In keeping with the conference theme of Practical Philosophy, or Making Conservation Work, Ariel O’Connor, currently at the National Air and Space Museum and formerly at the Walters Art Museum in Baltimore, presented a talk where an imperfect treatment method proved to be the best option. A nineteenth century Persian water pipe, or nargile, made of gilded silver and elaborately decorated with gemstones, was heavily tarnished and needed to be cleaned in preparation for an upcoming exhibit. The water pipe was first analyzed with XRF to determine the treatment; XRF indicated silver with copper, gold, and lead, but no mercury, suggesting the object was gilded with some process other than mercury gilding.
Test cleanings were done with several standard methods for reducing tarnish mechanically: cosmetic sponges, Duraglit polish, precipitated calcium carbonate, Long Shine silver polishing cloth, and Mars Erasers. Acidified thiourea, a chemical method of reducing tarnish, was not initially considered because of research showing the problems of thiourea – leaching of copper, microetching, and long-term detrimental effect of thiourea residues. (See for example, a recent article by Contreras-Vargas et al, “Effects of the cleaning of silver with acified thiourea solutions” in Conference Proceedings of Metal 2013.) While the mechanical methods, especially the erasers and sponges, were easy to use, the final appearance of the polished surface in the test cleaning areas was not as golden as expected.
Analysis with XRF led to a frightening conclusion: even the gentlest mechanical methods, such as a cosmetic sponge, removed gold from the surface. And as the object was gilded with a method other than mercury gilding, the gold layer was already very thing. Analysis after a test cleaning with thiourea, however, did not show the same loss of gold. The decision to clean the object with thiourea was reluctantly made. Steps were taken to use thiourea as safely as possible: dwell time was minimized, the surface was immediately rinsed with deionized water, and the work was performed in the fume hood because of the generation of poisonous gases. O’Connor introduced a novel technique for applying and rinsing the thiourea. Strips of non-woven cotton Webril Handi-pads were placed on the object then wetted with thiourea applied by dropper; rinsing was also performed with the handi-pads. Cotton swabs were used as little as possible because of the possibility of abrasion of the surface.
The treatment raised the important question of when is it acceptable to use a method that has been shown to be damaging. O’Connor articulated why, on this occasion, chemical cleaning with thiourea was the lesser of two evils. I look forward to seeing more research on the topic of cleaning gilded silver as well as discussion of the ethics involved in this treatment.