There are several factors that affect the results of kiln firing. Amongst them are temperature and atmosphere. In this article, we will address the critical roles both the firing temperature and resulting atmosphere play in the formation of Zisha, and how both can ultimately determine its overall appearance and performance.
Firing temperature primarily influences the level of sintering on the clay, which directly contributes to the sintering atmosphere of the kiln; the sintering atmosphere of the kiln plays an integral role in the overall coloring of the clay. Due to the compositional nature of zisha, firing temperatures are normally higher than average ceramic materials, ranging from 1000℃ to 1300℃, and as there are varying types of zisha, each will have their own specific firing requirements. For example, Zhuni is known for having the lowest firing tolerance, followed by Hong Ni, and with Duanni having the highest. (NOTE: the same kind of zisha will also differ in characteristics due to their varying origin, processing, storage, aging, etc.)
Zisha must be skillfully fired in order to achieve a desired effect, however, if the right temperature is not achieved, the clay may result to being “under-fired” or “over-fired”. An “under-fired” clay generally refers to clay that was not fired under optimal firing temperatures; pieces will have a muted appearance, a dull sound when tapped, and often accompanied by an undesirably intense water absorption when used, pores are not open enough, might affect the taste of the tea. On the other hand, an “over-fired” clay refers to clay that has been exposed to temperatures too high for its composition; pieces will appear deformed and noticeably enveloped by firing bubbles/bumps and dark iron crystals on the surface.
Theoretically, there is such a thing as the “perfect” firing temperature for each clay; however, in practice, the firing temperature will simply be split between “high” and “low”, as the “perfect” temperature cannot be realistically achieved due to the sometimes fickle nature of the kiln. Moreover, the color of the clay will likely vary despite pieces being exposed to the same kiln temperature at the same time, as the unique placement of each piece within the kiln certainly affects the extent of firing exposure.
Some features worth noting to determine if the clay has been fired properly is to observe the amount of light reflected by the clay - a well-fired clay will reflect light vividly and have a glossy sheen, whereas a poorly-fired clay will not. Additionally, a new/unused teapot with good clay will almost instantly develop a deep and noticeably bold pigment (a process called “color rendering” - which is a part of the overall patina development) only after several uses, whereas a poorly-fired clay will take much longer to achieve such an effect, if any at all!