# Degradation Products on Byzantine Glasses from Northern Tunisia
## 4 Discussion
In that area encrustations enriched in CaCO\({}_{3}\) have been evidenced: XPS analysis has highlighted how Ca enrichment takes place in the outermost layer (Ca/Si ratio determined by XPS is remarkably higher than that determined by EDS) and the presence of calcium carbonate is in agreement with the FT-IR spectra. Figure 11: Ca/Si e Na /Si ratios obtained from XPS and EDS analyses carried out on unaltered and altered glass areas for samples GLR011 (**a**) and GLB002 (**b**). Figure 12: (**a**) binding energy (BE) of XPS Si 2p peak versus cation/Si ratios. The shaded area represents the range of BE reported in literature for SiO2. [50] [ref: [[https://srdata.nist.gov/xps/selEnergy/Type.aspx](https://srdata.nist.gov/xps/selEnergy/Type.aspx)]([https://srdata.nist.gov/xps/selEnergy/Type.aspx](https://srdata.nist.gov/xps/selEnergy/Type.aspx))]; (**b**) Area % of non-bridging and OH component in O 1s peak vs Ca/Si and Na/Si ratios; (**c**) non-bridging/bridging oxygen area ratio from O 1s peaks vs Ca/Si ratios and (**d**) Na/Si ratios. Figure 13: Binary diagrams relative to the concentrations of some trace elements such as: Co vs Mn, Pb vs Mn and Ti vs Pb, detected on different morphological glass areas. It is worth noting that also Fe 2p signals recorded in the visibly altered regions of samples GLB002 and GLR014 reflect the higher amount of hydroxides, whose signals in O 1s peaks fall at the same binding energy values of NBO, being the components at about 712 eV more intense in the external encrustations (points 2). As far as sample GLR014, the presence of an intense component ascribed to carbonate in C 1s signal recorded on point 1, could be due the presence of sodium and calcium carbonate. The marked decrease of sodium, calcium (and also of other metals such as Co and Mn) and of the carbonate component in point 2 could be due to leaching processes. The LA-ICP-MS results highlight the different enrichment patterns observed in the black encrustation of GLB002 and in the iridescent patina of GLU034. Indeed, the black encrustation of GLB002 compared to the unaltered bulk glass is always enriched in Co, Mn, Ni, Pb and Ti. It is worth mentioning that Mn and Ti enrichment was also confirmed by SEM-EDS data. Enrichment in Mn has also been observed on encrustations in another study reported in the literature [40]. The iridescent patina of GLU034 sample, compared to the unaltered bulk glass, only shows a higher value of Pb. Considering that the Pb is calcium-vicariant, his enrichment is coherent with the observed presence of Ca compounds in both samples. Moreover, the higher values of other heavy metals (Co, Ni, Ti) are in agreement with the presence of different compounds (oxide and oxide hydrates, insoluble sulfides or manganese dioxide) and they are jointly responsible for the darkening of the glass [49]. The different chemical behavior of the two types of degradation layers is evident in the binary diagrams Co versus Mn, Pb versus Mn and Ti versus Pb (Figure 13) and in the histograms of Figure 14. The black encrustation of GBL002, instead, is depleted in Cu and Cr. Since the investigations have been performed only on a limited number of samples, advancing explanations for these phenomena might be speculative. Nevertheless, the increase in some elements such as Pb on the altered areas could be explained with depletion in other elements. Figure 14: Histograms of (**a**) Mn, Ti; (**b**) Co, Cr, Cu, Ni, Pb in both unaltered and altered areas of GLB002 and GLU034 samples.