REVERSIBLE FILLS FOR TRANSPARENT AND TRANSLUCENT MATERIALS
In general, the materials used for making molds and casts for filling gaps in transparent and translucent objects are the same in Europe and North America, i.e., Plasticine, modeling clay, dental wax, silicone rubber, epoxy, and polyester resins. However, the exact chemical composition of the materials will vary and it cannot be assumed, for instance, that epoxy resins or silicone rubbers available in one country will necessarily produce the same results as those available in another. Also, materials used in temperate climates may not be available for use in tropical climates. Thus it is appropriate to quote materials in terms of chemical rather than trade names in order that appropriate materials can be found in or near the country in which they are used. Discussion between the conservator and the manufacturer or supplier should be possible. One consideration is the age of the material, which may require that there be no prolonged storage or shipping times.
3.1 MODELING MATERIALS
Missing areas of glass may be modeled up in situ prior to molding using one of several commercially produced modeling materials. Those most commonly used are potters' clay and Plasticine. Plasticine is a putty composed of petroleum jelly, fatty acids, and whiting. These oily substances enable the Plasticine to be worked to a smooth surface. However, it does not adhere very well to glass and will contaminate the surface. Plasticine residues should be removed with cotton wool swabs moistened with a degreasing agent such as acetone before gap-filling with resin begins.
Newplast is a modeling material similar to Plasticine in texture but formulated especially for use against polyester resins. It is available in a buff or dark blue form. The dark blue variety has a tendency to discolor the resin cast and is therefore not recommended. Damp clay adheres very well to glass and is easily worked to a smooth surface with a spatula dipped in water. In addition, any fragments of glass that can be positioned accurately but do not actually join to the body of a vessel—i.e., floating fragments—may be held in position by placing them in situ on a clay former. The disadvantage of using clay is that moisture contained within it can separate cellulose nitrate adhesive (if used in the repair) from the glass after several hours. Backing joins with tape, rubber latex, or thin sheets of wax does not prevent this separation from occurring. In fact, adhesive on Sellotape and masking tape breaks down with moisture to form a messy substance, removal of the wax sometimes causes the joins to fail, and latex flows into tiny cracks and chips and is difficult to remove without dismantling the glass.
3.2 MOLDING MATERIALS
Materials used for making molds from glass artifacts should have the following properties. They should not harm the object physically by adhering too strongly to the surface, by pulling off glass projections, or by generating heat. They should not harm the glass chemically by contaminating or reacting with it. Molding materials should reproduce all the fine details or the original without distortion. The viscosity and thixotropic properties should be sufficiently variable by the manufacturer or conservator to allow the materials to be adapted to meet different requirements. They should preferably be available at reasonable cost and have an adequate shelf life. Molds must be able to withstand the heat of polymerization of the proposed casting material and must not react with that material.
For molding glass, the most suitable molding material is generally silicone rubber, which is available in several grades of thixotropicity. However, the cheaper materials—plaster of paris and dental wax—are frequently used with good results. The majority of silicone rubber products used for molding cure by catalytic elimination of alcohol to form cross-links between chains. When the alcohol evaporates, the rubber shrinks, but the amount of shrinkage is small (less than 1%) and occurs over a period of a few days. Shrinkage of 2.2% has been observed to occur over a number of years; however, this shrinkage is rarely a problem in the case of small objects restoration, where molds are unlikely to be reused after a period of time has elapsed. Silicones are insoluble in solvents but can be swelled considerably by the use of aliphatic, aromatic, and chlorinated hydrocarbons (Noll 1968). Wihr (1977) has suggested swelling silicone rubber back to size by exposing it to organic solvents, but this method seems to be unreliable.
In the majority of cases silicone rubber requires no release agent between it and the glass surface. However, instances of silicone rubber adhering to glass and porcelain have been known (Morgós et al. 1984), and therefore preliminary tests must be undertaken. A release agent such as petroleum jelly or an organic lacquer must be used if silicone rubber is to be cast against a cured section of silicone rubber or the two will adhere. Thin layers of silicone rubber may tear when peeled off an object, but thick layers are hard-wearing and the molds are reusable. If necessary, the rubber may be overcatalyzed to shorten its setting time, for instance when silicone rubber is being used to reattach a silicone rubber mold to glass. Overcatalyzing silicone rubber does result in its being extremely brittle, but brittleness is not a problem in this instance.
If silicone rubber is improperly mixed, it will not cure or will cure in patches. Uncured rubber is best removed with small paper tissues a little at a time to prevent it from being smeared over the object. The final residue may be removed with small cotton wool swabs soaked in acetone. If, however, the uncured rubber has become trapped within cracks or joins in the glass, the object may have to be dismantled to be cleaned thoroughly.
Inert fillers such as kaolin, talc, or aerosol silica may be added to thicken mobile grades of silicone rubber, which can then be applied over the first layer of rubber to produce a thicker, self-supporting mold that does not tear easily while being removed from the object. This procedure is particularly useful in cases where small molds are being made, since it obviates the necessity of buying two grades of silicone rubber (i.e., mobile and viscous grades). In the case of large molds the rubber mold may require the support of a mother mold made of plaster of paris or even resin and fiberglass matting.
Rubber latex shrinks too much at the time of cure and in the following weeks to be of use in molding such a precise material as glass. Even a small shrinkage will mean the details such as trailed threads on the glass will not match up with those on the cast. A mold must remain stable for several days or weeks while restoration is in progress.
However, rubber latex has been used for making molds on glass, and, as it tears easily, it is possible to find brittle remains trapped in cracks and chips. A repaired glass may have to be partially or wholly dismantled to release the traces of rubber latex.
Hot-melt preparations such as gelatine, Formalose, and Vinamold (PVC) should be avoided for direct use on glass since the heat may cause damage. However, Formalose, a gelatine material containing glycerine to keep it flexible (Wihr 1977), is useful for reproducing the interior shape of an object with a narrow neck where plaster or rubber cannot be introduced because of their viscosity and, on setting, their rigidity. The Formalose can be poured hot into a plaster mold and, on cooling, it sets and begins to shrink uniformly. The process must be watched, and when there is a gap between the Formalose and the plaster, representing the thickness of the glass vessel to be reproduced, resin can be cast into the gap. The Formalose core must be supported away from the plaster walls. The method is described by Petermann (1969).
Dental waxes are composed of a number of different waxes, contain an unidentified pink or red dye, and are often supplied as sheets measuring 180 × 82 × 1.5 mm. Dental waxes are available in various grades of hardness, in sheet sizes up to 305 × 203 mm and in thicknesses of 0.3 to 3 mm. For making molds the sheets or parts of them can be softened by gentle heating in warm air or water before shaping them over the glass object. Before casting, the wax mold should be coated with a proprietary solution of poly(vinyl alcohol) (PVAL) to act as a release agent between the wax and the resin to be cast against it. Without this separating layer, dye from the wax tends to be absorbed by the resin. The PVAL has a tendency to gather in pools, but this development can be prevented by adding a few drops of Synperonic N to the solution, which breaks the surface tension on the wax. The PVAL must be brushed continuously until it has almost set, then left to dry before the wax is attached to the glass. If the cast being made is large, it may be advisable to apply a second coat when the first has dried. The second coat must be brushed on with a light touch so as not to disturb the first.
Plaster of paris (calcium sulphate) is prepared by heating gypsum (CaSO4 · 2H2O) to drive off some of the combined water, forming 2CaSO4 · H2O. On adding more water, the calcium sulphate rehydrates, forming interlocking crystals, which set to a rocklike mass with very slight expansion, typically 0.5%. Various grades of plaster are available with different setting times, expansions, and particle fineness. Some dental plasters set to form very hard solids with very slight expansion. The material is available, cheap, and useful for the construction of case molds over silicone rubber. However, it requires release agents between plaster-to-plaster surfaces and plaster-to-resin ones. It is rigid and hence any undercuts on the glass must be molded first, preferably in silicone rubber. If incorrectly made, a plaster mold may prove very difficult to remove without causing damage to the glass inside.
3.3 RELEASE AGENTS
Release agents must prevent adhesion among objects, molds, and casts; the agents chosen will depend upon the materials being used. As previously mentioned, silicone rubber requires the use of a release agent such as petroleum jelly or organic lacquer only when it is being cast against a section of cured silicone rubber. The surface of plaster of paris mold pieces, however, must be sealed, as each is made to prevent it from adhering to the adjacent pieces.
Shellac in a solution of industrial methylated spirit (IMS) (U.S.: grain alcohol) or a solution of PVAL can be used to seal the surface of dry plaster of paris mold pieces, after which the surface is coated with soft soap, Vaseline, detergent, petroleum jelly, or a wax emulsion (see also Koob 1979).
It is worth noting that silicone release agents, which are available as liquids or in spray cans, can be used although they are not recommended. Traces of silicone oils will remain on the cast, and, if they are not removed completely, they can prevent paint or adhesives from bonding properly to the surface.
Polytetrafluoroethane (PTFE) dispersions in aerosol cans have also been used as release agents in the past. However, they are not recommended as the dispersions become embedded in the resin cast. PTFE dispersions are also thought to contribute to the destruction of the earth's ozone layer.
3.4 CASTING MATERIALS
Over the past few decades many epoxy and polyester resins, designed for different purposes, have been manufactured and distributed by hundreds of outlets. The majority of these cannot be used for glass restorations since they are opaque and/or colored, because they contain fillers, or because they cure only at high temperatures. The ideal resin for use on glass objects should be a water-clear, room-temperature curing product. It should be possible to pour the resin into molds; it should set with minimal shrinkage to form a hard solid; it should be crystal clear, not yellow with age, and remain colorless indefinitely. No materials meet this specification completely (Down 1984), although some come close to it. Before restoration work begins, the resin to be used should be tested by mixing and casting it into a mold of the same material to be used in the final reconstruction. This precaution will ensure that the resin's shelf life has not expired and its behavior is as expected, i.e., the formula of the product has not been altered by the manufacturer since it was last ordered. A further consideration is the possibility of future removal: epoxy and polyester resins cannot be dissolved but can be softened with dichloromethane and removed mechanically. Care must be taken that the swelling of the resin, which accompanies the softening, does not cause damage to the glass.
In chronological order the following epoxy resins have proved useful adhesives and fillers for small gap fills in glass: Araldite AY103/HY956; Ablebond 342-1 (marketed from 1979); HXTAL NYL-1 (made and marketed for conservation purposes from 1984); Epo-Tek 301 and 301-2 (U.S.); Fynebond (U.K.) and Ciba-Geigy XW396/XW397. The latter was so named during its experimental stage and is now marketed as Araldite 2020. The properties of this resin are well documented. Currently the author uses Araldite 2020, as it is readily available and has good working properties, whereas Fynebond is expensive and the hardener crystallizes at room temperature (as does Ablebond 342-1) and the use of HXTAL NYL-1 is impractical due to the high cost of importing the resin from the United States. Although developed for use as adhesives, epoxy resins have been used for small gap fills in glass. Large gap fills made in epoxy resin would be expensive compared with polyester resin. Epoxy resin casts do not polish as well as those made of polyester resins.
A number of articles have been written concerning the matching of colorless adhesives for glass to the glass itself (Tennent and Townsend 1984; Messinger and Lansbury 1989). Augerson and Messinger (1993) have experimented with mixtures of HXTAL NYL-1 and Ablebond 342-1 epoxy resins to alter and control refractive index.
There are also many clear polyester resins on the market, and whereas the epoxy resins mentioned above seem to be universally available to conservators, different polyester resins are used in different countries. The difficulty of transporting highly flammable materials may have a bearing on this fact. The author has had experience of Tiranti's clear embedding resin (U.K.), Trylon Shallowcast embedding resin EM400PA (U.K.), and Poly-pol PS-230 (Netherlands). All are clear or become clear with the addition of the organic peroxide hardener. These resins have been chosen as they are initially water-clear and cure in thin layers. Provided that they have been accurately mixed, the rate at which the cured resins yellow depends to some extent upon the conditions in which restored items are displayed or stored.
Disadvantages in the use of polyester resins are the shrinkage of 8% during curing (though this shrinkage can partly be compensated for by topping up the mold as the resin polymerizes); the emission of styrene for some considerable time after curing; and the fact that the resin surface often remains tacky for some time. Reasons for this final phenomenon are interference from atmospheric moisture, aging of the hardener, or, if the resins are stored under refrigeration, the use of the resin and hardener before they have reached room temperature, thus preventing the complete chemical reaction.
Hardening of the surface is aided by polymerizing the cast in a dry atmosphere such as a sealed cabinet containing trays of silica gel. Warming the cast in an oven is not recommended since it may cause premature discoloration of the resin. Polyester embedding resins abrade and polish easily.
Although cured epoxy and polyester resins can be reversed, they cannot be truly dissolved. Because solvents cannot break down and dissolve the cured polymer network, the resin merely swells to the point that it can be mechanically removed. This procedure has potential dangers. The swelling process may cause so much tension that damage is caused to the glass. Thus several applications of solvent (normally methylene dichloride), each of which is removed mechanically after a few minutes, are preferred.
The methacrylate Plastogen G with Lumopal hardener used by Wihr (1963) and Errett (1972) is transparent and mobile and is therefore normally cast into closed molds (see above). The liquid resin is mixed with 0.25 and 0.5 hardener (powder), which is difficult to assess in small quantities, and the addition of too much hardener may cause premature discoloration of the resin. Plastogen G has a 15-minute pot life but cannot be worked during that entire time because a skin quickly forms over the surface. It also has an extremely powerful, unpleasant smell. After mixing, and if necessary coloring, the resin should be covered and left to stand while air bubbles escape. Notman (1973) used a color acrylic resin to build up grozed edges in stained glass so that the pieces could be edge-joined.
Technovit 4004A (poly methacrylate) is occasionally used as a gap filler, but, being translucent, it can be used only with opaque glass. The polymer (powder) is mixed with the liquid monomer in the ratio of 5 parts to 3, but the proportions are not critical and the setting time may be varied by changing the amount of the powder. When mixed in the recommended proportions, Technovit 4004A sets at room temperature in 10–15 minutes but can be worked with a spatula during this time. This product is guaranteed by the manufacturer not to shrink or expand on curing. It emits heat if cast in large amounts. Technovit adheres well to glass, is relatively hard, and can be abraded and polished.
The thermosoftening property of Technovit 4004A has been exploited by Errett (1972) and Jackson (1983) to reshape small casts and thus dispense with the need to make complicated molds.
3.5 COLORING MATERIALS
Materials used for coloring fall into two categories: those used for mixing in with resin and those applied to the resin after it has cured. Colors for mixing in with the resin may be transparent or opaque depending upon the desired effect. When incorporating color with resin, enough colored resin must be produced to complete the restoration, and allowance must be made for areas that may need to have more resin added to them or others that may have to be cast more than once. Preparing sufficient color is important since a color can rarely be exactly matched a second or third time. Hardener is then added to small amounts of the colored resin as required for use, and the resin is allowed to stand before use to enable air bubbles to escape.
Most glass on display in museum cases is exposed to high levels of illumination, and hence light-resistant pigments are needed in any restored portions. Improvements in pigment technology have provided the conservator with a fairly wide palette of colors, and a list of light-resistant colors has been given by Thomson (1972). Other pigments are used by Wihr (1963), Davison (1981), Errett (1972), and Staude (1972). Occasionally pigments can produce adverse effects with some reactive polymers, and it is then more satisfactory to purchase ready-mixed colors. There are colors for polyesters, silicones, and epoxies; their light stability must be checked before use. Manufacturers or suppliers may issue data, but conservation scientists in major museums have tested many materials and published the results either in-house or in conservation literature.
Unfortunately, the transparent epoxy and polyester adhesive and casting materials often require the use of dyes rather than pigments to color them in order to retain their transparency, and there is a much smaller range of light-stable dyes available for use in polymers.
3.6 RETOUCHING LACQUERS
The lacquers usually employed for retouching the restored portions of glass objects are those used in the restoration of ceramics, porcelain in particular, and their availability varies from country to country. Transparency and retention of color are important, as is any short- or long-term interaction with the resin substrate. Enameled decoration may be copied on resin casts using any pigments and media used for ceramic restoration, provided that they adhere to the cast and the solvent does not damage it. Gold decoration may be copied in leaf gold applied on a size or as liquid metallic paints, though the latter will discolor on aging.