Development and trends in synchrotron studies of ancient and historical materials

Abstract

Synchrotron photon-based methods are increasingly being used for the physico-chemical study of ancient and historical materials (archaeology, palaeontology, conservation sciences, palaeo-environments). In particular, parameters such as the high photon flux, the small source size and the low divergence attained at the synchrotron make it a very efficient source for a range of advanced spectroscopy and imaging techniques, adapted to the heterogeneity and great complexity of the materials under study. The continuous tunability of the source — its very extended energy distribution over wide energy domains (meV to keV) with a high intensity — is an essential parameter for techniques based on a very fine tuning of the probing energy to reach high chemical sensitivity such as XANES, EXAFS, STXM, UV/VIS spectrometry, etc. The small source size attained (a few micrometres) at least in the vertical plane leads to spatial coherence of the photon beams, giving rise in turn to a series of imaging methods already crucial to the field. This review of the existing literature shows that microfocused hard X-ray spectroscopy (absorption, fluorescence, diffraction), full-field X-ray tomography and infrared spectroscopy are the leading synchrotron techniques in the field, and presents illustrative examples of the study of ancient and historical materials for the various methods. Fast developing analytical modalities in scanning spectroscopy (STXM, macro-XRF scanning) and novel analytical strategies regarding optics, detectors and other instrumental developments are expected to provide major contributions in the years to come. Other energy domains are increasingly being used or considered such as far-infrared and ultraviolet/visible for spectroscopy and imaging. We discuss the main instrumental developments and perspectives, and their impact for the science being made on ancient materials using synchrotron techniques.

Introduction

The study of the composition, structure, morphology and physico-chemical properties of materials from archaeology, cultural heritage and palaeontology is an essential component of the research in these fields [1], [2], [3]. It complements, when available, historical evidence and information retrieved from the geochronological context. Over the past years, progresses in the instrumentation available on site, at the laboratory and at large scale facilities led to considerable improvement in our understanding of these materials. Among them, synchrotron-based techniques are often unique in the brightness attained and the versatility of the source, allowing a very wide range of photon-based spectroscopy and imaging techniques.

Questions such as the understanding of the discolouration of smalt pigments in paint layers using synchrotron infrared and X-ray absorption spectroscopy techniques [4], the long-term alteration of bone materials in archaeological contexts through synchrotron small-angle X-ray scattering techniques [5], [6], improvements in the determination of the age at death of hominins through phase-contrast visualisation of incremental features in fossil teeth [7] demonstrate the diversity of approaches and questions in the field that benefited from synchrotron investigation in the recent years. The range of synchrotron techniques is very broad and includes hard X-ray, soft X-ray, EUV–VUV, UV/visible, near-, mid- and far-infrared techniques. Although the potentials of the source in the field of ancient and historical materials was identified as soon as the mid 80’s [8], [9], [10], [11], real developments started mainly from the years 2000 onwards [12], [13], [14] with major contributions from a restricted set of expert users of the technique. The field in some respect still lacks the strong collaborative effort that was put in place in some other fields of synchrotron studies such as for structural biology and nano-sciences.

The use of some of the core synchrotron radiation techniques for the characterisation of ancient materials has already been reviewed by several authors, including synchrotron microtomography for palaeontological specimens, synchrotron-based FT-IR and X-ray absorption spectroscopy for cultural heritage [14], [15], [16], [17], [18], [19]. However, the present article focuses on the main methodological developments and trends that are central to this field of research. We therefore recall here the physical principles of the most important synchrotron techniques, review the use of synchrotron techniques with examples of application from the open literature and the contribution of the authors in the field, and present adaptations in each of the core techniques to the specific constraints of ancient and historical materials.

Finally, we discuss and question in a full section the prominent trends that may lead to future novel developments in the synchrotron-based study of ancient and historical materials.