Glass is a relatively durable material in the

Glass is a relatively durable material in the buried environment (Jackson et al., 2012) and the morphology of solid glass objects and fragments often remain intact. However, surface corrosion of glass occurs in moist and wet soils leading to a loss of transparency and the formation of a surface crust rich in silica and depleted of basic ions. This process weakens the glass and this may accelerate shattering of thinner objects (Huisman et al., 2008). The rate of surface degradation in soil is strongly affected by the glass composition and not easily predicted (Van Giffen, 2014). The alkali type and content is critical: Roman and other ancient glass is generally more resistant to chemical attack than glass from the mediaeval Cyclopamine when wood ash containing potassium (K) replaced soda ash in its manufacture. Under acidic conditions and moderately alkaline conditions (pH < 9) alkali ions are leached from the glass matrix, while under more alkaline conditions hydroxyl ions disrupt silicon–oxygen bonds within the silica structure (Melcher et al., 2010). At more alkaline pH, laminar surface layers are more likely to form (Roemich et al., 2003) which may be iridescent. In all but the driest soils, surface coatings and other decoration on glass are expected to degrade quite quickly (< 100 y). The strong dependence of corrosion rates of glass objects on material composition and manufacture leads to uncertainty in any prediction of the relative rates of surface degradation in different soils: corrosion is expected to be least in very dry soils; rates of corrosion may be moderated in well-drained and neutral soils in drier regions; highly alkaline soils are anticipated to be the most corrosive.