Jessie Taylor
Candling a kiln is a process that involves heating pottery for several hours below 212°F (100°C), which is just below the boiling point of water. This process is used to slowly evaporate any residual moisture in the clay, preventing the pottery from exploding when the water turns to steam. During candling, the kiln lid is usually propped open a few inches to allow steam to escape, and the temperature is held for a specific length of time, which can be pre-programmed for electric kilns. Candling is an important step in the firing process to ensure the pottery is dry before it is subjected to higher temperatures.
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What You'll Learn
Pottery must be bone dry before candling
Candling a kiln involves heating pottery for several hours below 212°F (100°C), which is the boiling point of water. Typically, pottery is candled at around 185–200°F. During candling, the kiln does not reach high temperatures. This process is used to slowly evaporate any residual moisture in the clay, preventing the pottery from exploding when water turns to steam. As the kiln temperature approaches 212°F, any water remaining in the clay will turn to steam, expanding sharply in volume and moving into small air bubbles in the clay, putting pressure on these air pockets. This pressure can cause the pottery to crack or explode.
It is important to note that all greenware pottery, even if it feels dry, will contain some water. When drying clay, the moisture content will match the level of moisture in the surrounding air. This means that clay can only be dried to a point where it matches the dryness of the atmosphere it is in. Therefore, it is crucial to ensure that your pottery is as dry as possible before placing it in the kiln. If the clay is cool to the touch, it indicates the presence of mechanical water, which takes longer to evaporate and will only dry out to the extent that the surrounding atmosphere allows.
To check if the pottery is still emitting moisture during candling, potters may open the peepholes on the kiln and hold a mirror above the opening. If condensation forms on the mirror, it indicates that moisture is still evaporating, and the pottery requires more time to be adequately candled. This inspection is typically done at intervals of 2 to 3 hours.
The duration of candling depends on various factors, such as the humidity of the environment, the thickness of the pottery, and the desired level of dryness. For example, large clay sculptures with thick walls may require drying for 3–4 weeks before candling, while thinner pieces may take less time. Additionally, the outside temperature and exposure to sunlight can impact the drying time.
In conclusion, ensuring that pottery is bone dry before candling is crucial to prevent explosions and cracks caused by the rapid expansion of steam. Potters should take the necessary time to adequately dry their pieces before placing them in the kiln, as this is an essential step in the firing process.
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How to set the temperature
Candling a kiln involves heating pottery for several hours below 212°F (100°C), which is the boiling point of water. This process helps to slowly evaporate any residual moisture in the clay, preventing pottery from cracking or exploding when water turns to steam.
The temperature for candling a kiln should be set between 185-200°F (85-93°C). This temperature range is crucial as it ensures that the kiln remains below the boiling point of water, allowing the moisture in the clay to evaporate gradually.
If you have an electric kiln, you can pre-program the kiln to hold at this temperature for a specific duration. For example, you can set the kiln to candle overnight, allowing it to dry the pottery thoroughly. During candling, the kiln operates at relatively low temperatures, which some potters may prefer not to leave unattended.
Alternatively, you can set the kiln to candle for shorter intervals of a few hours and then check on its progress. This method helps save time and energy, as the pottery may not require the entire night to dry. To check if the pottery is still emitting moisture, open a peephole on the kiln and hold a mirror above it. If condensation forms on the mirror, it indicates that moisture is still escaping, and the pottery needs further candling.
Candling is a crucial step in the firing process, ensuring that the pottery is sufficiently dry before reaching higher temperatures. By setting the kiln temperature within the recommended range and adjusting the duration as needed, you can effectively remove moisture from the clay and prepare it for the subsequent firing stages.
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How long to hold the temperature
Candling a kiln involves setting the kiln to heat up to below the boiling point of water (212 °F or 100 °C) and holding it there. The length of time for which the kiln is held at this temperature depends on the type of kiln, the material being fired, and the desired outcome.
For electric kilns with computer control, the hold time can be pre-programmed. Potters using a manual kiln will candle by turning on only the bottom element of the kiln and leaving out a peephole or propping the lid ajar. In this case, the hold time will depend on the desired temperature range and the size of the piece being fired. For example, small pieces can normally be heated as fast as 800 °F/hr, while larger pieces may require a slower rate and additional steps.
The purpose of candling is to ensure that any remaining moisture in the clay is fully evaporated before the kiln reaches higher temperatures. This is crucial because moisture turning into steam can cause the pottery to crack or even explode. Typically, the hold temperature for candling is between 100 °F and 200 °F (38 °C to 93 °C) for several hours. This initial phase is especially important for greenware, which is unfired or raw clay.
The hold time can vary depending on the specific needs of the project. For example, when firing glass, holding the kiln at a temperature of 1450 °F for 30 minutes can produce the same results as heating it to 1480 °F for only 10 minutes. In some cases, a soak or hold at the end of firing can be useful to ensure even temperatures throughout the kiln, especially if it is densely packed. This final hold may last from 15 minutes to an hour or more, but it is important not to soak for too long to avoid overfiring the ware.
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How to check if pottery is dry
Candling a kiln involves heating pottery for several hours below 212°F (100°C) to slowly evaporate any residual moisture. This is important because if the clay contains moisture, it will turn to steam as the kiln reaches 212°F (100°C), causing the pottery to explode.
To check if your pottery is dry, there are several methods you can use:
Touch
One way to test is to touch the clay and see if it feels dry. If it is cool to the touch, it still contains moisture. However, this method only checks the surface dryness, and the clay might still have moisture within. A better way to test using touch is to hold the clay against your cheek. If it feels cold, it is still wet; if it feels like room temperature, it is dry.
Weight
Pottery will weigh less when it is dry because properly dried clay will lose around 20% of its moisture content.
Color
The colour of the clay will turn lighter as it dries.
Time
In a dry climate, pottery can take a week or two to dry completely. However, in a humid climate, it can take longer—sometimes up to three or four weeks. The thickness of the clay will also affect the drying time. For half-inch-thick or larger pieces, it is recommended to allow up to three weeks of drying time.
Mirror Trick
During candling, potters will often check if the pottery is dry by holding a mirror over the peephole. If condensation forms on the mirror, moisture is still present in the kiln. However, this method is not always reliable, as small pieces at the bottom of the kiln might not create enough moisture to mist up the mirror.
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How to avoid damaging your kiln
Candling a kiln involves heating pottery for several hours below 212 °F (the boiling point of water). This process helps to slowly evaporate any residual moisture in the clay, preventing pottery from exploding when water turns to steam. While candling is an important step in the process, there are several things to keep in mind to avoid damaging your kiln:
Don't Exceed Temperature Limits
Never allow your kiln to exceed the maximum temperature rating listed on the serial plate. This can not only damage the kiln but may also be dangerous.
Ensure Pottery is Dry
Before placing pottery in the kiln, ensure it is completely dry. Water turns to steam at 200 °F, and if there is still moisture in the clay, it can explode, causing damage to the kiln and other pieces.
Be Cautious with Electric Kilns
If using an electric kiln, you can pre-program it to hold a temperature for a specific duration. However, never leave the kiln unattended, especially for extended periods, as this can be a fire hazard and waste energy if the pottery is already dry.
Wear Protective Gear
When handling a kiln, always wear kiln mitts or gloves to protect your hands from burns. Additionally, wear IR and UV protective glasses to shield your eyes from the radiant heat, which can cause serious eye damage over time.
Handle with Care
When moving your kiln, avoid pushing it on the stand as this can damage the structure. Instead, lift it carefully and consider investing in a deluxe stand with casters for easier transportation. If moving the kiln over a long distance, create a crate using wooden pallets, Styrofoam, and cardboard to secure the kiln and prevent damage.
Regular Maintenance
Kiln upkeep is crucial. Regularly inspect and maintain the kiln, replacing any damaged, discoloured, corroded, or brittle electrical components. Ensure kiln lids have secure braces and follow the manufacturer's instructions for proper installation and maintenance.
By following these guidelines and staying vigilant during the candling process, you can effectively avoid damaging your kiln and produce high-quality pottery.
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Frequently asked questions
Candling a kiln involves heating pottery for several hours below 212°F (100°C), which is the boiling point of water. This process helps to slowly evaporate the residual moisture in the clay and prevents pottery from exploding when water turns to steam.
If you have an electric kiln, you can program the kiln to heat up to a temperature of 185-200°F and hold it there for a certain number of hours. You can leave the kiln overnight, but this is not recommended by suppliers due to safety concerns and energy wastage.
You can check if your pottery is still emitting moisture by opening one of the peepholes on the kiln and holding a mirror above it. If condensation forms on the mirror, it means that moisture is still evaporating and your pottery needs to be candled for longer.
