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.
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