Perhaps better known as a medium used in biological studies, agar is gaining traction in the conservation profession as an ingredient for building poultices. The material, a combination of the polysaccharides agarose and agaropectin derived from the cell walls of an alge, is widely available, both in a highly purified form through laboratory supply houses and in a food grade form. It is used as a rigid gel to make poultices for flat surfaces or as more liquid sol-gel for three dimensional surfaces.
Cindy Lee Scott’s study of this material and possibilities for its use in removing shellac from ceramic surfaces was inspired by work using agar gels to clean outdoor plaster busts presented in 2010 by Paolo Cremnesi and published by Anzani et al. In their work, deionized water was the primary ingredient in their poultices, but Cindy Lee wondered if solvents and other cleaning agents could be added to these mixtures to extend their versatility and how these additions might change the stability and working properties of the gel.
After reviewing various properties of the material, particularly the manner in which the porosity of an agar poultice could be modified by altering its concentration, and then the manner in which the gel is typically prepared, used and removed from a substrate (it peels off the substrate cleanly), Cindy Lee presented her own work, conducted in two phases.
The first phase of her work, a component of her thesis as part of her studies at the UCLA/Getty program, involved exploring agar as a gelling material for various cleaning agents on terracotta test tiles coated with a kaolinite-type slip with the goal of finding new ways to removing shellac from previous restored ceramics. She tested agar sol-gels mixed with ethanol, acetone and 5M sodium hydroxide, alone and in combination. She looked at efficacy of cleaning and clearance using visual analysis using a binocular microscope, UV-fluorescence microscopy, and FTIR spectroscopy.
She found that these agar gels performed beyond expectations particularly in comparison with other options tested for that study. She found that agar sol-gels had excellent working properties when they were mixed with ethanol and sodium hydroxide and had good clearance from the surface. Clearance improved for gels with a lower concentration of agar when Japanese paper was used as an intermediary layer.
The second phase of Cindy Lee’s work, undertaken during her internship at the Museums of New Mexico, extended the number of cleaning agents added to agar gels. Additives included solvents, surfactants, chelating agents, oxidizers, and acids. She noted working properties of each mixture, color changes to the gel that might lead to staining, and stability of the resulting gel. In this phase, access to analytical equipment was limited, so her analysis was generally limited to visual observation. Additionally, these mixtures were applied to plaster tiles coated with various materials including alizarin dye, shellac, various paints, PVA emulsion, and soil and artificially aged to test cleaning and clearance.
To summarize very quickly, as far as adding solvents go, best results for workability were achieved with ethanol, then Stoddard Solvent followed by acetone and xylenes. She noted that solvents added in too high a concentration could cause the gel to dissociate. Good working properties were also noted for gels made with the chelating agents, oxidizers, acids and bases tested. The surfactants tested caused complete dissociation of the gel. As far as cleaning efficacy, it appears that gels tested had a more difficult time removing dye and smaller particulates, but with regard to smaller particulates, concentration of agar within the gel appears to have a great influence. She found that efficacy could be improved by altering concentrations of agar and solvent, the temperature of the poultice on application, the length of application and number of applications of the poultice.
Cindy Lee concluded her presentation with pros and cons of using agar-based solvent gels for objects conservation. Since she was kind enough to provide me with her paper and slide deck to help me write this post, I will share them here (if you click on the image it expands):
In short, I found her presentation to be an excellent introduction to this material and I can’t wait to experiment with it myself. It was also excellent to see the kind of simple testing we all do regularly in our own labs presented in a RATS/OSG session, and I look forward to seeing this work published. I don’t presently use Agar, but her presentation has encouraged me to do some experiments to see if this material would be appropriate to use in my own practice.