1 INTRODUCTION

The conservation procedure proposed in this article was developed in response to the need to conserve an assemblage of excavated, wet, painted medieval window glass fragments. They appeared on art-historical grounds to date from a variety of periods. The treatment had to remove the water in which the fragments had been stored since excavation, consolidate the deteriorated pieces, and leave them in a fit state for long-term storage. While achieving these aims, the treatment also had to be inexpensive, require no specialized equipment, be relatively quick to perform, and involve a minimum of health and safety risks. The proposed treatment removes the water by replacing it with ethanol and consolidates the fragments with Paraloid B-72 (an acrylic polymer) dissolved in a mixed ethanol and acetone solvent system. Examination of the stored samples five years after this treatment indicates that the method appears successful. It may thus be worthy of further consideration and testing by others who need to conserve glass in a similar condition.

To explain some of the rationale behind the treatment proposed, the article includes a brief description of the mechanisms by which archaeological glass is attacked, the compositional and environmental factors influencing the rate of deterioration, and some of the features that these deterioration processes create. Although some of this information may be found elsewhere (for example, in Weyl and Marboe 1962–67; Newton and Davison 1989; Pollard and Heron 1996), other aspects of the model are more speculative and are the subject of current experimental work.

Fig. 1.

This reaction depicts the formation of glass from silica and an alkali metal oxide

Fig. 2.

Alkali metal ions are lost from the surface layer of the glass and replaced by hydrogen ions accompanied by water. Sodium ion migrates into the solution, which becomes progressively more alkaline, in effect becoming a solution of sodium hydroxide.