3D-printing takes China's ancient grottoes to world

Cave No. 12 of the Yungang Grottoes, the 1,500-year-old masterpieces of Chinese Buddhist art located in Datong, North China's Shanxi Province, has been recently "moved" to the Zhejiang University Museum of Art and Archaeology in East China by 3D printing technology, bringing the grottoes to the wider world.

The world's first mobile 3D-printed 1:1 replica cave of the Yungang Grottoes was recently completed in Zhejiang University and opened to the students and faculty on June 12. (Visit https://my.matterportvr.cn/show/?m=efLVXMG77MP)

The replica cave is jointly developed by the Cultural Heritage Research Institute of Zhejiang University and the Yungang Grottoes Research Institute. The joint project team has overcome technical difficulties such as data collection and processing, structural design, block printing and coloring in the past three years, introducing the immovable UNESCO World Heritage site to the world.

Cave No. 12, also known as the Music Cave, is 14 meters deep, 11 meters wide and 9 meters high. It was carved with rich images of heavenly figures and musical instruments from home and abroad. These figures represented the earliest royal symphony orchestra in ancient China and played a vital role in the history of Chinese music and dance.

Starting cooperation since August 2016, the joint project team spent three months on laser-scanning the cave and took 55,680 photos of it. After photogrammetric calculation and 3D processing, a high-fidelity colored 3D model was built.

The huge cave and images of the Yungang Grottoes were carved with complex techniques, and the complicated spatial layout of the heritages in the cave also posed great challenges for scanning and surveying. Diao Changyu, deputy dean of the Cultural Heritage Research Institute of Zhejiang University, noted that the data collection of cave No. 12 has touched the technical ceiling in this field.

The massive high-precision 3D data collection, as well as huge analysis and calculation always impede digital recording. According to Diao, such huge data cannot be processed by a single software system, so the cave and images must be divided into "blocks" based on the grotto's structure and then be combined together.

However, "block type" printing had no precedent or mature solution. To ensure successful printing, a special 3D printer was tailored for the project.

What came after 3D printing was the coloring process, during which the cultural heritages in the cave were expected to be covered with proper materials, textures and colors. The current 3D coloring and printing techniques were not able to reach desired effects, so the project team opted for artificial coloring to achieve maximum authenticity.

After rounds of discussion, the Yungang Grottoes Research Institute decided to color the cultural heritages based on scientific data and experts' understanding, rather than "copying" everything from cave No. 12.

"We must make the coloring plan under the guidance of archaeology. For instance, we have to figure out the relation between the current colors and their previous appearance, as well as what caused the change," said Li Zhirong, deputy dean of the Cultural Heritage Research Institute of Zhejiang University.

To some extent, the project team was "remaking" a grotto, Li said, who believes the replication is indeed a restoring study. The project team preferred bright hues which identify with the colors when the grotto was just finished, but maintained the real grotto's weathering effects on the replica. "We remained all effective core information related to time," Li said.

After eight months of coloring with mineral pigment in ancient techniques, the staff with the Yungang Grottoes Research Institute finally completed the grand and beautiful replica cave that covers over 900 square meters.

The replica cave No. 12 consists of 110 two-cubic-meter blocks in six tiers, and each of the block weighs about two tons. The blocks can be shipped by eight standard container vehicles and assembled in one week for exhibition.

The replica is hailed as a "walking" grotto, and a Silk Road concert hall that never rings down the curtain. Experts believe that the completion of the project marked technical breakthroughs and a very important step forward of China in digital protection and inheritance of cultural heritages.