- by vinux
In DingShu Town, there used to be “kiln households” specialized in firing, and “firewood shops” specialized in fuel. Masters of the workshop almost had no involvement in this process. A purple-clay pot is a product of the combination of earth and fire. Good purple-clay mud with appropriate firing techniques can create a good purple-clay pot.
The factors affecting the burning effect are temperature and atmosphere. The firing temperature mainly affects the sintering degree, and the firing atmosphere mainly affects the coloration effect. The firing temperature of purple sand is higher than that of ordinary ceramic raw materials. Depending on the different clay materials, the firing temperature can range from 1000℃ to 1250℃.
“Eating temperature” refers to the most suitable firing temperature that a certain purple sand clay material can withstand. Zhu Ni has the lowest eating temperature, followed by Red Ni, and then Tu Ni has the highest. For the same kind of clay material, due to different production areas, refining methods, aging time, etc., there will be different eating temperatures. Before using the same type but different batches of clay material, test pieces should be fired first to explore the effect of firing at different kiln temperatures in order to achieve the best results.
If the temperature differs from the most suitable one by 10℃, “under-firing” and “over-firing” will occur. “Under-firing” refers to the clay not absorbing enough heat to reach its desirable temperature, resulting in a dull grey surface with a sluggish sound, loose texture, and incomplete firing. “Over-firing” means that the kiln temperature is higher than what the clay can bear, causing deformation on the surface with bubbling and much iron crystal deposits.
Theoretically, the perfect firing temperature point for a certain kind of clay is unique, but in practice, the firing temperature is mostly in the upper and lower areas of this point. The color of the clay can vary greatly between the high and low points of the firing zone.
If the kiln temperature is too high, the pottery greenware will deform into waste products, which is commonly referred to as “kiln whitening” in the industry. When the temperature is too high, the quartz contained in the purple sand clay will be vitrified and there will be no reflection of various colors under natural light.
The richer the reflective color and light, the better the clay quality is. If the reflected color and light are rigid and single, it is definitely not a good clay quality. Another phenomenon of good clay quality is that when boiled water is poured into a pot, the color of the pot will darken, commonly known as “color displaying property”.