Candling a kiln refers to a slow and controlled heating process used to dry out greenware, which is unfired pottery. During this stage, the temperature in the kiln is slowly raised to less than the boiling point of water for several hours to allow moisture to escape without damaging the greenware.
Unlocking the Secrets of Kiln Candling
Unfired pottery, also known as greenware, can be dried out gently and slowly using a technique called candling a kiln. During this step, the temperature inside the kiln is slowly raised to between 200°F and 300°F for several hours. This lets the water evaporate without cracking or warping the greenware. The slow, controlled heat gives off a flickering effect that looks like a candle flame. This effect is called “candling.”
Candling is an important part of making pottery because if the greenware isn’t dried enough before firing, it can crack, warp, or even explode in the kiln because it has too much water inside. Candling is especially important for pottery that is thicker or bigger and can hold more water.
The kiln can be heated up to the desired firing temperature for bisque or glaze firing after candling. The first firing, called bisque firing, makes the pottery stronger. The second firing, called glaze firing, gives the pottery a glassy finish and finishes the piece. If you do it right, canning can help make sure that the whole firing process goes smoothly and that you end up with well-made, durable pottery.
What Are The Different Types Of Kilns?
There are many different kinds of kilns on the market, from small tabletop kilns to large industrial kilns, each made for a different task.
Most tabletop or hobbyist kilns are small, easy to move, and cheap. They are usually powered by electricity and can be used for small projects like making jewelry or firing small pieces of pottery.
Ceramic kilns are used to make pottery and come in many sizes, from small ones that fit on a table to big ones used in factories. Electric kilns are common and can be used to fire both the bisque and the glaze. Those who want a more traditional firing experience can also use gas kilns.
Glass kilns are made to fuse and slump glass, and they are often used to make one-of-a-kind works of art. They are usually small to medium-sized and can be powered by electricity or gas.
Industrial kilns are used for big jobs like making bricks or working with metal. Most of the time, these kilns are very big and expensive, and they can be made to fit specific production needs.
Aside from the different kinds of kilns, there are also different ways to fire them, like oxidation, reduction, and wood firing. The type of kiln and way it is fired will depend on the project and what is wanted to happen.
What Are The Different Types Of Materials That Can Be Candled In A Kiln?
Candling is a way to slowly dry out unfired pottery, also called “greenware,” before putting it in a kiln to be heated. Candleing is most often done to greenware, but there are other materials that can also benefit from this process.
Clay is the most commonly candled material, as it is used to create pottery and ceramic pieces. Porcelain, earthenware, and stoneware are all types of clay that can be candled to ensure that the greenware dries out slowly and evenly before firing.
Other Materials
Plaster, which is used to make molds for pottery and other sculptures, and wax, which is used to make molds for metal casting, are two other things that can be candled. Candling helps to gently dry out the material in both cases without damaging it or making it warp.
How Do You Candle An Electric Kiln?
- Load the kiln. Put the greenware on the shelves of the kiln, leaving space between each piece so air can flow. Don’t put too much in the kiln, because that can make the process of candling uneven.
- Before you start the candling process, make sure the kiln is securely closed and latched.
- Set the temperature. Set the temperature of the kiln to a low level, usually between 200 and 300°F. This temperature needs to be kept for a few hours so that the greenware can slowly dry out.
- Keep an eye on the temperature. Throughout the process of making candles, keep an eye on the temperature of the kiln to make sure it stays within the desired range. Use a pyrometer or another device that measures temperature to make sure you are right.
- Increase the temperature. After a few hours at the lower temperature, slowly raise the temperature of the kiln to the bisque or glaze firing temperature you want.
- Watch the kiln. Keep an eye on the kiln’s temperature to make sure it gets to and stays at the right firing temperature for the right amount of time.
- Cool down. After firing, let the kiln cool down slowly, and then take out the pieces that have been fired.
Knowing When Your Pottery Has Finished Candling
Figuring out when your pottery is done candling is an important part of making pottery because it helps make sure the greenware is dry and ready to be fired. When deciding if your pottery is done candling, there are a few things to keep in mind:
- The amount of time needed for candling depends on the size and thickness of the greenware as well as the temperature you want to fire it at. As a general rule, greenware should be candled at a low temperature for several hours before the firing temperature is slowly raised.
- Throughout the candling process, the greenware should be examined for moisture content. The level of moisture in the pottery can be measured with a moisture meter. The amount of water in the greenware should go down over time as it dries out.
- You can also tell if the greenware is ready for candling by how it looks. If the greenware is still wet or feels damp when you touch it, it is not ready to be fired. Look for a smooth, dry surface and signs of cracking or warping, which could mean the greenware needs more time to dry out.
- Lightly tap the greenware with your finger or something else. If it sounds flat or dull, there may still be some water in it. If it sounds sharp or clear, it’s probably dry and ready to be fired.
If greenware is candled for too long, it can become over-dried, which can lead to cracking, warping, or other issues during firing. Over-drying can cause the greenware to become brittle, making it more prone to breakage or other damage during firing.
I never candle overnight because candling overnight or for an extended period of time can be risky, as it may increase the likelihood of fire hazards or other safety issues. Kilns should always be monitored when in use to ensure that they are functioning properly and safely. I do not like to leave my electric kiln on unattended.
Using A Mirror To Check Moisture
Open a peephole on the kiln and hold the mirror above the peep hole and look for moisture on the mirror. It’s easy to position the mirror above the peephole without disturbing the temperature and humidity levels inside the kiln. To use this method, first make sure the peephole is clean and clear and gives you a good view of the inside of the kiln. Then, hold a small mirror or other surface that reflects light above the peephole and tilt it so that it shows what’s inside the kiln. This will let you see if there is moisture in the greenware by looking for condensation on the mirror.
I advise to avoid opening the kiln during the candling process unless absolutely necessary. Opening the kiln can cause significant fluctuations in temperature and humidity, which can affect the quality of the finished pottery. By using a peephole and mirror or other tools to monitor the moisture content of the greenware, you can help ensure that it is properly dried before proceeding with firing, without risking damage to the pottery or the kiln.
Location Of Pottery In Your Kiln And Your Kiln Load
It is possible for pottery at the bottom of the kiln to be dryer than pottery at the top of the kiln during the candling process. This is because temperature and humidity conditions can vary within the kiln, with the bottom of the kiln often being hotter and drier than the top.
During the candling process, the goal is to evenly and slowly dry the greenware to avoid cracking or warping during firing. However, if the pottery at the bottom of the kiln is significantly drier than the pottery at the top, it can result in uneven drying and potential damage to the greenware.
To help ensure even drying, it is important to use appropriate kiln-loading techniques, such as placing thicker or heavier pieces of pottery at the bottom of the kiln, and lighter or thinner pieces at the top. This can help to balance the temperature and humidity conditions within the kiln and promote even drying.
You should monitor the moisture content of the greenware during the candling process, using techniques such as the mirror or moisture meter methods described earlier, to ensure that all of the pottery in the kiln is properly dried before proceeding with firing. As a general rule, I recommend it is a good idea to check for moisture at least once per hour during the initial stages of the candling process, and then every few hours thereafter as the moisture level in the greenware decreases. However, keep in mind, if you notice significant condensation on the mirror during any of these checks, you may need to adjust the temperature or humidity levels in the kiln, or allow more time for the greenware to dry before continuing the firing process. This takes practice to get it right and this is where the art comes in, right?
Downdraft Kilns And Using The Mirror and Peephole Technique
If you are using a downdraft vented kiln the downdraft should be turned off.
A downdraft kiln vent is a type of ventilation system that is designed to help remove fumes and gases from the kiln during firing, and can have an impact on the candling process as well.
In a downdraft kiln vent system, the hot air and gases generated by the firing process are drawn down into a chamber beneath the kiln, where they could be forced through a series of filters to remove particulates and other impurities before being released into the outside air. This helps to improve air quality in the workspace and reduce the risk of health hazards associated with firing pottery.
Candle or Slow Bisque Mode
Available on kilns with a digital controller. Candle mode is a feature that is available on some electric kilns with digital controllers. However, not all electric kilns have this feature, and the specific models that offer candle mode can vary by brand and manufacturer. Some popular brands that offer kilns with candle mode or something similar might include Skutt, Paragon, and L&L Kilns.
Another option is to program your controller to gradually warm up for each firing. This is what I do using a Paragon kiln and controller. That way it’s built into each firing sequence. I do not know all the features of each manufacture so I say it’s always a good idea to check with the manufacturer or consult the product specifications to determine if a specific kiln model has candle mode or they may call it a pre-heat mode or slow bisque mode.
Using A Moisture Meter
I have not had much luck using a moisture meter due to the variations in readings. I guess you could get a general idea of the moisture content. If you place a moister meter on your clay and get a reading of say 10% moisture and then move it a couple of inches you can get an entirely different reading of maybe 50%. You pottery is going to give you a lot of different readings. So that’s why I say you can get a general idea. Basically it’s going to come down to factors like the type of clay you are working with, the thickness of the clay and if you used slip or added any water.
Water In Clay
There are some general signs that can help you figure out what kind of water is in your clay:
Most of the time, bound water is held by the clay particles more tightly and takes more energy to remove than mechanical water.
At lower temperatures, water is often lost through mechanical means, and it is easier to get rid of it by drying or heating.
Bound water is usually found in clays that can be molded and shaped more easily, while mechanical water is usually found in clays that can’t be molded and shaped as easily.
In general, clay that is hard to work with and needs a lot of water to become malleable is likely to have water that is bound to it. Mechanical water is more likely to be in clay that is less flexible and needs less water to be workable.
In the end, the type and amount of water in your clay will depend on a number of things, such as the type of clay, the amount of water it contains, and how it was processed.
What Is Dehydroxylation?
Dehydroxylation is what happens when you heat clay to a high temperature, usually between 600°C and 800°C. During this process, water molecules are taken away from the clay minerals, which changes the chemical structure of the clay. This makes the clay particles smaller and makes them stronger and more durable. This is an important step in the firing process, especially when it comes to drying and preparing clay for bisque firing and putting candles in a kiln.
When working with clay, it’s important to dry and prepare it properly before firing so that it doesn’t crack or warp when it’s heated. This is done by slowly drying the clay, which lets it slowly lose its water content. If the clay dries too quickly, the water inside the clay particles may expand, causing the clay to crack or break. To keep this from happening, potters and ceramicists often use both air drying and slow heating to get the clay ready for firing.
Once the clay has been properly dried and prepared, it is ready for bisque firing, which involves firing the clay at a low temperature to remove any remaining moisture and partially fuse the clay particles together. During the bisque firing, the water molecules are taken out of the clay minerals, which is called dehydroxylation. This makes the clay particles smaller and makes them stronger and more durable.
Candling a kiln is a common way to get the kiln and clay ready for bisque firing. To remove any remaining moisture from the clay and to help prevent thermal shock during the firing process, the kiln is gradually heated to a low temperature during the candling process. This slow, steady heating makes it possible to remove the water molecules from the clay minerals without cracking or warping the clay.
In short, dehydroxylation is a process that happens when clay is heated to a high temperature. The clay particles get smaller and stronger and last longer as a result. To keep the clay from cracking or warping during firing, it must be dried and prepared properly before firing. Bisque firing and canning a kiln are important steps in the firing process that allow for the gradual removal of moisture from the clay and the process of dehydroxylation.
What Is The Kiln Sitter And How Does It Help With Candling A Kiln?
A device called a “kiln sitter box” is used in ceramic kilns to help control the firing process and keep things from getting too hot. The kiln sitter box is usually a metal or ceramic box that is attached to the side of the kiln and has a trigger mechanism with a spring inside.
During the process of firing, a small piece of ceramic in the shape of a cone called a pyrometric cone is put in the kiln. The kiln sitter box is then set up to hold a rod that will set off the mechanism when the cone has reached its melting point.
Once the cone has melted, the trigger is pulled back, and the kiln sitter box turns off the power to the kiln. This stops the kiln from firing again. This keeps the kiln from getting too hot and makes sure that the ceramics inside get the right amount of heat.
The kiln sitter box has nothing to do with candling, which is the process of slowly raising the temperature of the kiln to dry out the ceramics before firing. But the kiln sitter box is an important safety feature that makes sure the firing process goes smoothly and keeps the kiln and the ceramics inside from getting damaged.
How Do High Fire Glazes Affect Candling A Kiln?
There are a few ways in which high-fire glazes can change the way a kiln is candled. Candling is a slow, gradual process of heating the kiln to remove moisture from greenware or unfired pottery before turning up the heat to start the firing process. In order to prevent the glaze from cracking or crazing during the firing process, high-fire glazes typically need a longer and slower firing cycle. As a result, the candling process may need to be lengthened.
Here are some ways that high-fire glazes can affect how a kiln is candled:
Glaze Compatibility
High-fire glazes are made to melt at high temperatures and stick to the clay body. But if the glaze doesn’t work with the clay body, it can lead to problems like cracking, crazing, or shivering. Before firing the pottery, the clay body and the glaze should be checked for problems like these. Candling a kiln with high-fire glazes can make sure the glaze is properly fused to the clay body and won’t crack or craze.
Drying Time
High fire glazes are usually thicker than low fire glazes and take longer to dry. Candling a kiln can help the glaze on the pottery dry slowly and evenly, which makes it less likely that the glaze will crack or blister during the firing process.
Firing Schedule
Glazes with a high temperature need to be fired for longer and more slowly than glazes with a low temperature. By lighting a candle in the kiln, you can make sure that the temperature rises slowly to the right level, which lets the glaze melt and join with the clay body.
Do Electric Kilns Require Different Techniques For Candling Than Other Types Of Kilns?
The basic principles of candling remain the same regardless of the type of kiln being used. However, making candles in electric kilns is not much different from making candles in other kinds of kilns.
What Is A Typical Firing Schedule When Candling An Electric Kiln?
A typical firing schedule when candling an electric kiln can vary depending on the size of the kiln, the type of clay or pottery being fired, and how much pottery or clay is being fired. The general principle of candling is to gradually heat up the kiln to remove any residual (or as much as you can) moisture from the pottery before ramping up to higher firing temperatures.
Here is an example of a typical firing schedule for candling an electric kiln. The times are approximate and will vary based on the factors I already mentioned above.
- I always place my pottery inside the kiln first. Load the pottery or greenware into the kiln on the lowest shelf, leaving space between each piece for proper airflow.
- then I Preheat the kiln. Turn on the kiln and allow it to preheat for about 30 minutes. This will help to warm up the kiln and prevent any thermal shock when the pottery is first introduced.
- Place pottery inside the kiln. Load the pottery or greenware into the kiln on the lowest shelf, leaving space between each piece for proper airflow.
- First candling phase. Set the temperature to 200°F (93°C) (your temp can vary and might be different) and let the kiln run for 2-4 hours to slowly and gently remove any moisture from the pottery.
- Second candling phase. Increase the temperature to 300°F (149°C) and let the kiln run for 2-4 hours. This will continue to remove any moisture from the pottery and begin the drying process.
- Final candling phase. Increase the temperature to 400°F (204°C) and let the kiln run for 2-4 hours. This will further dry out the pottery and prepare it for the higher firing temperatures.
- Ramp up the temperature. Once the candling process is complete, gradually increase the temperature of the kiln to the desired firing temperature over several hours or even days, depending on the size and type of pottery being fired.
- Hold or soak time. Once the kiln reaches the desired firing temperature, it should be held at that temperature for a set amount of time to allow the glazes to mature properly.
In summary, the firing schedule when candling an electric kiln involves gradually increasing the temperature over several hours or days to slowly remove any moisture from the pottery before ramping up to higher firing temperatures. The temperature increase should be gradual and consistent, and the process should take several hours or even a full day depending on the size of the kiln and the type of pottery being fired. What I mentioned above will gives you a general idea. you will have to experiment to find what works for you.
How Do Bisque Firings Differ From Regular Firings When It Comes To Candling A Kiln?
Bisque firings are different from regular firings but when it comes to candling a kiln the general principle remains the same. there would be some slight differences like a lower starting firing temperature. Bisque firings are generally fired at a lower temperature than regular firings, often around 1800°F (982°C). This means that the candling process for bisque firing will start at a lower temperature, typically around 200°F (93°C) or maybe a little lower, and then gradually increase to the desired bisque firing temperature over a longer period of time.
Also since bisque firings are fired at a lower temperature, they typically require a longer firing time to achieve the desired result. This means that the candling process for bisque firing may take longer, if could take several hours or even a full day to reach the desired bisque firing temperature.
And finally, be aware that objects placed near the bottom of the kiln can experience a slower rate of temperature rise, while objects placed at the top of the kiln may experience a faster rate of temperature rise. This can affect how the clay particles fuse together and the final texture of the piece.
Conclusion And Summary
Here are some of the most important things to know about canning a kiln:
Candling is the process of slowly and gently heating a kiln to get rid of any remaining moisture in pottery or clay before bringing the temperature up to higher levels for firing.
The candling process is especially important to prevent thermal shock, which can cause pottery to crack or break during the firing process.
Candling is typically performed at lower temperatures, starting around 200°F (93°C) and gradually increasing over several hours or even a full day to the desired firing temperature.
Depending on the size of the kiln, the type of pottery being fired, and the desired outcome, the candling process can take a different amount of time.
Different kinds of kilns might need different candling methods, like adjusting the airflow or where the pottery is placed.
Bisque firings, which are the first firing of clay before glaze is applied, take longer and are done at lower temperatures than regular firings.
Candling a kiln the right way can help keep the pottery from breaking and make sure the firing goes well.
In conclusion, canning a kiln is a unique and important process that involves slowly and gently heating a kiln to remove any remaining moisture from pottery or clay. By taking the time to properly light the candles in a kiln, potters and ceramicists can prevent thermal shock and make sure the firing goes well.
References
“Mastering Kilns and Firing: Raku, Pit and Barrel, Wood Firing, and More” by Lindsay Oesterritter
“The Kiln Book: Materials, Specifications & Construction” by Frederick L. Olsen
“Electric Kiln Ceramics: A Guide to Clays, Glazes, and Electric Kilns” by Richard Zakin
Photo by Vaibhaw Kumar on Unsplash