Jewels brought home - but it takes patience

The National Library of China has inherited former royal collections of books that can be dated to the Song Dynasty (960-1279).

"Perhaps it's a nice home for these books when they return from overseas," Zhao says. "What we really appreciate is that many Chinese auction houses put the national interest first. Whenever they get books from overseas, they usually contact us first to see whether we are keen to have them."

The library began to collect anything that might be regarded as a treasure from overseas shortly after the founding of New China. For example, Chen Qinghua, a banker and a well-known collector of ancient books, left the mainland for Hong Kong in 1949, taking many precious books, including rare Song Dynasty printed versions. His family later migrated to the United States.

In 1956 and 1965 the Chinese premier Zhou Enlai approved spending to get some of them back to China, and in 2004 the library received the rest from Chen's son. Some of the books Chen previously owned are on display in the current exhibition.

However, Zhao says the price of ancient books often makes them unaffordable to the library.

"But if some Chinese collectors can get them back, at least that brings them home."

Three million ancient Chinese books are estimated to be in public or private collections overseas, said Lin Shitian, director of the executive office of the National Center for the Preservation and Conservation of Ancient Books.

"We've undertaken a survey worldwide to establish a database for these books," Lin says, and in it there are about 500,000 entries from 40 public institutions.

Zhao regrets that some books may never return to China, at least physically.

He tells of once hearing about a Japanese collector who wanted to sell some important Chinese books, but when he arrived in Japan to make inquiries he found that the books had been listed as cultural relics that could not leave the country.

"Digitization is one way of sharing resources and beginning cooperative studies."

Thanks to that, 19 copies of Yongle Dadian have come home to the National Library of China from the University of Oxford.