A romance with stone

Whether in science or art, people tend to favor stories of sudden inspiration: an element discovered by chance or a flash of insight that gives birth to a masterpiece. But most of the time, things do not happen that way.

At least, that is not how it happened in Chengde, Hebei province, where a team of stone restorers has spent the past 15 years rescuing hundreds of cultural relics. Their work has rarely offered a definitive solution. Instead, it has been a long struggle against time, marked by experiments, failures and more experiments, as they search for ways for relics to coexist with the forces that are slowly destroying them.

What they have built in the process is not a single breakthrough, but a body of experience: an approach that can be tested, repeated, and passed on.

An unsolved problem

In 2011, when Li Li arrived in Chengde, about a three-hour drive from Beijing, she stood beneath an exquisite glazed pagoda and looked at its cracked stone base. Her mind went blank.

Chengde was once home to the Qing Dynasty (1644-1911) imperial court's summer retreat. The city is known for the Mountain Resort, built as an imperial summer residence, and a group of surrounding temples. The site was inscribed on UNESCO's World Heritage List in 1994.

Li, then a researcher who had just joined the China Academy of Cultural Heritage, was not responsible for preserving the grand buildings themselves. Her task was to protect the less conspicuous stonework that supported and surrounded them: pedestals, statues, steles, and other stone elements.

Stone, by conventional wisdom, should be among the easier materials to preserve. It is hard and heavy, built to withstand wind and rain. Its apparent permanence is precisely why people have relied on it for centuries.

But the craftsmen who worked here 300 years ago could not have known what their locally sourced stone would eventually endure.

The rock is a fragile volcanic material known as tuff. It contains salt-bearing minerals, is highly porous, and readily absorbs water.

Temperatures in Chengde can reach 40 C in summer and fall to around — 20 C near the ground in winter. The repeated cycle of freezing and thawing causes the stone to split into thin layers.

"Tuff is much less dense than marble or white marble," says Chen Dong, chief engineer at the Chengde Municipal Cultural Relics Bureau.

He compares the stone to a millefeuille, with layers constantly peeling away. Sometimes, he says, "You barely touch it, and it crumbles."

That was only one problem. The stones had also been battered by wind and rain, suffering from missing sections, sanding, hollowing, and other forms of damage.

Before the China Academy of Cultural Heritage took on the project, Chengde had invited experts from around the world to examine the problem, including specialists from the Getty Conservation Institute in the United States. "They all shook their heads and said it was too difficult, that there was no solution," Chen recalls.

The difficulty was partly rooted in the unusual properties of the stone. Tuff of this kind is rarely found outside northern China. Li and her colleagues traveled across the country looking for comparable examples, but found no other site with the same pattern of cracking. "We were starting from zero," Li says. "We had no idea where the road would lead. As the person in charge of the project, I didn't know where to begin."

In the early years, the team rented a small one-story house halfway up a mountain near a temple where they were working. It served as a laboratory, meeting room, dormitory, and kitchen. They painted the house blue. "Blue represents hope," Li says.