Journal Pre-proof Study on the coloring mechanism of the Ru celadon glaze in the Northern Song Dynasty Pei Shi, Fen Wang, Biao Zhang, Hongjie Luo, Jianfeng Zhu, Guiqiang Fei PII: S0272-8842(20)31798-3 DOI: https://doi.org/10.1016/j.ceramint.2020.06.139 Reference: CERI 25566 To appear in: Ceramics International Received Date: 5 June 2020 Revised Date: 11 June 2020 Accepted Date: 12 June 2020 Please cite this article as: P. Shi, F. Wang, B. Zhang, H. Luo, J. Zhu, G. Fei, Study on the coloring mechanism of the Ru celadon glaze in the Northern Song Dynasty, Ceramics International (2020), doi: https://doi.org/10.1016/j.ceramint.2020.06.139. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. 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Study on the coloring mechanism of the Ru celadon glaze in the Northern Song Dynasty Pei Shi a, Fen Wang a ∗, Biao Zhang a, Hongjie Luo b, Jianfeng Zhu a, Guiqiang Fei c a Shaanxi Key Laboratory of Green Preparation and Functionalization for Inorganic Materials, School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi'an 710021, PR China b Research Institute of Cultural Relics, Shanghai University, Shanghai 200444, PR China c College of Chemistry and Chemical Engineering, Shaanxi University of Science & Technology, Xi'an 710021, PR China Abstract The Ru kiln is one of the five most famous kilns in the Chinese Song Dynasty. To clarify the coloring mechanism of the Ru celadon glaze, the celadon samples from the Ru Guan kiln site of Qingliangsi were analyzed by spectrophotometer, X-ray fluorescence spectrum, Raman spectroscopy, scanning electron microscopy and X-ray photoelectron spectrum. The results indicated that the molar ratios of SiO 2/Al 2O3 and CaO/(CaO+Al 2O3) affected the formation of micro-bubbles, anorthite crystals and phase separated structures. A large number of bubbles and anorthite crystals formed special glossiness and opacifying effect in the Ru celadon glaze. And then, dense phase separation droplets in the amorphous region were in short-range order, but their diameters (31-46 nm) were too small to form visible structural colors on glaze surfaces. Only “opal effect” was formed by the light scattering, which added the aesthetic feeling for the Ru celadon. Besides, the phase separation droplets intensified the segregation of iron, and thus deepened the chemical color and made the Ru celadon glaze appear green-blue. Due to the neutral to alkaline soil at the Ru Guan kiln site, the water in the soil and its corrosion on the Ru celadon glaze resulted in the formation of Si-OH bond. ∗ Corresponding author: Tel.: +8602986131687; fax: +8602986168188. Email addresses: [email protected] (Fen Wang) 1 Keywords: Ru Guan kiln; Qingliangsi; Celadon glaze; Northern Song Dynasty; Coloring mechanism 1. Introduction Ru kiln, located in Qingliangsi, Baofeng Country, Henan Province, is one of the five most famous kilns (Ru, Guan, Ge, Ding and Jun kilns) in the Chinese Song Dynasty (A.D. 960-1279) [1]. It started from the earlier Song Dynasty, matured in the late Northern Song Dynasty and stopped development in the late Yuan Dynasty. In the later period of the Northern Song Dynasty, from Yuanyou to Chongning for more than 20 years, Ru kiln had been making celadon for imperial courts. The best of glaze color was sky-green, and the body color was the standard of fragrant grey. In the late Northern Song Dynasty, the Ru celadon was with more fresh color and more exquisite craftsmanship. At that time, it had the reputation of “Ru kiln is the chief” [2]. The researches on the chemical composition of the Ru celadon glaze and its raw material were systematic. Ding et al. [3] found that the celadon glaze of the Ru kiln unearthed in the Ru Guan kiln site of Qingliangsi had the characteristics of high calcium and aluminum contents, which provided the physical and chemical basis for the formation of anorthite microcrystalline cluster and thick glaze layer, and created the jade-like quality of the celadon glaze. Also, fibrous and radiating aggregates with optical properties similar to agate were observed in the Ru celadon glaze by polarization microscope. It showed that the Ru celadon glaze was not only made of glaze ash and feldspar materials with relatively high aluminum content but also mixed with agate. For coloring of the Ru celadon glaze, researchers had the same view that the glaze color was the result of the interaction of chemical color and structural color. Zhang et al. [4] investigated that the glaze color of the Ru celadon was mainly related to the concentration of structural iron ions, rather than that of the Fe 2O3 and Fe 3O4. As the Fe 2+ /Fe 3+ ratio gradually increased, the glaze color of the Ru celadon would gradually change from pea green to sky green. Li et al. [5] revealed that the Ru celadon glaze belonged to crystalline and phase separation glaze for the first time. A dual coloring mechanism was in effect for the Ru celadon glaze, covering chemical 2 compositional coloring by iron ion chromophore and structural coloring by structural inhomogeneities including nano-sized phase separation structures and micro-sized crystals, bubbles and inclusions. Yang et al. [6] confirmed the study of Li et al. and indicated that there were amount of crystallites and phase separation structures in the Ru celadon glaze, the former met Mie scattering conditions and the latter met Rayleigh scattering conditions. Nevertheless inspired by structural colors of amorphous arrays in the biological world, Yin et al. [7] proposed the phase-separated structures with short-range order could form the structural color in the glaze for the first time. It was the physical origin of opalescence blue in the Ru celadon glaze. However, until now, few studies have been made to interpret the relationship between composition and structure-property and to explore the coloring mechanism of the Ru celadon glaze. In this paper, the apparent performance, chemical composition, microstructure and chemical state of iron ions of Ru celadon glazes with different colors were compared, and the main factors affecting the appearance of Ru celadon glaze were studied. In addition, the interaction between the chemical color and the structural color, as well as the internal relationship between them and the glaze color were analyzed. Based on these, the color mechanism of the Ru celadon glaze in the Northern Song Dynasty was revealed comprehensively and systematically. 2. Experimental 2.1 Information about the samples All of the samples excavated from the Ru Guan kiln site of Qingliangsi, kindly loaned by the Palace Museum in Beijing. The photos and information of the samples were shown in Fig. 1 and Table 1. 2.2 Methodology The glaze color of samples was analyzed by spectrophotometer (X-Rite Ci7800, America). The measuring aperture was 6 mm. The chemical composition were tested by X-ray fluorescence (XRF, XGT-7200V, Japan) with Rh target. The molecular structure of the glass phase was studied by confocal Raman spectroscopy (Raman, Renishaw-invia, England). The laser wavelength was 532 nm, and the laser power 3 was 3.35 mW. And then, the crystalline phases were identified by X-ray diffraction (XRD, D/max 2200PC, Japan) with Cu K α radiation ( λ=1.5406 Å) under 40 kV and 100 mA. The scanning speed was 8º/min and the step degree was 0.02º. Microstructure and phase compositions were performed by using scanning electron microscopy (SEM, S4800, Japan) equipped with an energy dispersive spectrometry (EDS). Before the testing, the surfaces of samples were etched in 1 vol% HF for 40 s to expose the crystals and phase separation structures, and then sprayed platinum to make the glaze surfaces conductive. Finally, the valence and coordination state of iron ions in the glaze were analyzed by ultraviolet-visible-NIR (UV-vis-NIR) spectrometer (Cary 5000, America) and X-ray photoelectron spectrum (XPS, VG Scientific, England) with Al K α radiation. 3. Results and discussion 3.1 Relationship between chemical composition and apparent performance Table 2 and Fig. 2 show the colorimetric values (L*, a*, b*, C*, h*) of samples and the corresponding scatter diagrams of L*, C* and h*. From Fig. 2 (a), L* values of all samples were between 56 and 66, and increased in the order of R1, R3, R2 and R4. From Fig. 2 (b), the glaze colors of samples were located in the yellow-green-blue area and close to the green axis. By comparing h* of samples, the green-blue tone of samples decreased in the order of R2, R1, R4 and R3. Due to the small difference in body colors of the samples, their glaze colors were mainly determined by the chemical composition, microstructure and the content of different valence iron ions. Table 3 presents the glaze compositions of these samples. The content of SiO 2 changed obviously, from 70.65 wt% to 75.24 wt%. Next were Al2O3 and CaO with contents of 11.56-14.79 wt% and 6.52-8.26 wt%, respectively. And the difference of the Fe2O3 content was not huge. Also, the samples contained 1.49-2.71 wt% MgO+P 2O5+MnO 2, indicating that a certain amount of plant ash was used as a flux for the Ru celadon glaze [8].