LIU Guangfu 1, HE Xuanliang 2, FU Xiangyang 3
(1. Henan Key Laboratory of Research for Central Plains Ancient Ceramics, School of Ceramics, Pingdingshan University, Pingdingshan 467000, Henan, China; 2. School of Material Science and Engineering, Shaanxi University of Science & Technology, Xi'an 710021, Shaanxi, China; 3. School of Art and Design, Shaanxi University of Science & Technology, Xi'an 710021, Shaanxi, China)
Extended abstract:[Background and purposes] Lushan speckle porcelain was created in the early Tang Dynasty, while the kiln site was located in Duandian Village, Lushan County, Henan Province, China. Lushan speckle porcelain pioneered the phase-separated glaze technology in China, which has a great influence on the surrounding kiln and the porcelain industry in later generations. Lushan speckle porcelain glaze was of a double-layer structure, which was glazed on the body twice. Firstly, the bottom-glaze (loess raw material with high iron content) was applied on which different surface-glazes (paste mineral raw material) were applied after drying. When fired at a high temperature, the glaze could form mottled effects with different postures. These studies on Lushan speckle porcelain from the Tang Dynasty have mainly been focused on chemical composition, microstructure and coloring mechanism. However, there was few information on the diffusion of coloring elements in double-layer glaze, while the influence of chemical composition in bottom-glaze on the diffusion of coloring elements had not been fully studied. The main research object of diffusion in glass melt was alkali metal ions, but there was little research on the diffusion of chromogenic elements, especially iron. Black bottom-glaze with different chemical compositions were prepared by changing the contents of network formers (SiO2, Al2O3) and network modifier (CaO, Na2O) in black bottom-glaze. In addition, by analyzing the change of iron ion content, the diffusion mechanism of iron in black bottom-glaze with different components was studied and its diffusion coefficient was calculated.[Methods] X-ray fluorescence spectrum (XGT-7200V, Japan) was used to analyze chemical composition of the glazes. X-ray irradiation on the sample would excite the characteristic X-rays of elements, which could be qualitatively analyzed by comparing with the characteristic X-rays of each element. In addition, the intensity of characteristic X-ray was related to the content of elements in the sample, so that the sample could be quantitatively analyzed by detecting the intensity of these spectral lines. The sample should be kept dry during the test. The main test parameters include voltage of 30 kV, current of 1 mA and test depth of 10 μm.[Results] Samples with different chemical composition of bottom-glaze were selected, the interface between the two glazes was taken as the coordinate origin, 10 points were taken within 1 mm in the direction of surface-glaze and the content of iron was measured. The least square method was used for fitting. The average mass fraction of iron was changed at different depths of the surface glaze, when the SiO2 content in the bottom-glaze increased from 70 wt.% to 80 wt.%. With increasing content of SiO2, the straight line became flat obviously, while the slope after fitting decreased from 3.19 to 1.81, indicating that the diffusion rate of iron decreased with increasing content of SiO2. Meanwhile, the average iron concentration decreased at an accelerated rate. However, when the Al2O3 content increased from 3 wt.% to 9 wt.%, the slope first decreased from 3.93 to 1.26 and then increased to 2.47. With increasing content of Al2O3, the diffusion rate first decreased and then increased. With increasing content of CaO from 0 wt.% to 6 wt.%, the linear slope changed from 2.91 to 5.66, while the addition of CaO on glaze surface accelerated the diffusion rate of iron. When the Na2O content increased from 0 wt.% to 6 wt.%, the slope increased from 1.55 to 2.16, which had the same effect as CaO and promoted the diffusion of iron, but when the increase amplitude was the same, the slope increased from 1.54 to 1.74, where the increase amplitude decreased slightly.[Conclusions] When the content of SiO2 in the black bottom-glaze of Lushan speckle porcelain increased from 70 wt.% to 80 wt.%, the fitting slope decreased from 3.19 to 1.81, while the diffusion rate of iron decreased. With increasing content of SiO2, the average content of iron decreased faster. When Al2O3 content increased from 3 wt.% to 9 wt.%, the slope decreased from 3.93 to 1.26 and then increased to 2.47, while the diffusion rate of iron first decreased and then increased. When the content of CaO and Na2O increased from 0 wt.% to 6 wt.%, they both promoted the diffusion of iron. When the content of SiO2 and Al2O3 in the black bottom-glaze was too high, there were large negative ion groups in the glaze melt, which moved in a creeping way at high temperatures, so the viscosity of the melt was high and the diffusion of iron was hindered. However, when the content of CaO and Na2O was too high, the glaze melt mainly existed in the form of smaller negative ion groups, which moved in a rolling way at high temperatures, so the viscosity of the glaze melt was reduced, which was beneficial to the diffusion of iron. The diffusion of iron in glaze conformed to the law of unsteady diffusion. The network former was beneficial to the formation of network skeleton in glaze melt, reducing the diffusion channels of elements and hence decreasing the diffusion coefficient. The network modifier could break the network, increasing the number of element diffusion channels and hence contributing to the diffusion of iron.
Key words: Lushan speckle porcelain; black bottom-glaze; surface-glaze; chemical composition; iron diffusion