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Català- TIPS & INFORMATION
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The firing curve of ceramic clays is one of the topics that generates the most questions to our technical department. Beyond the maximum firing temperature (low, medium, or high), an adequate firing curve is essential to obtain a good result, minimising the risks of breakage and defects in the pieces during firing. Below, we explain the main phases or intervals of a conventional firing and the measures that you must take in each one of them.
Drying phase
Range: 25 – 100 ºC
After drying the pieces in natural conditions, they continue to retain a certain percentage of humidity in equilibrium with the workshop environment. Thus, it is advisable to start the firing with a drying phase to eliminate the remaining humidity and avoid possible breakages during the subsequent heating phase.
The drying phase is carried out at 100 ºC and is maintained for a period to allow the residual humidity of the pieces to evaporate. The drying time will depend on the type of clay, the size and thickness of the pieces and the natural drying conditions prior to firing. Since the kiln consumes practically no energy at 100 ºC, it is better to over-explode the time than to under-explode it. If the kiln has a damper, it should be open to allow moisture to escape.
Heating phase
Range: 100 – 600 ºC
Once the drying phase is complete, the heating phase begins up to 600 ºC, normally at 200 ºC/hour. Once the temperature has been reached, it is advisable to maintain a time interval to facilitate the allotropic transformation of the quartz and the evaporation of the structural water from the clay minerals. If your kiln's programmer does not allow maintenance at 600 ºC, set the heating to this interval at 100 ºC/hour. The kiln damper remains open during the heating phase.
Vitrification phase
Range: 600 ºC - T max
After finishing the heating, we close the damper and start the actual firing. In this stage, most of the physical and chemical changes that transform the clay material into a durable ceramic material occur. The temperature increase in this phase is usually 150 ºC/hour until reaching the maximum firing temperature.
Once the maximum temperature is reached, it is recommended to carry out a 30-minute maintenance or “palier” to ensure the complete and homogeneous firing of all the fired material.
Cooling phase
Range: T max - 100ºC
After finishing the maintenance at the maximum temperature, we open the damper and let the kiln cool naturally. It is recommended not to open the kiln door prematurely until it is below 100 ºC. In this way, we will avoid possible breakages caused by thermal shock and the cracking of the glaze.
Standard program
The graph shows the ideal firing curve together with a standard program from a kiln manufacturer, based on 2 simple ramps.
The standard program does not include the drying phase, which is very convenient, as explained above.
Temperature control
Temperature control in the kiln is essential to ensure successful firing. Modern kilns are equipped with sensors and controllers that monitor and adjust the temperature to the firing curve throughout the cycle, thus ensuring uniform and repeatable firing.
When in doubt, the use of pyrometric cones is recommended to verify the actual heat received by the pieces during firing. Heat depends on the kiln temperature, but also on the time in each temperature range, especially in the maximum temperature range. Pyrometric cones are designed to melt based on the heat received, so they begin by softening and end by bending. The final shape of the cone gives us a very precise visual indication of the actual heat reached inside the kiln. Three cones are usually used: the guide cone, which corresponds to a lower temperature and will completely bend when resting on the support; the firing cone, which corresponds to the desired temperature and which will bend, and the guard cone, which corresponds to a higher temperature and which must remain intact. If the fusion of the cones does not correspond to the expected result, it indicates that the actual heat received by the pieces has not been correct, either due to a defect (the firing cone has remained intact) or due to an excess (the guard cone has bent).
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