# Formation Mechanism of Crystal Spots in Jian Kiln Oil-Spot Glaze Revealed by Simulation Experiments
## 2 Experiments and Methods  
### Sample Preparation  
Based on the chemical composition analysis of Song Dynasty Jian kiln oil-spot glaze specimens [4,12], as well as previous studies [10], and in combination with field investigations of simulated Jian kiln firing techniques, the basic glaze composition (Table 1) and firing regime were determined. The experiments primarily utilized glaze stones, red clay, and plant ash sourced from Shuiji Town, Jianyang District, Fujian Province. Additionally, portions of quartz, calcined Longyan kaolin, potassium feldspar, talc, and industrial-grade iron oxide (>99 wt% purity) were incorporated to adjust the formulation, with the chemical composition detailed in Table 2.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Label** & **K\({}_{2}\)O** & **Na\({}_{2}\)O** & **CaO** & **MgO** & **Al\({}_{2}\)O\({}_{3}\)** & **Fe\({}_{2}\)O\({}_{3}\)** & **SiO\({}_{2}\)** & **TiO\({}_{2}\)** & **P\({}_{2}\)O\({}_{5}\)** \\ \hline J1 & 0.15 & 0.08 & 0.54 & 0.23 & 0.44 & 0.22 & 3.80 & 0.03 & 0.06 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Basic formulation of glaze (mol).  
The weighed raw materials were mixed evenly and wet-milled using a high-speed ball mill for 10 min (400 rpm), with a material-to-ball-to-water mass ratio of 1:2:0.8. The glaze slurry was then sieved through a 100-mesh (150 \(\upmu\)m) screen, ensuring a residue of less than 0.05% (by mass). The glaze was applied to bisque-fired samples pre-fired at 800 \({}^{\circ}\)C using a dipping method, controlling the glaze thickness to approximately 0.8-1.0 mm. The samples were then dried in an oven at 75 \({}^{\circ}\)C for 120 min. Finally, the samples were fired in an electric kiln (with a circular hole in the kiln door for sampling and inserting pinewood). Based on the firing experience, samples were taken from the kiln at eight key points during the firing process (labeled RC-1 to RC-8), and air quenching was performed to freeze the high-temperature state of the glaze. The ninth sample, RC-9, was taken after natural cooling following the end of the firing. Sample images and the firing schedule are shown in Figure 1. Starting from room temperature, the temperature was raised at a rate of 3 \({}^{\circ}\)C/min to 300 \({}^{\circ}\)C, then at 4 \({}^{\circ}\)C/min to 800 \({}^{\circ}\)C, followed by 2 \({}^{\circ}\)C/min to 1150 \({}^{\circ}\)C, and finally at 1 \({}^{\circ}\)C/min to 1315 \({}^{\circ}\)C. After maintaining the maximum firing temperature of 1315 \({}^{\circ}\)C for 60 min, 40 g of pinewood was added to the kiln every 5 min over a 30-min reduction period, after which power was cut, allowing for natural cooling. Pinewood, mainly composed of cellulose, resin acids, and lignin [10], undergoes incomplete combustion in the enclosed high-temperature kiln, producing the reductive gases H\({}_{2}\) and CO. During the reduction stage, kiln gases were sampled and analyzed using gas chromatography, yielding volume concentrations of H\({}_{2}\) and CO at 11.03% and 34.51%, respectively.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline
**Raw Material** & **SiO2** & **Al2O3** & **Fe2O3** & **TiO2** & **CaO** & **MgO** & **K2O** & **Na2O** & **P2O5** & **I.L** \\ \hline Glazed stone & 67.70 & 15.87 & 4.21 & 0.63 & 1.90 & 1.53 & 3.22 & 1.51 & & 4.06 \\ Red clay & 54.53 & 23.20 & 9.59 & 1.24 & & & 1.09 & & 10.59 \\ Grass ash & 35.85 & 11.51 & 4.11 & 0.49 & 18.45 & 5.57 & 9.17 & & 2.30 & 13.04 \\ Calciened kaolin & 57.44 & 39.93 & & 0.02 & & & 2.63 & & \\ Quartz & 99.68 & 0.32 & & & & & & & \\ Potassium feldspar & 66.86 & 17.60 & & 0.06 & & 15.09 & 0.45 & & \\ Talc & 17.00 & & & 0.01 & 2.53 & 41.94 & & & 38.53 \\ \hline \end{tabular}
\end{table}
Table 2: Chemical composition of raw materials (wt%).  
Figure 1: Firing curve and photographs of samples RC-1 to RC-9 taken at different temperature points from the kiln.