# Formation Mechanism of Crystal Spots in Jian Kiln Oil-Spot Glaze Revealed by Simulation Experiments
## 1 Introduction  
The Jian kiln is located in Shuiji Town, Jianyang District, Nanping City, Fujian Province, China. It was established during the late Tang and Five Dynasties and flourished in the Song Dynasty, becoming renowned for producing various types of black-glazed tea bowls [1]. Oil-spot glaze is one of the representative glaze colors of the Jian kiln from the Song Dynasty, named for the crystalline spots that resemble oil droplets floating on water. The crystalline spots of Jian kiln oil-spot glaze are mostly oval-shaped, with colors including reddish-brown, brown-yellow, and silver-white. The firing process requires precise control of temperature and atmosphere, making it technically challenging and therefore quite rare. According to ancient Japanese texts, such as the _Juntai Guan Zuoyou Zhangji_, oil-spot glaze be bowls were regarded as \"the second treasure\", valued at the equivalent of five thousand rolls of silk [2]. The oil-spot glaze bowls housed in the Osaka City Museum of Oriental Ceramics and the Seikado Bunko Art Museum have been designated as \"national treasures\" and \"important cultural properties\", respectively. Since the 1980s, research on the formation and coloration mechanisms of Jian kiln oil-spot glaze crystals has gradually increased. [PERSON] and colleagues at the Shanghai Institute of Ceramics, Chinese Academy of Sciences, were the first to conduct systematic studies on Jian kiln oil-spot glaze tea bowls. They discovered that the crystal patterns on the surface are formed by the \"boiling effect\" of iron oxide at high temperatures, leading to the oversaturation crystallization of iron oxides. The coloration of the crystal patterns is closely related to the type and size of the precipitated crystals [3,4]. [PERSON] argued that the formation mechanism of Jian kiln oil-spot glaze crystals is not solely due to the decomposition of iron oxide; rather, it involves the aggregation of multiple small crystal patterns under the influences of tension and gravity, resembling floating ducked. In contrast, the patterns of oil-spot glaze from northern kilns (such as those in Linfen, Shanxi, Zibo, Shandong, and Dingyao, Hebei) are formed by the rapid decomposition of iron oxide followed by cooling and crystallization [5]. [PERSON] et al. suggested that the formation of crystalline spots involves processes of phase separation, migration of iron-rich phases to the surface, and the growth of nanoscale magnetite and hematite crystals [6]. [PERSON] et al. were the first to identify the presence of metastable \(\varepsilon\)-Fe\({}_{2}\)O\({}_{3}\) crystals within the spots of Jian kiln oil-spot glazes, emphasizing the critical role of firing conditions in their formation [7]. Oil-spot glazes are iron-based crystallization glazes, and in recent years, many researchers have conducted extensive studies on how temperature regimes and atmospheric conditions influence their coloration [8, 9, 10]. Among them, [PERSON] prepared oil-spot glazes of varying colors by adjusting firing techniques, attributing the coloration of these crystalline spots to a coupling of structural and chemical colors [11]. Current research on the coloration mechanism of oil-spot glaze crystalline spots and their influencing factors has thus reached a relatively mature stage. However, the formation process and mechanisms of the crystalline spots in Jian kiln oil-spot glazes remain contentious and are not yet fully understood. Using simulated firing experiments allows for accurate replication of the glaze's entire reaction process. Sampling and quenching at critical temperature points can preserve the glaze morphology at high temperatures. This quasi-in situ method provides a more realistic representation of the glaze formation process. Therefore, this study aims to clarify the formation process and mechanism of crystalline spots in Jian kiln oil-spot glazes by sampling at key temperature points during simulated firing and air quenching to freeze the high-temperature morphology of the glaze layer.