparation. Furthermore, the low incident energies employed make the technique particularly sensitive to the surface layers. In the case of Famille Rose porcelains, this offers the opportunity to selectively study the enamel and glazes. The experimental configuration also allows the concurrent collection of XRF data which can greatly assist in interpretation of BR-EDXRD patterns. In this work BR-EDXRD data were collected from various positions on the piece which indicate the presence of various phases present in the enamel, including Au, Pb2Sn2O6, SnO2 and KPb4(AsO4)3. Furthermore, Co is confirmed to be present in several blue spots by the concurrent XRF measurement, though no Co phases were detected. The significance of these findings is discussed.

Open Access

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Abstract: Purportedly originating from the late Predynastic–Early Dynastic period of ancient Egyptian civilisation (about 3100 BC), the statuette known as “MacGregor Man” was acquired near the site of Naqada in southern Egypt by the Reverend William MacGregor at the end of the 19th century. The Ashmolean Museum (University of Oxford) subsequently purchased the statuette at the sale of MacGregor’s collection in 1922, and it remains one of the most important items in the museum’s Egyptology collection. However, the authenticity of the statuette has been the focus of fierce scholarly debate since it first came to light. At that time there was little with which the object could be compared and a forger would have little source material to copy: some of its features are consistent with securely provenanced statuettes and figurines that were discovered some time after the appearance of MacGregor Man. Several other aspects have caused others to question whether MacGregor Man is genuine. It is carved from what is widely believed to be a basaltic rock; rare in ancient Egypt and its hardness would have made sculpting such a detailed statue exceedingly difficult with the tools available to ancient stonemasons. Also, there are strong indications that MacGregor Man may have been deliberately damaged, possibly in an effort to artificially “antiquate” the statuette (though other explanations are feasible). Despite the sustained interest in MacGregor Man, no mineralogical analysis of the stone it is sculpted from has been carried out to date. Though it has been visually identified as being most likely a basalt, it has so far been impossible to ascertain even this basic information without removing a sample from and thus damaging the statue. Even a basic understanding of the mineralogy of the sample could prove informative since basaltic composition can be diagnostic for determining the region of its origin. Using energy-dispersive X-ray diffraction (EDXRD) in a back-reflection geometry, a non-destructive technique, high-resolution powder diffraction data from several regions were collected at Diamond Light Source (Didcot, Oxfordshire, U.K.) in February 2019. From these data, despite poor powder averaging (resulting from the large crystallite size), we plan to extract crystallographic information such as the minerals present and unit cell dimensions. This will be compared to published data from known Egyptian basalt samples in an attempt to ascertain whether it is Egyptian in origin. The results of this crystallographic analysis will be presented.