# Degradation Products on Byzantine Glasses from Northern Tunisia
## 3 Results  
### Sem-Eds  
Electron microscopy, in combination with energy dispersive X-ray microanalysis, has successfully contributed to the investigation of degradation phenomena on materials of art and archaeology [37; 38; 39]. In the present study SEM-EDS investigations made it possible to highlight mainly two kinds of surface degradation developed on the different samples, i.e., iridescent patinas and crusts.  
In Table 2 the results of SEM-EDS analysis on samples GLU034, GLR011 and GLB002 are reported. GLU034 and GLR011 samples (Figure 2) appear iridescent in the altered areas. The results of elemental analyses for sample GLR011 are shown also in Figure 3 to better evidence the differences between unaltered and altered area. In the altered areas an enrichment in silica and a depletion in alkali is observable. It is worth nothing that, as already reported in our previous work dealing with the characterization of these glasses [28], low sodium concentrations were generally detected, but this is in accordance with other cases reported in the literature as we have already underlined in [PERSON] et al. 2016 [28]. This has been observed for all the samples and it could probably be attributable to some initial depletion in sodium even if visually unaltered areas are clearly observable, as is well evident from the images reported in Figure 1.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**GLU0034**} & \multicolumn{2}{c}{**GLR011**} & \multicolumn{2}{c}{**GLB002**} \\ \cline{2-7}  & **Unaltered Class** & \multicolumn{2}{c}{**Altered Glass**} & \multicolumn{2}{c}{**Unaltered Class**} & \multicolumn{2}{c}{**Altered Glass**} & \multicolumn{2}{c}{**Unaltered Class**} \\  & **Iridescent Pating** & **Unaltered** & **Iridescent Pating** & **Iridescent Pating** & **Unaltered** & **Glass** & **External Crust** \\ \hline Na\({}_{2}\)O & 2.88 & 2.24 & 2.26 & 1.85 & 2.44 & 3.24 \\ MgO & 1.62 & 0.56 & 0.22 & - & 0.67 & 1.70 \\ Al\({}_{2}\)O  
The alteration phenomenon that we have observed, i.e., iridescent patina and crusts formation on the surfaces of our samples, have been already observed in the literature (see also the description reported in the introduction) [11; 20; 21; 22; 23; 24; 25; 26; 27; 40; 41; 42].  
A different situation has been observed for sample GLB002 (Figure 4) with respect to what was observed for samples GLR011 and GLU034; it showed on the surface a layer of dark brown encrustations (an iridescent area was also present in this sample, but it was not possible to analyze it by SEM-EDS because of the poor quality of the signal, due to the orientation of the surface/electron beam). For sample GLB002 a depletion in silica and an enrichment in alkali is present (Figure 5). Furthermore, the black encrustations are enriched in titanium, iron and manganese. It is worth noticing that these elements are those typically present in the glass composition [28].  
Figure 4: SEM images acquired on the dark-brown area of sample GLB002.  
Figure 3: Mean concentrations (wt%) detected by SEM-EDS of the elements present on the altered (iridescent area) and unaltered area of sample GLR011.