# Degradation Products on Byzantine Glasses from Northern Tunisia
## 4 Discussion
As degradation proceeds, new effects are produced on the surface, which are formed by interaction with pollutants and carbon and sulfur dioxide, which cause the precipitation of salts on the surface. Some weathering products can be observed on the glass surface in accordance with what is reported in the literature [37]. In particular in our case the presence of CaCO\({}_{3}\) has been observed by ATR FTIR (Figure S1) together with traces due to organic substances (signals at about 2920 cm\({}^{-1}\)) in accordance with what described by [PERSON] et al. [37] where it is reported that the weathering product could be calcium carbonate or some organic Ca-containing compound. The development of corrosion crusts complicates the analysis of historical glass pieces and the diagnosis of their deterioration [4]. Contrary to the iridescent areas, where a leaching of alkali and alkaline earth ions causes the modification of the glass network, browning corrosion is due to the migration of Mn, Fe and other metallic ions carried by the attacking aqueous solution, which enter in the glass from the surrounding soil or are depleted from the pristine glass core (this is the case of Mn and Fe) [46]. This is perfectly in accordance with what observed for sample GLB002 where an enrichment mainly in Mn, Fe and Ti has been observed on the encrustations together with an increase in Ca, K, Na, Mg, Al, Cl and S (Figure 5). In this case different compounds could be formed; for example, in the literature Fe (III) oxide and oxide hydrates, insoluble sulfides or manganese dioxide are reported and are in part responsible for the darkening of the glass [3]. Nevertheless on the base of elemental analysis by SEM-EDS, it is difficult to make hypotheses on the chemical nature of the species present on the surface. We can just presume that, for example, the observed increment on the encrustations of S, K, Ca, Na and Cl concentrations in case of sample GLB002 could be due to the formation of sulphates such as syngenite (K\({}_{2}\)SO\({}_{4}\)-CaSO\({}_{4}\)-H\({}_{2}\)O) or of chloride as NaCl in accordance to what is observed by [PERSON] et al. [37]. From the point of view of the surface morphology, sample GLB002 shows (Figure 4) the typical features characteristic of corrosion processes [48]. XPS has been already applied in the literature for the characterization of the alteration and corrosion phenomena occurring in Roman glass fragments [49] and for investigating the weathering process of glass surfaces with the formation of corrosion growth rings detected by optical microscope [46]. A comparison has been carried out between EDS and XPS results (Figure 11) in order to highlight differences in the chemical composition of the outermost layers compared to the bulk. To this purpose Ca/Si and Na/Si ratios have been considered (atomic percentages detected by XPS were converted into weight percentage in order to compare the results). For sample GLR011 it is confirmed also for the outermost layer what was already observed for the bulk, i.e., a decrease of both Ca/Si and Na/Si ratios in the altered areas confirming the formation on the surface of a silica gel depleted in alkali ions. The formation of silica gel is confirmed by XPS data: taking into account all the samples, the binding energy of Si 2p increases with sodium and calcium depletion towards values typical of SiO\({}_{2}\) (Figure 12a). Besides, in O 1s peaks, the component ascribed to non-bridging oxygen increases its intensity in the visibly altered regions, where cation/silicon ratios decrease (Figure 12b). If the non-bridging/bridging oxygen area ratio (NBO/BO) is plotted _vs_ Ca/Si (Figure 12c) and Na/Si (Figure 13) ratios it is evident that, in the cation depleted areas, NBO/BO increases substantiating the hypothesis of silicate depolymerization. The only exception is represented by point 2 of the GLB002 sample where NBO/BO increases with the Ca/Si ratio (Figure 12c).