The beauty of ancient mosaics comes from the collation of hundreds of tiny colourful tiles, or tesserae. These tiles are made of glass with the addition of different opacifying and colouring agents. Being able to understand which agents were used gives valuable information about the ingredients and the glassmaking techniques used thousands of years ago. Crucially, for archaeological objects like mosaic tiles, it's important to find a way to get this information without damaging them.
As part of her master's thesis in Applied Physics at Università degli Studi di Milano-Bicocca, Giulia Marcucci used neutron methods, alongside other techniques, to do just that. With her colleagues from Italy and ISIS beamline scientists, she studied a selection of colourful tesserae from archaeological excavations and mosaics under restoration from different archaeological sites and geographical areas in Italy, Greece and Syria, dating from the 6th to the 13th century AD.
Standard methods of investigating these tiles either just look at very small areas of the surface or require the tiles to be ground into a powder. Giulia started with preliminary characterisation using conventional methods like Raman spectroscopy and Particle-Induced X-ray/Gamma Emission (PIXE/PIGE). The Raman scattering was able to give information about the agents present but could only be used to investigate one point on the surface of the tile, and so multiple measurements in different sections were needed, with no access to the inner volume. The PIXE and PIGE experiments showed the quantitative elemental composition of a very small area.
However, the glass tiles do not have uniform composition, and therefore neutron techniques enabled Giulia to study the bulk of each sample without causing any damage. Using Neutron Resonance Capture Analysis (NRCA) on the INES beamline, she was able to study the elemental composition of the entire tile. NRCA was able to detect antimony (Sb), which is related to the presence of Sb-based opacifiers in the glass, and copper, which creates the colour in the red tesserae.
Carrying out Neutron Activation Analysis (NAA) using the ChipIr germanium detector, after irradiation on INES, enabled Giulia and her collaborators to quantify elements in the bulk of the tiles with a high neutron cross section, such as sodium, gold and arsenic. The results were consistent with those from the surface PIXE/PIGE measurements.
Using Neutron Diffraction on Polaris, they were able to see the crystalline phases corresponding to the opacifiers present in the entire volume of the glasses. By studying the sample intact, not only could they keep the historical object undamaged, but they gained extra information about the bulk of the tesserae, which would not be possible with surface techniques. The team could also see if there were metal based agents in the tiles, something not possible using Raman spectroscopy.
Only by combining all the different techniques was Giulia able to build a full picture of what ingredients were used to create the desired colour and opacity. Identifying which crystal structures are present gives valuable historical insight into the production of these mosaic tiles.
Since completing her PhD, Giulia is now a postdoctoral researcher supported by the Italy-UK agreement for neutron science and is using the same techniques to look at the composition of other historical objects, including meteorites!
Further information
DOI: 10.1140/epjp/s13360-021-01696-2; DOI: 10.13036/17533562.64.2.21
