Abstract
We present new transmission electron microscopy (TEM) based electron diffraction characterization techniques (orientation imaging combined with 3D precession electron diffraction tomography-ADT) applied on cultural heritage materials. We have determined precisely unit cell parameters, crystal symmetry, atomic structure, and orientation/phase mapping of various pigment/opacifier crystallites at nm scale which are present in green and white Roman glass tesserae. Such TEM techniques can be an alternative to Synchrotron based techniques, and allow to distinguish accurately at nm scale between different crystal structures even in cases of same/very close chemical composition, where is also possible to visualize between different crystal orientations and amorphous/crystalline phases. This study additionally demonstrates that although opacifiers in green and white tesserae are found to have average Pb2Sb2O7 cubic and CaSb2O6 trigonal structures, their pyrochlore related framework can host many other elements like Cu, Ca, Fe through ionic exchanges at high firing temperatures which in turn may also contribute to the tesserae colour appearance.
Highlights
The scientific study of ancient glass and ceramic pigment is very important in archaeometric research and involves a significant amount of various analytical techniques like e.g. XRF (X-Ray Fluorescence), EDS (Energy-Dispersive X-Ray spectroscopy), EPMA (Electron Probe Microanalysis), XRD (X-Ray Diffraction) and Raman to characterise various phases that may contribute to colour
As the Focused Ion Beam (FIB) sample holders are standardly made of Copper, to ensure that Cu is contained in the samples we performed EDS-SEM measurements (Additional file 1: Fig. S2a–c) and Additional file 1: Tables S1, S2) where can be seen that copper is detected in small quantities (~ 3%) in both glass matrix and in the opacifiers/precipitates
Is very important to note that the conclusion of our structure analysis that the precipitates should have the Pb2Sb2O7 structure is not based only on the combination of unit cell, COD and EPMA data; is the good fit between ED reflection intensities (46 in case of cubic structure) with the correct structure(s) that reveals that the atomic composition can only correspond to Pb2Sb2O7 structure
Summary
The scientific study of ancient glass and ceramic pigment is very important in archaeometric research and involves a significant amount of various analytical techniques like e.g. XRF (X-Ray Fluorescence), EDS (Energy-Dispersive X-Ray spectroscopy), EPMA (Electron Probe Microanalysis), XRD (X-Ray Diffraction) and Raman to characterise various phases that may contribute to colour. All such techniques either lack spatial resolution (micron instead of nm size probe) and/or their results are often not conclusive due to possible co-existence of many phases present within the analyzed volume area.
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