Claire’s talk highlights a topic that I’m glad we conservators are beginning to talk about more openly – treatments that didn’t necessarily work. This subject also fit in nicely with the overall conference theme of treatment and innovation. Although some of her treatment steps did not bring about the desired results, Claire was successful in safely and aesthetically preparing the ceramic so that it can be exhibited at the Freer Gallery of Art.
The earthenware dish features a reddish-buff ceramic body, off-white slip, and transparent glaze, with minimal brownish-black and red inscription decorations. The ceramic was previously broken into about 40 pieces and restored, with multiple campaigns of adhesive and overpaint. The museum has records of one treatment in 1964, and while removing old repairs, Claire found evidence of at least two previous restorations which likely occurred before the object was acquired in 1954. The major condition issue was various types to staining, including general yellow and gray stains. Additional disfiguring stain lines were present a few millimeters in from the edges of the sherds, including some areas with two or three separate rows of parallel staining.
Claire carried out analysis in an effort to determine the cause of the staining. Using SEM-EDS, among other tools, she determined that the staining contained little iron and was primarily organic in nature, meaning it is likely not from burial but from the various restoration campaigns. Adhesive identification was inconclusive, likely because multiple types are present; however, Claire noted the presence of hide glue, some type of acetone-soluble adhesive, and possibly shellac. During testing, the staining did not appear to be soluble in a range of solvents, likely because of crosslinking. Soluble salts were also identified with a microchemical test for chlorides, although no salt efflorescence was visible. An iron-containing accretion on the back within the footring was likely from burial and was mechanically reduced to avoid transfer to other areas of the ceramic during stain reduction treatment.
Claire proceeded with stain reduction tests, following Bruno Pouliot, Lauren Fair, and Richard Wolbers’ method of three rounds of poulticing using a chelator, bleach, and a final rinse.* She ultimately chose sodium citrate (2% at pH 8) as the chelator because it was mild yet effective, and she used carbamide peroxide (20% at pH 8) as the bleach. For both of these steps, she used agarose gel (2%) as the poultice material, favoring its ability for controlled, localized application. The gel poultices were only applied to stained areas on the front surface of the ceramic.
Agarose gel is made by mixing the powder with water, heating to a low temperature, pouring the mixture out to cast, and then cutting into blocks when cool. The gel blocks can then be soaked in solution; for instance, Claire soaked blocks in the chelator for one hour before applying to the ceramic. Plastic wrap was used to cover the gel blocks while on the ceramic to reduce evaporation. For the final rinse, the sherds were soaked in baths of deionized water, which served to clear the chelator and bleach as well as desalinate the ceramic.
To complete the treatment, the sherds were joined using Paraloid B-72, which was also bulked with microballoons and fumed silica to fill gaps and losses. These areas were inpainted as the dish will be displayed in an art museum, as opposed to an archaeological context, and the curator preferred to have the losses integrated.
Although the staining was somewhat lightened, its appearance was not sufficiently reduced by the poulticing steps. Claire carried out many poulticing trials, but the staining proved tenacious and she did not want to take the treatment so far as to risk causing damage to the object. Although improvements were made, the curator still did not find the dish to be in exhibitable condition, since the staining lines were still visible and particularly distracting against the overall stark, white appearance of the ceramic. At this point, Claire decided to try painting out the stains, over a barrier layer of B-72. Although she did not like the idea of painting over the original ceramic surface, this seemed to be the only reasonable option for preparing the object to be exhibited and accessible to public. Painting light over dark and matching the surrounding off-white glazed slip must have been a challenging task. But in the end, conservators and curators were both pleased with the results!
* For more information, I recommend: Bruno Pouliot, Lauren Fair, and Richard Wolbers. “Re-thinking the Approach: Techniques Explored at Winterthur for the Stain Reduction of Ceramics,” 2013 in Recent Advances in Glass, Stained-Glass, and Ceramics Conservation, pre-prints of the ICOM-CC Glass and Ceramics Working Group Interim Meeting and the Forum of the International Scientific Committee for the Conservation of Stained Glass, Amsterdam. pg. 211–223.