We’ve all been there: you pull a piece from the kiln only to find a heartbreaking crack. You thought you matched the COEs perfectly, so what went wrong? The truth is, a glass coe chart is just the first step. The real coe glass meaning involves more than a number; it’s about how different glasses behave when they melt. Viscosity—how “runny” the glass gets—can create hidden stress even when the glass coe matches. This guide will help you look past the numbers on a coe chart and understand why “tested compatible” is the secret to creating strong, beautiful art that lasts.

Key Takeaways

• Use one COE per project to prevent cracks: The Coefficient of Expansion (COE) measures how glass expands and contracts with heat. Using glass with the same COE ensures all pieces move together, preventing the internal stress that causes your art to break.
• Compatibility is more than just a number: While matching COEs is essential, factors like viscosity also affect a fuse. For the most reliable results, use glass from a single manufacturer that has been tested to work together seamlessly.
• Keep your glass organized to avoid mix-ups: Create a simple system in your studio by storing COE 90 and COE 96 glass in separate, clearly labeled containers. This small habit is one of the best ways to protect your projects from failing due to accidental mixing.

What is COE in Glass?

If you’ve spent any time in the world of glass fusing, you’ve definitely heard the term COE. So, what does it actually mean? COE stands for Coefficient of Expansion. In simple terms, it’s a measurement of how much a piece of glass expands when it gets hot and shrinks as it cools down. Think of it as the glass’s unique “breathing” rate. Every type of glass has a specific COE number, and this little number is one of the most important things to understand before you put your project in a kiln. Knowing your glass’s COE is the key to creating beautiful, strong, and lasting fused art. It’s the foundation of compatibility and the first rule of successful fusing.

Why COE Matters in Glass Fusing

Here’s the golden rule of glass fusing: you absolutely cannot mix glass with different COEs in the same project. All glass within the COE 90 family is designed to work together, and the same is true for all glass in the COE 96 family. Sticking to one COE family for a single project is essential because it ensures every piece of glass expands and contracts at a compatible rate. When you follow this rule, your glass pieces will fuse together seamlessly, creating a single, solid piece. Ignoring it is a recipe for disaster, leading to cracks and breaks that can ruin hours of your hard work.

How Thermal Expansion Affects Your Art

Imagine two friends trying to walk together while holding hands, but one is taking giant leaps while the other takes tiny steps. Eventually, the tension will cause them to pull apart. That’s exactly what happens to your glass in the kiln when you mix COEs. As the temperature rises, glass with a higher COE expands more than glass with a lower COE. As it cools, it shrinks more. This difference in movement creates immense internal stress. Your beautiful, fused piece might look perfect when you pull it from the kiln, but that hidden stress can cause it to crack or even shatter hours, days, or weeks later.

What Makes Different Glass Types Compatible?

While sticking to a single COE is a fantastic rule of thumb, it’s important to know that COE doesn’t always equal perfect compatibility. Factors like the glass’s viscosity (how it flows when molten) can also affect the final result. This is why some artists find that mixing glass from different manufacturers, even if they are all labeled COE 90, can sometimes cause issues. For beginners, the safest path is to use glass from a single manufacturer for your project, like sticking to all Bullseye or all Wissmach glass. This ensures that the glass was formulated to work together perfectly, taking any guesswork out of the equation.

COE 90 vs. COE 96: What’s the Difference?

When you first start fusing glass, you’ll quickly encounter the two most common options available to artists: COE 90 and COE 96. While the numbers are close, their properties are distinct enough that they can never be mixed in a single project. Think of them as two different languages; they might share some similarities, but they don’t communicate well together, and trying to force them will only lead to cracks and disappointment. This incompatibility is why understanding the difference is so fundamental to your success as a glass artist. Choosing between them isn’t about which one is “better” overall, but which one is better for your specific project. Your decision will likely come down to a few key factors: your project’s needs, your budget, and your personal preferences for color and workability. Do you need a specific, vibrant shade that only one type offers? Are you working on a large piece where cost is a major factor? Understanding the unique characteristics of each will help you select the right glass for your vision and set you up for successful, crack-free results every time. Let’s break down what makes each type special so you can make an informed choice for your next masterpiece.

When to Choose COE 90 Glass

Artists often gravitate toward COE 90 glass for its incredible and unique color palette. If you’re looking for vibrant shades like cranberry or hot pink, you’ll find them here. This glass typically has a slight texture, with one side feeling like an “orange peel” and the other featuring a pattern. A good tip is to always cut on the smoother side for a cleaner score. Be aware that COE 90 can be a bit more fragile and challenging to cut, which sometimes leads to more breakage. It’s also generally more expensive and sold by the pound, a reflection of its handmade production process.

When to Choose COE 96 Glass

For many artists, COE 96 glass is a go-to choice because it’s so user-friendly. It’s smooth on both sides, which makes cutting a breeze compared to its COE 90 counterpart. While it offers a fantastic range of beautiful colors, it does lack some of the more intense “hot” colors you’d find in the COE 90 family. One of the biggest draws for COE 96 is its affordability. It’s usually less expensive and sold by the square foot, making it a more budget-friendly option, especially for larger projects or for artists who are just starting to build their studio inventory.

What About COE 104 Glass?

While COE 90 and COE 96 are the mainstays for kiln fusing, you’ll likely come across another number: COE 104. This is often called “soft glass” because it has a much higher rate of expansion, making it behave very differently under heat. You won’t use it for the fused plates or slumped bowls you create in a kiln. Instead, COE 104 is the preferred glass for lampworking, an art form where artists use a high-temperature torch to melt and shape glass rods. Its unique properties make it ideal for creating intricate, detailed work like handmade beads and tiny sculptures, which is why it’s crucial to understand that it belongs in a completely different category from your fusing glass.

Primary Uses for COE 104

COE 104 glass is the undisputed champion for bead makers and artists who create small sculptural pieces at a torch. Because it’s a “soft” glass, it melts at a lower temperature and becomes pliable, allowing for incredible detail and control. This workability is what makes it a favorite in the lampworking community. The color palette is another huge draw. As noted by glass suppliers like Delphi Glass, the range includes everything from brilliant rainbow hues to unique, subtle shades. This variety gives artists the freedom to craft anything from simple jewelry beads to complex, miniature glass figures right in the flame.

Properties of COE 104 Glass

The key characteristic of COE 104 glass is its high expansion rate, which gives it that “soft” quality perfect for torchwork. This property also allows for a stunning array of colors, including transparents, pastels, and special reactive colors that create unique effects when heated. But just like with fusing glass, compatibility is non-negotiable. While all COE 104 glass is designed to work together, you can’t mix it with anything else. As one educational guide explains, artists working with soft glass are almost always told to stick strictly to COE 104. It’s a completely separate system from the COE 90 and COE 96 glass you’d use in a kiln, so keeping it separate is essential to avoid project-ruining cracks.

Comparing Firing Schedules for COE 90 & 96

When it’s time to fire your work, remember that COE 90 and COE 96 have different ideal firing schedules. It’s always best to consult the manufacturer’s specific guides for the exact temperatures and times they recommend for their glass. Here’s a helpful tip for saving energy and time: you can actually fire separate COE 90 and COE 96 projects in the same kiln at the same time. The key is to keep the pieces completely separate and use a slow, careful firing schedule that accommodates both. This allows you to maximize your kiln space without risking your work.

Understanding Your Kiln’s Firing Schedule

A firing schedule is essentially a recipe for your kiln, guiding it through a series of temperature changes to perfectly fuse your glass. It’s not just about cranking up the heat; it’s a carefully controlled process of heating, holding, and cooling that ensures your project comes out strong and beautiful. Every stage has a purpose, from preventing the glass from cracking under stress to achieving the exact texture and shape you want. While you can find many pre-programmed schedules in modern kilns, understanding what’s happening at each step gives you the power to troubleshoot issues and customize your firings. This knowledge transforms you from someone who just pushes buttons into an artist who is truly in control of the medium, allowing you to achieve consistent, professional-quality results every time you close the kiln door.

The Key Stages of a Firing Schedule

Think of a firing schedule as having three main acts: the initial climb, the main event, and the cool down. First, you slowly raise the temperature to avoid shocking the glass. Next, you hold it at specific temperatures—a process called soaking—to let the heat distribute evenly and allow the glass to fuse. Finally, you begin the critical cooling phase, which includes annealing to remove internal stress. Each stage is vital for the structural integrity of your finished piece. Skipping or rushing any part of this process is like trying to bake a cake by turning the oven to its highest setting and hoping for the best—you’ll likely end up with a cracked, unusable mess.

Initial Heating and Thermal Shock

The first part of the firing process is all about patience. You need to heat the glass slowly to prevent thermal shock, which is just a technical term for cracking caused by rapid temperature changes. As the glass heats, it expands, and you want every part of your piece to expand at the same rate. A key milestone is 1,000°F; you should approach this temperature gradually. Rushing this initial heating stage is one of the most common ways projects fail, as the uneven expansion creates stress that can cause the glass to crack before it even has a chance to fuse.

Soaking for Even Heat

Once you reach a target temperature, you’ll “soak” the glass by holding that temperature for a set amount of time. This step is crucial because it gives the entire piece, from the thin edges to the thicker center, a chance to catch up and reach a uniform temperature. Soaking ensures that the fusing process happens evenly across your project. The length of the soak depends on the size and thickness of your glass, as well as the effect you’re trying to achieve. It’s this holding pattern that allows the glass to soften, stick together, and smooth out just the way you want it to.

Annealing and the Strain Point

After your glass has reached its top temperature and fused perfectly, you can’t just turn the kiln off. The cooling process is just as important as the heating process. Annealing is the controlled cooling of the glass to remove internal stress. This usually involves holding the temperature steady around 960°F, which is the annealing point for most fusing glass. This pause allows the glass molecules to relax and align, creating a strong, stable piece. After the annealing soak, the kiln continues to cool very slowly until it’s back to room temperature, ensuring your artwork will last a lifetime without cracking.

Common Firing Techniques and Temperatures

The top temperature you fire to determines the final look of your piece. Do you want the individual pieces of glass to remain distinct and textured, or do you want them to melt into a single, glossy surface? By controlling the peak temperature and soak time, you can achieve a wide range of effects. These different levels of fusing each have their own names, like tack fusing, contour fusing, and full fusing. Learning the target temperatures for these techniques will give you precise control over your artistic outcomes. Remember that these are general guidelines; always start with the manufacturer’s recommendations for your specific glass.

Tack Fusing for Texture

If you want to join pieces of glass together while keeping a lot of their original shape and texture, tack fusing is the way to go. This technique involves heating the glass just enough for the pieces to stick firmly to one another, but not so much that they lose their individual edges. You’ll typically fire to around 1350°F. The result is a piece with a lot of surface dimension and a rougher, more defined look. Tack fusing is perfect for creating jewelry or decorative panels where you want the layers to be visible and add depth to the design.

Contour Fusing for Dimension

Contour fusing is a step up from tack fusing. You’ll take the temperature a little higher, somewhere between 1346°F and 1410°F. At this heat, the edges of your glass pieces will soften and round over, creating a more blended look than a tack fuse, but the piece will still retain some dimension and a raised “relief” effect. It’s a great middle-ground technique that gives you a fused look without completely flattening your design. This is ideal when you want a softer finish but still want to maintain the distinct shapes and layers you so carefully arranged.

Full Fusing for a Smooth Finish

When you want your glass pieces to melt together completely into one smooth, flat, and glossy surface, you’re aiming for a full fuse. This requires the highest temperatures, typically around 1450°F for COE 96 glass. At this heat, the glass becomes molten enough to flow together, erasing the boundaries between the individual pieces. The final result is a single, solid piece of glass that is usually about 6mm thick if you started with two standard 3mm layers. This is the most common technique for creating coasters, plates, and other functional items that require a level surface.

Slumping to Create Shapes

Slumping is a separate process that happens after your glass has already been fused flat. It involves placing your fused piece over a ceramic or stainless steel mold and heating it to a much lower temperature, usually around 1250°F. At this temperature, the glass becomes soft and pliable, allowing gravity to pull it down and take the shape of the mold. This is how you create bowls, plates, vases, and other three-dimensional forms. It’s a magical step where your flat design transforms into a functional or sculptural object with beautiful curves and contours.

Advanced: Creating Molten Effects

For artists who really want to push the boundaries, you can take the glass to even higher temperatures where it becomes a free-flowing liquid. This is an advanced technique that can create incredible organic patterns and effects, but it also comes with a higher risk. At these extreme temperatures, the glass can even boil, creating bubbles and unique textures. This level of heat requires a deep understanding of how glass behaves and careful control over your kiln, but it can produce stunning, one-of-a-kind results that are impossible to achieve with standard firing schedules.

How to Find Your Glass’s COE

Nothing is more frustrating than pulling a cracked or broken piece from the kiln. The good news is that most fusing failures are preventable by confirming your glass is compatible before you fire. If you have a studio full of unlabeled scraps or mystery glass, don’t worry. There are several straightforward methods to figure out the COE of your glass, ensuring your next project is a success. From simple label checks to more technical tests, you can confidently identify what you’re working with. Taking a few extra minutes to verify compatibility will save you from the disappointment of a project that cracks due to thermal shock or stress. Think of it as setting your artwork up for a long, beautiful life.

Check the Manufacturer’s Label

The easiest way to find a glass’s COE is to check its original packaging. Most manufacturers include a sticker or label that clearly states the COE. When you buy new sheets of COE 90 glass, it’s a great habit to keep a small piece of the labeled corner with your scraps from that sheet. This simple step saves you a lot of guesswork later. Remember, the COE, or Coefficient of Expansion, tells you how much the glass will expand and shrink when heated and cooled. Using glass with the same COE is the foundation of a strong, stable fused piece. So, before you start cutting, always look for that label first.

Run a Simple Compatibility Test

When you can’t find a label, the next best thing is to run a compatibility test. This involves fusing a small piece of your mystery glass to a piece of glass with a known COE. You can buy special molds for this, but a simple method works just as well. Place a small square of your unknown glass on top of a slightly larger square of known glass (like COE 96) and fire it in your kiln. After it cools, examine the piece. If the glass is compatible, the edges will be smooth and rounded. If it’s incompatible, you might see a sharp, stressed line or even a crack where the two pieces meet.

How to Test Unknown or Vintage Glass

Be extra careful with unknown scraps or vintage glass, even if you think you know the COE. As glass manufacturer Wissmach notes, even glass from different companies labeled as the same COE should be tested for compatibility. Production methods and formulas can vary slightly, leading to stress in the final piece. This is especially true for older glass, which may not conform to modern standards. Always run a small test fuse before incorporating that beautiful, one-of-a-kind vintage piece into a larger project. A few minutes of testing with a piece of known COE 96 glass can save you from discovering a compatibility issue after hours of work.

Check for Stress Using Polarized Light

For a more technical approach, you can use polarized light to spot stress that isn’t visible to the naked eye. You’ll need two polarizing filters, which you can find at a camera supply store. Place your cooled test piece between the two filters and hold it up to a light source. As you rotate one of the filters, look for bright lines or halos of light around the edges where the two glasses meet. This is called a stress halo, and it’s a clear sign of incompatibility. This method helps you catch subtle stress that might cause your project to crack days or even weeks after firing, ensuring your finished art is as strong as it is beautiful.

What Happens When You Mix COEs?

Mixing glass with different COEs is one of the most common and costly mistakes in glass fusing. When you combine glasses that expand and contract at different rates, you introduce a tremendous amount of stress into your project. This internal tension can have devastating effects on your artwork, either immediately or over time. Understanding what happens when you mix COEs is the first step to preventing these frustrating failures and ensuring your beautiful creations last a lifetime. It’s why sticking to a single COE, like using only COE90 glass for a project, is so important.

The Risk of Immediate Cracks and Breaks

When you fuse incompatible glasses in a kiln, they melt together just fine. The problem starts during the cooling process. As the temperature drops, the glass with the higher COE will shrink more than the glass with the lower COE. This difference in contraction creates a powerful internal tug-of-war. The stress can become so intense that the piece cracks or shatters right in the kiln. You might open the lid expecting a masterpiece only to find a puzzle of sharp, fractured glass. This immediate breakage is a clear sign that the internal forces were simply too strong for the glass to handle.

Why Projects Can Fail Days Later

Sometimes, a piece made with mixed COEs will survive the cooling cycle and look perfectly fine. This is where things get tricky. Just because it didn’t break immediately doesn’t mean it’s stable. The internal stress is still there, trapped inside the glass like a ticking time bomb. The piece is incredibly fragile, and any small change can trigger a break. A slight shift in room temperature, a gentle tap, or even just the vibrations from a passing truck can be enough to cause it to crack days, weeks, or even months later. This delayed reaction can be especially heartbreaking after you’ve already grown attached to your finished work.

What Incompatible Glass Looks Like

If a piece breaks, you can often spot the tell-tale signs of COE incompatibility. Look for sharp, clean cracks that often originate at the seam where the two different types of glass meet. Another visual cue is a “halo” or a hazy, cloudy line appearing along the fusion point, which indicates stress. You might also see unusual bubbling in that area. These are all symptoms of a project under extreme tension. To avoid this, always work with tested compatible glass, like a reliable line of COE96 glass, and keep your scraps carefully sorted to prevent accidental mixing.

How to Read a Glass COE Chart

Think of a glass COE chart as your map for fusing. At first glance, it might seem like a simple list of numbers and codes, but knowing how to read it properly is the key to creating strong, beautiful pieces that last. It’s about more than just matching a number; it’s about understanding the language of your materials. A chart tells you the fundamental properties of your glass, but it’s up to you to interpret that information to ensure your project is a success. Getting this right prevents the heartbreak of pulling a cracked piece from the kiln, sometimes even days after it has cooled.

Reading a chart correctly involves three main steps. First, you need to decode the basic information, like what the COE number actually means and how to identify your glass using manufacturer codes. Next, you’ll want to know how to compare different brands of glass, even if they share the same COE. Finally, and most importantly, you need to grasp what compatibility ratings truly mean, because it goes much deeper than a single number. Let’s walk through each of these so you can approach your next project with total confidence.

Decoding Symbols and Manufacturer Codes

Let’s clear up one of the biggest misconceptions in glass fusing right away: COE does not equal compatibility. As one expert puts it, this is a persistent kiln forming myth. COE stands for Coefficient of Linear Expansion, which is just a technical way of saying it’s a measurement of how much a specific piece of glass will expand when it gets hot. While it’s a critical piece of the puzzle, it’s not the whole picture.

The first step in using a COE chart is simply identifying your glass. Manufacturers often have their own symbols or product codes printed on labels. A chart helps you translate these codes into a COE number, so you know the basic expansion rate you’re working with. Think of it as step one: correctly identifying your materials before you even think about putting them in the kiln.

How to Compare Different Glass Brands

A common question artists ask is whether they can mix different brands of glass as long as they have the same COE. For example, is all COE 90 glass created equal? The safest answer is no. While two different brands might both be rated as COE 90 or COE 96, their chemical formulas can be slightly different. These differences can affect other properties, like viscosity (how runny the glass gets when molten).

Mixing brands can introduce stress into your finished piece because they may not cool and shrink in perfect harmony. For beginners, the best rule of thumb is to stick with one manufacturer per project. This way, you can be confident that all the glass has been designed and tested to work together seamlessly.

Understanding Compatibility Ratings

So if COE isn’t the final word, what is? True compatibility means that different glasses can be fused together and cool down without creating internal stress. As the experts at Bullseye Glass explain, the expansion properties of glass change as it heats and cools, so a single COE number doesn’t tell the whole story about glass compatibility.

Reputable manufacturers conduct rigorous tests on their own glass lines to ensure every color and style within a specific COE is compatible with the others. When you see a glass labeled “tested compatible,” it means the manufacturer guarantees it will fuse properly with other glass in that same product family. This rating is your most reliable indicator of success, as it accounts for the glass’s behavior throughout the entire firing and annealing process.

Rules for Fusing with Confidence

Creating beautiful fused glass art is incredibly rewarding, but a project that cracks in the kiln can be just as heartbreaking. The good news is that you can prevent most fusing failures by following a few fundamental rules. Think of these as your studio guidelines for making sure the glass you combine plays well together from the first firing to the final anneal. By understanding compatibility beyond the numbers and adjusting your process accordingly, you can approach your kiln with a lot more certainty and excitement for the final result.

Follow the 10-Point Compatibility Rule

Let’s clear up a common myth: Coefficient of Expansion (COE) does not equal compatibility. While it’s a critical starting point, COE is simply a measurement of how much glass expands and contracts when heated and cooled. Glasses with the same COE number, like all the beautiful varieties of COE90 glass, are manufactured to work together. However, other factors like viscosity (how the glass flows when molten) can differ between brands. The real rule is to treat COE as a guideline, not a guarantee. True compatibility means the glasses can be fused without causing stress that could lead to cracks later on.

Try These Safe Glass Combinations

When you’re starting out, the safest approach is to stick with glass from a single manufacturer within one COE family. For example, build your project using only Bullseye glass (COE 90) or only Wissmach glass (COE 96). If you want to mix COE96 glass from different companies, you should always run a small test first to ensure they are truly compatible. Fusing a small square of each glass together will show you if there’s any stress or cracking before you commit to a larger piece. This simple step can save you from the disappointment of a project failing due to incompatibility, thermal shock, or a bad annealing cycle.

Follow the ¼ Inch (6mm) Rule

Here’s a fascinating piece of glass physics that will save you a lot of surprises. When heated to a full fuse temperature, glass naturally wants to settle into a uniform thickness of about ¼ inch (or 6mm). Think of it as its comfort zone. If you start with a single layer of glass that’s thinner than this, the edges will likely pull inward and curl up as the glass tries to achieve that magic thickness. On the other hand, if you stack multiple layers that are thicker than ¼ inch, the molten glass will spread out until it settles at that same ¼ inch height, making your final piece larger than you originally planned. Understanding this rule helps you predict how your glass will behave and is especially important when working with molds where you want the glass to fill the space just right.

Keep a Detailed Firing Log

One of the best habits you can develop as a glass artist is keeping a detailed firing log. It might seem like a chore at first, but it will become your most valuable tool. Before you even start a project, you should have a firing schedule in mind. In your log, write down everything: the type of glass you used, the number of layers, the exact firing schedule, and which of your kilns you used. When you take the piece out, snap a quick photo and attach it to your notes. This record helps you replicate your successes and, just as importantly, learn from your failures. When a piece turns out perfectly, you’ll have the exact recipe to do it again. And if something goes wrong, your log will give you the clues you need to figure out why.

How to Adjust Firing Schedules for Different COEs

Different types of glass melt and cool at different rates, which is why you can’t use the same firing schedule for every project. COE 90 and COE 96 glasses have different recommended temperatures for fusing, slumping, and annealing. Always check the glass manufacturer’s website for their specific firing guides before you program your kiln. The size, thickness, and complexity of your project also play a huge role. For very large, thick, or unusual art pieces, it’s always wise to do a test firing with a smaller piece to make sure your schedule is just right.

Adjusting for Glass Thickness

A standard firing schedule is usually designed for glass that’s about a quarter-inch thick. If your project is thicker, you’ll need to adjust your timing. Thicker glass requires more time to heat up and cool down evenly, all the way to its core. Rushing this process can cause thermal shock and lead to cracks. To prevent this, slow down your heating and cooling speeds and add longer “soak” times at key temperatures. This gives the heat a chance to penetrate the entire piece uniformly. For projects involving more than two layers of glass, or when using thick elements, patience is your best friend. A slow, steady firing schedule ensures your masterpiece comes out of the kiln strong and stable.

Adjusting for Glass Viscosity

Viscosity is another key factor in compatibility that goes beyond the COE number. In simple terms, it’s how fluid or “runny” the glass becomes when it’s molten. Different brands of glass, even with the same COE, can have different viscosities. When you mix them, one might flow more readily than the other, creating subtle stress as they cool and settle together. This is why sticking with a single manufacturer for a project is always the safest bet. When you use only COE90 Bullseye glass, for example, you can be confident that every sheet has been designed and tested to melt and flow in perfect harmony, giving you the most reliable and stress-free results.

Common COE Mistakes to Avoid

Working with glass COE is a science, but it doesn’t have to be intimidating. Once you understand the basics, you can sidestep the common issues that lead to cracked or broken projects. Getting familiar with these pitfalls will save you a lot of time, materials, and heartache, letting you focus on the creative part of your work. Let’s walk through the four most common mistakes artists make and how you can steer clear of them.

Mistake #1: Assuming Same COE is Always Compatible

This is probably the biggest myth in glass fusing. While using glass with the same COE is the essential first step, it doesn’t automatically guarantee compatibility. COE, or Coefficient of Linear Expansion, only measures how much glass expands and contracts with temperature changes. However, other factors, like the glass’s chemical makeup and viscosity, also play a huge role. Think of COE as a starting point, not the final word. For true success, you need to use glass that has been tested for compatibility, which is why many artists stick to a single manufacturer, like Bullseye or Wissmach, for a project.

Mistake #2: Mixing Vintage and Modern Glass

It’s so tempting to use that beautiful piece of vintage glass you found, but mixing it with modern glass is a big risk. Even if an older piece is marked with a specific COE, its formulation is likely different from today’s COE 90 glass. Glass manufacturing has evolved, and those subtle differences can introduce a lot of stress when the pieces are fused together. If you’re set on using an unknown or vintage piece, it’s best to run a compatibility test first. Otherwise, you risk your project cracking hours, or even days, after it comes out of the kiln.

Mistake #3: Forgetting About Viscosity

Viscosity is just a simple way of describing how fluid or stiff glass becomes when it’s heated. Two types of glass can share the same COE but have very different viscosities. This means one might become soft and runny at a lower temperature while the other remains relatively stiff. When fused, this mismatch creates stress because the two glasses aren’t moving and settling together in the same way. This hidden tension is a primary cause of breakage. The best way to avoid this is by using glass lines that are designed and tested to work together, ensuring their viscosity and expansion characteristics are in sync.

Mistake #4: Ignoring Proper Annealing

You can do everything else right, but if you rush the cooling process, your project is likely doomed. Annealing is the critical step of slowly cooling your glass in the kiln to relieve all the internal stress built up during fusing. Every piece of glass, no matter the COE, needs a proper annealing cycle. Skipping this or using an incorrect schedule is one of the top three reasons for breakage, right alongside thermal shock and incompatibility. Always follow the glass manufacturer’s recommended firing and annealing schedules to give your artwork the strength and stability it needs to last.

Where to Find a Reliable Glass COE Chart

When you’re trying to prevent a beautiful project from cracking in the kiln, you need COE information you can trust. Sourcing your data from a random forum post or an outdated chart can lead to heartbreak. The good news is that reliable information is easy to find if you know where to look. Sticking to official and expert sources will give you the confidence you need to fuse different types of glass successfully.

Check the Manufacturer’s Website

Your first and most reliable source should always be the glass manufacturer. They are the ultimate authority on their products and provide the most accurate specifications. Companies like Bullseye Glass and Wissmach offer detailed charts and product descriptions right on their websites. For example, Wissmach provides extensive information on their various COE 96 glass types, ensuring you know exactly how their Opaque, Transparent, and Prisma Glass™ sheets are designed to work together. I recommend bookmarking the sites for the glass brands you use most often so you can quickly reference their official data whenever you start a new project.

Consult Professional Glass Suppliers

Professional glass suppliers are another excellent resource. Since we carry glass from many different manufacturers, we work hard to provide clear and consistent information for our customers. Here at ArtGlassSupplies.com, you’ll find the COE listed on every product page, from COE 90 sheets to stringers and frit. Other suppliers also offer helpful tools. For instance, some sites provide interactive color palettes that help you choose compatible colors within a specific COE, making it easier to plan your designs. Think of supplier websites as a convenient hub for comparing different brands and stocking up on supplies with confidence.

Use Trusted Educational Resources

Sometimes, you need more than just a number. Trusted educational resources can explain the nuances of glass compatibility that a simple chart might miss. For example, the experts at Kiln Frog have a fantastic guide explaining that COE does not always equal compatibility. They highlight that other factors, like viscosity, play a huge role, especially when working with different brands. These guides are invaluable for understanding the “why” behind the rules of fusing. Reading blogs and articles from experienced artists and suppliers helps you build a deeper understanding of how glass behaves in the kiln, turning you into a more knowledgeable and successful artist.

How to Organize Your Glass by COE

Working with different COEs opens up a world of creative possibilities, but it also introduces a critical need for organization. A little planning in your studio can save you from the heartache of a cracked project. Think of it as setting your future self up for success. By creating clear systems for your glass, you can work with confidence, knowing every piece is exactly where it should be. Let’s walk through a few simple but effective strategies to keep your glass collection sorted and your projects safe.

Set Up a Simple Storage and Labeling System

The easiest way to avoid mixing COEs is to keep them physically separated. If you work with both COE 90 and COE 96 glass, they should never live in the same bin or on the same shelf without clear dividers. A great practice is to dedicate specific storage areas for each type. You could use one rack for COE 90 and a completely different one for COE 96. As one expert advises, you should “keep them very well organized, sorted, and clearly marked.” This simple habit prevents you from accidentally grabbing the wrong sheet. Label everything clearly with a permanent marker or a color-coded sticker system so you can identify the COE at a glance.

Keep a Detailed Inventory of Your Glass

Once your glass is physically sorted, keeping a running inventory is your next best move. This doesn’t have to be complicated; a simple spreadsheet or a dedicated notebook works perfectly. For each piece of glass you own, log its details, including its size, color, and most importantly, its COE. When you add a new piece, make sure you “mark it clearly (e.g., ’90 COE’)” in your inventory list. This practice does more than just track your supplies. It reinforces the habit of checking the COE for every single piece of glass that enters your studio, making you more mindful as you work. An accurate inventory also makes it much easier to plan projects and shop for new stained glass without buying duplicates.

Simple Tips to Prevent Accidental Mixing

Your organization system is only as good as your daily studio habits. The most critical rule for preventing cross-contamination is to work with only one COE at a time. When you’re ready to start a new piece, “only get out the glass for that specific COE.” Before you even think about touching your COE 90 glass, make sure every last scrap of COE 96 from your previous project is cleaned up and put away. This is especially important for your scrap bin, which can easily become a danger zone of mixed COEs. Use separate, clearly labeled containers for your scraps. This discipline ensures that a stray shard won’t find its way into your kiln and compromise the beautiful piece you’re creating with your glass molds.

Related Articles

• What Does COE Mean in Glass? A Fuser’s Guide
• The Complete Guide to COE 90 Fusing Glass Supplies
• COE 90 vs 96: Which Glass Should You Choose?
• What is COE Glass? A Simple Guide for Fusers
• What Is COE 96 Fusing Glass? A Beginner’s Guide

Frequently Asked Questions

My piece cracked, but I’m positive I used glass with the same COE. What else could be the problem? This is a frustrating but common situation. While COE incompatibility is a major cause of breakage, it isn’t the only one. The most likely culprit is your annealing cycle. If the glass cools too quickly, stress gets trapped inside, causing it to crack later. Another possibility is thermal shock, which happens if you open the kiln when it’s still too hot. Finally, check the thickness of your project. A piece that is very thick or has uneven layers needs a much slower, more conservative firing schedule to heat and cool evenly.

Why are COE 90 and COE 96 the main standards for art glass? Think of them as two different families of glass developed by manufacturers to give artists a reliable system. Each family was formulated with specific properties in mind. COE 90, for example, was developed by Bullseye Glass to offer a unique and vibrant color palette. COE 96 became popular with other manufacturers as a slightly more forgiving and often more affordable option. Having these two distinct, tested-compatible systems makes it much easier for artists to build a collection of glass that they know will work together successfully.

Is it ever okay to use a tiny, unlabeled scrap of glass in a project? I know how tempting it is to use every last beautiful piece, but adding an unknown scrap is a huge gamble. Even a tiny piece of incompatible glass can create enough stress to crack your entire project. It’s just not worth the risk. The best practice is to run a compatibility test by fusing the mystery scrap to a piece of known glass. If you don’t have time for a test, it’s safer to set that scrap aside for a mosaic project where it won’t be kiln-fired.

Can I fire separate COE 90 and COE 96 projects in the same kiln load? Yes, you absolutely can, and it’s a great way to make your firings more efficient. The key is to keep the projects completely separate on different kiln shelves or with plenty of space between them on the same shelf. You will need to use a firing schedule that is slow and gentle enough to work for both types of glass. This usually means using the schedule for the more sensitive of the two, which ensures both projects heat and cool properly without causing stress to either one.

Is one COE better than the other for specific projects, like jewelry or large platters? There isn’t a hard rule, and your choice often comes down to personal preference and budget. However, some artists prefer COE 96 for larger projects because it’s generally more affordable per square foot, which can make a big difference in cost. For jewelry, many artists are drawn to the unique, vibrant colors found in the COE 90 palette, like bright pinks and reds. Since COE 96 is often described as easier to cut, beginners might find it more user-friendly for any type of project when they are just starting out.