10/03/2025
Here are two fired pyrometric cones—a heat-measuring device—and one has melted and bent over by 10 degrees, while the other is fully bent over during firing. A cone that has only slightly bent indicates that the target "heat work" for that cone number was not fully reached. The consequence is that the pottery or glaze is under-fired, with unideal results.
THE REASON THIS HAPPENED IN THIS KILN I SERVICED:
Improper cone sitter calibration: On older kilns with a cone sitter, incorrect calibration can cause the kiln to shut off too early. The cone sensing rod can also become bent or chipped, leading to inaccurate shutoff.
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If you are underfiring, there are...
POSSIBLE REASONS:
• Worn-out heating elements: The heating coils inside the kiln degrade over time, causing their electrical resistance to increase.
• Failed relays: Relays are electrical switches that turn the heating elements on and off.
• Inadequate voltage: Your kiln may not be getting the correct voltage due to wiring issues or problems from the power company.
• Faulty thermocouple: The thermocouple is the sensor that reads the temperature inside the kiln and sends the information to the controller
• Controller errors: A digital controller may display an error code (such as "Err1" or "Fail") if it detects a problem, like a failed thermocouple, and will prevent the kiln from completing the firing.
• Improper cone sitter calibration: On older kilns with a cone sitter, incorrect calibration can cause the kiln to shut off too early. The cone sensing rod can also become bent or chipped, leading to inaccurate shutoff.
THINGS TO CONSIDER WHEN UNDER-FIRING
Potential consequences of under-firing
• On clay
Lack of vitrification: The clay may not have reached its full maturity, leaving it more porous and less durable than desired.
Lower strength: The resulting piece may be weaker and more prone to breakage.
• On glazes
Improper melting: The glaze may appear stiff, dusty, or mottled, with dull colors and a gritty texture. A 10-degree bend likely means the glaze is not properly melted.
• Unstable glaze: The surface of the ware may not be food-safe if the glaze has not fully melted and fused with the clay body.
Crazing or crawling: The raw ingredients in the glaze may not have dissolved properly, leading to issues like crazing (fine cracks) or crawling (the glaze pulling away from the clay).
