Microstructure and Formation of Stellate Fe–Ca–Si Crystals on the Double-Layer Black-Glazed Ceramic from Song Dynasty, China

Glaze crystallization is a key surface decoration technique in black-glazed ceramics, reflecting the advanced craftsmanship of ancient ceramic production. In this study, a black-glazed ceramic sherd from the Song Dynasty (960-1279 C.E.), excavated in Nanjing, China, was systematically analyzed using optical microscopy, macro-X-ray fluorescence (Macro-XRF) imaging, and field-emission scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (FESEM-EDS) and Raman spectroscopy. The results reveal well-shaped stellate crystals within the cover glaze, which are comprised of oval crystals, brown prismatic crystals, fibrous crystals, and reddish-brown spherical crystals. These crystals are identified as α-cristobalite, rich-Ca and low-Mg hedenbergite [Ca(Mg_0.09Fe_0.89Mn_0.02)Si₂O₆] and rich-Ca spinel ferrite [(Ca₀.₆Mg₀.₄)Fe₂O₄], respectively. A potential growth mechanism for these structures is proposed. The α-cristobalite nuclei serve as a structurally compatible, low-energy substrate for hedenbergite nucleation. Local enrichment of Ca and Fe, combined with liberated SiO₄ units, supplies the components for hedenbergite growth, which proceeds radially as prismatic and fibrous aggregates around the cristobalite core. As crystallization advances, localized Fe enrichment and micro-oxidizing conditions at the hedenbergite-glass interface promote the precipitation of Ca-rich spinel ferrite as submicron spheres intergrown with hedenbergite. Prolonged cooling further enhances this intergrowth, ultimately producing the well-ordered, multilevel stellate structures observed.