For Tian Tingting, aged books sometimes resemble stale puff pastry. Often suffering from water damage, or having been snacked on by bookworms, the centuries-old pages are crispy and rotten, and in many cases, stuck together.

"They are too fragile," Tian says. "You can"t be too careful when separating them."

She is a book restorer at the National Library of China, the world"s biggest repository of ancient Chinese books. More than 2.7 million such books-defined as those published before the fall of monarchy in China in 1911-are now housed at the library.

Sometimes, the restorers behave more like chefs. For example, an old but efficient way to separate the pages is to steam them in a pot for six minutes, no longer and no less. Then a small bamboo stick and patience will help to split them apart.