Precious herbal medicines and black tea leaves are placed on the workshop floor, providing a beneficial aroma. Dyed in yellow, the paper not only takes on its former appearance, but pesky insects are kept at bay by the aroma.

"The additions I make are usually thinner and lighter in color than the original pages, enabling my restoration work to be easily recognized," Xie said.

He often finds earlier signs of restoration, and although his predecessors might have made some mistakes, the patches they placed on pages were often kept to retain historical information.

Xie feels he is much more fortunate than restorers in ancient times. A long-term exhibition of key books restored in recent years opened at the NLC last year. Some of the titles he worked on are on display, with his name highlighted on tags.

However, most of the time, Xie and his colleagues work quietly behind the scenes. Patience is required, especially as Xie is now in charge of restoring a series of documents in Tangut, an extinct language from West China that dates back 800 years.

Unlike his colleagues who sometimes read ancient books to relax during work, Xie does not understand the works he is restoring.

He said, "This job is OK, but who wouldn't get bored after doing the same work for years?

"But this is what I'm good at. I can hardly recall anyone on our team leaving their job at this library. Once you take up this work, it can mean a job for life."

Du Weisheng, 69, is testimony to such devotion, having worked as an ancient book restorer at the NLC since 1974.

"It's great to see younger people joining our team and choosing to stay. They have good educational backgrounds, a range of expertise, and thus bring a more scientific approach to restoration," Du said.

Zhang Zhiqing, deputy director of the NLC, said there were fewer than 100 full-time restorers of ancient books in China in 2007. To change this situation, the National Center for Preservation and Conservation of Ancient Books was established that year, followed by a series of nationwide projects to better care for ancient pages.

"When discussing ancient books, scholars used to mainly focus on their documentation value," said Zhang, also deputy director of the national center. "But many people do not realize that books are precious cultural relics as well, like bronzeware or porcelain. Ancient books record our lineage of civilization, and their value sometimes is incomparable to other cultural relics."

Zhang said there are now more than 1,000 professional restorers in China, and about 3.7 million pages of ancient books have been renewed since 2007. Over 10,000 conservators have been trained for some 2,000 venues housing ancient book collections nationwide.

In 2013, Du became a national-level inheritor of intangible cultural heritage on ancient book restoration, and a training center was established at the NLC to nurture more professional restorers. Led by the library, 30 more training centers were established nationwide to produce a younger generation of talent for the industry. Seven restorers at the NLC are listed as tutors in this program.

"To show respect, my apprentices call me 'master', but I would rather consider myself their teacher," Du said. "Masters in ancient times were unwilling to pass down all their skills to apprentices, but as a teacher, I want these young people to surpass me."