Plastic solution and particle accelerator revive faded Chinese figures.
A chemical treatment could prevent China's Terracotta Army from cracking up. Around 1,500 of the 2,000-year-old figures are fading as their protective glaze begins to crumble.
The new method effectively glues the lacquer together from the inside, explain Heinz Langhals and colleagues at the University of Munich in Germany. A handful of soldiers have been protected so far and thousands more may be treated.
The life-sized terracotta figures were found buried in underground chambers near Xi'an, China, in 1974. The army - thousands of warriors and horses - were interred in the mausoleum of the first Chinese Emperor Qin Shihuangdi, who died in 210 BC. Soon afterwards, looters set fire to the burial tomb and the ceiling collapsed. For the next two millennia, the clay effigies lay buried in water-soaked soil.
As the figures are exhumed, their sodden glaze starts to dry out. The brownish lacquer, covered with coloured pigments, begins to flake and fall off.
Langhals' team bathes the warriors in a solution containing hydroxyethyl methacrylate (HEMA). The organic molecule, which is commonly used to make plastics, is small enough to penetrate tiny pores in the glaze.
Next the soldiers journey to nearby Lintong, where they are bombarded with electrons in a particle accelerator. This converts the impregnated liquid into a robust polymer, bonding the fragile coating together like glue1.
The technique outstrips standard preservation methods. Cracked paint on artefacts is often treated with a solution that seeps into crevices and dries to form a tough film. But these molecules are too big to infiltrate the tiny, water-filled pores in the Terracotta Army's glaze.
Crucially, the new technique does not alter the figurines' appearance - other protective polymers can add a glossy sheen. And the research is timely: new excavations may reveal further legions of figures.
Langhals, H. & Bathelt, D. The restoration of the largest archaeological discovery - a chemical problem: conservation of the polychromy of the Chinese terracotta army in Lintong. Angewandte Chemie International Edition, 42, 5676 - 5681, (2003).
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Ball, P. Terracotta Army saved from crack up. Nature (2003). https://doi.org/10.1038/news031124-7