In March 2012, the Vancouver Art Gallery (VAG) hosted a three-day workshop on the cleaning of painted surfaces by Richard Wolbers, Associate Professor in the Art Conservation Department at the University of Delaware, where he is also working on a PhD on groundbreaking conservation techniques for works in acrylic paint. With a background in science as well as studio art, Wolbers has revolutionized the way paintings and, by extension, paper, textiles and objects are cleaned. At the VAG workshop, we received the benefit of his latest research; the mornings were spent at lectures and the afternoons were spent testing and experimenting with materials in the lab.
The first day was designated for reviewing basic cleaning chemistry: pH, buffers, surfactants and chelators. Day two dealt with gel delivery systems for aqueous solutions and organic solvents with a focus on gels made from xanthan gum (bacteria derived), agarose (seaweed derived), Pemulen TR2 (polyacrylic acid), and Velvesil Plus (silicone crosspolymer).
Xanthan gum (2% weight/volume in water) forms a viscous gel which is stable over a wide pH and temperature range. Additional materials can be added to make custom cleaning poultices. Xanthan gum gels can also hold non-polar solvents in intermolecular pockets (oil in water emulsion), a property which has the potential to greatly reduce the conservators exposure to solvent. These gels rinse well, which makes them suitable for use on paper and textiles. Agarose (purified agar) is most useful when used as a rigid gel (about 4% weight/volume in water). In this state it can be used as a molecular sponge to deliver, and then remove, aqueous cleaning solutions to and from porous substrates.
Pemulen TR2 is an alkyl acrylate crosspolymer that can be used to make oil in water emulsions. Organic solvents can be added up to about 20% volume/volume. Because of rinsing problems, Pemulen gels are not recommended for porous materials.
The final gel to be covered was Velvesil Plus, a real showstopper. It is a silicone polyether copolymer that can be mixed with both polar solvents (including aqueous solutions) and non-polar solvents, up to about 20% each. This very unique material is a thick waxy gel that can be painted on small areas with great precision. It can be used as a type of dry poultice to deliver and then remove tiny amounts of water, or aqueous solutions, to water-sensitive items such as parchment and acrylic paintings. It can also be used to draw out solvent-soluble materials such as ballpoint pen marks from solvent-sensitive surfaces.
Day three of the workshop covered solvent-based Carbopol (acrylic-alkene-ether polymer) gels, and a review of the Teas diagram, which is used to determine what materials a solvent is likely to soften or dissolve. Wolbers is actually moving away from these solvent gels as he feels that most cleaning can be done using the materials described above. His goal is to simplify conservation treatments to make them more safe for both the object and the conservator. His systems can drastically reduce the amount of organic solvents used in conservation, and he substitutes the most toxic solvents with safer alternatives. Benzyl alcohol, for example, can be mixed with mineral spirits to approximate the solubility parameters of xylene.
I thank the VAG conservation staff and Nadine Powers for organizing this extremely worthwhile event which has inspired me to retool my lab and to start experimenting with this brave new world of Wolbers.
Previously: Theatres of the World: the conservation of two murals in the Simon Fraser University Theatre, Part 2.
Next issue: Caring for Public Collections: A Condition Survey