Volume 8, Number 2, May 1986, pp.4-6

Cleaning Natural Resin Varnishes

At the March WCG meeting held at the S.I., Museum Support Center, Richard Wolbers, Associate Painting Conservator, Winterthur/ University of Delaware Art Conservation Program, spoke on "Alternative Cleaning Systems for Natural Resin Varnishes." Investigating alternatives to traditional cleaning methods is an ongoing project for Mr. Wolbers and in this lecture he presented some of his initial findings.

As we know, natural resin varnishes are initially soluble in a narrow range of solvents, but as they age this range becomes wider and, traditionally, stronger, more polar solvents are used to remove them. Mr. Wolbers' interest in finding alternatives to strong solvents for removing natural resin varnishes began, first of all, with a concern for the health of the conservator, and secondly with the desire to formulate emulsions for varnish removal which are based on an understanding of the complex mixtures which make up the natural resin molecule.

Three emulsions which he has formulated and used successfully were discussed. The first is as follows: 60 ml xylene, 12 ml Triton X-100, and 30 ml 1% Triethanolamine/water. Triton X-100 acts as an emulsifier for an aromatic solvent (xylene) and a polar solvent (water), and the physical-chemical emulsion arrangement of the two meets the solubility requirement of aged dammar. Triethanolamine is added for a slightly alkaline pH which helps wet out the acid groups in the natural resin molecule. A 1% solution of triethanolamine has a pH of about 8. The proportions of the solvents, the nature of the hydrocarbon and the percentage of triethanolamine can all be varied. The emulsion can also be made as viscous as desired by modifying the amount of Triton X- 100. A gel slows down the evaporation rate, facilitating the action of the solvents and decreasing exposure levels to solvent fumes. It also helps to restrict the water to the surface of the painting. To use the emulsion, Mr. Wolbers applies a small amount to the varnish layer, allows it to remain for an appropriate amount of time, and then removes it with a dry swab. The area is then cleared with a petroleum solvent. Unlike other detergents Triton X-100 is soluble in a variety of solvents, and is easily removed. Furthermore, it excludes water from the drying resin or oil surface on which it was applied, substantially reducing the tendency for these materials to entrap water and subsequently blanch on final drying.

Mr. Wolbers showed slides of a 19th-century Dutch landscape on which this emulsion had been used successfully. A continuous layer of varnish had been removed with solvents, but an older, discontinuous and highly oxidized layer remained in the interstices of the brushmarks. Stronger solvents affected the paint layer, but working with this emulsion he was able to safely remove the older varnish.

The usefulness of this emulsion suggested another alternative cleaning method, one based on a structure closer still to the material in aged dammar. Limited amounts of "resin soaps" are commercially available, but they may be used in aqueous preparations to emulsify the aged coating materials directly into water. A typical preparation of this kind might be: 2% resin soap, 1% Triton X-100, and 2% Hydroxy Propyl Methyl Cellulose. Triethanolamine Abietate is one resin soap which he has used. Triton X-100 again acts as a wetting agent, and the Hydroxy Propyl Methyl Cellulose is a colloid used to form an aqueous gel. The Abietate is a triethanolamine salt; in solution it rapidly dissolves to give a slightly alkaline pH, but concentration and pH can be varied to tailor it to a particular cleaning situation. A low concentration of benzyl alcohol may also be added as a "bridge solvent" to lower the overall solubility parameter of the mixture and to increase the potential swelling the applied film.

The third mixture discussed contains an enzyme to remove oil coatings: .05 TRIS, 2% Hydroxy Propyl Methyl Cellulose, .01% Triton-X, and Lipase. TRIS is a buffering agent; Hydroxy Propyl Methyl Cellulose is used to make an aqueous gel; Triton-X 100 acts as a wetting agent, and the Lipase is a "lipolytic" or triglyceride (drying oil) hydrolyzing enzyme.

Mr. Wolbers showed slides of several objects on which he had successfully used a lipolytic mixture to breakdown and remove an oil containing surface coating. These included a fancy chair (Baltimore, 1823) and a painting by John Vanderlyn. In both cases the oil content of the varnish layers had been determined by staining with Rhodamine B. The chair had multiple glaze layers. The enzyme mixture removed a discolored oil coating without damaging the under layers which were protected by a resin varnish. The Vanderlyn painting contained three varnish layers. The uppermost was an oil/resin layer which was very difficult to remove. The lower layers were primarily resin varnishes. The oil/resin layer was removed with the enzyme mixture, and then the lower layers were removed with solvents of a reduced solubility parameter. The enzyme mixture works less well when the oil layer is pigmented, and is slow as an aid in removal of overpaint. Slides were also shown of a landscape by Ernest Lawson on which a protein enzyme mixture was used to remove an aged residue of hide glue which had been left on the surface after it was used as a consolidant.

Reprinted with permission from the Washington Conservation Guild Newsletter, April 1986 (Vol. 10, No.2).