Whether a ceramic body is easy to fire depends not only on peak temperature, but on whether the heating rate, soaking time, maturation range and body reaction rate are properly matched.
· Excessive heating may prevent moisture and decomposition gases from escaping.
· Insufficient soaking may leave pores open and densification incomplete.
· Excessive temperature or soaking can create over-vitrification, deformation and glaze flow.
· A well-designed kiln curve allows gas release, phase development and densification to occur within a repeatable production cycle while maintaining dimensional stability.
A kiln firing curve is the planned relationship between temperature and time during ceramic firing. It normally includes preheating and gas release, controlled heating, high-temperature soaking and controlled cooling. An “easy-to-fire” body reaches the required water absorption, density, strength and dimensions within a reasonably wide process window. Small temperature or timing variations should not cause large changes in product quality. |  |
1. Heating rate controls safe gas release
During low- and middle-temperature stages, residual moisture, organic decomposition products and gases from mineral reactions must leave the body. When heating is too fast, the outer body or glaze may begin to seal while gases remain inside. This can contribute to pinholes, blisters and internal defects. Products with thick walls, complex waterways or large dimensions generally require a more carefully controlled gas-release stage. |  |
High-temperature soaking provides time for liquid-phase formation, particle rearrangement and pore closure.
A study of firing temperature, time and quartz particle size in sanitary ware bodies found that these variables jointly affect phase formation, thermal expansion and water absorption.
Insufficient soaking may leave the body under-fired, with higher water absorption and lower strength. Excessive soaking may promote grain growth, excessive shrinkage and pyroplastic deformation.
3. Body formulation defines the firing window
The proportions and characteristics of clay, quartz and feldspar determine when sintering begins, when liquid phases form and when densification is completed.
Research on feldspar and firing-cycle effects in sanitary ware bodies found differences in phase composition and densification behavior when feldspar composition, particle size and time-temperature profiles changed.
A kiln curve should therefore never be optimized independently of the body formulation.
4. Firing maturity must be verified with product data
ISO 10545-3 specifies methods for determining water absorption, apparent porosity, apparent relative density and bulk density in ceramic products.
A factory should evaluate firing performance using:
Firing shrinkage;
Water absorption and bulk density;
Warping and cracking rates;
Glaze pinholes and color variation;
First-pass yield at different kiln positions.
Gas-release stage: Allow sufficient time for moisture and gases to escape.
Heating stage: Control temperature differences according to wall thickness.
Maturation stage: Set an appropriate peak temperature and soaking time.
Cooling stage: Avoid excessive gradients and rapid thermal shock.
Body matching: Validate mineral composition, particle size and curve together.
Kiln-position tracking: Compare density and deformation across the kiln car.
Industrial firing curves do not have one universal fixed value and must be set for the product and production-line configuration.
A kiln curve determines firing performance by matching heat input with the body’s reaction rate. Heating rate controls gas release and temperature gradients. Soaking time controls densification. Formulation defines the maturation window. Cooling controls final stress. A stable firing curve is not simply a hotter curve. It is a repeatable balance between yield, dimensions, water absorption, energy use and production capacity.
Q1: Why can a product remain under-fired even when peak temperature is reached?
A: The soaking time may be insufficient, the body may require a higher maturation temperature, or the product interior may not reach the displayed kiln temperature.
Q2: Can longer soaking reduce water absorption?
A: It may help within the correct firing window. Beyond that range, the improvement can decline while deformation, excessive shrinkage and energy use increase.
Q3: How can a factory verify that a kiln curve is stable?
A: Compare water absorption, density, shrinkage, deformation and first-pass yield across multiple batches and kiln positions instead of relying only on the displayed temperature.