The saying "three parts making, seven parts firing" among potters highlights the decisive role of the firing process in ceramic making. This is because the firing process completely reshapes the physical and chemical properties of ceramics, and it involves complex and high-risk control of the kiln fire. Let's explore this from three aspects: the physical and chemical transformations in the kiln, the technical barriers in firing, and practical case studies.

I. The Physical and Chemical Transformations in the Kiln

Ceramic firing is a process of material reconstruction at high temperatures. When the clay body enters the kiln, it undergoes stages such as dehydration (80-400℃), oxidation (400-900℃), and vitrification (900-1300℃), during which the components of the body and glaze change dramatically:

Body Densification: Kaolinite in the clay decomposes into mullite crystals, and feldspar melts to form a glassy phase that fills the pores. The shrinkage rate of the body can reach 8-12%. If the temperature is too low, the body will be porous and weak; if it's too high, the body will deform and soften.

Glaze Vitrification: Quartz and feldspar in the glaze melt at high temperatures to form a smooth glassy layer. For example, the milky effect of the celadon glaze from Ru kiln comes from tiny bubbles and unmelted quartz particles in the glaze. This "opaqueness" requires precise control of the kiln temperature around 1280℃.

Atmospheric Coloring: In an oxidizing flame (rich in oxygen), iron elements turn reddish-brown, while in a reducing flame (oxygen-deficient), they produce grayish-blue or copper-red colors. The "kiln change" effect of Jun ware is achieved by adjusting the oxygen levels in the kiln at different stages.

II. Technical Barriers in the Firing Process

The firing process is fraught with challenges:

1. Temperature Control Precision:

Ceramics of different types are highly sensitive to temperature. For example, ordinary pottery is fired at 700-1000℃, while Yixing purple sand needs 1170-1200℃ to form its unique double-pore structure. Jingdezhen's Qingbai porcelain requires temperatures above 1320℃ for complete vitrification. A temperature deviation of more than ±20℃ can lead to glaze cracking or underfiring.

Traditional wood-fired kilns rely on "fire chicken" (a column of glaze mud) to judge the temperature. When the 12th fire chicken melts in sequence to the 10th, the kiln temperature is close to 1300℃. Modern electric kilns, despite having temperature control systems, can still have temperature differences of over 50℃ inside the kiln."Temperature cones" are used repeatedly for calibration.

2.Art of Atmosphere Regulation:

The gas composition inside the kiln directly affects the glaze color. 

Critical value of reducing flame: When firing copper-red glaze, the oxygen content in the kiln must be reduced to 1-2% at around 1250℃ and maintained for over 30 minutes. Otherwise, copper ions cannot be fully reduced to red. The firing failure rate of Jingdezhen's Jihong glaze is as high as 70% because this process is extremely difficult to control.

The wisdom of traditional kilns: Traditional kilns like Dragon Kiln use the natural draft from the hillside terrain. By adjusting the speed of fuel feeding and the opening and closing of the flue, a segmented atmosphere can be created inside the kiln—oxidizing at the front, neutral in the middle, and reducing at the back. This complex airflow management requires decades of experience from the kiln master.

3. Unpredictable Risks:

Even with perfect clay bodies and glazes, firing can still fail due to:

Cracking caused by thermal stress: Uneven drying of the body or too rapid cooling can cause inconsistent contraction between the inside and outside. For example, thin-walled porcelain from Jingdezhen must be cooled slowly in the 750-650℃ range; otherwise, it will shatter easily.

Temperature variations within the kiln: In the same kiln, the top temperature can be 50-100℃ higher than the bottom. When the Ming Dynasty imperial kiln factory fired large dragon jars, the body had to be placed in a specific position in the kiln, and "fire test pieces" were used for real-time monitoring.

Fuel characteristics: Pine wood in a wood-fired kiln contains resin, producing long and stable flames. Coal-fired kilns, on the other hand, easily generate sulfur compounds, causing "smoke yellow spots" on the glaze. The unique "jade-like luster" of traditional Jingdezhen wood-fired porcelain comes from the special ash deposits on the glaze surface from pine wood combustion.

III. Practical Case Studies on the Decisive Role of Firing

1. The Pinnacle and Challenges of Ru Kiln:

The Northern Song Dynasty's Ru wares with celadon glaze is considered as the epitome of celadon. Its firing requires three stringent conditions:

Body and glaze formulation: The clay contains trace amounts of iron, and the glaze is mixed with agate powder (mainly silicon dioxide) to enhance the milky effect.

Kiln structure: A connected kiln design is used, with a fire chamber and kiln room separated by a partition to control airflow and create a stable reducing atmosphere.

Firing technique: Tiny sesame-shaped supports (only 1-2 mm in size) are used to hold the body, allowing full glaze firing. Even with all these measures, the success rate of Ru kiln products is less than 10%, with over 90% of the excavated items being defective.

2. Challenges of Colored Glazes in Jingdezhen:

Sacrificial Red Glaze: It requires precise control of the oxidation-reduction reaction of copper ions at 1320℃. A temperature fluctuation of more than 5℃ can lead to color failure. The Qing Dynasty's imperial kiln supervisor Tang Ying once recorded, "There is no fixed method for sacrificial red glaze; it must be obtained by chance in the fire."

"Three Yangs Bring Prosperity": A single piece displays three glaze colors—red, blue, and moon white. This requires creating temperature differences in different parts of the kiln and adjusting the kiln position and fuel-feeding rhythm. While this can be processor-controlled in gas kilns, kiln master's experience in "fire watching" is still relied on in wood-fired kilns.

IV. Integration of Modern Technology and Traditional Experience

Despite the fact that modern kilns (such as electric and gas kilns) have achieved digital control of temperature and atmosphere, the core challenges of firing still remain:

Dilemma of Imitating Ancient Porcelain: To replicate the jade-like texture of the Song Dynasty official glaze, it is necessary to simulate the "fire pricking" effect of wood-fired kilns in electric kilns—by intermittently cutting off electricity to create temperature fluctuations and produce tiny bubbles in the glaze.

Exploration of Innovative Materials: New types of nano-ceramics require firing at ultra-high temperatures (above 1800℃) and in a vacuum environment, which poses higher demands on the kiln's refractory materials and temperature control systems.

Summary

The saying "three parts making, seven parts firing" essentially acknowledges that the firing process ultimately shapes ceramic art. From the open-air stacking firing in the Neolithic Age to modern intelligent kilns, humans have always been exploring how to tame the kiln fire. As the Ming Dynasty's The Exploitation of the Works of Nature said, "The fired fine porcelain has the texture like pure skin and warm jade." The kiln fire is not only a technique but also the magic that gives life to clay. Its secrets lie both in the high temperatures of 1300℃ and in the potter's focused gaze at the flames.