## 3 Results and Discussion
### Archaeometric Analyses: Defining the Analytical Approach Based on Material Features
When dealing with archaeological and historical glasses, it is important to identify \"recipes\" that can be linked to primary production furnaces and, therefore, provide information on geographical areas of production and provenance [3]. However, for deeply coloured glass, a further degree of difficulty is due to the \"contamination\" of the compositional data of the matrix with materials added as colouring and opacifying agents. To investigate the base glass, an integration between EPMA and LA-ICP-MS can work as a suitable compromise. Especially in the last few years, EPMA has been gradually being replaced by LA-ICP-MS for the determination of minor and major oxides, calculated by difference given a known oxide (generally SiO\({}_{2}\)). Recent research shows, indeed, that close correspondence is generally observed between the data achieved by EPMA and \"new generation\" LA-ICP-MS equipment when analysing colourless or naturally coloured glasses [31; 32]. LA-ICP-MS can perform major to trace element analysis of almost all elements within a sample during a single run, due to specific quantification protocols such as Internal Standard Independent (ISI) and Sum Normalization (SN) methods [33]. Although the potentialities of this technique are significant, its application to the study of deeply coloured opaque glass still needs to be thoroughly explored. In particular, it should be noticed that the most commonly used quantification method (the Sum Normalization) assumes that glass is almost exclusively comprised of oxides in known oxidation states and that the sum of the concentration of all oxides should equal 100% [33]. The first statement is quite difficult to verify when dealing with deeply coloured and opaque glasses such as tesserae, when the addition of several compounds is responsible for the colour shades and the opacity. Recent research also states that EPMA analysis of major and minor elements has several important advantages compared with other techniques, such as LA-ICP-MS as  
Figure 5: Example of nanometric copper-based inclusions detected in a red mosaic glass tessera: (a) SEM–BSE image; (b) EDS spectrum; (c) X-ray diffraction pattern showing the presence of metallic copper (Card No. standardreference4-0836). well as Secondary Ion Mass Spectrometry (SIMS): higher spatial resolution (from one to several \(\upmu\)m), a well-established matrix correction procedure, and a lower cost [34]. Given the current state of knowledge, further studies should, therefore, be conducted on the specific application of LA-ICP-MS for the quantification of not only trace, but also major and minor elements in opaque deeply coloured glass before a complete admission of EPMA for quantitative analysis of major and minor elements can be proposed. When dealing with opaque deeply coloured glasses, EPMA data can, however, be affected by the addition of materials acting as colourants and opacifiers. To better compare the composition of the base glass with the categories reported in the literature for naturally coloured glass, EPMA data should, thus, be adequately processed in order to, if not completely neutralise, at least minimize this effect of contamination. Following the method proposed by [PERSON] [35], reduced composition can be obtained by subtracting the oxides of elements presumably due to additives from the total sum of all those measured and by normalising the remaining data. However, the main concern associated with [PERSON]'s method remains the following: when the recalculation is carried out, how can we be sure not to incur arbitrary subtractions? Previously performed in-depth characterisation of colouring and opacifying phases can provide considerable help and a treasurable guide in preventing any subjective subtractions, as we do know what materials are responsible for the hue and opacity of the samples under study and this information can guide us in the recalculation of data.