1 INTRODUCTION

As with all aspects of restoration, there are a number of important factors to be addressed when considering gap-filling of incomplete glass objects. First, the condition of the glass itself has to be assessed, since restoration is essentially a molding and casting operation that will entail the application of wax or silicone rubber directly onto the glass. It therefore stands to reason that the glass and any applied decoration must be in a robust condition; loose flakes and low-fired or unfired painting and decoration cannot be subjected to the molding and casting processes. Second, an assessment needs to be made as to whether restoration is possible, given the shape, amount, and thickness of glass remaining. Third, it is important to decide whether restoration is desirable on ethical grounds, particularly if no full profile exists. Restoration may be undertaken to improve the stability of an object such as a glass vessel or simply to improve its appearance for display purposes. If a great deal of the vessel is missing, it may not be worth restoring in terms of the time taken to carry out the work, unless the glass is historically important or is of particular value to a collection (bearing in mind, however, that glass that is of less importance to one collection may be valuable to another). Last, consideration has to be given to the availability, properties, cost, and appearance of synthetic materials with which to carry out the restoration (Davison 1981, 1984; Jackson 1984; Fisher 1988, 1992; Eckmann 1995). (See the discussion in section 3, Materials, below.)

Before the adoption of synthetic materials by the conservation profession some 30 years ago, gap-fills in glass were carried out using natural materials such as beeswax and with plaster of paris, which was often painted silver in an attempt to improve its appearance. It was also not unknown for pieces of glass from similar vessels to be cut and married to produce an apparently complete object. This type of restoration was especially suited to archaeological glass, where joins between the fragments could be disguised with flakes of iridescence and with mud.

Currently, gap-fills in glass objects are effected by making molds formed from dental wax sheets or in silicone rubber, whether on existing areas of the object or from a modeled clay or Plasticine form representing the missing area. The molds are secured over the areas to be replaced, and a clear resin cast into them. Once the resin has cured, the molds are removed and the casts trimmed with a sharp scalpel and, if necessary, smoothed with fine abrasives and polished or given a coat of clear lacquer.

If a simple cast turns out to be distorted or there are significant differences in the levels of the glass and resin (i.e., steps), it is normally less time-consuming to remove the cast and start again. On more complicated casts excess resin can be removed by the careful use of a dental drill, and the resin cast can, if necessary, be filed and abraded to some extent, though care must be taken not to crack the cast by applying too great a pressure. After using progressively finer grades of abrasive paper, the surface may be polished using, for example, Solvol Autosol (kieselguhr or “infusorial earth,” a methylated soap, white spirit, and ammonia) on a tiny buffing wheel held in the drill head. However, care must be taken in the use of abrasive pastes, since they may become irremovably trapped in small deficiencies in the cast, such as air bubbles, or at the junction between the cast and original glass. During any operation involving the use of a dental drill, it is wise to step down the current with a rheostat to prevent friction from melting the surface of the resin and the restored object from being accidentally spun out of the conservator's hands. Final finishing can be undertaken using successively finer grades of abrasive papers such as Micro-Mesh, and polish applied with cotton wool swabs. It is rarely possible to obtain the original clarity of the cast after working on it. If all else fails, the cast can be given a coat of clear lacquer—although, of course, there are then two materials that may discolor (resin and lacquer). Some restorers do, in fact, always cast colorless resin in colored glasses and apply color in a sprayed lacquer.

It is part of a conservator's task to adapt restoration techniques as required since each restoration is an individual undertaking; indeed, the processes described in this article are the result of such adaptation. However, restoration processes may be conveniently classified as follows:

• gap-filling with casts from molds taken from the glass itself
• replacement with casts from a mold taken from a modeled section or a previous restoration
• gap-filling where the interior of the vessel is inaccessible for working
• gap-filling with preformed casts
• partial gap-filling.