## 3 Results and Discussion
### Archaeometric Analyses: Defining the Analytical Approach Based on Material Features  
Once the selection of materials to be analysed has been completed, the setup of the analytical protocol may vary according to the features of the materials themselves. An in-depth discussion of the potential and limits of all possible analytical techniques applicable to the study of archaeological glass would be beyond the scope of this article. Therefore, in the following discussion, reference will be made to the most commonly used analytical techniques for archaeometric studies on glass to foster data comparability. The first criterion influencing the choice of analytical protocol is whether the materials to be analysed are naturally or intentionally coloured glasses. The archaeometric approach to the study of nonintentionally coloured glass (which means without the intentional addition of colouring and opacifying agents to the base glass) is simpler than deeply coloured (and often opaque) glass. The latter is, in fact, characterized by greater heterogeneity in terms of microstructure and compositional features, due to the addition of specific raw materials aimed at imparting the desired colour and degree of opacity. The setup of a more articulated analytical approach is needed based on the interrelation of different investigation techniques to achieve an in-depth characterization of colouring and opacifying agents. When dealing with naturally coloured glass, the archaeometric approach is based on the use of fewer analytical techniques, the primary objective being the determination of the chemical composition of the base glass and, if needed, the provenance of the materials used as vitriying agents. Figures 1 and 2 show block diagrams of the two different analytical approaches applicable to the study of naturally and intentionally coloured glass; it is always advisable to carry out an observation and documentation of the samples in optical microscopy, preliminary to the filing of the fragments/objects under study. Figure 1: Block diagram of the proposed analytical approach for naturally coloured archaeological glass. When archaeometric analyses have to be performed on intentionally coloured glasses, it is highly recommended to start with an in-depth study of the colouring (and opacifying) phases. As more extensively discussed elsewhere [8], since deeply coloured glasses show highly heterogeneous microstructures and microtextures, a thorough characterisation of colouring and opacifying phases should be carried out before the bulk composition of the vitreous matrix to avoid misunderstandings in data processing and interpretation. A combined Standard Colour System Chart (such as NCS Index or PANTONE) and Visible Reflectance Spectroscopy (VIS-RS) approach is here proposed as a starting point. It can support, in fact, an objective definition of the chromatic hues and shades of deeply coloured glasses avoiding any subjective nomenclature. The example par excellence is represented, in this case, by a particular category of glass-based artefacts: mosaic tesserae. For these small cubes of coloured glass, a chronotypical study is unfeasible; therefore, colours and opacity are the only macrofeatures that can be used to methodically select among copious assemblages of samples to be studied. NCS coordinates can be used to effectively separate the tesserae into preliminary chromatic macrocategories (i.e., yellow, green, blue, red, and black). These groups can be defined based on NCS coordinates and, more precisely, by taking the second part of the NCS-notation into account, which describes the hue by means of a numerical code. For example, a tessera with NCS-notation S 2030-G70Y will be described as of a yellow colour and code G70Y indicating a colour shade described as a yellow (Y) with 70% resemblance to yellow and a 30% resemblance to green (G). It is possible, in this way, to avoid the use of definitions such as \"greenish yellow\" or \"yellowish green\", eliminating a first important degree of subjectivity in the definition of colour.