CHEN Yulai ¹, WEN Rui ², ZHANG Maolin ³, WANG Lihua ⁴

(1. Shaanxi Academy of Archaeology, Xi'an 710054, Shaanxi, China; 2. School of Cultural Heritage, Northwest University, Xi'an 710127, Shaanxi, China; 3. Research Center of Ancient Ceramic, Jingdezhen Ceramic University, Jingdezhen 333403, Jiangxi, China; 4. Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China)

Extended abstract:

[Background and purpose] Sky-blue glaze is one of the most celebrated glaze colors in the history of Chinese celadon, which represents a major technological and aesthetic achievement after the secret-color wares of the Yue Kiln. The Yaozhou Kiln, one of the most important ceramic production centers in northern China, began producing sky-blue glazed porcelain during the Five Dynasties period, therefore occupying a key position in discussions of the origin, formation and transmission of sky-blue celadon technology. Although the chemical composition and coloring mechanism of Ru ware have been systematically studied, the glaze recipe, technological pathway and color-forming mechanism of the Five Dynasties sky-blue porcelain from the Yaozhou Kiln remain insufficiently clarified. In particular, it is necessary to explain how Yaozhou potters transformed earlier celadon glaze into sky-blue glaze through recipe modification and firing control and how this technological route relates to the later development of sky-blue glaze at the Ru Kiln. Based on the chemical composition, Fe valence state and glaze microstructure of Yaozhou sky-blue porcelain shards, the present study was aimed to clarify the glaze formulation and color-generating mechanism of this ware, thus reconstructing the technical innovations involved in the transition from celadon to sky-blue porcelain during the Five Dynasties. Yaozhou sky-blue glaze was also compared with Ru ware in order to reveal both the common principles and the kiln-specific features of their technological development.

[Methods] Fourteen shards of sky-blue glazed porcelain from the Yaozhou Kiln were selected for scientific analysis, including eleven specimens dated to the Five Dynasties and three dated to the transitional period from the end of the Five Dynasties to the beginning of the Northern Song dynasty. The samples include bowls, dishes, plates, rims and zhan, preserving technological features, such as decorative slip, spur marks, crackles and palm-eye marks. The major and minor elemental compositions of the glaze layers were determined by using energy-dispersive X-ray fluorescence spectroscopy with a BRUKER ARTAX 400 micro-XRF instrument. Measurements were performed under Rh target radiation at 30 kV and 900 μA in a helium atmosphere for 300 s with a beam diameter of 1 mm, while Corning glass standards were used for calibration. To determine the oxidation state of iron in the glaze, X-ray absorption fine structure spectroscopy was conducted at beamline BL14W1 of the Shanghai Synchrotron Radiation Facility, using Fe, Fe₂O₃ and Fe₃O₄ as reference standards. In addition, polished glaze cross-sections were examined by using SEM-EDS after cutting, polishing, HF etching, rinsing and ultrasonic cleaning, so as to reveal the crystallization and phase-separation features of the glaze. The analytical results were then compared with literature compositional data for the Five Dynasties Yaozhou celadon, official Ru ware and civilian Ru ware. Parameters, such as the SiO₂/Al₂O₃ molar ratio and the RO coefficient were used to identify the direction of glaze reformulation from celadon to sky-blue glaze.

[Results] Chemical composition of the Yaozhou sky-blue glaze is relatively stable, suggesting a controlled and consistent glaze recipe. The average composition is 63.34 wt.% SiO₂, 12.65 wt.% Al₂O₃, 2.58 wt.% Fe₂O₃, 14.35 wt.% CaO, 1.18 wt.% MgO, 3.54 wt.% K₂O and 0.42 wt.% Na₂O. Relatively high contents of MgO, MnO and P₂O₅ indicate that plant ash was an important fluxing component in the glaze. Compared with the Five Dynasties Yaozhou celadon, the sky-blue glaze contains less SiO₂ but more Al₂O₃, CaO and K₂O. The average SiO₂/Al₂O₃ molar ratio decreases from 11.17 in celadon to 8.54 in sky-blue glaze, indicating a relative decrease in high-silica components or in the SiO₂ /Al₂O₃ ratio, while clay-related components were increased and the flux proportion was adjusted. At the same time, the RO coefficient reveals a tendency for the glaze system to evolve from calcium glaze toward calcium-alkali glaze, which would have broadened the melting range, increased the high-temperature viscosity of the melt and hence helped maintain a thicker and more uniform glaze layer. The Yaozhou sky-blue glaze is a crystallization-phase separation glaze. During firing, anorthite crystals precipitated in the glaze, which were commonly accompanied by phase-separated structures around crystal margins and in intercrystalline spaces. These phase-separated domains often appear as droplet-like particles with diameters of about 100 nm, satisfying the conditions for Rayleigh scattering, whereas the crystal size falls within the range for Mie scattering. Such a combined crystallization-phase separation structure provides the physical basis for the glaze’s opalescent appearance and jade-like texture. This also implies that Yaozhou potters had mastered a controlled cooling process that allowed sufficient time for both crystal precipitation and phase separation. XAFS analysis results indicate that iron in the glaze exists in mixed valence states. Based on the absorption-edge position, Fe²⁺ is estimated to account for about one-third of total iron. This proportion is lower than that reported for Ru glaze, in which Fe²⁺ accounts for about one-half of total Fe, indicating that although Yaozhou also employed reduction firing, its reducing atmosphere was less effectively controlled than that of the Ru kiln. Comparison with Ru official ware and civilian Ru ware shows that Yaozhou and Ru sky-blue glazes ultimately converged toward very similar compositional proportions, but possibly through different compositional adjustment pathways: Yaozhou’s reduced content of quartz on the basis of its celadon recipe versus Ru ware’s increased proportion of clay materials on the basis of the Linru ware recipe. In firing technology, the two kiln systems also shared several notable features, including wood firing instead of coal, full-glaze support firing and secondary firing.

[Conclusions] The production of Five Dynasties sky-blue glaze at the Yaozhou kiln was the result of deliberate glaze reformulation and sophisticated firing control rather than accidental variation. Firstly, potters adjusted the proportion of fluxing agents, so that the glaze evolved from a calcium glaze toward a calcium-alkali glaze, while simultaneously reducing quartz input and lowering the SiO₂/Al₂O₃ ratio. This promoted the formation of crystallization-phase separation structures and provided the material basis for the opalescent, jade-like texture and sky-blue appearance of the glaze. Secondly, the glaze contains mixed-valence iron, with Fe accounting for about one-third of the total Fe, confirming that reduction firing was essential to color generation but also showing that the reducing atmosphere in the Yaozhou Kiln was weaker than that in the Ru Kiln. Thirdly, although the Yaozhou and Ru Kilns followed different glaze-reformulation routes, they arrived at highly similar compositional characteristics and shared several firing practices, such as wood firing, full-glaze support firing and secondary firing. These findings not only clarify the technological mechanism of sky-blue glaze of the Yaozhou Kiln, but also provide important evidence for understanding the developmental relationship between the Yaozhou and Ru Kilns in the history of Chinese celadon technology.

Key words: sky-blue glaze; Yaozhou Kiln; Ru Kiln; glaze recipe; XAFS; crystallization-phase separation