# Formation Mechanism of Crystal Spots in Jian Kiln Oil-Spot Glaze Revealed by Simulation Experiments
## 3 Results and Discussion
### Formation Process of the Crystal Spots  
Based on the analysis results of air-quenched samples, we have illustrated a schematic diagram of the crystalline spot formation process (Figure 8). The formation process of the crystalline spots includes the following five stages: First, the glaze powder begins to melt at approximately 1190 \({}^{\circ}\)C, and at the subsequent temperature of 1220 \({}^{\circ}\)C, the Fe\({}_{2}\)O\({}_{3}\) in the glaze undergoes intense decomposition, forming localized iron-rich areas. Iron-rich particles migrate to the glaze surface along with bubbles generated from the decomposition, resulting in the crystallization that forms silver-white spots. Second, with the continued increase in temperature and the effect of heat retention, the crystals precipitated on the glaze surface undergo re-melting, causing the initially formed crystalline spots to disappear, while Fe\({}_{2}\)O\({}_{3}\) continues to decompose, and iron-rich particles keep migrating to the glaze surface, leading  
Figure 7: **(a)** shows an optical photograph of the glaze surface of sample RC-7; **(b)** SEM image of region A in Figure 7a; **(c)** SEM image of region B in Figure 7a; **(d)** SEM image of region C in Figure 7a; **(e)** Mapping image of region C in Figure 7a; **(f)** RC-7-A, RC-7-B, RC-7-C are the Raman spectra of the A, B, and C regions of sample RC-7, respectively.  
to the saturation of surface iron concentration. Third, when the iron concentration on the glaze surface reaches saturation, the high-temperature reducing atmosphere intensifies the decomposition of Fe\({}_{2}\)O\({}_{3}\), generating a large number of bubbles while increasing the concentration of Fe\({}^{2+}\) and decreasing the viscosity of the glaze melt. The bubbles rapidly migrate to the glaze surface and burst, resulting in the precipitation of Fe\({}_{3}\)O\({}_{4}\) nanoparticles due to the oversaturation of iron content. Meanwhile, under the influence of surface tension, the glaze surface contracts, causing the small crystalline spots to coalesce into larger spots. Fourth, as the temperature naturally decreases to approximately 1150-1180 \({}^{\circ}\)C, the crystals within the spots are gradually oxidized, precipitating leaf-like \(\varepsilon\)-Fe\({}_{2}\)O\({}_{3}\) crystals. Since the leaf-like \(\varepsilon\)-Fe\({}_{2}\)O\({}_{3}\) crystals require more iron sources for growth, the iron content in other areas of the crystalline spots decreases, resulting in the formation of nanoparticles in these regions. Simultaneously, liquid phase separation occurs outside the crystalline spots, enriching iron and forming intermediate spherical crystals. Fifth, the leaf-like \(\varepsilon\)-Fe\({}_{2}\)O\({}_{3}\) crystals within the crystalline spots continue to increase and fill the entire spot, resulting in a yellow-brown appearance. With the increase and densification of the crystals, the crystalline spots ultimately exhibit a reddish-brown color. At the same time, to support the growth of the leaf-like \(\varepsilon\)-Fe\({}_{2}\)O\({}_{3}\) crystals within the crystalline spots, iron sources migrate into the spots, gradually depleting the external areas of iron, which ultimately appear black.