THE CONSERVATION OF WET MEDIEVAL WINDOW GLASS: A TEST USING AN ETHANOL AND ACETONE MIXED SOLVENT SYSTEM
D. R. GRIFFITHS, & A. M. FEUERBACH
4 THE CONSERVATION OF THE BEDFORD GLASS ASSEMBLAGE
4.1 CONSERVATION BACK-GROUND
At the time of conserving the Bedford glass, the approach of dewatering with a hydrophylic solvent followed by the application of a consolidant system was already established (Newton and Davison 1989). The use of Paraloid B-72 dissolved in toluene as a consolidant system for excavated glass had been suggested by Newton and Davison (1989), and this method is still in use today (Johnson 2001). The novel aspect of the method reported here was the use of the mixed ethanol-acetone solvent system for the Paraloid B-72 (Davison 1994).
4.2 INITIAL WASHING
As the first stage of the long-term conservation procedure reported in this article, the glass fragments were removed from between the foam and plastic of the temporary storage. Although the glass was all damp, it was not all wet in the sense of being surrounded by excess water. The fragments were therefore immersed in tap water to soften any attached soil deposits and facilitate removal of the visible areas of greenish, slimy organic growth. Washing prior to long-term storage was also important to dissolve any soluble salts that may have remained on the surface of the glass or in cracks or that may have accumulated due to ion exchange following the original desalination. (In general terms it may be noted that some form of equilibrium state may have been approached over the long term during burial but that the drastic change in immediate environment occasioned by excavation may result in rapid reactions under the new environment.) The preconservation washing was also employed to dilute and remove any localized volumes of highly alkaline solution that may have formed in the small volumes of water surrounding most of the individual pieces during temporary storage.
The fragments were weighed, drawn, measured for thickness, and examined under a binocular microscope for condition and traces of paint or staining. The pieces were kept wet during the recording process by repeated immersion in the tap water. The fragments were then placed individually on a piece of finely woven nylon netting. The netting allowed the pieces to be suspended in the solution so that all surfaces were in contact with the liquid. The netting also supported the fragments and facilitated moving them from one solution to another.
4.3 CHOICE OF MATERIALS
The overall rationale behind the treatment proposed in this article was to wash the glass in water to make it clean and then: (1) to replace the water in the friable parts of the glass samples with another liquid (one that would be miscible with a solution of a desirable consolidating polymer) without allowing the glass to become dry; (2) to introduce the consolidant in solution; and (3) to cause the consolidant to be precipitated within the friable glass structure as its solvent evaporated.
The consolidant chosen for this application was Paraloid B-72, reportedly a 70:30 molar poly(ethyl methacrylate/methyl acrylate) copolymer (Horie 1987, 106). It was chosen for its good yellowing resistance and particularly for the fact that it has a sufficiently high glass transition temperature (roughly speaking, a high enough softening temperature) to avoid the surface's being in any way sticky or likely to entrap extraneous particulate matter (dirt and dust) by the particles sinking into the surface. Many alternative consolidants, including some that can be prepared as water-based emulsions, have low glass transition temperatures that would lead to undesirable accretion of dirt and dust over the course of time.
As the chosen consolidant is not water soluble, it was necessary to select a dewatering liquid that would on the one hand be miscible with the water in the wet glass and on the other hand be miscible with a solvent suitable for dissolving and introducing the consolidant into the glass. The liquid chosen for the dewatering was ethanol.1
It is important for the water to be removed from the glass without allowing the pieces to become dry. The extent to which the ethanol will replace the water in the hydrated layer and so postpone shrinkage and new cracking is uncertain. Observations by Alten (1988) on highly deteriorated glass, which from her description was probably very similar to the highly deteriorated Bedford glass, suggest that no damage is done by immersing the glass in ethanol and that new cracking occurs only on air-drying, be that from a water bath or from an ethanol bath. A most important aspect in preserving the appearance of the wet glass is to prevent any access of air to any cracks, whether ancient or modern. In the approach proposed in this article, the water needs to be replaced with another liquid that will allow a dissolved polymer to be brought into the cracks and be precipitated there to maintain structural integrity and visual appearance. Simply drying the glass in air would cause air to enter cracks and voids in the glass, and this air would impede subsequent introduction of the consolidant. Air in the cracks would also make the cracks far more visible and might give the glass a frosted, cloudy, or iridescent appearance dissimilar to the original appearance of the glass. (The refractive index difference across a glass-air interface is far greater than that across a glass-liquid or glass-consolidant interface and is thus far more likely to result in reflections that make the cracks visible.) Acetone would have been an alternative to ethanol as a dewatering agent but might have evaporated too quickly in moving between solutions and so allowed air into cracks. Most workers would consider it less pleasant to work with acetone than with ethanol. (Acetone has a degreasing effect on the human skin and can cause headaches.)
The choice of solvent for the consolidant was a third consideration. Ethanol does dissolve Paraloid B-72 slightly but not sufficiently for it to be effective in a single introduction of consolidant. Acetone will dissolve far more of the chosen consolidant (well over 4 wt%) but tends to cause the consolidant to migrate to the surface of the piece being consolidated as the acetone evaporates. It can also sometimes leave a bloom on the surface. A mixture of these two solvents was tested on a small sample and then, after apparently satisfactory results, on all the glass.
The consolidant solution containing approximately 2 wt% of Paraloid B-72 in 50% acetone and 50% ethanol was produced. A 4 wt% solution of B-72 in acetone was first produced, and this mixture was diluted with an equal volume of ethanol to give the (approximately) 2 wt% solution. Paraloid B-72 will dissolve in ethanol to some extent but not at anywhere near the 2 wt% level. It can readily be observed in a beaker that any concentration of ethanol significantly higher than 50 vol% causes the polymer to start to precipitate out of solution. The 2 wt% solution was thus seen to be very close to saturation, a feature particularly desired for the reasons given below. It was this near saturation loading of consolidant that determined the particular consolidant concentration and the solvent ratio, but other ratios could be tested in future work.
Acetone was deliberately chosen as the primary solvent in the mixture because it would evaporate faster than the ethanol (see discussion in section 6 below). The ethanol will slow the evaporation of the acetone, inhibiting the tendency to form a surface bloom. The ethanol should also reduce the tendency for the consolidant to be carried to the surface during drying.
4.4 DEWATERING AND CONSOLIDATION
To effect the dewatering after the initial washing, examination, and recording of the glass, the fragments and netting were in the first instance immersed in a mixture of 50 vol% water and 50 vol% ethanol held in a glass container. This stage was designed to begin the removal of the water by displacing it with ethanol. A glass cover was placed over the container to retard evaporation. As a fume cupboard happened to be available, the container was kept in it in order to remove any traces of ethanol vapor from the workplace more effectively. The use of a fume cupboard is not necessary if the lid fits well or the container is kept in a well-ventilated place. After three days the pieces of glass and their supporting nylon netting were removed and placed into 100% ethanol, where they were left for a further three days. Occasional gentle agitation would minimize the possible development of concentration gradients. In addition to displacing the water, the ethanol would also kill any organic growth that may have remained on or within the fragments.
After three days in the 100% ethanol, the fragments were removed and placed into the consolidant solution described above. After three days in the consolidant solution, the fragments were removed, one by one, with a pair of plastic tongs. The fragments were immersed in a bath of 100% acetone for a few seconds to remove any excess Paraloid B-72 solution from the surface. The pieces were then allowed to dry in the fume cupboard, but a well-ventilated, relatively dust-free area would probably suffice. (The use of a solvent bag in order to slow evaporation did not appear necessary, further contributing to the ease of application of this conservation procedure.) After drying for a few days, the fragments were packed in individual resealable polyethylene bags, which were placed in storage boxes.
The fragments were kept in the dewatering and consolidant solutions for three days to ensure the penetration into cracks deep within the very deteriorated glass fragments, but this time could be shorter for less-deteriorated pieces. In practice, the dwell times of batches in the solutions allowed plenty of time for the next group to be documented.