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Ancient city wall faces 'mighty' invader

(China Daily)
Updated: 2009-06-16 08:09

XI'AN: An ancient Chinese city wall has survived the ravages of wind, rain, warfare and natural disaster for more than 14 centuries - but its toughest foe yet may be tiny mites eating it away from the inside.

The 1,400-year-old Hanguang Entrance is under attack by tiny mites that are eating it away from the inside. Inset: A magnified image of one of the mites. [File photo]

While doing research recently on ways to protect the wall in Xi'an, capital of northwestern Shaanxi province, experts at the Hanguang Entrance Remains Museum were surprised to find it being undermined by mites.

"The discovery is the world's first of this kind. The mites have seriously damaged the wall," said Li Yuhu, an archaeologist at Shaanxi Normal University.

The Hanguang Entrance is one of 18 entrances in the city wall in Xi'an, which served as the nation's capital for 13 dynasties. The eight-meter-high entrance with one 5.5-meter-wide gate in the middle and two 5.3-meter-wide gates on either side, is the best preserved, and was built at the beginning of the seventh century.

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The wall created a suitable environment for the bacteria which mites feed on, said Li, who believes the mites had built their dens and lived in the wall for a thousand years, and had eaten away at it from the inside.

Li and his colleagues have begun killing the mites by spraying an insecticide into the wall every 20 days.

"The insecticide is widely used across the world and has proven harmless to human beings, vegetation and the environment," said Li.

The effects of the insecticide could last seven to 14 years, he said.

"It is just the beginning. We know very little about the mites and we want to further investigate them," Li Yuhu said.

Meanwhile, experts have also developed a material to reinforce the wall against other mite-related problems, including bacteria, loosening and cracks, Li said.

The material, invented by Li in 1990, has been effective in the protection of the terracotta warriors and Banpo Ruins in the province during the last decade.

A protection team set up last year carried out a one-year study on Li's method and looked for ways to improve it for use with the wall, said Wang Su, head of the protection bureau of the Hanguang Entrance Remains Museum.

The experts tested the method on the earth floor from the Tang Dynasty (618-907) before using it on the wall. The method proved effective in making the earth stronger and more permeable, said Wang Danhu, an expert with the technological section of the State Administration of Cultural Heritage.

If it proves successful, the method will have a bright future as most of China's historic buildings were built with earth, which has posed a difficult problem for archaeologists from around the world, said An Jiayao, an archaeologist at the Institute of Archaeology, Chinese Academy of Social Sciences.

Relics in Japan and the Republic of Korea also face the same problem, An said.

Xinhua