Tyler Brooks
Candling is a process in kiln firing that involves heating pottery for several hours below 212°F. This is done to ensure that the pottery is dry before it is fired at higher temperatures, as moisture in the clay can turn to steam and cause the pottery to explode. The duration of candling depends on factors such as the size and shape of the kiln, the ventilation method, and the initial moisture content of the pottery. While some potters candle overnight, others may candle for several days or even a week, especially for ware with a thick cross-section. After candling, the kiln is fired, and the duration of firing can vary depending on factors such as the power supply and the density of the kiln load. It is important to monitor the firing process and make adjustments as needed to ensure the desired results.
Characteristics and Values of Candling a Kiln
| Characteristics | Values |
|---|---|
| Temperature | Typically 185–200°F, below 212°F |
| Purpose | To remove residual moisture and mechanical water in clay to prevent pottery from exploding when water turns to steam |
| Duration | Several hours, or overnight, for days or a week depending on the thickness of the cross-section |
| Venting | Lid propped open a few inches with a kiln brick or similar item; upper peephole plug removed |
| Safety | Limit timer set to turn kiln off after a certain amount of time to prevent catastrophe |
| Post-firing | Wait until kiln is below 125–250°F before opening; make notes on firing log |
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What You'll Learn
Pottery must be dry before firing
Candling involves heating pottery for several hours below 212°F. Typically, pottery is candled at around 185-200°F, which is just below the boiling point of water. This causes any residual moisture and mechanical water in the clay to evaporate slowly. If your clay contains moisture, it will turn to steam as the kiln reaches 212°F. As water turns to steam, its volume expands sharply. The steam will move into any small air bubbles in your clay, putting these air pockets under pressure. This can cause your pottery to explode.
Therefore, it is important that your pottery is completely dry before it is fired in a kiln. On average, it takes 1 to 2 weeks for pottery to dry completely, but this can vary depending on conditions such as climate, size, thickness, and the type of clay used. For example, clays with high plastics and finer particles, such as porcelain, have a higher water content and will take longer to dry. The shrinkage rate will also be more significant, increasing the chance of cracking.
To test whether your pottery is dry, you can try the condensation test. Check with a mirror at regular intervals of 2 to 3 hours. Once the mirror no longer fogs up with condensation, this suggests that the pottery is dry enough to fire. However, not all potters trust this method, as a small piece of pottery at the bottom of the kiln might not create enough moisture to mist up the mirror. This is why some potters choose to candle overnight, to ensure that everything is fully dry.
It is also important to dry your pottery slowly and evenly to prevent cracking. If you dry your pottery too quickly, it might crack. Therefore, it is best to dry your pottery at room temperature, and in the shade if drying outdoors.
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Candling involves heating pottery for several hours below 212°F
Candling involves heating pottery for several hours at temperatures below 212°F, typically between 185-200°F. This is done to slowly evaporate the residual moisture and mechanical water in the clay, which can prevent pottery from exploding when water turns to steam. As water turns to steam, it expands in volume and moves into small air bubbles in the clay, creating pressure that can cause cracking or explosions.
The process of candling helps to remove chemically combined water from the clay, also known as dehydroxylation, which typically occurs at temperatures between 842-1112°F. This stage of the firing process is sometimes referred to as water smoking. While it is considered good practice to ensure that pottery is bone dry before it is placed in the kiln, it is not uncommon for potters to load their kiln with pieces that are still slightly damp. By candling for a longer period, potters can ensure that the clay is completely dry before firing.
During candling, the kiln does not reach high temperatures, and some potters choose to leave their kiln on overnight to ensure that their pottery is fully dry. However, this practice is not recommended by suppliers, as it can be a waste of energy if the pottery is already dry. To check if the pottery is still emitting moisture, potters can open the peephole on the kiln and hold a mirror above it. If condensation forms on the mirror, it indicates that the pottery needs to be candled for longer.
The length of time required for candling can vary depending on the thickness of the ware. For sculptural pieces with a thick cross-section, candling may take several days or even a week. In contrast, typical gas firings of functional ware may only require overnight candling on very low heat before continuing with normal firing the next day.
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Pottery is typically candled at 185-200°F
Candling is a process in which pottery is heated at a temperature below 212°F (the boiling point of water) for several hours. This process is crucial as it slowly evaporates any residual moisture and mechanical water in the clay, preventing the pottery from exploding when water turns to steam. The expansion in volume as water turns to steam can cause steam to move into small air bubbles in the clay, putting these air pockets under pressure. Thus, it is essential to dry out the pottery as much as possible before placing it in the kiln.
The duration of candling depends on the size and shape of the kiln, as well as the method of venting used by the potter. For kilns with thicker cross-sections, such as those used for sculptural pieces, candling can take several days or even a week. After candling, the temperature is held for a period known as a "soak," which helps to ensure even heating throughout the kiln. The soak can last from 15 minutes to an hour or longer, depending on the kiln's density.
It is important to note that the power supply can impact the firing process, and voltage drops may occur if the kiln is too far from the breaker box or during high-demand periods like hot summer days. Thus, it is recommended to be near the kiln while it is firing, especially towards the end, to ensure it turns off as scheduled. A limit timer can also be set as a safety measure to automatically turn off the kiln after a specified amount of time.
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The kiln lid should be propped open during candling
Candling a kiln is the process of heating pottery for several hours below 212°F (typically at 185-200°F) to slowly evaporate residual moisture and mechanical water in the clay. This prevents pottery from exploding when water turns to steam. During candling, the kiln does not reach high temperatures.
If you are using a manual kiln, you can candle by turning on only the bottom element of the kiln and leaving out a peephole or propping the lid open ajar by an inch. If you have an electric kiln, you can program the first segment of the firing schedule to pre-heat by setting the temperature to up to 200°F and holding it for a certain number of hours.
Propping the kiln lid open during candling is necessary to allow moisture to escape as the kiln heats up. This is especially important if your kiln does not have a ventilation system installed. By creating a cross-draft with a fan and an open window, you can help the moisture escape through the open kiln lid.
It is important to note that if you have a downdraft vent, you should turn it off before opening a peephole, as opening a peephole while the vent is on will draw cold air into the kiln, which is detrimental to both the kiln and the pottery. Additionally, if you are using a kiln with a vent, you do not need to prop the lid open during candling, as the vent will provide a sufficient escape route for the moisture.
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Soaking at the end of firing can even out kiln temperatures
Candling is a process in which pottery is heated for several hours below 212°F, typically at around 185-200°F, which is just below the boiling point of water. This causes residual moisture and mechanical water in the clay to evaporate slowly, preventing pottery from exploding when water turns to steam. As water turns to steam, it expands sharply in volume, and this process ensures that the steam does not move into any small air bubbles in the clay, putting pressure on these air pockets.
The duration of candling varies depending on the thickness of the pottery and the size of the kiln. For sculptural pieces with a thick cross-section, a kiln might be candled for days or even a week. For functional ware, it is common to candle overnight on very low heat and then continue with normal firing the next day.
Now, onto the topic of "soaking" at the end of firing:
Soaking, or holding the kiln at a specific temperature for an extended period, is a technique used to even out temperatures throughout the kiln, ensuring that all pieces achieve the desired temperature. This is particularly beneficial for densely packed kilns, as it helps distribute internal stresses and allows the glaze to flow and heal imperfections. The duration of soaking depends on the specific ware being fired and can range from 15 minutes to 2 hours or more. However, it is important to note that soaking for too long can lead to overfiring, so it is crucial to monitor the process closely.
Soaking is an essential step when firing glazes with a stiff melt, such as low-temperature zirconia whites, and for porcelains requiring translucency, density, and glassy surfaces. It is also advantageous for heavier kiln loads, as it aids in heat penetration. Additionally, in hi-tech industries, such as those firing almost pure calcined alumina bodies, soaking plays a key role in developing the desired matrix properties.
In conclusion, soaking at the end of firing is a critical step in the kiln firing process, as it ensures even temperature distribution, enhances glaze quality, and aids in achieving the desired properties for various types of ware. By adjusting the duration of soaking based on the specific requirements of the ware, potters can optimize the firing process and produce high-quality ceramics.
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Frequently asked questions
Candling a kiln involves heating pottery for several hours below 212°F. This causes residual moisture and mechanical water in the clay to evaporate slowly and prevents pottery from exploding when water turns to steam.
Candling typically takes place at around 185-200°F for several hours. Some potters candle overnight to ensure that everything is fully dry. For sculptural pieces with a thick cross-section, candling may take days or even a week.
One way to check if your pottery is dry enough to stop candling is to use a mirror. Check the mirror at regular intervals of 2-3 hours. Once the mirror no longer fogs up with condensation, this suggests that the pottery is dry enough to fire.
After candling, the kiln can be fired. The length of the firing process depends on the type of kiln and the material being fired. A typical gas firing may take around 7.5 hours, while a soak at the end of the firing process may last from 15 minutes to an hour or more.
