## 3 Results and Discussion
### Archaeometric Analyses: Defining the Analytical Approach Based on Material Features
For NCS evaluations to be accurate, it is important that observations are made in controlled repeatable daylight conditions avoiding any artificial source of light. After preliminary NCS-aided discrimination between chromatic macrocategories, further data on optical properties (L*a*b* numerical coordinates and the reflectance for each wavelength in the visible spectrum) can be collected by VIS-RS. Figure 3 shows a comparison between reflectance curves acquired on tesserae belonging to Red, Yellow, Green, and Blue chromatic macrocategories identified by taking NCS-notations into account. Reflectance curves of tesserae belonging to the NCS-Red macrocategory displayed a very flat behaviour in the wavelength range between 400 and 580 nm, followed by an increase in reflectance intensity for the wavelengths above 580 nm. NCS-Blue and NCS-Green tesserae showed bell-shaped reflectance curves; the reflectance peak was located in the region between 440 and 540 nm for the blue tesserae, while, for the green tesserae, it was slightly shifted between 470 and 540 nm. Last, for the NCS-Yellow tesserae, reflectance curves were characterised by an increase in reflectance intensity for the wavelengths above 560 nm. Though further research is needed, the potentiality of VIS-RS seems to go far beyond the description of colours by means of reflectance curves and L*a*b* numerical coordinates. In particular, the shapes of the reflectance curves in the visible spectrum and  
Figure 2: Block diagram of the proposed analytical approach for coloured archaeological glass. the percentages of reflectance can deliver preliminary qualitative information relating to the colouring and opacitying agents [6,20,21]. After preliminary OM documentation, sampling, embedding, and polishing, SEM-EDS should be performed on deeply coloured glass. Back-scattered electrons (BSE) signals allow detecting and documenting of the different morphologies of the crystals precipitated into the glassy matrix, with EDS spot measurements also ascertaining their elemental composition. SEM-EDS is undoubtedly suitable to carry out high magnification morphological inspection of the inclusions dispersed in glassy matrix, as well as providing a qualitative and semiquantitative analysis of their elemental composition. However, to provide a more in-depth characterisation of these inclusions, necessary to identify raw materials responsible for the colour and opacity of the tesserae, SEM-EDS inspection needs to be integrated with other analytical techniques. Especially if coloured and opaque, archaeological glasses are strongly heterogeneous materials; it is, thus, quite challenging to define what is the most suitable analytical technique for providing a full characterisation of the inclusions. It would, maybe, be more correct to claim that there is not only one. The choice of the technique/s is highly dependent upon the nature of the inclusions that we want to investigate. An integration of at least one molecular (such as Raman Microscopy) and one mineralogical analysis (X-ray Powder Diffraction--XRPD, micro-XRD or SEM equipped with an Electron Backscattered Diffraction detector--EBSD, if available) is recommended, offering an appropriate compromise to achieve a thorough characterisation of the inclusions responsible for the colour and opacity of glass. The characterisation of lead-tin-antimony-based compounds found in ancient glasses as colouring and opacitying agent can be taken as an example. As discussed elsewhere in more detail [8], lead-tin-antimonate crystals have been attested to in several assemblages of opaque coloured glasses [22, 23, 24, 25]. The most credited hypothesis is that the presence of lead-antimonate inclusions doped with tin could be related to the use of tin-rich metallurgical scraps [26,27]; therefore, an exact characterisation of these inclusions could represent the starting point for providing insights into the identification of a possible area of origin of the raw materials used as colouring and opacitying phases.