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Microstructure and Formation of Stellate Fe-Ca-Si Crystals on the Double-Layer Black-Glazed Ceramic from Song
Shasha Long1, Blythe McCarthy2, Juping Gong3
1Key Laboratory of Archaeological Sciences and Technology (Shandong University), Ministry of Education, Shandong University, Qingdao 266237, China.
Abstract:
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(Mg0.09Fe0.89Mn0.02)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.
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