A novelty for cultural heritage material analysis: Transmission Electron Microscope (TEM) 3D electron diffraction tomography applied to Roman glass tesserae
Introduction
The scientific study of colors and constituent materials in ancient glasses, ceramics, decorated pottery etc., is an issue of great importance in archaeometric research due to its association with manufacturing and production information and finally to the so-called chaîne opératoire choices and processes.
To tackle the above issues, the field of archaeometry makes use of the analytical information of an array of instrumentations, (e.g. X-ray Fluorescence (XRF), Raman spectroscopy, X-ray Diffraction (XRD), Secondary Image Mass Spectrometry (SIMS) or Electron Probe Microscopy Analysis (EPMA); in all such techniques when the research aims are towards the study of the various phases and their association to the final coloration there is lack of straightforward answers due to the complexity and heterogeneity of the historical materials exhibit. All the previous mentioned analytical techniques operate only at 1–0.3 micron resolution scale, where the acquired data are usually not conclusive due to the possible co-existence of many (nm size) phases present and probably interfering within the analyzed (micron size) volume; as a result, data/signals coming from a particular crystal location may in fact be influenced from a number of other surrounding crystals contribution.
Archaeological glass is an overall homogeneous material and even small fragments may generate secure results for the chemical composition of a finished product. However, glass mosaic tesserae and especially the opaque ones, are often complex in nature; several phases can be identified in the glass matrix of a single specimen. Mosaic tesserae were cut from various materials including glass; the advantages of using glass versus stone or ceramic material being the ability to offer a range of colors and a glittering quality which could heighten bold contrasts in pictorial representations. Though the earliest evidence for the use of glass tesserae was identified on the Greek island of Delos, dating back to the second century BC [1], their widespread use in mosaics is significantly noticed as of the mid-first century AD and onwards [2]. Hellenistic till Late Roman glass tesserae was a soda-lime-silica base glass, having initially antimony-based opacifiers while tin based opacifiers initially introduced during the second century BC and gradually prevailed by the Late Roman/Early Byzantine period. In addition, calcium phosphate was occasionally used since then and onwards. By adding opacifying agents and with the combination of coloring elements, such as iron, copper, manganese and cobalt, a very wide range of colors was achieved [3].
A large collection of Late Roman colored mosaic glass tesserae, excavated in 2008 at the sanctuary of Isis and Sarapis, within the archaeological site of ancient Messene (Peloponnese, Greece) was analyzed by SEM/EDS aiming at the characterization of its base glass composition and pigments [4]. The tesserae studied within the present work, cover the yellow and blue colors while their composition and structure is representative of the Late Roman glass tesserae technology [3], [4]. The aim of the present work is to exploit a novel TEM and electron diffraction (ED) based technique (Fig. 1), which allows obtaining accurate information regarding the crystal structure of the pigments/opacifiers of the glass mosaics and more specifically the study of their composition, crystal structure and its relationship with the glass matrix.
Section snippets
Sample preparation
Three different thin/electron beam transparent lamellae (4 × 4 micron size and approximately 100 nm thick) were removed by using the Focused Ion Beam (FIB) sample preparation technique (Fig. 1S). Those three slices were FIB cut using Ga ions, during a time-consuming process, from larger tesserae fragments having yellow, deep blue and light blue colors. All samples were lifted out from FIB to specific TEM grids for subsequent observation and examination. For SEM observation and EPMA analysis several
Technique and instrumentation
For FIB specimen preparation the FEI Dual Beam Helios NanoLab 600 was used at LMA Zaragoza using Ga + ions source at 30 keV. For electron diffraction tomography experiments, a TEM Jeol 2010 LaB6 (200 keV) was used equipped with “spinning star” precession system [5] at the University of Patras. TED EDS measurements were performed with TEM Jeol 2100F equipped with Jeol Centurio detector at Si Map Grenoble University. SEM measurements were performed at the University Rey Juan Carlos Madrid (model
Yellow tesserae
In this work we analyzed FIB lamellae from different colored glass tesserae (yellow, deep and light blue). Regarding the yellow color sample, SEM examination study revealed the presence of small precipitates (200–400 nm) embedded in an amorphous glassy matrix (Fig. 2).
We performed ADT analysis on 3 different crystallites and reconstructed reciprocal space as shown in Fig. 3a and b respectively. For ADT analysis, TEM goniometer was tilted about ± 80° to collect × 160 ED patterns for every 1° of
Concluding remarks
The introduction of novel precession electron diffraction tomography technique (ADT) in the field of cultural heritage allows to obtain very precise crystallographic data (unit cell, symmetry and atomic positions) of different phases that may exist in ancient glass and pottery This technique can be used easily in any TEM (100–300 kV) having sufficient angular tilt (± 45°), precession electron diffraction hardware, and 3D electron diffraction tomography software. Although TEM has been scarcely
Acknowledgements
NZ acknowledges financial support from the CULTTECH MSc Program of the University of the Peloponnese (KA-ELKE293) and RA from the Spanish Ministerio de Economia y Competitividad (MAT2016-79776-P). Authors acknowledge assistance with TEM measurements by Dr. M. Kollia at the Interdepartmental Laboratory of Electron Microscopy and Microanalysis of Patras University. The authors are very grateful to the Director of the Society for the Messenian Archaeological Studies, Prof. P. Themelis, for
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