JAIC 1998, Volume 37, Number 1, Article 4 (pp. 35 to 47)
JAIC online
Journal of the American Institute for Conservation
JAIC 1998, Volume 37, Number 1, Article 4 (pp. 35 to 47)

REVERSIBLE FILLS FOR TRANSPARENT AND TRANSLUCENT MATERIALS

SANDRA DAVISON



2 METHODS


2.1 GAP-FILLING WITH CASTS FROM MOLDS TAKEN FROM THE GLASS ITSELF

Where tiny fragments of glass are missing from a vessel—for example, part of a rim or edge of a foot—their shape may be copied by taking a mold from a similar area of glass and then repositioning the mold behind the area to be filled. The molding material may be dental wax in the form of sheets or silicone rubber, which takes fine detail. Resin may then be applied to the mold in thin layers, each one being allowed to set in turn. When cured, the resin can be abraded and polished as necessary.


2.2 REPLACEMENT WITH CASTS FROM A MOLD TAKEN FROM A MODELED SECTION OR A PREVIOUS RESTORATION

In the case of glass vessels having extensive, or more complex, missing areas, such as a raised design to be copied, a form is constructed of modeling clay or Plasticine onto which the existing fragments are placed. The design may be modeled, and the whole is then molded in silicone rubber and the modeling material removed so that resin may be cast into the areas of missing glass. If a previous restoration needs replacement for aesthetic reasons, but is dimensionally sound, a silicone rubber mold may be taken from it before removal (as in the case of the Auldjo jug—see section 2.3 below). This procedure will dispense with the need to model the missing area.


2.3 GAP-FILLING WHERE THE INTERIOR OF THE VESSEL IS INACCESSIBLE FOR WORKING

If the vessel to be restored has a narrow neck, so that the interior cannot be reached, it will be necessary to adopt special methods of restoration. If the vessel is damaged only in the body— that is, the neck has not become detached—the fragments can be taped in position and epoxy resin introduced to all the cracks except those around the perimeter of the damaged area. Once the resin has set, the damaged area can be removed as one piece, missing areas within it backed with tape, wax, or silicone rubber, depending upon their size, and filled with resin. When the resin has hardened and any necessary cleaning and polishing carried out, the repaired section may be taped in position on the vessel and the perimeter cracks sealed with adhesive. If the fragments were simply bonded and the gaps filled out of the body of the glass, any slight misalignment would prevent their eventual reinsertion.

If, however, the vessel is very damaged and has a large area of glass missing from the body, it may still be possible to reconstruct it with epoxy resin as described above, leaving a large area unbonded around the edges so that it can be removed. This area allows access to the interior of the vessel for ease of working. When gap-filling of the missing glass is complete, the glass can be replaced and the perimeter cracks sealed with adhesive.

A more complicated version of the silicone rubber molding technique had to be devised in order to restore the Auldjo jug, a Roman cameo glass on display in the British Museum. The jug has a narrow neck, and therefore access to the interior for molding purposes was severely restricted. The jug had been broken at some time in the past, and a considerable amount of the body was missing. It had previously been repaired, probably with animal glue, and gap-filled with plaster of paris colored dark blue to match the remaining glass. The plaster restoration had become damaged and unsightly, so it was decided to remove the plaster in order to effect a more accurate, lightweight, and aesthetically pleasing restoration. The previous plaster restoration was dimensionally correct, and it was possible to take a silicone rubber mold from it after some repair had been done. Two silicone rubber molds were produced, one conforming to the inner and one to the outer profiles of the jug. The outer mold was supported by constructing a two-part resin and fiberglass mold, which could be bolted together during the casting process. The inner mold was supported by filling the interior with vermiculite particles. The details are published in Newton and Davison (1989).


2.4 GAP-FILLING WITH PREFORMED CASTS

An alternate method of replacing missing areas of glass that has been suggested (Gedye 1968) is to cut the shapes from preformed acrylic sheets such as Perspex (U.S. Plexiglas), bend them to the required curve after softening with a hot air blower, and attach them to the glass with an adhesive. However, this lengthy process requires accuracy in cutting and filing, and the finished result is in no way as aesthetically pleasing as casting in the missing fragments with a clear resin; it is therefore not recommended for general use.

A method of gap-filling glass with preformed cast of epoxy resin was devised at the British Museum (Hogan 1993). A glass bottle required considerable conservation work to make it stable enough for exhibition. The green glass vessel, which is approximately 25 cm high, had an unusually thin base (1 mm approximately) in comparison to a thicker, and hence heavier, neck and rim (3 mm approximately). About a third of the original vessel is missing.

The vessel was reconstructed in two parts. The main body of the vessel and the neck and shoulders were reconstructed using a UV-curing acrylic (Loctite 350 engineering adhesive). The joins between the top and bottom of the vessel were not substantial, and some filling was necessary to give support. A method of support with minimum use of strategically placed resin infills was devised. Detachable resin fills were constructed away from the object and stuck into position once cured. This procedure had the advantage over casting the resin in situ of not creating undue stress on the small join areas during molding and casting of the resin. It also minimized contact with the surface of the glass during the finishing processes such as sanding and polishing.

Sheets of epoxy resin AY103/HY956 were cast to correspond to the varying thickness of the glass vessel in simple hexagonal molds made from sheets of dental wax with added support from wax sides (any suitable molding material could be used). When the cured resin was removed from the mold, a thin film of wax remained on the surface of the resin. This film was removed with a spatula, and the resin surface was then cleaned with white spirit.

A sheet of dental wax was placed across the gaps in the glass to be filled, and the outline of the gap was scribed onto the wax using a pin vise. The wax shapes were then cut out and placed in situ in the vessel to ensure a good fit, and they were secured with Sellotape. Once this process was completed, each wax shape was removed in turn, using acetone to release the Sellotape, and placed onto a resin sheet of appropriate thickness. Its outline was scribed onto the resin with a pin vise. The resin was then gently heated with a hot air blower to make it pliable enough for the shape to be cut out with scissors. The edges of the resin shards were filed where necessary with needle files to ensure close contact with the glass. Where appropriate the resin sherds were reheated and the curvature was modified to comply with the contour of the glass vessel. The finished pieces were placed in position and secured with HMG (cellulose nitrate) adhesive. The resin sherds were then painted with one coat of Rustin's clear plastic coating (urea formaldehyde) colored with Maimeri Restoration Colors to match the color of the glass.

As this method of gap-filling proved to be very successful, variations upon this technique using alternative resins were used to restore other glass vessels. A piece of Anglo-Saxon glass was gap-filled using the same methods but substituting clear HXTAL NYL-1 epoxy resin for the AY103 epoxy resin. The resin was heated more gently by immersing the pieces in warm water. The pieces were then bonded into position with HMG.

Another small delicate flask needed support for its neck in order to connect it to the body and base. An internal mold of the neck was made using a coil of dental wax. The wax was removed and sealed at one end, and superfine casting plaster was poured into it to produce a solid core of the internal shape of the neck. A sheet of Ciba-Geigy Araldite 2020 (epoxy resin) was made into a wax mold as previously described. Before the resin had completely cured, it was removed from the wax mold. While still flexible, it was formed around the plaster core using cling film as a release agent and left to cure fully. Once cured, the plaster core was removed by splitting the resin with a scalpel blade. The resin backing, now in two sections, was inserted into the neck of the flask, and adjustments were made to secure a good fit. The backing was then bonded with HMG, giving full support to the neck. To prevent discoloration of the epoxy, the HXTAL NYL-1 and the Araldite 2020 must be fully cured before being bonded in position with HMG.

The method proved to be extremely effective, not just in the final appearance of the glass vessels but also in the support it provided. Being able to work on the resin fills away from the glass surface, avoiding the need for individual molding and casting of sherds, makes this a quick and safe method of filling fragile glass. The fact that the resin pieces are bonded into position with cellulose nitrate makes future removal easy and safe for archaeological glass. A disadvantage may be in the heating of the resin, which could accelerate yellowing. Yellowing was not a problem in the case of AY103 epoxy as the glass was green and a perfect color match was not essential. In the cases of HXTAL NYL-1 and Araldite 2020, although no yellowing occurred in the instances described, excess heating of the resin should be avoided. A previous case of heating HXTAL NYL-1 epoxy with a hot air blower caused yellowing of the resin. In view of this effect, the use of warm water to soften the resin is preferred. This method could, in theory, be used as a complete gap-fill on a vessel, depending on the intricacy of the missing shapes.


2.5 PARTIAL GAP-FILLING

Where a glass vessel has large areas missing but does not warrant total reconstruction because, for example, it will remain in storage, it may be partially restored for safe handling, e.g., during study, photography, or drawing for publication. Strips of fine glass fiber tissue cut to size and impregnated with cellulose nitrate adhesive or epoxy or polyester resins are used to bridge gaps in the glass and to hold floating fragments in their correct positions. Total reconstruction of small vessels is also possible by this method. The result is not aesthetically pleasing but may be useful as a temporary measure.


Copyright 1998 American Institute for Conservation of Historic and Artistic Works