Firing represents the critical final stage in sanitaryware manufacturing, where the green body is transformed into a high-strength, durable, and chemically resistant product through high-temperature treatment in shuttle or tunnel kilns. Peak temperatures can reach approximately 1220°C, depending on the specific body formulation.
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Fundamental Influencing Factors of the Firing Process
A thorough understanding of the physico-chemical changes occurring at different temperature stages is essential for optimizing the firing curve and minimizing defects. These transformations are primarily governed by four key parameters: raw material particle size distribution, body and glaze composition, heating and cooling rates, and soaking times at critical temperatures.
Kiln Types and Their Applications
The industry predominantly utilizes tunnel kilns and shuttle kilns. Tunnel kilns operate continuously and are typically employed for the initial bisque firing. Shuttle kilns, operating in batch mode, are often used for lower-volume production or glaze firing (re-firing).
Key Physico-Chemical Reactions During Firing
The body undergoes a complex sequence of changes during its passage through the kiln, including dehydration, decomposition, oxidation, solid-state reactions, glass phase formation, dissolution, and crystalline phase transformations.
| Reaction Progression During the Heating Cycle | Phase Transformation Management During Cooling | |
Ambient to 150°C: Removal of mechanically combined water. Preheating is often used to facilitate this step. The ware must contain less than 1.5% moisture initially, and heating must be gradual to prevent cracking, achieving a final moisture content below 1%. 150°C to 500°C: Initiation of organic material oxidation and decomposition. Carbonaceous matter within the body begins to burn out. The heating rate must be regulated accordingly and is inversely related to ware thickness. 500°C to 700°C: Dehydroxylation and quartz inversion. Clay minerals like kaolinite lose their chemically combined water. Quartz undergoes the α to β phase transformation at around 573°C. Decomposition of carbonates and micaceous minerals also occurs within this range. 700°C to 1050°C: Carbonate decomposition and initial sintering. Magnesium and calcium carbonates decompose, releasing CO₂. The body begins to shrink and initial sintering commences. 950°C to 1100°C: Glaze maturation and body densification. The glaze starts to melt, and significant body shrinkage occurs as sintering progresses. 1100°C to 1250°C: Vitrification and high-temperature soaking. Silica melts, forming a glassy phase that promotes drastic shrinkage and densification. Sufficient soaking time is crucial at the peak temperature to ensure homogeneity, complete body-glaze reactions, and allow for gas escape. | 1250°C to 1200°C: Slow cooling for gas escape. A reduced cooling rate allows trapped gases to escape before the glass phase solidifies. 1200°C to 800°C: Rapid cooling. Fast cooling through this range helps achieve a high-gloss, glassy glaze surface. 800°C to 600°C: Controlled cooling. A steady, uniform cooling rate is maintained to minimize thermal gradients before the quartz inversion point. 600°C to 500°C: Quartz inversion zone. Quartz reverts from the β to α form, accompanied by significant volume change, necessitating careful cooling to prevent dunting. 500°C to Room Temperature: Unrestricted cooling. The finished ware can safely cool to ambient temperature. |
Analysis and Remedies for Common Firing Defects
Bloating and Blackening: Often results from firing in a reducing atmosphere or insufficient kiln ventilation.
Black Core: Caused by inadequate soaking time at peak temperature or the use of raw materials with high organic content, leading to incomplete carbon burnout.
Reduced Gloss: Arises from under-firing, insufficient glaze maturation, or devitrification.
Bloating: Occurs in thick-walled sections where gases cannot escape before the surface seals during vitrification.
Cracking/Dunting: Can be caused by excessively rapid heating below 700°C or significant temperature gradients across the ware.
Conclusion
Firing is the transformative core process that defines the ultimate properties of sanitaryware. Mastery of the underlying physico-chemical principles and precise control over the entire thermal cycle are paramount for achieving high quality and production efficiency. This analysis provides technical professionals with a systematic framework for understanding firing technology, and further discussion on these points is welcomed.