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In 1974 archaeologists came across a vast army numbering in the thousands in Lintong, China. The soldiers were made of terra-cotta, and they were part of the mausoleum of the first Chinese emperor, Qin Shihuangdi. So far, more than 1,500 life-size warriors have been excavated. But once the figures are removed from the pits, their pigment fades--a problem that established methods of stabilization have failed to address. Now a novel technique described in the current issue of the journal Angewandte Chemie International may help the soldiers retain their color.
"The lacquer base-coat has changed while in storage," explains Heinz Langhals of the University of Munich. "The foundation cracks, comes loose, rolls together, and falls off as soon as the relative humidity gets below 84 percent." Langhals and his colleagues developed the new protective treatment using a common chemical used to produce plastics. They treated the excavated terra-cotta pieces with hydroxyethylmethacrylate (HEMA), which is water soluble and can penetrate the wet surface (the figures have been housed in damp soil for the past 2,000 years). When the pieces are then exposed to radiation that penetrates the overlying lacquer layer but not the terra-cotta itself, the HEMA molecules join to form long chains of polymers in a process known as curing. The polymer compound is stable and does not affect the pigments. Langhals also notes that "the reaction is stopped by oxygen from the surrounding air at the surface, preventing it from becoming shiny, which would greatly disturb the natural look of the clay warriors."
New excavations continue to unearth additional archaeological treasures at Lintong. The researchers note that a number of details of their method remain to be worked out, but its success so far suggests that it could soon become the method of choice for conserving the terra-cotta army.
