Deciphering the composition and microstructure of Jin-Yuan period oil-spot glazes from the archaeological site of Jining Lu of China

This study investigates the structure–property relationship in Jin-Yuan period (1115–1368 CE) oil-spot glazes from the archaeological site of Jining Lu to elucidate the scientific principles governing their distinctive coloration patterns. Through multi-scale characterization of 20 ceramic fragments using ultra-depth microscopy, SEM–EDS (Scanning Electron Microscopy-Energy Dispersive Spectrometer), XRD (X-ray Diffraction), WDXRF (Wavelength-dispersive X-ray fluorescence) and XPS (X-ray photoelectron spectrometer) , we establish fundamental correlations between processing conditions, crystalline phases, and optical properties. The ceramic bodies exhibit a characteristic high-alumina composition (Al2O3: 24.5–34.0 wt%), reflecting regional clay sources and manufacturing traditions. Glaze chemistry reveals intentional flux optimization, with high CaO content (4.5–7.2 wt%) and Fe2O3 enrichment facilitating liquid-phase sintering. Most significantly, we demonstrate that the silver/red dichroism is predominantly governed by atmospheric conditions modulating Fe2+/Fe3+ ratios and crystal polymorphism:(i) In a strong oxidizing atmosphere, vapor-phase deposition produces specular α-Fe2O3 (hematite) microcrystals (2–5 μm), yielding metallic luster; (ii) In a weak reducing-weak oxidizing atmosphere, co-precipitation of dendritic α-Fe2O3 and magnetoelectric ε-Fe2O3 (luogufengite) is promoted, generating distinct chromatic contrast. These findings provide mechanistic insights into ancient craftsmen's empirical control of phase transformations in iron-oxide systems.