THE CONSERVATION OF WET MEDIEVAL WINDOW GLASS: A TEST USING AN ETHANOL AND ACETONE MIXED SOLVENT SYSTEM
D. R. GRIFFITHS, & A. M. FEUERBACH
3 THE BEDFORD GLASS ASSEMBLAGE
The window glass for which the following treatment was first conceived was excavated in Bedford, England, in the vicinity of a medieval ecclesiastical building. The glass appeared on art-historical grounds to date from a variety of periods. The glass fragments had been excavated from damp soil under what had become a sand and salt dump for the Bedfordshire Council road maintenance section.
Many of the features of glass deterioration described above were seen in the Bedford glass assemblage. Knowledge of some of the causes and effects of the deterioration process has influenced the choice of conservation treatment. The condition of the glass varied from well preserved to highly deteriorated. These two states are well described by Alten (1988) in her experiments on waterlogged medieval window glass from York, although the milky areas she describes were not noted in the Bedford glass. The highly deteriorated glass was black and opaque. Some pieces had surface pits (presumed to originate from the time the glass spent in a window prior to burial), which often contained white powdery deposits. A couple of pieces of the black Bedford glass were allowed to dry in air to determine the result of this approach. After air-drying, the black deteriorated glass was very fragile. On handling, the black corroded glass tended to crumble into sugary fragments, exposing a thin central core of translucent glass with hemispherical depressions on its surface. Some of these hemispherical depressions and most of the corresponding parts of the black corroded glass that encased it exhibited pronounced iridescence. The better-preserved glass retained a good proportion of its transparency, and this was only slightly reduced by a superficial cloudiness on some pieces after air-drying.
Many of the pieces had retained their paint, while others had only indirect evidence of painting in the form of differently deteriorated areas resembling painted designs in their distribution. These patterns most probably reveal the decoration that was once painted onto the surface. It may be envisaged that under different circumstances the painted area may be more or less resistant to decay than the surrounding unpainted glass. Factors influencing the effect of painted decoration on deterioration of the glass include the composition of the glass, the composition of the glass paint, the degree of vitrification of the paint, the degree of fusion of the paint to the substrate, and the environmental history of the sample. For example, a paint that is poorly fused to the surface of the glass may trap a small volume of water in contact with the glass, which may quickly become alkaline by ion exchange and then attack the glass network.
It may be of interest to note that where paint has vanished and no ordinarily visible trace of its position remains, it may still be possible to discover the position it formerly occupied by a number of other approaches, none of which were employed with the Bedford glass assemblage but some of which are currently being investigated. For example, areas that were formerly painted would often display subtle differences in surface chemistry compared to surrounding glass that had never been painted. These differences might be revealed by causing a liquid to condense from the vapor state onto the surface of the glass, the tendency to condensation being influenced by surface state and the painted design being transitorily revealed during the condensation process.
3.2 TREATMENT IMMEDIATELY AFTER EXCAVATION
The glass had been excavated a few years before the involvement (ca. 1994) of the present authors. According to a report, the pieces had been repeatedly immersed in baths of tap water after being excavated to remove soluble salts. The fragments had then been placed between wet foam and plastic sheets in resealable plastic boxes. By the time long term conservation was undertaken about two years later by one of the present authors (Feuerbach), two of the boxes contained visible greenish organic growths. More than 300 fragments of painted medieval window glass needed to be removed from their short-term storage containers, documented, and conserved for longterm storage. The work had to be completed within a six-week period. This task was easily achieved, and the conservation process required only occasional attention.
An intermediate stage of temporary wet storage between excavation and conservation obviously carries the risk of mold and algal growth as well as the risk that, unless conscious action is taken to avoid it, damagingly alkaline solutions may form in cracks within the glass structure or in small uncirculating volumes of solution held in scratches or in thin films between the glass and some other surface. Antifungal and antialgal agents might help the situation but may have unknown effects on the glass and its deterioration. The use of pH buffering substances in the holding solution might mitigate the damage due to alkaline pH but may have unknown complexing effects that might detrimentally influence the state of the bulk-leached glass interface, the leached glass layer, or the glass-water interface.
In many cases it may be possible to apply long-term conservation procedures directly from the time of excavation and so avoid the complications of how exactly to arrange adequate interim wet storage. It would of course be necessary to ensure that any long-term conservation on-site was undertaken in an informed manner and with proper care.
When long-term conservation is to be postponed, excavators and site conservators may wish to consider interim storage in ethanol rather than in water. Ethanol would avoid the risks of algal and microbial growth, would arrest aqueous attack, and would probably be a reversible process. The possible fire hazard associated with the transport and storage of baths of ethanol would, of course, have to be considered.