The Art of Kiln Candling

Candling is the initial phase in the firing process, particularly crucial for greenware, which is unfired or raw clay. During candling, the kiln is set to a low temperature, usually between 100°F to 200°F, for several hours. The primary purpose of this step is to ensure that all the remaining moisture in the clay is fully evaporated before the kiln reaches higher temperatures. This is vital because moisture turning into steam can cause the pottery to crack or even explode. Candling is like the warm-up before the big game; it prepares the clay for the subsequent phases of firing, reducing the risk of damage and improving the overall quality of the final piece.

The Benefits

Moisture Removal

Moisture removal is a crucial step in the kiln firing process, especially when dealing with clay-based artworks or functional items. It’s all about ensuring that every last bit of moisture trapped in the clay is completely evaporated. Why is this so important, you ask? Well, if any moisture remains in the clay when you ramp up the kiln to higher firing temperatures, you’re asking for trouble. That lingering moisture will turn to steam, and steam needs to escape. If it can’t, it’ll create internal pressure within the clay piece.

This is where things can get messy, literally, yes I am speaking from experience, ha! The steam can cause the clay to crack or, even worse, explode inside the kiln, damaging not just the piece in question but potentially other pieces in the kiln as well. Nobody wants to open their kiln to find a clay grenade has gone off, ruining hours of hard work and wasting energy.

So, moisture removal is like the unsung hero of the ceramic-making process. It ensures that your piece can move on to the higher firing stages safely, maintaining its structural integrity and setting the stage for successful glazing and finishing. In summary, I don’t underestimate the power of effective moisture removal; it could be the difference between a masterpiece and a big mess.

Improved Structural Integrity

Improved structural integrity is another major benefit of the candling process in kiln firing. Greenware, which is essentially unfired clay, is notoriously fragile and prone to cracking or breaking if not handled with utmost care. Yes, I am speaking from experience, I have messed up many pieces. Candling offers a gentle introduction to the world of high temperatures for these delicate greenware pieces. By slowly heating up the kiln during the candling phase, the clay undergoes subtle but significant changes in its structure.

Now, you might be wondering why this slow and steady approach makes such a difference. Well, the gradual heat helps to tighten the clay particles, making the piece stronger and less susceptible to damage. This means that when you’re ready to crank up the heat for higher temperature firing, your artwork or functional item is more robust and better prepared to handle the thermal stress.

So, if you skip the candling stage, you’re basically throwing your greenware into the thermal deep end, and the likelihood of it surviving unscathed is much lower. But with candling, you’re giving your piece a fighting chance to withstand higher temperatures without cracking or warping. It’s like a mini boot camp for your clay, prepping it for the intense workout that lies ahead in the kiln! Well, that sounded kind of strange, but you get the point, I never thought about working out inside a kiln, ha!

Reduced Risk of Cracking

Reduced risk of cracking is one of the standout benefits of candling your clay pieces in a kiln. So, what’s the big deal about thermal shock? Imagine plunging your hand into a bucket of ice water and then immediately into hot water. You’d feel a jolt, right? Almost as good as an energy drink, right, ha! The same thing can happen to your clay pieces if they’re exposed to rapid temperature changes, leading to cracks or even breaking.

Candling acts as a buffer against this thermal shock. By slowly heating the clay, you’re allowing it to gradually acclimate to the higher temperatures it will face later in the firing process. This slow introduction of heat is crucial for reducing the internal stress that can lead to cracking. It’s like a warm-up before a workout; it prepares the clay for the more intense conditions to come.

So, if you’re looking to create a ceramic piece that’s not just beautiful but also durable and crack-free, don’t underestimate the power of the candling process. It’s a simple yet effective way to ensure that your clay pieces remain intact, from the kiln to the showcase.

Better Glaze Adherence

Better glaze adherence is another fantastic reason to consider candling your clay pieces before the main firing process. You might wonder, “Why does moisture matter when it comes to glaze?” Well, think of glaze like paint. If you’ve ever tried to paint over a damp wall, yes I’ve done that too, you know it doesn’t adhere well. The same principle applies to glazing clay.

When the clay is bone-dry, it becomes more receptive to the glaze, allowing it to stick better and create a more uniform coat. This is especially crucial for intricate designs where you want the glaze to go on smoothly without pooling or running. Candling helps to ensure that every nook and cranny is moisture-free, setting the stage for flawless glaze application.

So, if you’re aiming for a piece with a stunning, even glaze that truly brings out the beauty of your work, don’t skip the candling step. It’s like prepping a canvas for a masterpiece; the extra effort upfront leads to a more polished final product.

Quality Assurance

Quality assurance in pottery is a non-negotiable, especially when you’re investing time and resources (money) into your pieces. This is where candling steps in as an invaluable pre-firing procedure. Think of it like a final check-up for your greenware before it goes into the high-stress environment of higher firing temperatures.

By gradually raising the kiln’s heat, you’re essentially giving your pieces a “trial run” of sorts. This allows you to identify any last-minute issues like lingering moisture or structural weaknesses that could compromise the integrity of the piece during the more intense firing processes. Any issues detected can be corrected before committing to the full firing, saving you not just time but also the heartbreak of losing a piece you’ve worked hard on.

So, in a nutshell, candling is your quality control superhero. It’s that extra step that ensures you’re not rolling the dice when you load your carefully crafted items into the kiln for the main event.

Energy Efficiency

Energy efficiency might not be the first thing that comes to mind when you’re focused on the artistic aspects of pottery, but it’s an important consideration. Candling offers a green thumbs-up in this regard. By slowly heating your greenware at low temperatures, you’re methodically evaporating any remaining moisture. This is crucial because moisture is a major culprit behind cracks and even explosions during higher firing stages.

Now, why does this matter for energy efficiency? Well, if a piece cracks or explodes during the main firing process, not only is the piece likely ruined, but the energy used in that firing is essentially wasted. Plus, you’ll have to expend even more energy to re-fire a new piece from scratch. Yes, I’ve done that too, I’ve fired many times just to get one clay body that did not break. Sounds like a lot of this is from my experience, hm…

Candling helps you dodge these issues. By ensuring that each piece is in optimal condition before it undergoes higher firing, you’re reducing the likelihood of having to start over, saving not just your piece, but also valuable energy. It’s a win-win for both the artist and the planet.

The Multi-Million Dollar Question – But I have an Electric Kiln With a Controller

OK, here is why you pay me the big bucks, to answer questions like this and save you years of trial and error, ha!

The good news is that modern kilns with electronic controllers often have the capability to include a candling stage as part of your regular firing program. You can program the controller to start at a low temperature and hold it there for a certain amount of time, effectively accomplishing the candling process. After that, the program would proceed to ramp up the temperature according to your specified schedule for the rest of the firing.

This built-in approach means you don’t have to manually monitor the kiln during the candling stage and then reset it for the higher firing stages. Everything can be automated, making the process more streamlined and less labor-intensive. So yes, with an electronic controller, the candling process can be integrated into your regular firing program, saving you both time and effort. Just make sure to consult your kiln’s manual to ensure you’re programming it correctly for your specific needs.

What Is The Kiln Sitter Box And How Does It Help With Candling A Kiln?

The next best thing, right? The kiln sitter box is a mechanical device often found in older kilns, and it serves as a sort of “automatic shutoff” system. It works by holding a small pyrometric cone or bar that bends or melts at a specific temperature. When the cone or bar bends enough, it triggers the kiln sitter to shut off the kiln, indicating that the desired temperature has been reached.

When it comes to candling a kiln, the kiln sitter box can be less precise than electronic controllers, but it still offers some utility. You can set up a low-temperature cone or bar that corresponds to the candling temperature you want. Once the kiln reaches that temperature and maintains it long enough for the cone to bend, the kiln will shut off, effectively completing the candling process. After confirming that the moisture has been eliminated, you can then proceed to the higher firing stages, either manually or by inserting a new cone for the next temperature target.

The kiln sitter box does require more manual involvement compared to an electronic controller, but it can still help ensure that your candling process reaches the intended low temperature and holds it long enough to evaporate the moisture from your clay pieces.

What Is A Typical Firing Schedule When Candling A Kiln?

A typical firing schedule for candling a kiln is designed to gradually raise the temperature, allowing for the safe removal of moisture from the clay pieces. The schedule generally starts with a low-temperature soak, often between 100°F to 200°F (38°C to 93°C), for a few hours. This initial phase ensures that any remaining moisture in the clay evaporates slowly, reducing the risk of cracks or explosions during higher firing stages.

Here’s a basic outline of a typical candling schedule:

1. Temperature Ramp – Start by slowly ramping up the kiln to the candling temperature, often around 100°F to 200°F.
2. Soak Time – Hold or ‘soak’ at this temperature for several hours (often 4 to 8 hours, depending on the size and thickness of your pieces).
3. Temperature Assessment – Check the kiln and clay pieces to ensure that all moisture has been removed. If you see any signs of condensation or moisture, extend the soak time.
4. Cool Down – Once you’re confident that the moisture is gone, allow the kiln to cool down naturally to room temperature before opening the lid.
5. Ventilation – During the candling process, it’s often advised to slightly vent the kiln to allow moisture to escape. Some kilns have built-in venting systems for this purpose.
6. Readiness for Next Stage – After the kiln has cooled and you’ve confirmed the pieces are dry, they’re ready for bisque firing or glaze application, depending on your project needs.

The above is a general guideline, and individual needs may vary based on the specific type of clay, the thickness of the pieces, and the kiln’s characteristics.

How Do High Fire Glazes Affect Candling A Kiln?

High-fire glazes are formulated to mature at temperatures that usually range from cone 5 (around 2167°F or 1186°C) to cone 10 (around 2345°F or 1285°C). These temperatures are significantly higher than the temperatures used for candling, which are typically low and range from 100°F to 200°F (38°C to 93°C) to slowly remove moisture from the clay.

Because high-fire glazes require such high temperatures to mature, they don’t directly impact the candling process. Candling is a preparatory step done before any glaze is applied to ensure that the clay body is completely dry and ready for the bisque firing. Only after the bisque firing is complete, and the piece is confirmed to be moisture-free, is the high-fire glaze applied.

That said, it’s important to note that high-fire glazes often have specific application requirements that can be affected by the condition of the clay body. If you skip the candling process or rush through it, you risk having a clay body that’s not fully dry, which could lead to issues like cracks or poor glaze adherence in the subsequent high-temperature firings.

So, while high-fire glazes themselves don’t affect the candling process, the candling process can certainly impact how well these glazes adhere and mature during the high-temperature firings that follow.

Are There Particular High Fire Materials That Should Be Avoided When Candling A Kiln?

When it comes to candling a kiln, the focus is usually on the moisture content of the clay rather than the specific type of materials you’re using. However, certain high-fire materials may require special consideration, here is my list:

1. Porcelain – This material can be less forgiving when it comes to moisture content. Because porcelain often contains fewer impurities and less grog, it can crack more easily if not properly candled.
2. High-Fire Glazes – If you have already applied a high-fire glaze to your piece, it’s important to ensure that the glaze itself does not contain moisture or solvents that could evaporate and cause cracks during candling.
3. Specialized Clays – Certain types of clay that are formulated for high-fire processes might have special requirements for drying or candling. Always check the manufacturer’s guidelines.
4. Metallic Inclusions – Some high-fire clays and glazes contain metallic elements that could be sensitive to slow heating. Be sure to follow specific guidelines for these materials.
5. Advanced Compounds – Materials like silicon carbide, which are used in very high-fire applications, usually have their own specific handling and firing requirements that you should consult.
6. Thick or Layered Pieces – High-fire materials often come in thicker dimensions and may require extended candling to ensure that moisture is completely removed.
7. Glass or Fused Elements – If your piece includes elements like glass that are meant for high-temperature firing, be aware that their thermal properties might differ from the clay, affecting how you should candle the piece.

How Do Various Stages Of Clay Affect The Process Of Candling A Kiln?

Oh, yes!!! The process of candling a kiln is significantly influenced by the stage of clay you’re working with. Here’s how:

1. Wet Clay – If your clay is still wet or very damp, candling can actually speed up the natural drying process in a controlled manner, reducing the risk of cracking.
2. Leather-Hard Clay – At this stage, the clay is firm but still contains some moisture. Candling helps to uniformly evaporate this remaining moisture, making the piece more stable for the next steps in the firing process.
3. Bone-Dry Clay – For clay that feels dry to the touch, candling serves as an extra precaution to eliminate any residual moisture that may not be apparent. This is crucial because any leftover moisture can cause the piece to crack or explode in later high-temperature firings.
4. Bisque-Fired Clay – If you’re dealing with bisque-fired pieces that have absorbed moisture over time, a gentle candling can help ensure they are completely dry before applying glazes or undergoing a glaze firing.
5. Glaze-Applied Clay – If you’ve applied a glaze that has a significant moisture content, candling can help to gently dry the glaze before you proceed to the higher-temperature glaze firing. This can improve the glaze’s adherence and reduce defects like crawling.
6. Recycled Clay – Sometimes potters reclaim and recycle clay. This reclaimed clay can have varying moisture content, and candling helps to standardize it before reuse.
7. Mixed Media – If your piece involves mixed media elements, like paper clay or added inclusions, the candling process can help you assess how these materials react to slow heating and whether they increase the risk of cracks or other defects.

Understanding the specific needs of the clay at its particular stage is crucial for a successful candling process. Always remember to consider these variables to ensure that your ceramic pieces are prepared optimally for their next stage!

How Do Bisque Firings Differ From Regular Firings When It Comes To Candling A Kiln?

Bisque firings and regular (glaze) firings have different objectives and therefore require different approaches when it comes to candling a kiln.

1. Objective
   • Bisque Firing – The primary goal is to remove all mechanical water and chemically bound water from the clay, making it porous and ready for glaze application.
   • Regular Firing – The aim is to mature the glaze and clay body, solidifying the piece into its final, glass-like state.
2. Temperature
   • Bisque Firing – Generally conducted at a lower temperature range (cone 04 to cone 02).
   • Regular Firing – Requires higher temperatures to mature the glaze and vitrify the clay body (up to cone 10 in some cases).
3. Duration of Candling
   • Bisque Firing – Candling may last longer as the primary goal is to ensure all moisture is removed.
   • Regular Firing – Candling might be shorter or even skipped if the piece has been bisque fired and stored in dry conditions.
4. Moisture Sensitivity
   • Bisque Firing – The clay is more sensitive to moisture, making candling crucial.
   • Regular Firing – Since the piece has already been bisque fired, it’s less sensitive to moisture, but candling can still be useful for glazes with high moisture content.
5. Rate of Heating
   • Bisque Firing – Slow heating is essential to avoid cracking or explosions due to moisture.
   • Regular Firing – The rate of heating can generally be faster as the clay body has already been stabilized through bisque firing.
6. Ventilation
   • Bisque Firing – Venting is crucial to allow moisture to escape.
   • Regular Firing – Venting is less critical but still important for even heat distribution and to burn off any remaining organics.

I say, while candling is often more critical for bisque firings to ensure all moisture is removed, it can also be beneficial during regular firings, especially if you’re working with fresh glazes or if the piece has been exposed to moisture.

Can You Achieve Slightly Higher Cone Numbers By Adjusting Your Technique While Candling The Kiln?

No, sorry, candling the kiln is not a technique to achieve higher cone numbers or to affect the final maturity of the clay or glaze. The purpose of candling is primarily to remove residual moisture from the clay pieces at a low temperature before proceeding to bisque or glaze firing. Achieving a higher cone number relates to the peak firing temperature and the heat work done during the actual firing process, not the candling stage.

Cone numbers are specific and correspond to certain temperature ranges and atmospheric conditions within the kiln. Attempting to achieve a higher cone number during candling could lead to defects like cracking, warping, or even explosions due to rapid moisture loss or thermal shock. Therefore, it’s essential to follow the recommended temperature guidelines for the specific type of clay and glaze you are using.

Keeping The Kiln Lid Open During Candling Pros And Cons

Choosing between keeping the kiln lid slightly ajar and using a built-in venting system depends on several factors, including the type of kiln you have, the specific requirements of your work, and personal preference. Here are some of my pros and cons of each method:

Kiln Lid Slightly Ajar

Pros

• Simplicity – No need for additional equipment.
• Flexibility – Easy to adjust the lid’s position as needed.

Cons

• Inconsistent – Potentially less even heat distribution.
• Risk – Potential for foreign objects to enter the kiln.

Built-in Venting System

Pros

• Consistency – Generally provides more even heat and moisture removal.
• Safety – Reduces the risk of contaminants entering the kiln.

Cons

• Complexity – May require additional setup or equipment.
• Cost – Built-in systems can add to the overall cost of a kiln.

Both methods aim to achieve the same result: the removal of moisture to prepare the ceramic pieces for higher-temperature firing. The “best” method often comes down to the artist’s (your) specific needs and the type of work they are doing. For example, if you’re working on a critical project where even heat distribution is key, a built-in venting system might be preferable. On the other hand, if you’re doing a quick, less critical project, keeping the lid ajar could be sufficient. I always use the built-in venting system.

How Should I Load A Kiln For Candling?

Loading a kiln for candling requires careful attention to detail to ensure even heat distribution and proper moisture removal. Here are my pro tips to help you get the best results:

1. Space Wisely – Place your pieces with adequate space between them. This ensures that heat circulates evenly around each item, aiding in moisture removal.
2. Use Kiln Shelves – If you’re using multiple layers, kiln shelves can help distribute weight and heat evenly. Don’t forget to use kiln wash or shelf paper to prevent sticking.
3. Height Matters – Try to keep the height of the pieces relatively uniform across the kiln to aid in even heating. If you have taller and shorter pieces, consider their placement carefully.
4. Central Positioning – Place pieces toward the center of the kiln shelves when possible, as the edges of the kiln are generally hotter. This is especially important for delicate or thin pieces that could be prone to cracking.
5. Check for Obstructions – Make sure pieces are not too close to the thermocouples, kiln sitters, or other internal kiln components. Obstructions can cause inaccurate temperature readings and uneven heating.
6. Use Kiln Posts – If your pieces are flat, consider using kiln posts to elevate them slightly off the shelf. This allows for better air circulation and more even drying.
7. Venting – Consider whether you will keep the kiln lid slightly ajar or use a built-in venting system for moisture removal. Adjust your loading strategy accordingly.
8. Test Pieces – If you’re uncertain about how your pieces will react to candling, consider doing a test run with similar pieces to observe how they respond to the process.
9. Pyrometric Cones – Though not necessary for candling, some artists like to use pyrometric cones to monitor the kiln’s temperature. This can be especially useful if you’re new to the candling process.
10. Safety First – Always wear appropriate safety gear like gloves and a mask when loading and unloading the kiln, and make sure your kiln is located in a well-ventilated space.

FAQ Section For Candling a Kiln

Q: What is candling a kiln?
A: Candling a kiln is the process of slowly and gently heating a kiln to a low temperature, typically between 100°F and 200°F, to remove any remaining moisture from the ceramic pieces inside. This is usually done before higher-temperature firings to prevent cracks or explosions.

Q: How long should I candle my kiln?
A: The length of time needed for candling can vary based on the moisture content in the pieces and the size of the kiln. Typically, candling can last between 4 to 8 hours. However, it’s important to check the pieces for signs of remaining moisture to determine if additional time is needed.

Q: What does candling mean in ceramics?
A: In ceramics, candling refers to the initial, low-temperature firing process that removes moisture from clay pieces. This helps to prevent cracks, explosions, or other defects in later, higher-temperature firing stages.

Q: What is fire candling?
A: Fire candling is not directly related to kiln candling in ceramics. It’s a term more commonly associated with forestry and fire management, where small, controlled fires are set to remove underbrush and reduce the risk of larger wildfires.

Q: Why do you preheat a kiln?
A: Preheating or candling a kiln helps to remove residual moisture from the ceramic pieces, which is crucial for preventing cracks or explosions during the higher-temperature firing phases. It also prepares the pieces for better glaze adherence.

Q: Do all types of clay need candling?
A: While most types of clay benefit from candling, the process is especially important for clay with higher moisture content or for pieces that are particularly thick. Always check the manufacturer’s guidelines for specific clay types.

Q: Is candling necessary for bisque firing?
A: Candling is often considered an essential step before bisque firing, especially for pieces that might have absorbed moisture during storage or handling.

Q: Can I skip candling if I’m in a hurry?
A: Skipping the candling process can put your pieces at risk for cracks, explosions, or other defects during higher-temperature firing. It’s generally not recommended to skip this important step.