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Català- TIPS & INFORMATION
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The firing curve of ceramic bodies is one of the topics that generates the most questions for our technical department. Beyond the final firing temperature (low, medium or high), the final result depends on the total heatwork received by the piece, which is determined by both the temperature reached and the time spent during the firing cycle. For this reason, an appropriate firing curve is essential to achieve the desired results while minimising the risk of cracks and firing defects.
Below, we explain the main stages of a conventional firing cycle and the most important recommendations for each of them.
Drying Phase
Range: 25–100 °C
Even after drying naturally, ceramic pieces still retain a small amount of moisture in equilibrium with the workshop atmosphere. For this reason, it is recommended to begin the firing cycle with a drying phase to remove this residual moisture and prevent cracking during the subsequent heating stage.
The drying phase is carried out at approximately 100 °C, maintaining this temperature for long enough to completely evaporate any residual moisture from the pieces. The required time will depend on the type of clay body, the size and thickness of the pieces, and the drying conditions prior to firing. Since this is a relatively low temperature, extending the drying phase generally has only a minor impact on energy consumption compared with the benefits it provides. If the kiln is fitted with a vent or ventilation system, it should be left open to allow moisture to escape.
Heating Phase
Range: 100–600 °C
Once the drying phase is complete, the heating stage begins, raising the temperature to approximately 600 °C, typically at a rate of between 150 and 200 °C per hour.
Once this temperature has been reached, it is advisable to include a short soak to allow the quartz inversion and the release of chemically bound water from the clay minerals to occur more uniformly, thereby reducing internal stresses within the pieces. If the kiln controller does not allow a soak to be programmed at 600 °C, a similar effect can be achieved by reducing the heating rate within this range to approximately 100 °C per hour.
Throughout this stage, the kiln vent should be left open to allow moisture and gases to escape.
Maturing Phase
Range: 600 °C – Final Firing Temperature
Once the heating stage is complete, the kiln vent is closed and the actual firing begins. During this stage, most of the physical and chemical changes take place, transforming the clay body into a strong and durable ceramic material.
The heating rate during this stage is typically between 100 and 150 °C per hour. It is also advisable to reduce the heating rate slightly during the final stage of the firing cycle to promote a more even distribution of heat throughout the kiln load.
Once the final firing temperature has been reached, a 30-minute soak is recommended to ensure that all pieces mature evenly. For small firings or lightly loaded kilns, the soak time can often be reduced, whereas heavier kiln loads may require a slightly longer soak.
Cooling Phase
Range: Final Firing Temperature – 100 °C
Once the soak is complete, open the kiln vent and allow the kiln to cool naturally.
Important changes continue to take place within the ceramic material during cooling, particularly as it passes through the quartz inversion. For this reason, excessively rapid cooling should be avoided.
It is recommended not to open the kiln until the temperature has fallen below 100 °C, helping to prevent thermal shock and reducing the risk of glaze crazing.
Standard Firing Program
The diagram shows a recommended firing curve alongside a standard firing program from a kiln manufacturer, based on two heating segments and a final soak.
This standard program does not include the initial drying phase, which is strongly recommended, particularly when firing thick pieces or work that has not been allowed to dry thoroughly under natural conditions.
Many modern kilns include automatic firing programs with multiple heating segments that automatically adjust the heating rate during the final stage of the firing cycle, providing a more even distribution of heat throughout the kiln load and greater consistency from one firing to the next.
Temperature Control
Accurate control of both temperature and hold time during each stage of the firing cycle is essential for achieving consistent results. Modern kilns are equipped with electronic controllers and temperature sensors capable of following the programmed firing curve with great precision, ensuring uniform and repeatable firings.
Even so, the most reliable way to verify the actual outcome of a firing is still the use of pyrometric cones.
The heatwork received by the ceramic pieces depends not only on the temperature reached but also on the length of time that temperature is maintained, particularly during the final stage of the firing cycle. For this reason, two kilns reaching exactly the same final firing temperature may produce different results.
Pyrometric cones are designed to deform according to the amount of heatwork received. During firing they gradually soften and bend. Their final position provides a highly accurate visual indication of the actual heatwork achieved inside the kiln.
Three cones are typically used: the guide cone, which represents a slightly lower firing and should bend completely until it rests on the plaque; the firing cone, which represents the desired firing and should bend to approximately the position recommended by the manufacturer; and the guard cone, which represents a higher firing and should remain virtually undeformed.
If the cones do not bend as expected, it indicates that the heatwork received by the ware has been either insufficient or excessive, regardless of the temperature indicated by the kiln controller.
The firing temperature equivalents published by Orton for pyrometric cones are based on different heating rates. In studio ceramics, the firing programs used by most kiln, clay body and glaze manufacturers correspond to a final heating rate of approximately 60 °C per hour, which serves as the standard reference for interpreting these temperature equivalents.
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