Multi-Technique Analysis and Research on the Deterioration Features of Cave 29 in the Yungang Grottoes

The Yungang grottoes (5th to 6th centuries AD) were one of the largest and most artistically valuable ancient grotto complexes in China and were a precious World Cultural Heritage site. Influenced by the natural environment and human factors, various deterioration features appeared in the Yungang grottoes. Taking Cave 29 as the research object, field investigations were conducted to analyse the deterioration features and their distribution within Cave 29. To further analyse the formation mechanisms of deterioration features and the preservation conditions, multi-technique testing methods were used. Infrared thermal imager clearly identified hollowing, cracks and surface salt efflorescence zones. Leeb hardness testing on Cave 29’s walls gave average rebound values of 283–308 HLD, all values were lower than the average rebound values of fresh rock (661.4 HLD), indicating weathering-induced surface hardness reduction. Ultrasonic testing showed the east wall had the highest wave velocity (avg. 1928 m/s), indicating the best preservation, while the south wall had the lowest (avg. 1378 m/s), indicating the most severe weathering. Microscopic observations, ion chromatography measurements, and p-XRF were preliminarily determine that CaSO4 could be preliminary inferred as the dominant soluble salt phase present at this site. All these findings provided scientific data to support preventive protection and scientific conservation.