The site, which, archaeologists estimate, dates back to the Northern Wei Dynasty (386-534), shares cultural and artistic consistency with the Mogao Caves. The Dunhuang Academy also oversees the Yulin Caves.

As the mural vividly portrays, some listeners slightly lean forward, thirsty for knowledge. Some are puzzled, some look around. Some whisper to others, while some doze off.

It was Li's most relaxed moment during the interview. He's always been astonished at the ability of the ancient painters to precisely portray the mental state of the characters, and to present dynamic scenes with static images.

"It's a satisfying undertaking rather than a job that I'm obliged to get done. It fills me with joy to think that I have contributed to their survival and inheritance," Li says.

He adds that, as his knowledge accumulates and his understanding toward the artworks deepens, it becomes even more insufferable when his hands cannot keep up with his mind, like a doctor who knows what disease a patient has, but fails to treat it.

In March, he won first prize among mural and painted sculpture restorers at a national vocational skills competition related to cultural relics, which was held in Shanxi province. Nonetheless, there's always the motivation to further improve his skills, he says.

Mural doctors

Commonly seen "diseases" affecting the murals include cracking and flaking, as well as efflorescing that is caused by changes in temperature and humidity of the caves, and the deposition of salts.

To restore a mural requires an all-rounder. The restorer should know painting, master the skills of a mason and have some knowledge of chemistry and physics, to be able to recognize the problems, their corresponding causes and deliver a solution. They must also select proper materials and tools, and conduct experiments, before formally carrying out the restoration and evaluating the effect afterward.

For example, dealing with cracked murals with small flakes curling up usually involves removing the dust, injecting adhesives, backfilling the painted layer, then rolling and pressing to flatten the edges.

Even at the height of summer, when temperatures soar to nearly 40 C, the 58 year old and his colleagues have to wear quilted coats and pants in the caves to keep warm. Arthritis is a hazard for some.

However, for Li Bo, it's often not until he steps out of the caves after several immersive hours that he starts to feel the exhaustion.

Li Lingzhi says Li Bo shares all he knows without reserve.

He says, he's always impressed by the awe displayed by the older generations toward the cultural heritage, and the more comprehensive point of view they have of the tasks.

Li Bo admits that compared to  green hands, an experienced restorer spends less time forming up a solution, and meanwhile, they take a greater number of factors consideration.