Repairing by trial and error
It was during those difficult years, from 2011 to 2015, that Li's team began making progress in developing conservation materials.
The process was painstaking. Researchers first tested materials under simulated environmental conditions in the laboratory, making sure they would not damage the relics. Only then did they conduct small-scale experiments on the actual stone.
A single test sample might need to be observed for two years. A material could pass laboratory tests and appear ready for use, only to expand after its first winter. Meltwater from snow would seep into the stone as temperatures rose, exposing weaknesses that laboratory tests had missed.
"What's even more frustrating is that a material might survive the first winter, and crack in the second," Li says. When that happened, the researchers went back to the beginning. The cycle repeated so many times that Li could no longer count the failed experiments.
In 2012, drawing on techniques passed down by earlier generations of craftsmen, the team settled on a traditional lime-based material whose main component is calcium carbonate. Similar materials have been found at archaeological sites dating back more than 5,000 years.
But identifying the core material was only the beginning. The researchers still had to repeatedly test firing temperatures and experiment with additional ingredients to determine how the material would interact with different relics.
"No material can be called a once-and-for-all solution," says Xu Dong, who joined the Chengde team after graduating from university in 2012.
Even on the same relic, conditions could vary. A surface exposed to direct sunlight could experience greater temperature swings and deteriorate differently from a shaded side.
Repairing a carved stone dragon about 50 centimeters long can require five different color samples, says Wang Ziyi, who joined the team in 2014.
Xu and Wang were not simply researchers selecting materials. They also worked directly on the relics, carving patterns and carrying out repairs themselves. In their daily work, scientific research and hands-on conservation occupied roughly equal portions of their time.
To this day, the team continues to conduct thousands of experiments to find suitable materials and conservation methods. It has developed a systematic approach that includes cleaning and dust removal, desalination, preliminary consolidation, anchoring, grouting, pattern repair, and color matching.
In 2015, the former director of the Getty Conservation Institute visited Chengde. After seeing the damage, he expressed surprise. For Li, however, the significance of the comment was not simply the authority behind it. "It represented a voice," she says.
The team now has more than 50 members, with specialists drawn from fields ranging from science to art. Since 2011, it has worked on 15 stone conservation projects in Chengde, rescuing hundreds of valuable relics and developing more than 10 patented technologies and materials.
The team calls its approach the "Chengde experience". It rests on three principles: putting preliminary research first and carrying scientific research through the entire conservation process; integrating research, design, field conservation, and monitoring; and using technology and interdisciplinary collaboration to strengthen heritage conservation. The team has also moved beyond stone, applying what it learned to murals and other forms of heritage across China.
