# Degradation Products on Byzantine Glasses from Northern Tunisia
## 4 Discussion  
It is acknowledged that ancient glasses exposed for long time to unfavorable conditions undergo transformations and alterations. The degradation processes strongly depend on the chemical composition and environmental parameters of the glasses (such as soil humidity, temperature and pH). Exchange reactions take place between the so-called network modifier ions (Na+, K+, Ca2+ and Mg2+), added to the raw materials in the form of oxides or carbonates to decrease the melting point and making the glass more resistant towards chemical attack and water [37]. It has to be noticed that this study area in Northern Africa is subject to the semi-arid Mediterranean climate with few rainfalls, although the site of Ain Wassel in the Arkou valley has a sub-humid microclimate. Soils are in general clayy, but permeable enough. Worth noting in the region is the presence of salt, originating from the weathering of local diapiric structures, which partly erodes the land surface [44; 45]. Systematic studies on the corrosion of glasses, i.e., the attack of aqueous solutions onto the glass surface, have been often carried out in the literature by means of SEM-EDS [37]. This technique has allowed us to highlight morphological and elemental distribution differences between altered and unaltered areas. In the case of iridescent regions an increase of Si and alkali depletion was detected (see samples GLR011 and GLU034). The weathered glass is a complex system characterized by the presence of very fragile multilayered silica, where the number of layers and their thickness depend upon time. The weathering is mainly due to the effect of water that, through the mechanisms of diffusion and ionic exchange, produces a general loss of modifier ions on the surface of glass, generating almost pure silica iridescent layers [46] like those observed for samples GLR011 and GLU034. In general, three different layers can be identified in weathered glasses: the unaltered bulk glass, the gel layer composed by a hydrated silica network (typically, samples GLR011 and GLU034) and the layer of corrosion salts externally formed (GLB002). Samples GLU034 and GLR011 (Figure 2) show the characteristic lamellar structure formed following the leaching process of alkali ions. Elemental analysis of altered and not altered surfaces, carried out by SEM-EDS analysis on these samples, has shown that the degraded surface is characterized by alkali depletion and an increase in silica, compared to less unaltered surface. This ion exchange causes the formation of a thin layer near the glass surface, which is relatively enriched in silica and contains a low amount of other cations. Alkaline-earth ions (M2+) could also leach out by a similar mechanism. Indeed, often the leaching of Na from soda-silica-like glass results in the formation of an outer so-called gel layer, mainly consisting of the remaining glass components (SiO2 and CaO) [47]. Curious growth rings develop on the surface of the glass. This characteristic phenomenon has been observed for sample GLU034 (Figure 2). The alteration layer shows the presence of flakes (Figure 2): in fact, cracks form as the degradation increases, and they become a vehicle for the passage of water for a subsequent attack. The leaching phase is followed by a corrosion process in which the silicon oxygen bonds (Si-O-Si) are broken. This phenomenon is called glass depolymerization and progresses gradually to greater depths up to 150 um. Cracks have been clearly observed for samples GLU034 and GLR011. In particular, in sample GLU034, the presence of craters highlights the beginning of the corrosion process. Many authors describe that the transition from the degradation phase, defined as leaching to the corrosion phase, can be highlighted on the surface by observing these typical craters; in fact, over time they tend to widen (as the degradation proceeds) until their complete interconnection [10; 14; 16; 18; 23]. The layers produced are fragile and inconsistent and this leads to the thinning of the surface.