The Chemistry, Corrosion and Provenance of Archaeological Glass

Abstract

Archaeological glass is a non-crystalline solid, predominantly based on a polymerized network of silica tetrahedra modified by alkali and alkaline earth ions. This chapter reviews the Continuous Network Theory for the structure of such materials, and shows how the properties of glass such as working temperatures and chemical durability are explained by this model. In particular, a brief explanation is given of how the colour of glass is controlled not only by the presence of transition metal ions, but also by the interaction between these ions and their position within the network via crystal field theory. Two examples are given of the corrosion of glass, one of atmospheric corrosion, illustrated by some work on the medieval glass in York Minster, and the other considering the more complex situation of the decay of buried archaeological glass. Chemical analysis has resulted in the identification of characteristic chemical patterns over large areas and long time periods, associated with common traditions of use of specific sources of alkali, such as plant ash, evaporite deposits (natron), or ‘forest glass’. As with other common archaeological materials, provenance determination by trace element analysis has been a major objective for many years. For glass this has been largely unsuccessful, but newer isotopic techniques including strontium, neodymium and boron have given new impetus to these enquiries.