# Degradation Products on Byzantine Glasses from Northern Tunisia
The main forms of degradation that occur are:  
(i) formation on the surface of an iridescent patina that affects only the outermost layers, produced by the changed reflection conditions of the light due to the presence of the exfoliated surface [11; 14; 15; 16; 17; 18; 19];  
(ii) formation on the surface of crusts, that make the glass opaque and determine a decrease in gloss; the alteration layers modified the surface appearance. A nearly stratification forms on the surface, which tends to fragment over time [11; 20; 21; 22; 23; 24; 25; 26; 27]. The alkaline earth metals form weak ionic bonds with the network oxygen and are replaced by H\({}^{+}\) ions of the aqueous solution. The first step consists of the ion-exchange of the glass alkaline ions (M\({}^{+}\) = Na\({}^{+}\) and sometimes K\({}^{+}\)) with H\({}^{+}\) ions from water [6]. \[\mathrm{SiOM}+\mathrm{H_{3}O^{+}}\mathrm{\rightarrow}\mathrm{SiOH}+\mathrm{ M}^{+}+\mathrm{H_{2}O} \tag{1}\]  
As a consequence, within the network a depletion of alkali ions (Na\({}^{+}\) and K\({}^{+}\)) and alkaline earths ions and an enrichment of hydrated silica (i.e., a layer without alkali is formed on the surface) are observable. This is a diffusive phenomenon that only involves the outermost layers and depends significantly on the composition of the glass. In general, K\({}^{+}\) ions exchange much more easily than Na\({}^{+}\) ions and both tend to exchange even more easily than alkaline earth ions. The new layers formed are often brittle and can be easily detached even by small mechanical or friction impacts. All these alterations partly result from the action of adsorbed water on the glass surfaces and partly from the deposition of other atmospheric components. In the early stages of exposure, alteration consists mainly of the crystallization of salts on the glass surface, referred to as neocrystallisations [5; 7; 21]. These neocrystallisations result from the reaction between the anions and the cations present in the water or in the atmosphere in the contact with the glass [5]. In a previous paper we have thoroughly investigated by means of numerous analytical techniques [28] a group of Roman and Byzantine glass shards collected during an excavation campaign at Ain Wassel in the surroundings of the ancient city of Thugga in northern Tunisia [29; 30], a territory belonged to the Roman province of _Africa Proconsularis_ and particularly flourishing during the Roman and Byzantine periods. The study has been carried out with the aim to chemically characterize the objects and to study the production technology including the chromophores responsible for the different color shades. The main constituents used for ancient glass fabrication were the raw material, the flux and the coloring agents. As far as Thugga's glasses, for the raw materials a local origin of the sand has been hypothesized due to the homogeneity of the samples from the compositional point of view and in accordance with the presence of sand deposits of important rivers of that area. Concerning the flux, in most of the cases a mixture of natron and plant ash was used [28]. With regard to the glass chemical composition, it was found [28] that the chemical elements responsible for the different glass shades were generally Fe for the green shades, copper and cobalt for the blue ones and manganese (used as discoloring agent) for the colorless samples. Roman and Byzantine glasses generally show a good standard of technology, particularly in terms of color control. Transition metal ions, such as iron (Fe\({}^{3+}\)/Fe\({}^{2+}\)), cobalt (Co\({}^{2+}\)) and copper (Cu\({}^{2+}\)/Cu\({}^{+}\)) acted as coloring agents. In the present work we have investigated the degradation and corrosion products present on the surface of some of these shards.