Archaeological glass alteration products in marine and land-based environments: morphological, chemical and microtextural characterization

A study was performed on the natural degradation of glass from Italian archaeological sites, both marine and land-based, with the aim of understanding the role that the chemico-physical conditions of the environment had on the morphology and composition of the alteration products. Marine samples shows two different textural morphologies of alteration: multiple layers of iridescent lamellae in colored glass, or an opaque white crust on the surface of colorless glass, characterized by a spongy, distorted, ramified structure extending more and more deeply into the bulk of the glass. Buried glass presents only iridescent lamellae as alteration products. These are mainly composed of a hydrated 'silica gel', differing essentially in the extension of alteration lamellae in both submerged and buried glass. The principal events in the process may be summarized as ion exchange reaction, which causes selective leaching of alkalis and protons entering the glass to produce a hydrated, alkali-deficient layer, and congruent dissolution of the glass matrix, with destruction of the silica network and subsequent precipitation of hydrous silica-gel layers as secondary alteration products.