XU Yi'nan ¹, YU Huan ², JIANG Caishui ³, LIU Kun ⁴, CHENG Zhipeng ^{3, 5},

FANG Yuan ⁵, TAN Liruize ¹, ZHOU Jianʼer ³

(1. Jingdezhen Comprehensive Inspection and Testing Center, Jingdezhen 333032, Jiangxi, China; 2. Jingdezhen University, Jingdezhen 333400, Jiangxi, China; 3. Jingdezhen Ceramic University, Jingdezhen 333403, Jiangxi, China; 4. Key Laboratory of Inorganic Coating Materials, 

Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050,Shanghai, China;

 5. Jingdezhen Vocational University of Art, Jingdezhen 333099, Jiangxi, China)

Extended abstract:[Background and purposes] The Yixing uniform pottery is a kind of glazed pottery fired near the Dingshu Town area of Yixing City, generally known as "uniform pottery" or "Yijun". Yijun has not been interrupted since it was fired in the middle and late Ming Dynasty, among the five "golden flowers" of Yixing ceramics. Yijun was first recorded in the Ming Dynasty in the Jiajing period in the book "Museum Essentials". There are many kinds of Yixing pottery, including sky blue, sweet white, light green and dozens of glaze colors, among which the gray-blue glaze is outstanding. Gray-blue glaze is a kind of split-phase glaze with yellowish-white markings on the surface caused by kiln changes. Yijun gray-blue glaze kiln change is mainly attributed to the use of lime kiln sweat as the raw materials. The lime kiln sweat in the CaO and P₂O₅ components of the glaze surface has a corresponding role in promotingYijun gray-blue glaze phasing, in line with the Rayleigh scattering and Mie scattering laws, resulting in glaze milky blue and milky white.At the same time, in the oxidizing atmosphere, glaze melts contained Fe³⁺, Cu²⁺, Co³⁺, leading to glaze melt color in the visible light region. Therefore, the gray-blue glaze coloration is mainly produced, due to the coupling of structural coloring and ionic coloring. This work was aimed to study the effect of divalent alkali metal oxides, with different MgO/CaO molar ratios, on phase precipitation and crystallization of the Yixing pottery gray-blue glazes, so as to analyze the internal mechanism of their color changes. When the multilevel phase precipitation is generated, both the nucleation and the crystal growth are influenced.[Methods] Yixing mud, lime kiln sweat, Longyan kaolin, quartz, limestone and chemically pure iron oxide, copper oxide, zinc oxide, dicobalt trioxide were used as the raw materials.The two main components had particle size of 397 μm and 81 μm, with the mass ratio of 1:3.Then, other raw materials were weighed and mixed according to the ratio of material, ball, and water of 1.0:2.0:0.8, by using ball milling(YDM-1 rapid ball mill)at 1400 r·min⁻¹ for10 min.The glaze slurry was passed through a 100-mesh sieve. The molar ratios of MgO/CaO included 0.10/0.78, 0.15/0.73, 0.29/0.59, 0.44/0.44, 0.59/0.29. The color parameters of the samples were analyzed by using colorimeter, while the sample glaze was subjected to stereomicroscopic analysis under 250× magnification with a VHX-6000 super depth-of-field three-dimensional microscope. X-ray diffractometer and laser Raman spectrometer were used to analyze phase compositions of the samples.A micro-area X-ray fluorescence spectrometer (M4 TORNADO) was employedto examine surface elemental profiles.The samples wereetched in 5 wt.% HF solution for 35s, followed by ultrasonic cleaning for 20 min and drying at 80℃ for 2 h. Microstructures ofthe glaze were observed using a field emission scanning electron microscope (JEM-6700FSEM), while the microstructure map was constructed using an Image-Pro Plus image processing software.[Results] With the change of MgO/CaO molar ratio from 0.10/0.78 to 0.44/0.44, the overall L* value shows a decreasing trend and surface brightness of the glaze gradually decreases. Meanwhile, with the increasing MgO/CaO molar ratio, the split-phase droplets gradually become smaller and the glaze Rayleigh scattering was weakened, so the blue part of the glaze surface gradually becomes smaller.For the mottled part of the glaze surface, with decreasing MgO/CaO molar ratio, the split-phase droplets also gradually become smaller.The glaze surface mottled at the Mie scattering is correspondingly weakened, with lower the degree of milkyness.When the MgO/CaO molar ratio is 0.59/0.44, the L* value shows an overall decreasing trend and the brightness of the glaze surface gradually decreases. At ratio of 0.59/0.29, the glaze surface has silver blocky mottling, with metallic luster, resulting in a higher L* value. Due to the high temperature viscosity of the glaze, the generated O₂ escapes on the surface of the glaze and forms a certain number of pits.In this case, the glaze surface has a high content of iron, which is enriched with iron phases in the pits.After cooling due to the supersaturation, it precipitates into crystals in the form of magnetite, hematite and other forms,creating crystalline films that produce a full-wave band of strong reflections on the visible light, thus resulting in silvery-white patina. With increasing molar ratio of MgO/CaO, the degree of glassiness of the enamel gradually decreases, leading to a gradual decrease in the gloss of the glaze.[Conclusions] The molar ratio of MgO/CaO has a significant effect on the coloration of gray-blue glaze of Yixing uniform pottery. In oxidizing atmosphere, the glaze coloration is mainly produced through the co-coupling of structural color and ionic coloring. With increasing molar ratio of MgO/CaO, the generation and growth of split-phase droplets are suppressed, while the glaze color changes from blue-green to blue to brown, due to the effects of the Rayleigh scattering and the Mie scattering. When the molar ratio of MgO/CaO is 0.59/0.29, the glaze forms uneven silvery-white mottles, which is ascribed to the high temperature viscosity of the glaze caused by the high molar ratio of MgO/CaO, making the glaze surface uneven. High molar ratio of MgO/CaO inhibits the generation and growth of the phased droplets. The structure of the glaze surface produces multi-stage phase separation, which promotes the nucleation and crystal growth of the glaze, making the glaze surface easier to precipitate the crystals of iron oxide, the formation of crystal thin film and strong reflection of light, thus resulting in silver-white color.

Key words:Yixing pottery; gray-blue glaze; split-phase emulsion; precipitation of crystals; coloration mechanism