Novel characterization techniques for cultural heritage using a TEM orientation imaging in combination with 3D precession diffraction tomography: a case study of green and white ancient Roman glass tesserae

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

Fast electron diffraction tomography

International audienceA fast and fully automatic procedure for collecting electron diffraction tomography data is presented. In the case of a very stable goniometer it is demonstrated how, by variation of the tilting speed and the CCD detector parameters, it is possible to obtain fully automatic precession-assisted electron diffraction tomography data collections, rotation electron diffraction tomography data collections or new integrated electron diffraction tomography data collections, in which the missing wedge of the reciprocal space between the patterns is recorded by longer exposures during the crystal tilt. It is shown how automatic data collection of limited tilt range can be used to determine the unit-cell parameters, while data of larger tilt range are suitable to solve the crystal structure ab initio with direct methods. The crystal structure of monoclinic MgMoO4 has been solved in this way as a test structure. In the case where the goniometer is not stable enough to guarantee a steady position of the crystal over large tilt ranges, an automatic method for tracking the crystal during continuous rotation of the sample is proposed