# Formation Mechanism of Crystal Spots in Jian Kiln Oil-Spot Glaze Revealed by Simulation Experiments
## 3 Results and Discussion
### Microstructure and Phase Composition
Figure 5: (**a**) shows the microstructure of the crystalline spot region on the glaze surface of sample RC-5; (**b**) illustrates the magnified microstructure of the rectangular area in (**a**); (**c**) presents the Raman spectrum of the crystalline spot region of sample RC-5. Figure 6: (**a**) shows the fitted peak spectrum of the Fe 2p orbital for sample RC-4 glaze; (**b**) displays the fitted peak spectrum for sample RC-5 glaze. Figure 7a presents an optical micrograph of the glaze surface of sample RC-7, which is distinctly divided into the yellow-brown crystalline region A within the crystalline spots, the black region B, and the yellow-brown flowing pattern C outside the crystalline spots. The yellow-brown region A within the crystalline spots is filled with palm leaf-like crystals measuring approximately 100-300 \(\upmu\)m (Figure 7b), while the black region B contains nano-sized grains measuring 60-80 nm (Figure 7c). The yellow-brown flowing pattern consists of large crystalline spheres measuring about 1 \(\upmu\)m, composed of smaller crystalline fragments measuring 40-80 nm (Figure 7d). The EDS analysis of spot 3 results (Table 4) indicate that the yellow-brown leaf-like crystals have an Fe\({}_{2}\)O\({}_{3}\) content of 64.58 wt%, the nano-sized grains in the black region B have an Fe\({}_{2}\)O\({}_{3}\) content of 8.48 wt% (spot 4), while the crystalline spheres (spot 5) contain 28.39 wt% of Fe\({}_{2}\)O\({}_{3}\). The elemental distribution map of the crystalline spheres (Figure 7e) further confirms that they are iron-rich, oxygen-rich, and silicon-poor iron oxide crystals. The formation of the crystalline spheres may result from liquid phase separation in the glaze melt during cooling, leading to the precipitation of iron-rich phase crystals. The Raman spectrum in Figure 7f indicates that the yellow-brown leaf-like crystals in region A exhibit characteristic peaks of \(\varepsilon\)-Fe\({}_{2}\)O\({}_{3}\) at 125, 174, 244, 356, 446, 464, 510, 584, 698, 781, and 1398 cm\({}^{-1}\)[16; 24; 25; 26]. The Raman spectra of regions B (black) and C (yellow-brown flowing pattern) also display peaks at 122, 174, 240, and 1373 (1384) cm\({}^{-1}\) similar to those of the reddish-brown crystals, albeit with weaker intensity, indicating that all three regions contain an \(\varepsilon\)-Fe\({}_{2}\)O\({}_{3}\) crystalline phase. The Raman spectra of regions B and C also exhibit weak T\({}_{2g}\) and A\({}_{1g}\) vibrational bands of Fe\({}_{3}\)O\({}_{4}\) at 339 and 685 cm\({}^{-1}\), respectively. Additionally, the Raman spectrum of region C shows the E\({}_{g}\) vibrational mode of \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) at 620 cm\({}^{-1}\). The coexistence of these characteristic peaks indicates that the precipitated crystalline phases are in an unstable state, with incomplete crystal development, still in the transitional phase of phase transformation [27]. As the temperature inside the kil gradually decreases, the crystallization time also correspondingly extends. The continuous growth of the leaf-like \(\varepsilon\)-Fe\({}_{2}\)O\({}_{3}\) crystals precipitating within the patches requires additional iron sources (the iron required for the crystallization of the glaze is collectively referred to as iron sources, which include iron ions or iron-rich particles). This leads to a decrease in iron content in other areas of the patches, resulting in relatively smaller crystal sizes within the black regions. However, liquid phase separation occurs outside the patches, serving to enrich iron and continuously supplying iron sources to the interior of the patches for the growth of leaf-like \(\varepsilon\)-Fe\({}_{2}\)O\({}_{3}\) crystals. As the iron sources from the exterior of the crystal spots gradually migrate into the interior, the spots eventually become filled with micron-sized leaf-like \(\varepsilon\)-Fe\({}_{2}\)O\({}_{3}\) crystals, exhibiting the inherent yellow-brown color of \(\varepsilon\)-Fe\({}_{2}\)O\({}_{3}\) crystals.