Caring for your Objects:
The American Institute for Conservation
of Historic & Artistic Works
1156 15th Street NW, Ste. 320
Washington, DC 20005
This article was first presented as a poster at the 1998 AIC Annual Meeting.
"Some Ideas for a Better Coating"
The Deller Conservation Grp.
This article brings some new ideas towards maintaining the traditional varnishes used for ‘French polishing’ while introducing modern materials that will hopefully prolong the life of the film-forming materials and improve their workability. The technique of French polishing has been around since the very late 18th century. Over the years, this traditional application of shellac has changed little. Various recipes can be found in many finishing books that call for a variety of different ingredients that were believed to enhance the qualities of the finish. While studying old recipe books, it became clear that most were concoctions based mainly on hearsay and unfounded qualities attributed to different materials. Even today, woodworking magazines are publishing articles based on these old recipes and flawed concepts. Furniture conservators will continue to need to use the technique of French polishing as it produces the appearance that one expects on certain types of furniture. As furniture conservation matures, a more detailed look has been taken at the individual resins used in early varnish making. The resin of coniferous trees (pines) typically known as rosin, was often listed as one of the main ingredients. Under a variety of different names, (Venice Turpentine, Burgundy Pitch, Colophony, and Elemi,) rosin is now known to degrade rapidly and severely. Linseed oil and other drying and non-drying oils are often listed as components. With its cross-linking capabilities, linseed oil can cause the film to become tenacious and to darken. Moreover, such incompatible materials as oils, rosins (and even gum Arabic), can lead to an internal breakdown within the film itself.
Shellac, the key ingredient in the preparation used for French polishing is a very versatile and desirable natural resin. It is the addition of all the cheaper incompatible resins in the recipes that caused concern. After years of following the old recipes and several failures based on them, I attempted to create a more stable and acceptable coating for historic furniture that kept the visual beauty of traditional French polished surfaces. I began to eliminate those cheaper resins prone to rapid degradation that have always been listed in the traditional literature as important ingredients.
I also wished to create a clear polish, which meant I needed to eliminate the naturally occurring wax in the shellac. (The wax tends to cloud the polish). While there are commercially available shellacs that have the wax already removed, they tend to be chemically processed to the point of instability, most notable the white, bleached or “blond” shellacs. I prefer to use button shellac primarily due to the limited amount of processing it has gone through. But the wax serves as a natural plactisizer allowing the brittle shellac resin to remain somewhat flexible. With the wax eliminated, I needed to replace the wax’s placticizing effects with another material. Among the softer plant resins I have looked into are Manila Copal, Benzoin, sandarac, damar and mastic. I have chosen benzoin based on it’s traditional success as a varnish additive and the pleasant odor the benzoin imparts. However, it is added in small quantities in relation to the shellac. Typically, for a 500 ml solution, I grind 50 gr. of button shellac, 5 gr. of benzoin. I also prefer to use 95% ethanol (grain alcohol) as opposed to denatured. It may be my imagination, but I do notice a difference between the two alcohols as I use the polish. All of this is warmed in an ultra sonic bath (the temperature of the water jacket reaches 100 ºC) which accelerates the time the resins takes to go into solution. Once in solution, the wax and debris are filtered out using a medium fast filter cone. Because the solution is warm when first filtered, some wax passes through. I filter a second time for the desired clear polish.
I use a semi-closed filtering system, which reduces the amount of water the warm ethanol absorbs. Now that I had a “clean” traditional recipe for the polish, I began to experiment with adding a higher molecular weight synthetic resin.
I added 10 gr. of B 72 and immediately noticed an improved workability of the polish. The grain filling properties and lack of shrinkage caused by the presence of the B 72 makes the polish quicker to use. The added benefit of such a stable material as the ethylmethacrylate copolymer, makes this polish warrant further investigation.
I also experimented with B 67 (isobutyl methacrylate polymer), often listed as a varnish resin, but
I found that B 67 made the polish drag and stick as it was being padded out. In addition, B 67 has a tendency to crosslink. Now with an improved version of polish, I proceeded to the next step in creating an even more stable surface coating.
Ultraviolet damage to furniture coatings has long been a problem, and the preparation used in French polishing is no different. The two UV resistant additives in the coatings industry that would best suit my needs were Tinuvin 292, A hindered amine light stabilizer (HALS), and Tinuvin 328, an UV Absorber. The purpose of the 292 is to help prevent the ultraviolet light from breaking down the film itself, the 328 is meant to absorb the UV, preventing it from fading the underlying wood. According to Ciba-Giegy, the manufacturer, they work in a synergistic way. However, the UV absorbing effect of the 328, would also prevent using an ultraviolet lamp to identify the shellac’s orange fluorescence, which is one of the unique and desirable characteristics of shellac. With the 328 eliminated, only 292 is added to the polish in a quantity of 3% to the weight of the total resin content. It should be pointed out that when the polish is filtered, as much as 20% of the weight of the original resin is removed in the form of the wax and debris, which should be accounted for when the Tinuvin is measured out.
method of preparing varnish for French polishing has only been in use for a short
period (just a few years) and is subject to further investigation and
research. It is my
intention with this article to spark debate and improvements.
The Deller Conservation Group, Ltd.
Raw Amber Sandarac Seedlac