The formation of oil spot patterns on Jianzhan ceramics during the Song Dynasty is attributed to the enrichment of iron oxide on the surface of the glaze, followed by the crystallization of hematite and magnetite upon cooling. The formation process can be described as follows:
1. Formation of bubbles in the glaze:
Oil spots are traces left by bubbles that emerge from the glaze. These bubbles crystallize around iron trioxide (Fe2O3). The formation of bubbles results from the interaction between the glaze and the cup body.
In the case of black glaze porcelain, the high content of Fe2O3 leads to its decomposition into magnetite (Fe3O4) and oxygen gas when the kiln temperature reaches above 1000°C. A significant production of oxygen gas occurs when the temperature reaches 1200-1250°C.
From the firing process of oil spot Jianzhan, it can be understood that after the oil spot glaze melts at high temperatures, the glaze surface begins to bubble.
As the temperature rises, the bubbles in the glaze increase in size. Subsequently, the bubbles burst, creating depressions.
If the heating is stopped at this point, the cooled glaze surface will appear pitted. It is necessary to continue heating to allow the surface to become smooth again. The bubbling of the oil spot glaze results from the decomposition of iron oxide, producing oxygen gas.
2. Growth of bubbles:
The decomposition reaction of Fe2O3 in the oil spot glaze occurs when there is a high Fe2O3 content and enrichment. This reaction leads to the production of bubbles.
Therefore, the iron content in the oil spot glaze must be as high as 6-8% to generate sufficient bubbles. Around the bubbles, the concentration of iron oxide is higher than in other areas.
As the decomposition reaction proceeds, the bubbles gradually grow, or smaller bubbles merge to form larger ones, with an increasing concentration of iron oxide around them.
3. Upward movement of iron oxide to the glaze surface:
When the bubbles have grown to a certain size, the enrichment of iron oxide in the vicinity alters the composition of the glaze layer, reducing its viscosity and surface tension.
As a result, the bubbles rise to the glaze surface and burst. Afterward, the glaze surface becomes smooth again. During this process, the molten iron oxide-rich melt around the bubbles cools and precipitates as hematite or magnetite, forming the oil spot patterns commonly seen on Jianzhan ceramics.
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