1. Low-Temperature Stage: Moisture and Gas Removal
At the beginning of firing, residual moisture, organic matter and volatile components must be gradually removed from the ceramic body. If the heating rate is too fast, internal vapor cannot escape safely. This may cause hidden cracks, bursting, pinholes or glaze defects. Thick-walled products, large basins, one-piece toilets and wall-hung toilets are especially sensitive because their internal structures are more complex.
Therefore, the low-temperature stage should not focus only on speed. Its main purpose is to remove moisture and gases safely before high-temperature sintering begins.
2. Middle-Temperature Stage: Quartz Inversion and Thermal Stress
Ceramic bodies often contain quartz and other silica-based minerals. During firing, quartz can undergo phase transformation with volume change. Digitalfire explains that quartz inversion can create sudden volume changes and affect the thermal expansion behavior of the fired body. If heating or cooling is too fast, uneven temperature distribution may create thermal stress, leading to cracks or hidden structural defects.
For sanitary ware products, waterways, bowls, mounting surfaces and wall-thickness transition areas are especially vulnerable. Therefore, middle-temperature control should focus not only on the set temperature, but also on thermal uniformity inside the kiln.
3. High-Temperature Stage: Densification Determines Water Absorption and Strength
The high-temperature stage is where firing most directly affects product performance. As temperature rises, ceramic particles bond more closely and the body becomes denser. Digitalfire describes sintering as a densification process in which particles pack more tightly and bond into a stronger matrix. If peak temperature is too low or soaking time is insufficient, the body may remain porous, causing higher water absorption and lower strength. If temperature is too high or soaking time is too long, the product may deform, over-fire, change color or suffer from excessive glaze flow.
The BS EN ISO 10545 ceramic testing series includes methods for water absorption, apparent porosity, bulk density, modulus of rupture, breaking strength and linear thermal expansion. These indicators show why firing control is directly linked to measurable product performance.
4. Cooling Stage: Color Stability and Crack Prevention
Cooling is not just the end of firing. It is a critical stage for final quality. Cooling too fast can cause thermal stress and delayed cracking. Uneven cooling may lead to color variation, glaze tension and dimensional change. Glaze gloss, color depth, glaze-body fit and surface stability are all influenced by the cooling curve.
SACMI states that its TWS tunnel kiln can be used for initial firing and re-firing of sanitary ware with firing cycles of 10–25 hours. This reflects the need for industrial firing to balance cycle time, energy consumption, temperature stability and product consistency.
Conclusion
The firing curve determines not only the appearance of ceramic sanitary ware, but also its internal structure and long-term performance. The low-temperature stage affects defect risk. The middle-temperature stage affects thermal stress. The high-temperature stage determines density and strength. The cooling stage influences color stability and cracking risk. For ceramic sanitary ware factories, a stable firing curve means stable water absorption, strength, glaze quality and brand consistency.