Microanalysis of Dust Deposition inside Emperor Qin's Terra-Cotta Warriors and Horses Museum

Indoor dust deposition, including long-term (>10 years) deposition, short-term (similar to 0.5 year) deposition, and pigment flake samples from partially restored warriors were collected in Emperor Qin's Terra-cotta Warriors and Horses Museum in Xi'an, Shaanxi Province, China. Morphological and elemental analyses of individual particles were performed with scanning electron microscopy and energy dispersive X-ray spectrometry to investigate their composition and potential to damage the statues. Most of the indoor dust was composed of clay minerals, fly ash, and biogenic particles. Particles in 51.5% of the short-term deposition, and 49.5% of the long-term deposition contained elemental sulfur. Particles which contained sulfur were mostly associated with calcium sulfate in an internally mixed state with clay or quartz. Crystals of calcium sulfate were also found near interconnected pits and cracks on the outer surface of pigment flakes, revealing an acidic chemical reaction between sulfur dioxide and pigment material or deposited particles as the cause of pits and cracks on the statues' faces.