Environmental protection and sustainable development have attracted recent attention. Especially, the lead-free reproduction of ancient ceramic glazes with rich historical and cultural values becomes a common concern for both academia and industry. Jizhou kiln is one of the well-preserved ancient kiln sites in China, and is one of the famous folk firing kilns during the Song and Yuan dynasties. Jizhou kiln green glaze is colored in addition to the chemical composition as well as coloring mechanism. The sintering temperature is also one of the key factors influencing the ceramic glaze color, microstructure, and physical properties. However, the current research on the sintering process of copper-green glaze is still insufficient. The lead-free preparation of green glaze from the Jizhou kiln was achieved using the conventional ceramic raw materials in the Jingdezhen area and Cu²⁺ coloring. The influence of sintering temperature on the formation of glaze color, microstructure, and color-presenting mechanism of lead-free green glazed ceramics, as well as the mechanism of the effect of sintering temperature on the glaze's physical properties were systematically investigated. This study provides a scientific guidance and a technical support for the lead-free reproduction of green-glazed porcelain of Jizhou kiln to further promote the protection and inheritance of traditional ceramic culture, and to provide an important reference basis for the sintering behavior of other types of glazes.

A base glaze was prepared using 23% (in mass fraction, hereinafter the same) wollastonite, 9% talc, 20% feldspar, 26% quartz, 10% kaolin, 8% smelter's ash, and 4% zinc oxide (ZnO) from mineral raw materials originating in the Jingdezhen area of Jiangxi Province, China, with 4% CuO as a coloring agent. ZnO (99% analytically pure), CuO (99% analytically pure), and glazes were applied to blanks by an impregnation method (blanks were already 900 ℃ plain firing), and the impregnation time was 20 s. Subsequently, the samples were put into a box-type resistance furnace and sintered in air. Six lead-free green glaze samples sintered at different temperatures (i.e., 1 250, 1 260, 1 270, 1 280, 1 290 ℃ and 1 300 ℃) were obtained.

The chemical composition of the mineral raw materials was analyzed by energy-dispersive X-ray fluorescence spectrometry. The phase structure of the sample glazes was analyzed by X-ray diffraction and micro-confocal laser Raman spectrometry. The optical photography of the sample surfaces was carried out using a super depth of field microscope. Scanning electron microscopy was used to observe the microstructure of the samples after etching with 5% HF solution for 20 s as well as the fractional phase structure in the glaze. The chromaticity values and reflectance spectra of the glazes were determined by benchtop spectrophotometry as well as a UV-Vis-IR reflectance spectrophotometry. The elemental valence states were analyzed by X-ray photoelectron spectroscopy.

The optical analysis indicates that the glaze surface changes from greenish blue to light green and green with the increase of sintering temperature, and reaches the optimum effect of glaze surface at 1 280 ℃, at which time the glaze surface of the samples is smooth and uniformly colored, with a strong sense of glassy feeling. The results of XRD patterns and Raman spectra show that the glaze layer is composed of a glassy phase and trace SiO₂ crystals, and the phase of glaze layer is not a main factor of color presentation. The structural analysis shows that the glaze layer is not a homogeneous material, it is composed of two kinds of phase separation structures, i.e., round and worm-like. The phase separation structure shrinks and grows further to form a large-size round phase separation droplet, and the incident light undergoes Mie scattering with the increase of sintering temperature and the gradual decrease of high-temperature viscosity of the glaze layer, which produces a white opalescence and deepens the glaze surface coloration. Lead-free green glaze is mainly due to the effect of Cu²⁺ color presentation. As the sintering temperature increases, the Cu²⁺ concentration gradually increases, and copper ions and six bridging oxygen (O^b) form a six-coordinated structure, mainly presented in green. The coloration of lead-free green glaze is a result of the synergistic coupling of the phase separation structure and the valence state of copper ions, and the lead-free green glaze with a stable coloration and a good glaze quality can be prepared via optimizing the sintering temperature.

Conclusions The viscosity of the glaze layer gradually decreased, and the glaze surface showed greenish blue, light green, and green in turn, and the optimum effect of the glaze surface reached at 1 280 ℃ as the sintering temperature increased. The glaze was mainly dominated by the green coloration of Cu²⁺, and the molar ratio of Cu⁺/Cu²⁺ gradually increased, while the glaze changed from blue-green to lime green as the sintering temperature increased The round and worm-like phase-separated structures both appeared in the samples, and the phase-separated structures contracted at higher sintering temperatures and grew further into large-sized round phase-separated droplets, with Mie scattering occurring to produce a white opalescence that deepened the glaze coloration; The color-presentation mechanism of the lead-free green glaze was a result of the synergistic coupling of copper ion valence and phase-separated structures.