Degradation processes in colourless Roman glass: cases from the Bocholtz burial

Abstract

A group of Roman glass objects from the Bocholtz burial in the SW of Limburg (The Netherlands) was found to have been subject to varying degrees of degradation. Many of the 25 colourless glass objects were fragmented to pieces <0.1 cm (“sugared”), whereas the three transparent blue-green glass objects were in near-pristine state. Analyses using SEM, XRF and EDS revealed that the fragmentation was the direct result of the intense leaching of Na 2O from the glass structure and its replacement with water. The resulting gel layers with low-Na 2O contents and large amounts of water are sensitive to cracking when they dry out. Thin-walled glass appears to be less sensitive to cracking from the resulting stress than thick-walled glass. Local differences in the moisture regime during burial also influence the severity of the glass degradation. Glass with low concentrations of CaO seems to be the most sensitive to this form of degradation. The typical blue-green Roman glass is less sensitive as it generally has considerably higher concentrations of CaO. “Sugared” glass has – until now – only been reported in a small number of high-status burials. However, glass fragments that are degraded as strongly as some of the objects in the Bocholtz burial may have been overlooked in other archaeological settings (e.g. settlements), especially in coarse-grained soils or soils containing debris or rock fragments. As a result, the archaeological record for Roman glass may be biased towards the “typical” blue-green transparent glass objects.