Analyzing the surface black discoloration of oracle plastrons from the Taijiasi Site, China

Abstract Several Shang and Zhou Dynasty oracle plastrons from the Taijiasi Site (Funan, Anhui, China) exhibit random flake- or patch-like surface black deposits unlikely induced by burning or pyromancy. Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) and X-ray photoelectron spectroscopy (XPS) identified the elements causing the black color as Fe and Mn. The chromogenic mechanisms were explored via micro-X-ray fluorescence spectroscopy (μ-XRF) mapping analysis of the distribution of Fe, Mn, Al, etc. The Fe ions likely existed as trivalent oxide precipitates and Mn 4+ in its typical coordination mode in oxides. Thus, the black discoloration resulted from the enriched Mn and Fe, oxidation-reduction reactions, and microbial activities in the burial environment. Colloidal form Mn 4+ and Fe 3+ adsorbed on the plastron surface and complexed with humic acids from organic matter degradation to significantly darken the color. The results could effectively support the research, conservation, and restoration of oracle plastrons.