A NEW TECHNIQUE FOR THE CASTING OF MISSING AREAS IN GLASS RESTORATION
Erik Rissere.risser@ucl.ac.uk
MSc Conservation for Archaeology and Museums
Department of Conservation, Institute of Archaeology, University College London, 31-34 Gordon Square, London WC1H 0PY, United Kingdom
Abstract
This article traces the development of a new method of glass restoration for fragmentary and incomplete vessels. The technique involves making a blank of the missing area and then moulding and casting away from the object as an alternative to conventional gap-filling in situ.
Introduction
Traditional glass restoration techniques involve the casting of missing areas in situ (Newton & Davison 1996; Penelope & Norman 1987; Black & Wilson 1983; Larsen 1979; Davison 1978; Fiorention & Borelli 1975; Errett 1972; Staude 1972; Wihr 1968; Wihr 1963). Such techniques often involve moulding from a similar part of the glass object and then placing this over the missing area and casting; moulding from a modeled area and then placing this over the missing area and casting; or moulding from existing gap-fills, removing them, and then placing the mould over the missing areas. Although highly effective, such techniques also involve the cutting back of excess resin and the finishing of the surfaces of the casts, increasing the potential for damaging the glass and possibly creating stresses between the newly casted area and the glass itself. The following is a brief account of conservation measures undertaken to restore a large missing area of a second century A.D. Romano-British glass amphora at the Institute of Archaeology in London. It is hoped that the techniques described may provide a useful example of an alternative casting technique in which all work is done away from the glass object, consequently reducing the potential for any damage.
Conservation and Restoration
The vessel to be restored is a green/blue, mould-blown glass Roman amphora which dates from the first quarter of the second century A.D. On receipt, the object was in 132 pieces, varying in dimensions from 6 cm2 to slivers of less than 0.2 cm2 (Figure 1).
Aside from the numerous fragments, the glass itself was in relatively good condition. There was some surface weathering but no actual flaking. Based on research and photos of similar type vessels, the fragments were sorted into their believed categories: base, body, shoulder, neck, rim, and handle. Eventually, the vessel was found to preserve the entire rim, neck, handle, and one flat wall. Two walls were only a quarter and a half present. The base was a little under half preserved. Together, the fragments were found to comprise about sixty percent of the vessel which stood roughly 355 mm high and was 190 mm in diameter (Figure 2). Figure 1: Object prior to conservation. (72K)
In the case of such an incomplete glass artefact, full restoration involved the consideration of whether such a procedure was desirable on ethical grounds, as well as its overall feasibility. After considering the possibility of improving the stability of the object, as well as improving its aesthetic appearance for display, reconstruction was decided upon. Consultation with owners determined the amount of reconstruction desired. The amount of time and resources available, as well as the bottle's size, shape, and thickness determined the feasibility of reconstruction. In the end, it was decided that a full reconstruction was both possible and appropriate. Figure 2: Views of object after conservation. (29K)
Since enough of the vessel was preserved so that an accurate profile could be determined, the first step was to weld several sheets of medium thickness dental wax together. This was then heated with a hot-air blower and impressed into the interior of the vessel, moulding the one existing flat wall and the curvatures of the shoulder and neck, the curves to the other two flat walls, and the curve to the base. The wax sheet was allowed to harden and removed from the inside of the vessel. All the exposed glass break edges were then covered in aluminium foil to seal them from any potential contamination. The wax impression was inverted and positioned onto the inside of the object (Figure 3). The sheet was fixed to the aluminium foil with a hot spatula inserted through the neck.
A blank of the missing portions of the base, walls, and shoulder was then made with Basic Alpha Plaster (Figure 4). Once the plaster reconstruction was set (about 4 hours) the plaster model was removed. This was wet sanded to remove any inconsistencies and to mimic the relatively smooth surface of the original glass. In order to achieve a proper mould, it was necessary to produce a two-piece silicone rubber mould which conformed to the interior and exterior profiles of the square bottle. The position of the funnels used to introduce resin into the area occupied by the plaster was decided upon. To form them plastic drinking straws were attached to the plaster with small lumps of Aloplast modeling compound. Figure 3: Dental wax impression of interior contours. (66K)
Figure 4: Plaster blank. (79K)
Both parts of the mould were formed with Dow Corning Silastic E RTV silicone rubber. The outer silicone rubber mould was made in one section. After a wall of Aloplast was constructed to completely surround the plaster and to prevent any uncured silicone rubber from escaping (Figure 5), a thin layer of silicone rubber was brushed over the plaster surface. When the silicone rubber had cured, silicone rubber thickened with Santocel silicone matting agent was poured on until the area delineated by the Aloplast had been was completely filled. The whole thing was then inverted and polyvinyl alcohol was painted along the exposed silicone rubber to act as a separating agent between it and the second part of the mould which was made just like the other side, by adding layers of gradually thickened silicone rubber to the surface of the plaster. In this way, a two-piece mould conforming to the interior and exterior profiles of the fill was made (Figure 6).
Afterwards, a batch of Araldite 20/20 epoxy casting resin was colored with translucent pigment to match the glass. The bulk of the resin was catalyzed and introduced into the mould from a syringe through the plastic straws incorporated in the outer mould. A small amount of resin was left uncatalyzed in order to encourage it to rise and escape through other straws acting as air holes by gently tapping the mould. The resin was then allowed to cure fully for nine days before being removed. Figure 5: Plaster blank during moulding. (44K)
Figure 6: Two-part silicone rubber mould. (35K)
Excess resin on the outer surface of the epoxy cast in the form of seam-lines and stumps which had formed in the pour and air holes was removed with a small grinding wheel attached to a flexible dental drill. Tiny holes caused by trapped air-bubbles were filled using colored resin applied with a syringe. Finally, the surface was polished to a glossy finish using a felt polishing buff attached to a dental drill and Solvol Autosol (Figure 7). The cast was then attached to the glass vessel with Araldite 20/20 (Figure 8) placed at each corner to allow for easy removal if needed.
Figure 7: Finished cast. (26K)
Figure 8: Object after conservation. (44K)
Conclusion
The method described above illustrates a technique for making glass casts with little risk to the glass itself. With the exception of the wax and plaster stages, all work was done away from the glass. At no point were any modeling, moulding, or casting materials in contact with the glass surface, and at no point was any sanding, cutting back, or surface finishing done in contact with the glass. Consequently, it is hoped that this serves as a helpful and more ethical alternative to the traditional techniques of glass restoration.
Materials and SuppliersBasic Alpha Plaster - Alec Tiranti Ltd. 27 Warren Street, London W1P 5DG, U.K.
Aloplast® - Alec Tiranti Ltd. 27 Warren Street, London W1P 5DG, U.K.
Dow Corning Silastic E RTV Silicone Rubber® - Alec Tiranti Ltd. 27 Warren Street, London W1P 5DG, U.K.
Araldite 20/20® -B&K Resins Ltd. Ashgrove Estate, Ashgrove Road, Bromley, Kent BR1 4TH, U.K.
AcknowledgementsI would like to thank Olympia Theofanopoulou for her advice and guidance.
References and Bibliography
Black, J. and Wilson, A. (eds.) 1983. Making Replicas of Museum Objects: Moulding, Casting, and Finishing. London: Institute of Archaeology.Davison, S. 1978. "The problems of restoring glass vessels." The Conservator 2, 3-8.
Larsen, B. 1979. Moulding and Casting of Museum Objects. London: Institute of Archaeology.
Newton, R. and Davison, S. (eds.) 1996. Conservation of Glass. Cambridge: Butterworth-Heinemann.
The methods, techniques, and conclusions found in individual papers are the work and responsibility of the author of the paper, and should in no way be thought to represent the opinion or endorsement of either the Journal of Conservation & Museum Studies, the Institute of Archaeology, or University College London. No liability or contract is accepted or implied by the publication of these data.
Copyright © Erik Risser, 1997. All rights reserved.
http://www.ucl.ac.uk/archaeology/conservation/jcms/
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