6 DISCUSSION

The use of the mixed solvent for consolidating the dewatered glass is a novel (Davison 1994) and apparently successful approach to conserving waterlogged medieval window glass, but the exact mechanism of its action has yet to be determined. Several ideas as to why the mixed solvent produced good results may be suggested, although they are still a matter of speculation until more research is undertaken.

One speculation is this: As the consolidant solution starts to diffuse into the ethanol-saturated glass cracks, it will soon reach an ethanol concentration at which the consolidant starts to precipitate. As more consolidant solution diffuses into the cracks, the consolidant may be partially or wholly redissolved with the “precipitation front” moving deeper into the glass pore structure. Depending on time of immersion in the consolidant solution, some precipitated consolidant may already be present within the glass fragments when they are removed from the consolidant solution and the excess consolidant solution is rinsed off the surface.

Solvent evaporation may be expected to begin with the acetone component starting to evaporate preferentially at a solution front just beneath the surface. As soon as the acetone concentration in the solution drops, the consolidant will start to be precipitated. Although evaporation of the acetone will occur from the surface of the solution, diffusion of the acetone from the consolidant solution within the glass cracks and pores should cause the consolidant to be precipitated wherever the solution has penetrated. Acetone will diffuse from within the glass through the solution and the precipitated polymer, but its concentration may not be high enough to carry the consolidant with it to the surface. The decreasing consolidant solubility in the increasingly ethanolrich solvent that will be created during drying should reduce the tendency for the consolidant to be carried to the surface of the object, as might occur if a single solvent such as acetone or toluene had been used. At present this mechanism and its desired effect are unverified hypotheses, but further research in this area will be undertaken in the near future.

The end result of consolidation with the mixed solvent system may thus be that both impregnation and evaporation stages favor the deposition of consolidant at some depth within the crack and pore structure of the glass. The end results of the procedure seem sufficiently promising to be worth presenting for others to consider and investigate further.

6.1 CAUTIONARY NOTE

Despite the good showing in this initial trial, it is clearly necessary to exercise caution before applying a procedure that has been tested on only one occasion on one assemblage of glass. Particular caution might be necessary in the case of crizzling glasses that have thick hydrated layers on the surface. These layers lose water, shrink, and crack when exposed to low relative humidity. Simply observing that cracks appear on the surface of a glass as it dries does not of itself indicate whether these are pre-existing cracks revealed by the loss of the liquid that previously filled them, cracks created by shrinkage of a hydrated layer or a combination of both. It is not clear to what extent water in a fairly thick initially hydrated layer might be replaced by ethanol or acetone, but such a replacement is at least plausible. It is, however, less likely that a hydrated layer could be penetrated by a large polymer in solution. It might therefore be expected that a hydrated layer would shrink or crack as the water, alcohol, and/or acetone evaporated from it. Nevertheless, the presence of a polymer precipitating around the hydrated layer as it shrank might inhibit the appearance and growth of cracks by filling them as they develop. For the present, this sequence of events is pure speculation, and this method should be carefully assessed before risking any application to crizzling glasses. The possibility of crizzling appearing slowly over the course of time from stresses in the formerly hydrated layer should also be considered.