Submitted by bborghese on
In March 2012, the Vancouver Art Gallery in Vancouver, B.C. Canada, hosted a three-day workshop titled ‘Cleaning of Painted Surfaces’, given by Richard Wolbers, Associate Professor of Science and Paintings Conservation at the University of Delaware’s Winterthur Art Conservation Training Program.
At the Vancouver Art Gallery workshop, participants received the benefit of Richard Wolbers latest research, including the use of gels, rigid gels, silicone emulsions and a new (to conservation) solvent of extremely low polarity. The following is a summary of this workshop, from a paper conservator’s point of view.
Mornings were spent at lecture, and afternoons in the lab, testing the materials on items brought in by class participants. The first day was spent reviewing basic cleaning chemistry: pH, buffers, conductivity and surfactants. As a paper conservator trained in the 80’s, some of the concepts were familiar to me, and some were not.
Ph as the measurement of an aqueous solution’s acidity or alkalinity is, of course, standard knowledge to paper people. But the consideration of conductivity and how it applies to a solution’s ability to clean was new to me. The ability of a material to conduct electric current is related to the concentration of ions in solution. Every material has some amount of ionic compounds on its surface and, in the case of porous materials such as paper and textiles, within its body. This can be measured by taking samples (wicked from the item being tested by small pieces of dampened filter paper or rigid agarose gel) and using a hand held micro conductivity meter.
A hypotonic solution such as de-ionized water has conductivity less than that of the item being cleaned. It will draw ions out of the material being cleaned, and drive water in (swelling). This will clean the material, but if the difference in conductivity is too great, damage can occur. In the case of acrylic paint for example, the top of the paint layer can be blown off. An isotonic solution, where the conductivity of the cleaning solution matches the conductivity of the material being cleaned, is safe, but does not have much cleaning power. A hypertonic solution, where the conductivity of the cleaning solution is greater than the substrate, will push ions into the substrate and draw water out (shrinking). Unless one actually wants to put ions into the substrate, as in the case of depositing an alkaline reserve in paper, hypertonic solutions would be used with caution.
By knowing the conductivity of the material to be cleaned, solutions can be designed to maximize cleaning while minimizing the potential for damage.
As a general rule, a cleaning solution should not, for safety’s sake, be more than 10 times the conductivity of the item being cleaned, and much less is often quite effective. Ionic material that can be added to the solutions include, but are not limited to, buffers, chelators, and surfactants. The types and properties of buffers, chelators and surfactants commonly used in conservation were reviewed.
Day two covered gel delivery systems for aqueous solutions and solvents, with the focus on xanthan gum, agarose, Pemulen TR2 , and Velvesil Plus. A water solution of xanthan gum (2% w/v) and triethanolamine (TEA) (5% v/v) forms a viscous, pH 8.5% gel that is stable over a wide pH and temperature range. Additional materials can be added to make custom cleaning poultices. Oxidizing agents, such as bleach, and most cationic materials, such as ammonia, cannot be used as they cause the gel to collapse. 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 conservator’s exposure to solvent. These gels rinse well making them suitable for use on paper and textiles.
Agarose (purified agar) is most useful when used as a rigid gel (about 4% w/v in water). In this case it can be used as a molecular sponge both to deliver solutions to, and withdraw solutions from, the substrate. Polar solvent gels can be made from agarose gels by immersing them in acetone or ethanol, where the solvent molecules exchange with water molecules in the gel. The resulting gels can be used to soften some types of adhesive, or weaken solvent soluble materials from the substrate.
Pemulen TR2 is an alkyl acrylate cross polymer that can be used to make oil in water emulsions. Organic solvents can be added up to about 20% v/v. A gel made from 1.0% w/v Pemulen TR2 in water with 5% TEA gives gel with pH 8.5; the same gel made with 1% TEA has a pH of 6.5. Below pH 6 the gel collapses into slime. Because of rinsing issues, Pemulen gels are not recommended for porous materials. Xanthan gum gels are a better choice for paper and textiles that can be washed, and rigid agarose gels for water sensitive items.
The final gel to be covered was Velvesil Plus, a real showstopper. It is a silicone polyether co-polymer, that can be mixed with both polar solvents (including aqueous solutions) and non-polar solvents, up to about 20% each. This very unique gel is a thick, non-polar, waxy gel that can be painted on small areas with great precision. It can be used a type of “dry” poultice, delivering (and then removing) 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, even from solvent sensitive surfaces. Velvesil Plus can also be used as a mask to protect water sensitive media in the manner of cyclododecane. It is certainly more easily applied than cyclodocecane, but as always, testing is need to make sure application and removal do not disturb sensitive media. Velvesil Plus is removed with cyclomethicone a solvent with such low polarity that it is right off the lower right corner of the Teas diagram. (Cyclomethicone can also be used as a mask by itself, depending on the particulars of the piece being treated). In theory, cyclomethicone has such low polarity that it cannot cause unwanted solubility problems, but should still be tested before using. (I found that it did leave a very light tideline in an aged brown dyed paper). One also needs to remember that the mechanical action of application and removal could damage fragile media.
Day three-covered solvent based Carbopol gels, and a review of the Teas diagram. Wolbers is actually moving away from these solvent gels, feeling that most cleaning can be done using the above-mentioned gels and solutions. His health and safety goal is to drastically reduce the amount of organic solvents used in conservation, and to substitute the most toxic solvents with safer alternatives. Benzyl alcohol, for example, can be mixed with mineral spirits to approximate the solubility parameters of xylene. His goal as far as treatments are concerned is to simplify the process by reducing the number of chemicals and delivery methods needed.
The materials used in the workshop are, with one exception, available from chemical suppliers, and web-stores catering to small-scale producers of personal care items and cosmetics. As a paper conservator who would use large amounts of the very expensive agarose powder, I chose to experiment with plain food grade agar and this seems to be an acceptable substitution. Unfortunately, Velvesil Plus is a special order item, available only, at the time of writing this review, in 350 lb. lots, with a total cost of US$10,000 (£6200).
The workshop was an extremely worthwhile event, inspiring me to re-tool my lab, and start experimenting with this brave new World of Wolbers. I thank the conservation staff of the VAG, and Nadine Powers for organizing this event; it was a true inspiration to this “a bit past mid career” conservator.
Rebecca Pavitt has been a conservator in private practice since 1987, specializing in paper and flat textiles. She is a graduate of the Buffalo State College Art Conservation Program in NY, and a member of the Canadian Association of Professional Conservators. Her website is: www.fineartconserve.com