The terra-cotta warriors buried near the tomb of the first Chinese emperor, Qin Shihuangdi, present a fierce challenge–to modern-day chemists. Since the site’s discovery near Xi’an, China, in 1974, archaeologists have unearthed more than 1,500 of the life-size figures. But once the warriors see the light of day after more than 2,200 years of burial, their paint disappears, sometimes within minutes of exposure.
With an estimated 8,000 more figures still buried, scientists have been looking for ways to lock the paint in place. Now, a group of chemists in Germany has a technique that just might work.
The warriors were originally coated with polychrome–a material consisting of a lacquer base topped by a layer of pigment, explains Heinz Langhals at the University of Munich. Because water-saturated soil at the site has altered the lacquer, he says, the coating cracks and peels off once the warriors are removed from their soil encasements. Researchers have tried different polymer-based materials to strengthen the polychrome and secure it to the terra-cotta surface, but the polymer molecules have been too big to penetrate the coating.
Langhals and his colleagues decided to use hydroxyethyl methacrylate–an organic monomer used to make many plastics. The researchers saturated cotton compresses with the monomer and a polymerization agent and applied the preparation to terracotta fragments from a broken warrior. The water-soluble monomer diffused through the lacquer coat, partially replacing the water in the coat’s tiny pores.
Then, using an electron accelerator, the researchers irradiated the fragments with electron beams. The electrons activated the polymerization agent, which stitched the monomers together into polymers, consolidating the polychrome. The researchers describe the restoration technique in the Dec. 1 Angewandte Chemie.
“It’s a great idea,” says Pamela Vandiver, acting director of the Smithsonian Center for Materials Research and Education in Suitland, Md. So far, the researchers have tested their technique on small fragments and shown that the polychrome remains stable for several years.
Working with the Bingmayong Museum in China, Langhals hopes to use his method to preserve an entire warrior, starting in early 2004. Since many of the warriors are in pieces, he and his colleagues will have to treat fragments individually before reassembling them.
Several potentially enormous sites at the tomb complex were discovered in December 2002. As archaeologists gear up to unearth more buried treasures from these, Langhals hopes to use his technique to preserve those new finds as well.
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