**4. Conclusions**

The meticulous connection and correlation between archaeological, typological, and compositional data is a key requisite when approaching the study of archaeological glass. An actual coparticipatory approach between archaeologists and archaeometrists stands, therefore, as a fundamental requisite for understanding objects beyond their chemistry and recognising them as witnesses of human material culture.

In the attempt to outline a best practice approach for the study of archaeological glass, the reasons for comprehending the contexts as the first step toward a tailored selection of materials have been discussed. Understanding if we are in a context with sound evidence of production/working activities will lead to selection criteria other than those that will guide the selection of objects from multilayered contexts (or where no univocal evidence of production has been ascertained).

A proper discernment of the archaeological context will, therefore, support a wellfitted selection of the type(s) of finds to be subjected to chronotypological study and archaeometric investigation. The analysis of compositional and technological features cannot, in fact, be separated from a careful and detailed chronotypological study, aimed at framing the object and its morphology in the related chronological and geographical context of diffusion.

Regarding archaeometric analyses, it can be stated that the setup of the analytical protocol may vary depending on the features of the objects to be analysed. When dealing with naturally coloured glass, the analytical approach can be 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 vitrifying agents. Otherwise, deeply coloured (and often opaque) glass shows heterogeneity in terms of microstructure and compositional features, due to the addition of colouring and opacifying agents. The setup of a more articulated analytical approach is, thus, needed, based on the interrelation of different investigation techniques to achieve an in-depth characterisation of colouring and opacifying agents.

When dealing with opaque coloured glasses, the NCS System proved to be a suitable tool for a preliminary documentation of colours among assemblages to be studied: since NCS-coordinates describe the hue of opaque coloured glass, they can provide a first split of objects/fragment to be analysed into what can be defined as chromatic macrocategories (i.e., green, blue, red, and black), avoiding any subjective classification and denomination of the colours and, consequently, facilitating comparison among objects from different assemblages. By carrying out VIS–RS measurements, more in-depth information on optical properties related to different chromatic shades among coloured glasses belonging to the same chromatic macrocategory can, then, be achieved.

Since opaque coloured glasses show extremely heterogeneous microstructures, their microstructural and microtextural features should be investigated before the composition of the glassy matrix, to reduce the risk of interferences and misunderstandings in data evaluation. SEM–EDS is the starting point for the characterization of microstructural and microtextural features: BSE observations enable one to investigate and document the morphologies of the crystals precipitated into the glassy matrix, and EDS spot measurements provide preliminary data on their elemental composition. SEM–EDS is, thus, undoubtedly suitable to carry out high-resolution morphological inspection of the inclusions dispersed in a glassy matrix, as well as a qualitative and semiquantitative analysis of their elemental composition. However, if a more in-depth characterisation of these inclusions is needed to identify raw materials responsible for the colour and opacity of the samples under study, SEM–EDS inspection needs to be integrated with other techniques. As discussed in this paper, the choice of the most suitable technique/s can vary, depending upon the nature of the inclusions to be investigated. It has, for instance, been demonstrated that μRaman can be extremely suitable for investigating the composition of lead–tin–antimony-based phases, but the same cannot be said for Cu-based nanoparticles, where mineralogical analyses can be more informative.

In-depth examination of the microstructure needs to be followed by a determination of the bulk chemistry of the tesserae under study. Major and minor oxides, whose measurement is aimed at identifying both the fluxing agent and the "recipes" used in the glass-making process, have been analysed by EPMA. LA–ICP–MS analysis was carried out to determinine trace elements to draw inferences on the provenance of the sands used as vitrifying agents.

To identify, with as much precision as possible, the compositional categories of the base glass the tesserae belong to, EPMA data should always be recalculated (and normalised) to minimise any effect caused by any other intentionally added compound. A characterisation of the colouring and opacifying agents carried out prior to the analysis of the base glass will, therefore, also allow avoiding (or at least minimising) any subjective subtraction.

Last but not least, it should be remembered that, when dealing with archaeological materials, the study approach can be susceptible to implementations and variations evaluated on two fundamental criteria: the material features of the objects under study, which will never be identical to one another, as they are the product of ancient technologies and not of industrial processes; technological advancement and all that the progress of research will offer us which could be suitable to further deepen our knowledge of this material category.

**Author Contributions:** Conceptualisation, S.F. and T.C.; data curation, S.F.; methodology, S.F. and T.C.; resources, M.V.; supervision, M.V.; validation, M.V.; writing–original draft, S.F. and T.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The study does not report new data.

**Acknowledgments:** The authors sincerely thank the anonymous reviewers for their comments and valuable suggestions, which have contributed to increase the quality of the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

