Atmospheric deterioration of Qin brick in an environmental chamber at Emperor Qin's Terracotta Museum, China

Potential mechanisms for the changing appearance of Qin terracotta was evaluated on simulated bricks with, SO2, NH3, O3, and a NH3/O3 mixture for total dosages of 2.2–5.4 ppm-years. Changes in the surface composition and appearance were evaluated with scanning electron microscopy/energy dispersive X-ray analysis (SEM–EDX) and X-ray photoelectron spectroscopy (XPS). No observable changes with O3 exposure were found. Sulfate as sulfur and ammonium as nitrogen increased by 1.2 at% and 0.8 at% of the surface deposit as determined by XPS. O3 did not have a detectable effect by itself, but when combined with NH3, NO3 as nitrogen increased by 1.5 at%, indicating a strong oxidation of NH3 to nitric acid. The combination of outdoor O3 infiltration with indoor NH3, presumably from the visitors, appears to have a greater potential for damage than either pollutant by itself. SEM–EDX mapping of the surface showed sulfur associated with calcium and magnesium compounds after SO2 exposure, but not before. More detailed microscopic examination showed this primarily in the form of gypsum, especially near cracks and pits in the surface. Both of these mechanisms are probably among the causes of changes in terracotta appearances since they were unearthed in 1974.