Fusing glass is both an art and a science. Whether you're a seasoned artist or just firing up your first kiln, the process of heating glass to transform it into something new is full of possibilities—and potential pitfalls. While the magic of melting glass into beautiful forms is undeniably rewarding, small missteps can lead to unwanted results like bubbles, cracks, or cloudy finishes.
At Hollander Fusing and Art Glass Center, we’re all about helping artists succeed—so we’ve put together this guide to help you troubleshoot the most common issues that arise during glass fusing and how to fix or avoid them.
1. Air Bubbles in Glass
Air bubbles are one of the most common frustrations in glass fusing. While some bubbles can add character, most of the time they’re uninvited guests.
Why bubbles form:
- Air gets trapped between layers of glass
- Uneven kiln shelves create pockets for air
- Rapid firing doesn’t allow trapped air to escape
What to do about it:
- Level the kiln shelf: Use a ruler or straight edge to ensure your shelf is perfectly flat. Even a slight tilt can allow air to sneak in.
- Adjust your firing schedule: Try firing at a lower temperature with a longer hold time. This gives air more time to escape before the glass seals.
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Create space between glass layers: A small gap between stacked pieces helps trapped air find a way out.
Add a bubble squeeze step: Add a hold in the fusing cycle at around 1150°F to 1250°F for 20–60 minutes. This step slows things down and encourages air to escape before the glass fully fuses.
Double-check your kiln’s suggested settings: Always refer to your kiln’s user manual to find optimal firing schedules for your glass and project type.
2. Thermal Shock Cracks: The Sneaky Culprit Behind Fractures
One moment, your fused piece looks perfect—then crack! A thermal shock crack can sneak up on you when you least expect it.
What causes thermal shock:
- Uneven heating or cooling
- Parts of the glass heating at different speeds
- Using an inappropriate kiln for the size or shape of the piece
Prevention tips:
- Use a kiln with both top and side elements: This ensures a more even heat distribution, especially important for thick or tall pieces.
- Slow down the firing process: Rushing through the ramp-up or cool-down phases can cause internal stress that leads to cracking.
- Know your glass thickness: Thinner pieces heat faster. For wide, flat pieces, top heat works well. For thicker or taller pieces, both top and side heat are essential.
- 3. Glass Sticking to the Kiln Shelf
Nothing ruins a project quite like lifting it out of the kiln only to find it’s glued to the shelf.
Common causes:
- Uneven kiln wash application
- Not enough separator (kiln paper, shelf primer, etc.)
- Over-firing causes the glass to melt too much and adhere to the shelf
Solutions:
- Apply kiln wash evenly: Uneven coating can cause weak spots. Use a soft brush or sprayer for smooth, consistent application.
- Don’t overdo it: Too much kiln wash can also backfire, causing pieces to adhere or leave residue.
- Use shelf paper or fiber paper for a clean release, especially with intricate pieces.
4. Annealing Too Quickly: The Silent Stressor
Annealing is all about letting your glass relax. Skip or rush this step, and your masterpiece may shatter days or weeks later.
What happens when you anneal too fast:
- The outer layer cools faster than the interior
- Uneven stress builds up, eventually causing breaks
- Larger or thicker pieces are especially vulnerable
How to get it right:
- Use a proper annealing soak: Most glass needs to hold at 900°F for a period based on its thickness.
- Fire down when necessary: Gradually adding heat during the cooling phase helps thicker pieces cool more evenly.
- Invest in a kiln with precise control: Pre-programmed cycles are great, but always double-check the ramp-down rate—especially for big or complex projects.
5. Glass Compatibility: Matching COEs to Avoid Breakage
It’s tempting to mix glass colors and textures, but compatibility is key. Mismatched glass can fuse beautifully—then crack days later.
What to check:
- Coefficient of Expansion (COE): Glass with different COEs expands and contracts at different rates. When cooled, this creates tension that can break the piece.
- Stick to the same COE: If you're using 96 COE glass, make sure all components (base, frits, stringers) are 96 COE.
- Label your glass: If you're working with different brands or batches, labeling helps prevent mix-ups.
6. Devitrification: That Unwanted Cloudy Finish
Devitrification happens when the surface of your glass crystallizes during firing, resulting in a dull, cloudy, or rough texture.
Why it happens:
- Dirty or contaminated glass
- Over-firing
- Too much time at high temperature
How to avoid it:
- Clean your glass thoroughly before firing—dust, oils, or even fingerprints can trigger devit.
- Don’t overfire: More heat doesn’t always mean better fusion. Stick to the temperature and time needed for your glass thickness.
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Use clear caps or frit: Adding a layer of clear glass frit or a devit spray can protect against surface crystallization.
7. Warping and Sagging: When Shapes Lose Their Structure
Glass is naturally fluid at high temps—but sometimes it flows a little too freely.
Causes of warping:
- Over-firing
- Unsupported shapes during slumping
- Uneven kiln shelves or molds
Fixes:
- Support your glass: Use slumping molds or kiln furniture to hold your piece in shape as it softens.
- Stick to firing schedules that match your form and size. Avoid pushing the temperature beyond what’s needed.
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Check that kiln shelves are level: This affects everything from shape to structural integrity.
A Few Final Tips from Hollander’s Studio
Here are a few general tips from us at Hollander to set yourself up for success:
- Keep a kiln log: Document your firing schedules, glass types, and results. This helps you tweak future projects and avoid repeating mistakes.
- Test small: Before committing to a large piece, test your glass combination and schedule a small sample.
- Stay curious: Trial and error is part of the journey. Every project teaches something new.
Let Your Glass Fusing Shine
Fusing glass is a rewarding, creative process—but it helps to know how to steer around the common bumps in the road. Whether it’s bubbles, cracks, or a stubbornly stuck shelf, the key is understanding the root causes and how to fix them. With the right knowledge (and a little patience), you'll turn challenges into confidence—and confidence into art.
Need help with materials, tools, or just want to talk shop? Visit Hollander Fusing and Art Glass, where passion and glass meet.