REGALREZ IN FURNITURE CONSERVATION
5 POSSIBLE USE OF REGALREZ 1126 IN FURNITURE CONSERVATION
5.1 CHARACTERISTICS OF REGALREZ 1126
Apart from their molecular weight, the three highest molecular weight grades of Regalrez—1126, 1128, and 1139—are chemically identical to Regalrez 1094 and produced a varnish that did not get tacky when touched (table 3, nos. 10-12). Until now these grades have never been tested for conservation purposes. Because the molecular weight of 1126 is closest to 1094, we decided to explore its benefits. Regalrez 1126 dissolves more slowly in nonpolar solvents, is more viscous, and retains its solvent a little longer than Regalrez 1094 (see fig. 1). Nevertheless, when it still contains 16% of the original amount of solvent, the varnish, while soft, is not tacky.
For tests, several colleagues kindly provided scrap parts of furniture with heavily crazed and yellowed varnishes that would normally be considered unsalvageable. On these degraded varnishes, several concentrations of Regalrez 1126 were applied and subsequent gloss and saturation were examined.
Another important aspect of Regalrez is its compatibility with wax. Wax can be an effective means of lowering the gloss of a varnish, and after having French-polished or varnished a piece, many furniture conservators apply a final coat of wax for this purpose. The irregular crystalline structure of wax tends to scatter light in different directions, producing a surface that is more matte. Because Regalrez has a very hard gloss, it was mixed with beeswax as a matting agent. Finally, several mixtures of Regalrez 1126 with beeswax were tried as a better alternative for the traditional wax-resin finishes, which contain less stable resins such as dammar.
5.3 REGALREZ 1126 FOR SATURATION AND AS A VARNISH
A range of 10%, 20%, 30%, and 40% (w/w) of Regalrez 1126 in Shellsol TD was brushed on the degraded varnishes, and the subsequent gloss and saturation were observed and discussed with several colleagues. To explain the term “saturation,” we agreed on these four definitions:
1. The irregular surface of a degraded varnish causes most light to reflect in many directions, thus producing a whitish matte look. A new varnish applied on top smoothes out these irregularities, causing part of the light to be reflected in one angle only while the rest of the light penetrates the varnish layer.
2. A degraded varnish can have many internal voids and fissures, scattering the light both as it passes into the finish and as it returns through the finish after being reflected off the wood surface. Thus the degraded varnish has a somewhat opaque or even crazed appearance. A new varnish that has penetrated and filled the porous structure of a degraded varnish improves the transparency.
3. Degraded varnishes are often only partly bonded with the wooden substrate, so the wood is only partly visible. With the penetration of a new varnish the interface between varnish, and wood can be reestablished so the light gets all the way to the wood. The wood then shows up more vividly.
4. The fibrous surface of the wood is highly irregular and reflects light in all directions. When the new varnish gets through the degraded varnish and penetrates into the wood, there is a comparable refractive index between the now-saturated degraded varnish and the wood itself. Less light will be reflected off the wood, and more light will penetrate into the wood. Of the wavelength frequencies that penetrate the wood, all colors are absorbed except the wood's own color, being a shade of brown in the case of most antique furniture. This results in the absorption of most frequencies, and the wood looks considerably darker after saturation.
In the senses defined above, Regalrez 1126 was found to be very effective in saturating degraded varnishes. The low relative evaporation rate of the aliphatic solvent and the low viscosity of the mixture may cause the good penetration. To control the final amount of darkening, however, one should start with a 10% concentration or lower for the first application. In areas where the old varnish is missing, Regalrez 1126 may cause additional darkening that will not match the surrounding areas. Regalrez 1126 is therefore best used on surfaces that are more or less uniform.
To build up a top varnish, 20-40% (w/w) solutions were found to most workable. Probably because of the low viscosity of the mixtures and the low evaporation rate of the solvent, they flow out very evenly, producing a very smooth surface. A Regalrez 1126 top varnish can have a high gloss, whereas the addition of up to 10% beeswax helps to lower the gloss.
The brittleness of the higher molecular weight grades of Regalrez has not been tested. Nevertheless, on furniture even more so than on paintings, it is important to lower the brittleness of Regalrez. Regalrez 1094 without Kraton G1650 is as brittle as dammar, which is far too brittle to be used as a top finish on furniture. De la Rie (1993) showed that adding 10% Kraton G1650 made Regalrez 1094 as resistant to scratching as Paraloid B-72. Thus, when using Regalrez 1126 as a top finish, 10% Kraton G1650 should be added. Adding more Kraton G1650 is not recommended since the elastomer is the less stable component.
Schoeler and Stoian (1994) found that the hydrocarbon resin Regalite R91, mixed with 33-50% of the high molecular weight Plexigum PQ610, gave a durable topcoat. Regalite is not as stable as Regalrez because it is not fully hydrogenated. It would be useful to undertake more research into the stability of a combination of Regalrez 1126 and Plexigum PQ610.
It is difficult if not impossible to apply a final coat of wax on top of Regalrez because Regalrez immediately dissolves in the solvents present in paste wax formulations. This characteristic makes Regalrez 1126 problematic as a top varnish for privately owned pieces because there is no control over how the piece will be treated. However, one can cover the Regalrez 1126 with French polish or shellac or apply a coat of Paraloid B-72 (in polar solvents) with a brush. This procedure makes it possible to keep a degraded finish, saturate it with Regalrez 1126, and finish it with the more traditional French polish or the more stable B-72.
Because Regalrez is very compatible with waxes, it can be mixed with waxes, such as beeswax. Thus one can prepare a wax-resin finish without having to use the faster-aging dammar. For our purpose, a mixture of Regalrez 1126, beeswax, and white spirit (10:30:60 g, respectively) was brushed on and evened out with a cotton pad, which was dipped in the same white spirit. When the finish was dry, it was buffed up like a normal wax coat. Adding up to 25% of Regalrez 1126 to the beeswax produces a satin gloss comparable to a wax coat. If more is added, the finish starts to behave like a varnish and cannot easily be further buffed up. As explained earlier, the saturation on a piece of furniture is sometimes regained by melting in a new coat of wax with a hot-air gun. The wax-resin mixture described above can give a comparable saturation without having to use any heat on the object.