# Degradation Products on Byzantine Glasses from Northern Tunisia
## 3 Results
### Small Area XPS  
Samples labeled GLR014, GLB002 and GLR011 were selected for the present investigation since they show various kinds of weathering: iridescence and incrustations. X-ray photoelectron spectroscopy, which is a non-destructive technique, may be operated in various modes and within them small-area XPS was selected in the present investigation. The reason for this choice is that, despite a decrease in the signal intensities, this technique allows the gaining of chemical information only of the altered regions as well of the regions that morphologically exhibit changes in color or the presence of deposits, provided that the analyzed region is at least three times larger than the beam diameter selected for the analysis. In this way, it is possible the characterization of selected regions of the sample and thus a comparison of the composition between morphologically different areas for obtaining information is useful in the identification of the degradation process. Through XPS the surface concentrations of the main chemical elements have been obtained and, in particular, the oxidation states of the elements, the presence of carbonate salts and concentration trends of some elements in the outermost layers. As far as GLB002 sample, three different regions were characterized, as pointed out in the photographs of Figure 6: point 1 in Figure 6a, external crust labeled as point 2 in the same figure and an iridescent area (point 3 in Figure 6a). Figure 5: Mean concentrations (wt%) detected by SEM-EDS of the elements present on the unaltered and altered areas (showing the presence of a crust) of sample GLB002. A detailed inspection of the survey spectra (Figure 6b) allows the identification of the main constituents of the samples' surfaces. Oxygen, silicon, aluminum, calcium and sodium were detected on the surfaces of all the analyzed areas. Potassium was only revealed in point 1 and small amounts of iron were found on the external crust (point 2) and on the iridescent portion of the sample (point 3) (Figure 7). A curve fitting of Fe 2p peaks according to [43] was performed and allowed the observation that in the external crust the % of the peak area ascribed to Fe (II) is lower (13%) than in the iridescent areas (35%). Surface composition is given in atomic percentage (Table S1). The external crust and the iridescent patina were clearly depleted in alkali: K is not detected in point 2 and 3 and Na/Si ratio decreases from 0.3 in point 1 to 0.2 in the external crust, and it is found to be 0.03 in the iridescent patina (point 3 Figure 6c). Ca/Si ratio is higher in the external crust than in the point 1 while it is lower in the iridescent patina. Two different regions were analyzed in the GLR011 sample: point 1 (unaltered) and point 2 (iridescent patina) in Figure 8a. Figure 6: **(a)** The pictures show the analyzed areas in sample GLB002; **(b)** It reports X-ray photoelectron survey spectra acquired on the three different areas with a monochromatic Al K\(\alpha\) X-ray source; and **(c)** compares the surface cations-to-silicon ratios. Figure 7: Fe 2p peak obtained on the sample GLB002 in point 1, in the external crust (point 2) and in the iridescent patina (point 3). The detailed surface composition in atomic percentages of the GLR011 sample is reported in Table 2. Si, O, Al, Ca and Si were the only detectable elements together with C. The presence of this element might be ascribed to contamination (Figure 8b). The cations-to-Si ratios showed a pronounced depletion of Na (Figure 8c) in point 2, which exhibits an iridescence compared to point 1 and a minor decrease of the Ca/Si ratio. Also, in the case of sample GLR014 two different areas were characterized, i.e., point 1 and point 2 of Figure 9a. The inspection of the survey spectra acquired on sample GLR014 allows the identification of iron, cobalt and manganese together with oxygen, silicon, aluminum, sodium and calcium in point 1 of this sample (Figure 9b); Table 2 lists the atomic percentages and Figure 9c shows the atomic ratios. (Table 2 and Figure 9c).