Your kiln is the heart of your studio, but it can feel like a mysterious black box. You arrange your glass, press some buttons, and just hope for the best. Mastering your Bullseye firing schedules is how you take control and turn that hope into certainty. If you landed here looking for a bullseye coe 90 firing schedule pdf free download, you’ve found something even better. We’ll provide a solid Bullseye fusing schedule, but more importantly, we’ll explain the *why* behind each step—so you can work with your kiln, not against it, for stunning results.

Key Takeaways

• Master Glass Compatibility: Prevent stress cracks by exclusively using glass and materials with the same COE in a single project. For Bullseye glass, this means ensuring every sheet, stringer, and piece of frit is labeled COE 90.
• Adapt Your Firing Schedule: Use published schedules as a baseline, not a strict rule. Adjust your heating rates and soak times based on glass thickness and mold size, and keep a detailed firing log to fine-tune your process for consistent results.
• Perfect Your Annealing Process: Create strong, durable glass by controlling the cool-down. Follow a proper annealing soak and slow cooling rate, and resist the urge to open the kiln until it’s at room temperature to avoid thermal shock.

Your Guide to Bullseye COE 90 Glass

If you’re getting into kiln fusing, you’ll hear a lot about Bullseye COE 90 glass. Think of it as a complete system of glass designed specifically to be fused together. It’s a trusted favorite among artists because it’s reliable and predictable. When you use Bullseye products together, you can be confident that your project will survive the heat of the kiln without cracking. This compatibility removes the guesswork, letting you focus on your creativity. Whether you’re working with sheets, frit, or stringers, if they all share that “COE 90” label, you know they’re designed to play well together.

First, What Is Coefficient of Expansion (COE)?

Let’s break down one of the most important terms in glass fusing: COE, or coefficient of expansion. In simple terms, COE is a measurement of how much glass expands when it gets hot and shrinks when it cools. Every material does this, but in glass, the exact rate is critical. This rate is assigned a number, like 90 or 96. Glass with a higher COE expands and contracts more than glass with a lower COE. It might seem like a tiny detail, but it’s the key to making sure your fused glass art doesn’t crack.

Why COE 90 Is a Must for Glass Fusing

Imagine fusing two beautiful pieces of glass, only to pull a cracked mess from your kiln. This often happens when glasses with different COEs are combined. Because they expand and contract at different rates, they create stress against each other, and the glass eventually breaks. To avoid this, you must use glass with the same COE throughout a single project. Sticking to a single family of COE90 glass is the secret to achieving strong, crack-free results every time. It’s the golden rule of glass fusing.

What Can You Create with COE 90 Glass?

The creative possibilities with COE 90 glass are practically endless. Its reliability and vast range of colors make it perfect for artists of all skill levels. You can create stunning jewelry, beautiful custom dishes, or vibrant suncatchers that light up a room. Many artists use it to make intricate wall art, decorative tiles, and unique ornaments. By using different molds, you can also shape the glass into bowls, plates, and sculptural forms. Whatever your vision, COE 90 glass provides a dependable foundation for bringing it to life.

Special Considerations for Bullseye Glass

Working with Bullseye glass is a joy because of its consistency, but to get the best results, you need to understand its specific needs inside the kiln. It’s not just about temperature; it’s about the journey the glass takes to get there and back. According to the experts at Warm Glass, there are four main ways to fire glass: full fuse, slump, tack fuse, and fire polish. Each technique requires a different firing schedule to achieve a specific look and feel. Mastering these schedules is what separates a good result from a great one, ensuring your glass behaves exactly as you intend, whether you’re melting it flat or shaping it over one of our molds.

Working with Striking and Curious Glass

Some of the most exciting types of COE 90 glass are the “strikers” and “curious” varieties. These are the chameleons of the glass world—they go into the kiln one color and come out another. For example, a sheet that looks pale amber before firing might “strike” to a deep, rich red. As noted by Kiln Frog, questions about how these glasses change color are common. The final shade depends on the heatwork—the temperatures and times in your firing schedule. Because of this, it’s a great idea to run small test tiles before committing a special sheet to a large project. This helps you predict the final outcome and repeat it consistently.

Is Fused Bullseye Glass Food-Safe?

If you’re dreaming of making your own custom plates, bowls, or serving platters, you’re probably wondering if fused Bullseye glass is food-safe. It’s one of the most common questions artists ask. The simple answer is yes, with a few important conditions. When Bullseye glass is heated to a full fuse temperature in a kiln, it forms a smooth, non-porous surface that is considered safe for contact with food. However, this applies to the glass itself. If you add other materials like metallic lusters or certain decals, you must verify that those specific products are also food-safe. Always check the manufacturer’s guidelines for any decorative elements you add to your functional pieces.

Key Firing Schedules for COE 90 Glass

Think of a firing schedule as a recipe for your kiln. It tells the kiln how quickly to heat up, how long to hold a specific temperature, and how to cool down safely. Getting this recipe right is the secret to creating beautiful, strong, and crack-free glass art. While the schedules I’m sharing here are excellent starting points for COE 90 glass, remember that every kiln has its own personality. Some run a little hot, others a bit cool. The size of your kiln, the size of your project, and even the ambient temperature of your studio can have an effect.

That’s why I always recommend keeping a detailed firing log. Note the schedule you used, the type and thickness of the glass, the project’s dimensions, and the final result. Was the fuse perfect? Did it slump exactly as you wanted? This log will become your most valuable tool, helping you fine-tune these basic schedules to get consistent, predictable results from your specific kiln. Let’s look at the three most common firing schedules you’ll use for your COE 90 projects: the full fuse, the tack fuse, and the slump.

The Full Fuse Schedule

A full fuse is what most people think of when they imagine glass fusing. The goal is to heat two or more layers of glass until they melt completely into a single, smooth piece with soft, rounded edges. For a standard full fuse with two layers of 3mm COE 90 glass, a great target is to bring your kiln to 1490°F (810°C) and hold it there for about 10 minutes. This gives the glass enough time to flow together and heal its edges. This schedule is perfect for creating coasters, pendants, or the base layer for a more complex piece that you plan to slump later.

The ¼-Inch Thickness Rule

Here’s a fascinating principle of glass fusing that will save you a lot of headaches: the ¼-inch thickness rule. When you heat COE 90 glass to a full fuse temperature, it has a natural tendency to settle at about a quarter-inch thick. This is why the standard practice is to stack two layers of 3mm glass for most projects. The combined thickness is just under ¼ inch, which helps your piece hold its original shape and size without spreading out too much. If you want to contain the glass and build up more thickness, you can use kiln dams or other molds to control the flow. By working with this natural property of glass instead of against it, you’ll get more predictable and structurally sound results every time.

The Tack Fuse Schedule

What if you want to preserve some texture and dimension in your work? That’s where a tack fuse comes in. This schedule heats the glass just enough to stick the layers together firmly, but not so much that they completely lose their individual shapes. You’ll see defined edges and a layered look. To achieve this, you’ll use a lower process temperature, typically between 1350°F and 1380°F (732°C and 749°C). A tack fuse is wonderful for adding elements like frit or stringers on top of a base, creating pieces with a more sculptural and tactile quality. It gives your work a distinct, dimensional feel that a full fuse just can’t replicate.

The Slumping Schedule

Slumping is the process of taking a previously fused piece of glass and draping it over or into a form to give it a new shape, like a bowl or a plate. This is always done in a separate firing after your initial fuse. Since the glass only needs to soften enough to bend with gravity, the temperatures are much lower. A typical slumping schedule for COE 90 glass ranges from 1180°F to 1250°F (638°C to 676°C). You’ll place your flat, fused glass on top of one of your favorite molds, and the kiln will do the rest. The key is to watch it closely, as slumping can happen quickly once the glass reaches the right temperature.

The Fire Polishing Schedule

Sometimes after a fuse, or if you’ve done some coldworking, your glass might have a matte finish or slightly sharp edges. Fire polishing is the perfect fix. It’s a quick firing process designed to give your piece a beautiful, glossy surface without altering its shape. For COE 90 glass, a common schedule is to heat your kiln to around 1300°F (704°C) and hold it for just 10 to 15 minutes. The key is to watch your piece closely through the peephole. You’re looking for the moment the surface becomes shiny and smooth. This schedule is a lifesaver for restoring brilliance and creating a professional finish on your work.

The Kilncasting Schedule

If you want to move beyond flat pieces and create three-dimensional art, kilncasting is your next adventure. This technique involves melting glass into a form to create solid, sculptural objects. The firing schedule is more complex because you’re dealing with a larger volume of glass. A typical kilncasting schedule for COE 90 starts with a slower heat-up to around 1300°F (704°C) to allow the glass to start flowing into the details of the mold. Then, you’ll increase the temperature to about 1450°F (788°C) for a full melt. The most critical part is the slow cooling, or annealing, which can take many hours. This prevents internal stress and ensures your beautiful 3D creation comes out strong and whole.

How Glass Thickness Changes Your Firing Schedule

The thickness of your glass project is a major factor in your firing schedule. A simple rule of thumb is: the thicker the piece, the slower you go. For a project made of 6mm glass (or two 3mm layers), you might slow your initial heating rate to around 300°F (149°C) per hour. This gentle heating prevents the outside of the glass from getting hotter than the inside, which can cause stress and cracking. The same logic applies to cooling. Thicker pieces need longer annealing soaks and slower cooling rates to ensure the entire piece cools evenly. Rushing this process is one of the main reasons for cracks appearing days after a firing.

Breaking Down the 4 Firing Phases

Think of a firing schedule as a recipe for your glass. While every project is a little different, the process always follows four key phases. Mastering these steps is the secret to getting consistent, beautiful results from your kiln every single time. Each phase has a specific job, from gently warming the glass to building its final strength. Let’s walk through what happens inside your kiln during each stage of the journey.

Phase 1: Ramping Up the Heat

The first phase is all about slowly and safely bringing your glass up to temperature. You want to heat the kiln at a controlled pace, or “ramp rate,” to avoid thermal shock. This happens when glass heats too quickly, causing stress that can lead to cracks. For most projects using two layers of standard 3mm COE 90 glass, a ramp rate of 400°F (205°C) per hour is a great starting point. If your piece is thicker, slow it down. A gentle warm-up ensures your entire project heats evenly, setting the stage for a perfect fuse.

Phase 2: Soaking at Your Target Temperature

Once your kiln reaches its target temperature for fusing, it’s time for the “soak” or “hold.” During this phase, the kiln maintains a steady temperature, allowing the heat to penetrate the glass completely. This is when the magic happens. The glass softens, air bubbles trapped between layers get squeezed out, and the pieces begin to fuse together. The length of your soak determines the final look of your piece, whether you’re aiming for a full fuse where the edges are completely rounded or a tack fuse where the layers are just stuck together.

Phase 3: Annealing to Build Strength

This is arguably the most important phase for creating a durable piece of art. After the glass has fused, it needs to cool down in a very controlled way to relieve internal stress. For Bullseye COE 90 glass, you’ll rapidly cool the kiln to the annealing point, which is around 900°F (482°C). You then “soak” it at this temperature, allowing the entire piece to stabilize before cooling slowly. This process strengthens the glass on a molecular level, preventing it from cracking later on. It’s especially crucial for thicker pieces or those with complex shapes made using molds.

Phase 4: The Final Cool Down

The final phase requires one key ingredient: patience. After the annealing soak is complete, your kiln will continue to cool down to room temperature. It’s tempting to peek, but you must keep the door closed until the glass is completely cool, typically around 70°F (21°C). Opening the kiln too early can cause a rush of cool air, creating thermal shock that will crack your finished piece. Trust the process and let your kiln do its work. Your patience will be rewarded with a strong, stable, and beautifully fused creation ready for its next step.

Understanding the Brittle Zone

As your glass cools, it passes through a critical stage known as the brittle zone. Think of this as a danger zone, typically between 800°F and 1100°F (427°C and 593°C), where your project is incredibly fragile. In this temperature range, the glass isn’t fully solid yet, making it extremely vulnerable to thermal shock. This is why the number one rule is to keep the kiln door closed. Opening it even for a quick peek can introduce a rush of cool air, causing a sudden temperature drop that will almost certainly crack your piece. Respecting the brittle zone is a key part of the annealing process, which is vital for creating strong, durable art. As we’ve covered, this controlled cooling is what strengthens the glass on a molecular level, ensuring your beautiful creation doesn’t develop stress cracks long after it’s out of the kiln.

How Does Glass Thickness Affect Your Firing Schedule?

When you’re working with glass, one of the most important variables to consider is its thickness. A standard firing schedule is often designed for a project made of two layers of 3mm glass, for a total thickness of 6mm. If your project is thicker, thinner, or has uneven layers, you’ll need to adjust your firing schedule accordingly. Think of it like baking: a cupcake needs a very different baking time and temperature than a dense fruitcake. The same principle applies to glass.

The core issue is heat distribution. Glass is a poor conductor of heat, which means the surface heats up and cools down much faster than the center. For thicker pieces, this temperature difference between the outside and inside can create stress, leading to cracks or even a full-blown break. To prevent this, you need to give the heat enough time to penetrate the entire piece evenly. This means slowing down your ramp rates and extending your soak times to ensure your beautiful COE 90 glass project comes out of the kiln in one perfect piece.

How to Adjust Your Ramp Rates

The ramp rate is the speed at which your kiln’s temperature increases per hour. When you’re working with thicker glass, you need to slow this rate down. A slower ramp gives the heat time to travel from the surface to the core of the glass, preventing the outer edges from getting too hot too quickly. For a standard 6mm piece, a ramp rate of 300°F (149°C) per hour is a good starting point. This slower climb helps you avoid thermal shock, a common problem when the temperature difference within the glass becomes too great, especially as you approach 1000°F (538°C). For every additional layer of glass, consider slowing your ramp rate even more.

Differentiating Ramp Rates for Project Size

It’s not just the thickness of your glass that matters; the overall size of your project plays a big role, too. A large, platter-sized piece, even if it’s only two layers thick, needs a more cautious heating schedule than a small coaster of the same thickness. Why? Because the larger the piece, the greater the potential for temperature differences across its surface. The edges of a big project will always heat up faster than the center. By slowing down your ramp rate for larger pieces, you give the heat a chance to spread out evenly, preventing the center from lagging behind and creating stress. This is especially important in larger kilns where the heating elements might be further apart. As a general rule, if your project is bigger than 10-12 inches in any direction, start thinking about dialing back your ramp speed to ensure a safe and successful firing.

Extend Soak Times for Thicker Glass

Soaking means holding your kiln at a specific temperature for a set period. Thicker glass projects need longer soak times to ensure the entire piece reaches a uniform temperature. If the center of your glass isn’t as hot as the surface when the cooling phase begins, you’re setting it up for failure. Even a small temperature difference can introduce stress and lead to cracking. This is especially critical for larger or uneven pieces. Giving your project a longer soak allows the heat to fully saturate the glass, creating a stable foundation for a successful and stress-free annealing and cooling process.

Keep a Close Eye on Your Kiln’s Temperature

While pre-set schedules are fantastic starting points, every project and every kiln is unique. It’s so important to monitor your kiln’s temperature throughout the entire firing process. Don’t just set it and forget it. Your kiln’s own user manual is your best friend and should always be your first reference for recommended schedules. As you gain experience, you’ll learn the specific quirks of your equipment. Keeping a detailed firing log where you note the glass thickness, schedule used, and final results will help you fine-tune your process and consistently produce stunning, flawless pieces.

Common Firing Mistakes (and How to Avoid Them)

Opening the kiln after a firing cycle should feel like Christmas morning, but sometimes you’re met with a disappointing surprise. A crack, a cloudy surface, or an unexpected break can feel frustrating, but don’t let it discourage you. Most firing issues are common and, more importantly, preventable. Understanding what went wrong is the first step toward making sure it doesn’t happen again, turning a moment of disappointment into a powerful learning experience.

Learning to read your results and connect them to a specific part of the firing process is a skill every glass artist develops over time. It’s like becoming a detective for your own art. Was the temperature too high? Did it cool too quickly? Was the glass compatible? Answering these questions helps you refine your technique for the next project. Let’s walk through some of the most frequent problems you might encounter when firing Bullseye COE 90 glass and, most importantly, how to avoid them. By paying close attention to your schedule and materials, you can achieve consistently beautiful results.

Preventing Overheating and Devitrification

Have you ever pulled a piece from the kiln to find a hazy, scummy film on the surface? That’s called devitrification, or “devit” for short. It happens when the glass is held at a high temperature for too long, causing crystals to form on the surface. Devitrification can start to occur at temperatures from 1300°F (704°C) and up. While some artists use it for textural effects, it’s usually an unwelcome guest. To prevent it, make sure your firing schedule doesn’t hold the glass in this upper temperature range for longer than necessary. A reliable kiln that accurately follows your programmed schedule is your best defense against devit.

Fixing Pinholes and Blisters

Finding tiny bubbles or blisters on your glass is usually a sign of trapped gases. This happens when air is caught between layers, or when organic materials like glue residue or even fingerprints don’t burn off completely before the glass seals itself. The fix is to give those gases an escape route before that happens. You can do this when you adjust your firing schedule by adding a “bubble squeeze” soak. Try holding your kiln around 1250°F (677°C) for 20 to 30 minutes. At this temperature, the glass is soft enough for bubbles to rise and pop, but not so hot that the edges seal and trap them inside. It’s a simple adjustment that makes a huge difference in the clarity of your finished piece.

How to Stop Cracks from Poor Annealing

Nothing is more heartbreaking than a perfect piece that cracks during or after cooling. These fractures are almost always caused by internal stress from an improper annealing cycle. As glass cools, it contracts, and if one part of your piece cools faster than another, stress builds up. Even a temperature difference of just 10°F can be enough to cause a crack. This is why the annealing phase is so critical. It holds the glass at a stable temperature long enough for the entire piece to equalize before cooling slowly. Never rush the annealing process, especially with thicker pieces, and resist the urge to peek before the cycle is complete.

What Is Thermal Shock and How Do You Prevent It?

Thermal shock is a specific type of crack or break that happens when the temperature of the glass changes too quickly. It’s a risk during both the heating and cooling phases of your firing schedule. Ramping up the heat too fast can crack the glass before it even has a chance to fuse. The risk is highest below 1000°F (538°C), so it’s wise to use a slower ramp rate, like 300°F per hour, in this initial stage. The same rule applies to cooling. Don’t open your kiln until the internal temperature is close to room temperature. That sudden rush of cool air can easily shatter your warm masterpiece.

The Dangers of Mixing Incompatible Glass

This is one of the most fundamental rules of glass fusing: never mix glass with different Coefficients of Expansion (COE) in the same project. All glass expands when heated and contracts when cooled, but different types of glass do so at different rates. If you fuse a piece of COE 90 glass to a piece of COE 96 glass, their conflicting rates of contraction will create immense stress as they cool. This will inevitably cause your project to crack. To prevent this, always ensure every single component in your piece, from the base sheet to the tiniest bit of frit, has the same COE.

The Risks of Re-firing and COE Shift

Sometimes a project doesn’t come out of the kiln exactly as you planned, and you might be tempted to re-fire it to add details or fix an issue. While you can re-fire a piece, it’s not without risks. Each time you fire glass, you can cause a slight change in its properties, a phenomenon known as “COE shift.” The more times you send a project back into the kiln, the greater the risk that its COE will drift, potentially creating stress that leads to cracking. This is especially dangerous if you’ve accidentally mixed glasses with different COEs; even if it survived the first firing, a second trip to high temperatures is practically guaranteed to cause a break. While re-firing once or twice is often fine, pushing it to four or more times is a serious gamble. Your best bet is to always start with compatible COE 90 glass to minimize issues from the very beginning.

How to Perfect Your Annealing Technique

Think of annealing as a spa day for your glass. After the intense heat of fusing, your project is full of internal stress. Annealing is the crucial cool-down phase that allows this stress to relax, resulting in a strong, stable piece that will last a lifetime. Skipping or rushing this step is one of the most common reasons for cracks that appear hours, or even days, after you take your project out of the kiln.

Perfecting your annealing technique isn’t about a secret formula; it’s about understanding how glass behaves as it cools. By controlling the temperature and timing, you can ensure your beautiful creations are as durable as they are stunning. Let’s walk through the key steps to get this process just right.

Aim for Even Temperature Distribution

The first rule of a successful anneal is to make sure your entire piece is the same temperature before you start the slow cooling process. This is why firing schedules include an “annealing soak” or “hold.” During this time, the kiln holds the glass at a specific temperature, allowing thicker areas to catch up with thinner edges. This step is vital because if one part of the glass cools faster than another, it creates tension. Understanding each step of the firing process is important for success. Allowing the glass to reach an even temperature before slow cooling is what prevents this stress and the heartbreaking cracks that come with it.

Why You Need to Control Your Cooling Rate

After the annealing soak, you need to manage the cooling rate carefully. This isn’t a simple, steady drop. The goal is to move through the “annealing zone” (roughly 900°F to 750°F for COE 90) very slowly. This controlled descent gives the glass molecules time to arrange themselves into a stable, stress-free structure. The thicker your project, the slower this cooling segment needs to be. A programmable kiln is your best friend here, as it can execute these precise temperature changes for you. Once you are below the stress point, you can let the kiln cool more quickly to room temperature.

When in Doubt, Anneal Longer

It’s a mantra you’ll hear from experienced glass artists, and for good reason: when in doubt, anneal longer. Adding extra time to your annealing soak is a simple insurance policy against future cracks. While it might test your patience, that extra hour in the kiln is a small price to pay to protect the hours of work you’ve already put into your project. This is especially true for thicker projects or pieces with uneven shapes, where hidden internal stress can build up if the glass doesn’t cool perfectly evenly. A longer hold gives the entire piece more time to stabilize, significantly improving its final durability and giving you peace of mind.

How to Check Your Glass for Stress

The best way to deal with stress is to prevent it from happening in the first place. The number one cause of stress-related cracks is mixing incompatible glass. To prevent this, only use Bullseye COE 90 glass with other Bullseye 90 products in a single project. Even a tiny chip of a different COE can introduce enough stress to break your piece. If you suspect a piece has stress, you can check it using a pair of polarizing filters. When you look at the glass through the filters, any stressed areas will show up as glowing white or rainbow-colored lines, giving you valuable feedback for your next firing.

Get the Best Results From Your Kiln

Your kiln is the heart of your glass studio, and giving it a little TLC goes a long way toward creating stunning, successful pieces. Think of it less as an appliance and more as your creative partner. When you understand its quirks and keep it in great shape, you can achieve more predictable and beautiful results every time you fire. A well-maintained kiln is a reliable one, which means less stress for you and fewer “happy accidents” that aren’t so happy. Taking a proactive approach helps you move from hoping for the best to knowing what to expect.

The Importance of Kiln Calibration and Maintenance

Just like any essential tool, your kiln needs regular maintenance to perform its best. Start by getting into the habit of cleaning your kiln shelves and applying a fresh coat of kiln wash before each firing. This simple step prevents your glass from sticking and gives you a clean surface for your project. Periodically, you should also inspect the heating elements for any signs of wear and check that your thermocouple is functioning correctly. Creating a simple maintenance checklist can help you stay on top of these tasks. A well-cared-for kiln is the foundation of consistent and successful glass fusing.

Accounting for Kiln Load and Environment

Your kiln doesn’t exist in a bubble. The environment of your studio and what you put inside the kiln can significantly change how it fires. Think about the kiln load: a kiln packed with shelves and multiple projects will heat up and cool down much slower than one with a single small piece. The sheer mass of glass and furniture absorbs a lot of energy. How you arrange your pieces also matters, as it affects how heat moves and circulates. External factors play a huge role too. A kiln firing in a chilly garage during winter will perform differently than one in a warm studio on a summer day. Cold weather, drafts, and even fluctuations in your power supply can alter your results. This is why keeping a detailed firing log is so critical. By noting these variables, you can start to understand your kiln’s unique behavior and adjust your schedules for consistent success.

How to Monitor and Adjust Kiln Temperatures

Every kiln has its own personality; even two kilns of the same model can fire slightly differently. That’s why it’s so important to get to know yours. Always refer to your kiln’s user manual to find the recommended firing schedules for your specific glass and project type, as this is your best starting point. You can also use pyrometric cones occasionally to verify that your kiln’s controller is reading the temperature accurately. As you gain experience, you’ll learn how your kiln heats and cools, allowing you to make small adjustments for even better results.

Choosing Your Programming Method: Ramp/Hold vs. Time-to-Temp

When you program your kiln, you’re essentially giving it a recipe to follow. Most modern digital controllers, like the ones on the kilns we carry, offer two main programming methods. The most common and precise method is Ramp/Hold. This is where you tell the kiln exactly how fast to heat up (the ramp), what temperature to stop at, and how long to stay there (the hold). It gives you complete control over every phase of the firing. The other method, Time-to-Temp, is less common for glass fusing. It focuses on reaching a target temperature within a set amount of time, which gives you less direct control over the ramp rate. For the predictable and consistent results needed in glass art, the Ramp/Hold method is always the way to go.

Troubleshooting Common Kiln Problems

Even with careful planning, issues can pop up. One of the most common frustrations is trapped air bubbles. To help prevent them, try creating a small gap between stacked glass layers, which gives the air an escape route as the glass heats. Another critical point is glass compatibility. To prevent cracks, only use Bullseye COE 90 glass with other COE 90 products in a single project. Finally, keep a detailed firing log. Documenting your schedules, glass types, and outcomes helps you identify what works, turning every firing into a valuable learning experience.

Firing COE 90 Glass with Molds

Using molds opens up a whole new world of shapes and forms for your COE 90 glass projects. From bowls and plates to intricate sculptures, molds help you guide the glass into its final three-dimensional form. But working with molds isn’t just a matter of placing your glass on top and hitting “start” on your kiln. The mold itself becomes a critical variable in your firing schedule. Its material, thickness, shape, and size all influence how your glass heats and cools. Getting this right is the key to achieving that perfect slump without cracks or unwanted textures. Let’s walk through how to select and prepare your molds for a successful firing.

How to Choose the Right Mold for Your Project

The first step is picking the right mold for your project. The shape and size of the mold are more than just aesthetic choices; they directly impact your slumping schedule. A shallow, simple plate will require a very different firing cycle than a deep, complex bowl. As the experts at Camp Copeland Studio note, “Slumping schedules vary greatly depending on the mold’s shape and size.” A larger or more intricate mold will need slower ramp rates to ensure the glass conforms to the shape evenly without stretching too thin or trapping air. When you’re browsing for new creative molds, keep in mind how each one will interact with your glass inside the kiln.

Adjusting Your Firing Schedule for Molds

Once you have your mold, you can’t just use a generic slumping schedule and hope for the best. Each mold is unique, and your firing schedule should reflect that. Many manufacturers provide recommended firing schedules specifically for their products. As the team at Warm Glass advises, “It’s best to use the firing schedule that comes with that specific mold.” This is your best starting point because the manufacturer has tested it extensively. If your mold doesn’t come with instructions, you’ll need to do some testing. Start with a conservative schedule and keep detailed notes in your firing log so you can make precise adjustments for future projects.

The Right Way to Apply Kiln Wash and Primer

Before you even think about placing your glass, your mold needs proper preparation. This means applying a separator, like kiln wash or a specialized primer, to its surface. This coating creates a barrier that prevents the molten glass from sticking to the mold, ensuring a clean release after firing. It also helps your piece achieve a smooth finish. When you’re ready to fire, you’ll want to “use slumping molds or kiln furniture to hold your piece in shape as it softens,” as Hollander Fusing Center explains. Proper support is crucial for helping the glass take on the desired form without collapsing or warping unexpectedly. Take your time with this prep work; it’s a small step that makes a huge difference in your final result.

After the Firing: An Introduction to Coldworking

The moment your kiln is cool enough to open is always exciting, but the journey of your glass piece isn’t quite over. Coldworking is the essential next step, where you refine and polish your creation to give it a professional finish. This is the process of working with the glass after it has been fired, using techniques like grinding and polishing to smooth sharp edges, remove small imperfections, and truly bring out the beauty of your design. Whether your piece was fused flat or slumped into a mold, this stage is what transforms a good piece into a great one. It’s your chance to address any minor issues and ensure the final product is not only beautiful but also safe and functional to handle.

Finishing Edges and Removing Imperfections

One of the most common tasks in coldworking is dealing with the edges. After a full fuse, the bottom edge of your piece can sometimes be quite sharp, creating a small “shelf” that needs to be smoothed. Grinding is the best way to handle this, creating a clean, beveled edge that’s pleasant to the touch. You might also notice small bits of kiln wash or other imperfections that need to be removed. This is where you get to be a detective for your art, carefully inspecting your work and deciding what needs a little extra attention. Whether you’re using a simple hand-lapping block or a benchtop grinder, the goal is to create a flawless finish that elevates your work from a hobby project to a piece of fine craft.

My Go-To Tips for Consistent, Beautiful Results

Achieving stunning results in glass fusing isn’t just about having a great design; it’s about creating a process that you can rely on time and time again. While the magic happens inside the kiln, the real work starts with the habits you build in your studio. Developing a consistent workflow helps you replicate your successes and, just as importantly, learn from your failures without repeating them. Think of it as your personal science, where careful observation and good practices lead to beautiful, predictable art.

By adopting a few key habits, you can take much of the guesswork out of the firing process. Keeping a detailed log of your firings, ensuring your glass is compatible, and implementing a pre-firing quality check are three simple steps that make a world of difference. These practices don’t just save you from frustrating cracks or disappointing slumps; they empower you to push your creative boundaries with confidence. When you know your process is solid, you can freely experiment with new designs, colors, and forms, knowing you have a strong foundation to fall back on.

How to Temporarily Secure Glass Pieces Before Firing

You’ve just spent an hour perfectly placing tiny stringers and bits of frit for your design. Now comes the nerve-wracking part: moving your masterpiece to the kiln without everything sliding out of place. To prevent this, you can use a special fusing glue to temporarily hold your elements. These glues are designed to burn off cleanly in the kiln without leaving any residue. Use it sparingly—a tiny dot is all you need to tack a piece down. The most important step is to let the glue dry completely before firing. If it’s still wet, the moisture can turn to steam and create unwanted bubbles or even shift your design.

Remember, this is just a temporary hold to get your project safely into the kiln. It won’t fix fundamental issues like incompatibility. No amount of glue can save a project if you’ve accidentally mixed different COEs, so always double-check that all your components are COE 90. Never use household glues, as they contain chemicals that can release fumes, leave a permanent mark on your glass, and damage your kiln. Stick to products made specifically for glass fusing to ensure your design enters the kiln exactly as you intended.

Why You Should Always Keep a Firing Log

Think of a firing log as your glass art diary. It’s the single most effective tool for understanding what works and what doesn’t in your specific studio with your specific kiln. Every time you fire a project, take a few moments to jot down the details. Note the firing schedule you used, the types and colors of glass, the thickness of your project, and any molds or inclusions. I even like to note the room temperature or if it’s a particularly humid day. This practice helps you track your results and allows for precise adjustments in future projects. Over time, this log will become your personal encyclopedia for success, helping you troubleshoot issues and perfectly recreate your favorite effects.

Don’t Skip This Step: Test for Compatibility

This might be the golden rule of glass fusing: always use compatible glass. To prevent cracks and ensure a successful project, it is crucial to only use Bullseye COE 90 glass with other Bullseye COE 90 products. The Coefficient of Expansion (COE) measures how much glass expands and contracts when heated and cooled. If you mix glass with different COEs, they fight against each other during the cooling process, leading to stress fractures. This rule applies to everything that goes into your project, including sheet glass, frit, and stringers. Sticking to one compatible family of products for each piece takes the guesswork out of your art and is fundamental to creating strong, stable work.

Create Your Own Quality Check Process

A great piece begins with great preparation. Before you even think about loading the kiln, create a simple quality check routine. Start by inspecting your glass for any scratches or imperfections and make sure it’s squeaky clean, as even a fingerprint can leave a permanent mark after firing. Double-check that all your components are the correct COE. If you’re using a mold, ensure it’s properly prepared with a fresh, even coat of kiln wash. This simple, pre-flight checklist ensures that every sheet, frit, and stringer is ready for a successful firing. It’s a small investment of time that prevents the heartache of a failed project and helps maintain consistency in your work.

Related Articles

• The Complete Guide to COE 90 Fusing Glass Supplies
• COE 90 vs 96: Which Glass Should You Choose?
• How to Choose the Best Glass Kiln for Your Art
• What Does COE Mean in Glass? A Fuser’s Guide
• COE Glass Meaning: A Beginner’s Guide to Fusing

Frequently Asked Questions

Can I really only use COE 90 glass together? What if I just have a tiny piece of something else? Yes, this is the most important rule in fusing, and there are no exceptions. Even a tiny shard of incompatible glass, like COE 96, will cause your project to crack because it shrinks at a different rate during cooling. This creates immense stress that the glass simply cannot handle. To guarantee a strong, stable piece, you must ensure every single element in your project, from the sheet glass down to the smallest sprinkle of frit, is all COE 90.

My piece looked perfect when I took it out of the kiln, but it cracked a day later. What happened? This is a heartbreaking but common experience, and it almost always points to a problem in the annealing phase of your firing schedule. When glass cools too quickly, its surface contracts faster than its core, building up invisible stress. The piece might look fine at first, but that internal tension can cause it to crack hours or even days later. To prevent this, make sure your schedule includes a proper annealing soak and a slow, controlled cooling rate, especially for thicker projects.

When should I choose a tack fuse instead of a full fuse? The choice comes down to the texture and dimension you want in your final piece. A full fuse melts the glass layers together into one smooth, flat surface with soft, rounded edges, which is perfect for creating coasters or a base for a plate you plan to slump later. A tack fuse, on the other hand, uses a lower temperature to just stick the layers together. This preserves the individual shapes and edges of your glass elements, creating a piece with noticeable texture and a layered look. It’s a great choice when you want details like stringers or frit to stand out on the surface.

Do I need a brand new firing schedule for every single project I make? Not at all. You don’t need to reinvent the wheel every time you load your kiln. The schedules provided in this post are excellent starting points for most standard projects. Think of them as your base recipes. You will, however, need to make small adjustments based on the thickness of your glass or the shape of a mold. This is why keeping a detailed firing log is so valuable. By noting your schedule and your results, you’ll learn how to make small, confident tweaks rather than starting from scratch.

Why is it so important to keep the kiln closed until it’s completely cool? Patience at the end of a firing is just as important as the schedule itself. Even if the kiln display reads 150°F, your glass is still quite hot. Opening the door too soon causes a rush of cool room-temperature air to hit the glass, creating a sudden temperature change. This is called thermal shock, and it can easily crack or even shatter your beautiful, newly fused piece. It’s always best to wait until the kiln’s interior is at room temperature before you open it.