## 3 Results and Discussion
### Archaeometric Analyses: Defining the Analytical Approach Based on Material Features
SEM-BSE images (Figure 4a) provide information on the morphology of the inclusions, showing the occurrence of micro-crometric anhedral crystals in the vitreous matrix, frequently clustered together; EDS spot measurements carried out on the crystals gave preliminary information on the elemental composition of the crystals, demonstrating that they were mainly made of antimony and lead, although tin could also be detected (Figure 4b). To achieve an exact characterisation of the compound, Raman microscopy was directly performed on the inclusions (Figure 4c). Figure 3: Comparison between reflectance curves acquired by VIS–RS on NCS-Red, Yellow, Green, and Blue tesserae. Acquired spectra showed the typical features assigned to lead antimonate doped with tin: in addition to the shifted Pb-O lattice mode at 140 cm\({}^{-1}\), a peak at about 450 cm\({}^{-1}\), an increase in the band at about 330 cm\({}^{-1}\), and a collapsed band at 510 cm\({}^{-1}\) were observed, indicative of the partial replacement of the Sb\({}^{+5}\) species by a larger Sn\({}^{4+}\) cation [28; 29]. In the above example, Raman microscopy can be considered as the most suitable analytical technique to gain data on the composition of the lead-tin-antimony-based compounds used to impart yellow colour and opacity to the glass. There are, however, cases in which the use of this analytical technique may not be informative. An example is represented by the opaque red coloured glass containing metallic copper inclusions. SEM-BSE images (Figure 5a) showed, at high magnification, nanometric rounded particles exclusively made of copper as EDS spot analysis demonstrate (Figure 5b). The nanometric size of these inclusions did not allow Raman microscopy analysis, where the magnifications of the most commonly accessible instruments hardly exceeds 1000\(\times\). Diffractometric analysis (Figure 5c) can, in this case, be decisive, allowing one to distinguish the presence of metallic copper within the glass matrix. It should, of course, be emphasized that, unless instruments such as microdiffractometers and/or BSDE detectors interfaced with SEM are available, diffractometric analysis usually requires powdering the sample and, therefore, is destructive. Once the investigation of colouring and opacifying phases was completed, we analyzed the bulk chemistry. To investigate the base glass, both in terms of compositional recipes and provenance of raw materials, a combination of Electron Probe Micro Analysis (EPMA) and Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) analyses was proposed, together with specific data processing. Major and minor oxides, aimed at identifying both the fluxing agent and the \"recipes\" used in the glass-making process, can be determined by EPMA, while LA-ICP-MS analysis needs to be carried out for measuring trace elements and, thus, drawing inferences on the provenance of the sands used as vitriifying agents. Several analytical techniques can be employed for the quantification of major, minor, and trace elements in archaeological and historical glasses, such as Wavelength Dispersive X-ray Fluorescence spectrometry (WDXRF), Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES), Ion Beam Analysis (IBA), and Neutron Activation Analysis (NAA) [30]. However, when dealing with opaque coloured  
Figure 4: Example of lead–tin–antimony-based inclusions detected in a yellow mosaic glass tessera: (**a**) SEM–BSE image; (**b**) EDS spectrum; (**c**) Raman spectrum with bands at 140, 332, 450, and 510 cm\({}^{-1}\) suggesting lead antimonate doped with tin [28; 29]. glasses the prime advantage of combining EPMA and LA-ICP-MS for quantification of major to trace elements is the possibility of performing both analyses on the same mounted and polished samples where the study of colourants and opacifiers had previously been carried out.