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Mites make meal of a city's history

A CITY wall that is more than 1,400 years old is being eaten away by a species of mite in Xi'an, capital of the northwestern Shaanxi Province, archaeologists have said.

Experts were surprised to find it had been mites that were undermining the wall at the Hanguang Entrance Remains Museum.

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

The Hanguang Entrance is one of the 18 entrances in the city wall in Xi'an, which served as the nation's capital for 13 dynasties. The 8-meter-high entrance with three gates, one 5.5-meter-wide gate in the middle and two 5.3-meter-wide gates in the east and west, is the best preserved one, and was built at the beginning of the seventh century.

The wall had created a suitable environment for the bacteria which mites feed on, Li said.

The mites had lived and built their dens in the wall for a thousand years, and had been eating it away from the inside, Li said.

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

Because of its involatile nature, its effect 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 said.

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

The material, invented by Li in 1990, had proven effective in protecting the terracotta warriors and the Banpo Ruins, the largest and best-preserved primitive village ever discovered in China, Li said.

A team set up last year is trying to figure out a way to improve it for use on 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 AD) before using it on the real wall, which proved effective, said Wang Danhu, an expert with the technological section of the State Administration of Cultural Heritage.

The new material made the earth stronger and more permeable, Wang said.

If successful, the method will have a broad future as most of China's historic buildings were built using earth, which had been a difficult problem for the world's archaeologists, said An Jiayao, archaeologist of Institute of Archaeology, Chinese Academy of Social Sciences.


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