Complementary use of monochromatic and white-beam X-ray micro-diffraction for the investigation of ancient materials

Abstract

Archaeological artefacts are often heterogeneous materials where several phases coexist in a wide grain size distribution. Most of the time, retrieving structure information at the micrometre scale is of great importance for these materials. Particularly, the organization of different phases at the micrometre scale is closely related to optical or mechanical properties, manufacturing processes, functionalities in ancient times and long-term conservation. Between classic X-ray powder diffraction with a millimetre beam and transmission electron microscopy, a gap exists and structure and phase information at the micrometre scale are missing. Using a micrometre-size synchrotron X-ray beam, a hybrid approach combining both monochromatic powder micro-diffraction and Laue single-crystal micro-diffraction was deployed to obtain information from nanometre-and micrometre-size phases, respectively. Therefore providing a way to bridge the aforementioned gap, this unique methodology was applied to three different types of ancient materials that all show a strong heterogeneity. In Roman terra sigillata, the specific distribution of nanocrystalline hematite is mainly responsible for the deep-red tone of the slip, while the distribution of micrometre-size quartz in ceramic bodies reflects the change of manufacturing process between pre-sigillata and high-quality sigillata periods. In the second example, we investigated the modifications occurring in Neolithic and geological flints after a heating process. By separating the diffracted signal coming from the nano-and the micrometre scale, we observed a domain size increase for nanocrystalline quartz in geological flints and a relaxation of the residual strain in larger detritic quartz. Finally, through the study of a Roman iron nail, we showed that the carburation process to strengthen the steel was mainly a surface process that formed 10-20 mm size domains of single-crystal ferrite and nanocrystalline cementite.

From Archaeological Sites to Nanoscale: The Quest of Tailored Analytical Strategy and Modelling

“…In addition the electronic diffraction can provide structural Fig. 2 Histogram of the apparent size of the coherently diffracting domains obtained after refinement of the powder data, showing a slight increase after the heating process of natural flints (Dejoie et al 2015). Such analysis is used to prove the performing of a heat treatment by the ancient men to improve the flint knapping behaviour's analyses on the compounds observed.…”

“…Last a recent study used XRD under synchrotron to determine particle sizes on a range from the micrometric to the nanometric scale on various ancient materials like Roman terra sigillata, Neolithic flint (Fig. 2) or Roman iron nails (Dejoie et al 2015). This was achieved thanks to a micrometric and monochromatic beam combined to the exploitation of Laue diffraction.…”

Full-Field Transmission X-ray Microspectroscopy (FF-XANES) Applied to Cultural Heritage Materials: The Case of Ancient Ceramics

“…(1-2) cutting using a wire saw in order to obtain (3) two strips with black decors, (4) sandwich making using glue, (5) cutting thin lamella (500 μm), (6) protection with glue, (7) mechanical polishing down to 40-50 μm, (8) adding a reinforcing Cu washer, and (9) finally optical micrograph of central area.…”

“…The study of larger crystals relative to the beam size requires polychromatic beam in order to record a large number of reflection on the 2D diffraction pattern to be able to identify the nature and the orientation of the crystallite (or a few crystallites) irradiated by the beam; X-ray Laue diffraction conditions. This coupled diffraction approach applied to the cultural heritage materials is detailed in [5]. In this chapter, we will present only the approaches based on X-ray absorption spectroscopy, which currently give the best spatial resolutions and are much less time-consuming.…”

A concise synchrotron X-ray microdiffraction field guide for the Earth scientists

“…A four-bounce constant-exit monochromator consisting of two identical channel-cut Si(111) crystals can be inserted in the path of the beam for an easy and rapid switch between polychromatic (white) beam and monochromatic beam, while illuminating the same spot on the sample. This is one capability which is rather unique to BL12.3.2, the possibility to conduct both white and monochromatic X-ray experiments on the same micron area of the sample (Dejoie et al, 2015). The available photon energy range is between 5 and 24 keV.…”