Microstructural and spectroscopic characterization of ancient ivory from the Sanxingdui site: insights into taphonomy and ritual burning

The recent discovery of over 700 ivory tusks at the Sanxingdui sacrificial pits (Sichuan, China) has garnered global attention. These artifacts exhibit unique preservation states and can be categorized into three types based on their appearance and coloration: white, green, and black. This study employs a multi-analytical approach—combining SEM-EDS, XRF, FTIR, Raman spectroscopy, and XRD—to elucidate the microstructure, composition, and taphonomic history of these artifacts. Results indicate that all ivory samples have undergone significant diagenesis, characterized by the complete hydrolysis of the organic collagen matrix and the deformation of dentinal tubules, while retaining a mineral phase of poorly crystalline carbonated hydroxyapatite (Type B). We determine that the coloration mechanisms are distinct and environmentally conditioned. The “green” ivory owes its hue to the infiltration of copper ions released from co-deposited bronze artifacts, distinguishing it from the manganese-stained osteological remains found at other ancient sites. For the “black” ivory, spectroscopic evidence (prominent Raman D and G bands) confirms the presence of graphitic carbon, indicative of burning. However, the absence of high-temperature mineral phase transformations and the retention of low crystallinity suggest a process of incomplete combustion. We propose that this resulted from the “Liaoji (燎祭)” ritual, where ivory was rapidly buried alongside hot ash, undergoing carbonization under hypoxic conditions. These findings provide critical material evidence supporting the archaeological hypothesis of “burning sacrifices outside the pit and burial inside the pit,” offering new scientific insights into the sacrificial practices of the ancient Shu civilization.