The graceful ceramic pedestal was coated with a thin layer of dirt, some of it perhaps lodged in the artifact's clay pores for thousands of years.
Wielding a soft brush, Marci Burton applied a blob of clear gel to the object's surface, then removed it moments later, taking a bit of grime with it. Bit by bit, the pedestal's grayish hue gave way to a milky beige.
Burton is part of a massive conservation effort at the University of Pennsylvania Museum of Archaeology and Anthropology, which is preparing a new exhibit of nearly 1,200 finds from ancient sites in the Middle East that opens April 21.
To prepare these works of art and history for display, the conservators apply the tools of science. A laser was used to lift centuries of grit from limestone carvings. A 3-D printer produced a new head for the statue of a bull. A technique called X-ray fluorescence revealed the type of metal alloys used to make certain relics, guiding the choice of treatment.
And for delicate cleaning, the preferred option is gel. The viscous substance that Burton dabbed on the pedestal did not look like much, but it represented some sophisticated chemistry.
Depending on the composition of dirt to be removed, Penn Museum's conservators use one of six gel formulations that gently clean without harming the surface beneath -- chemically binding, dissolving, or simply lifting off unwanted residue. The staff was trained last fall by the man who pioneered the concept: Richard C. Wolbers, an associate professor of art conservation at the University of Delaware.
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"It's a meditatively slow process," Burton said, working the gel in a circular motion on the surface of the 4,400-year-old relic. The pedestal, unearthed in 1927 on a joint expedition by the Penn Museum and the British Museum, is thought to have been a stand for holding a ceremonial vessel of some sort.
Wolbers started using gels in the 1980s to treat paintings, and they also have been used to clean historic papers. Their use on ancient ceramics is a relatively recent development. The last time someone treated the pedestal that Burton was working on, in 2012, dust was removed with a soft brush, a vacuum, and swabs moistened with de-ionized water.
That procedure was only partly effective, said Lynn A. Grant, Penn Museum's head conservator. And the goal is to avoid using water where possible, lest it carry dirt into the object's pores.
The gels contain water, too, but it is mixed with food-grade xanthan gum -- a substance used in some ice creams, salad dressings, and sauces. The resulting jellylike texture allows the conservator to control how much of the cleaning agent touches an object's surface, with no unnecessary wetting, spreading, and dripping, Wolbers said.