The Science of Fusing: What Actually Happens in the Kiln

One of the most common questions I get is: “So… how do you make these?”

It’s such a good question, because fused glass feels a little like magic when you first see it. But behind the magic, there’s some fascinating science happening every single time I fire up the kiln.

From Room Temperature Glass to Hot Flowing Movement

When I start a project, I’m cutting, stacking, and arranging pieces of glass that feel hard and unchanging. Then I close the kiln, press start, and wait. Inside, the transformation begins.

Here’s the amazing part: glass doesn’t just suddenly “melt” like ice turning into water. Instead, it softens slowly across a range of temperatures. That gradual shift is what allows me to control the results — from a textured tack fuse, to a perfectly smooth full fuse, or even shaping the glass into a mold to create bowls and plates.

Key Temperatures in the Kiln

To give you a peek behind the scenes, here’s what’s happening as the kiln heats up:

• Room Temperature to ~1000°F: The kiln warms slowly so the glass doesn’t crack from heating too fast. This is called the “ramp up” stage.
• 1000°F–1250°F: The glass begins to soften. If I stop here, I get a tack fuse — the pieces stick together but you can still see their texture.
• 1350°F–1500°F: This is where the magic of full fusing happens. The glass flows just enough to merge into one smooth surface.
• Above 1500°F: The glass becomes very fluid — that’s typically for casting techniques or for racking a design through the glass; a bit hotter than for the flat fusing projects I usually make.

Then comes one of the most important (and overlooked!) parts: the cool down.

The Importance of Annealing

Cooling the glass is every bit as important as heating it. If the kiln cools too quickly, the glass holds onto stress inside — and even if it looks fine, it can crack days, weeks, or even years later.

That’s why I make sure to “anneal” each piece. The kiln holds at a certain temperature long enough for the molecules to realign and settle into balance. The result? A piece that isn’t just beautiful, but also strong and durable.

Why I Love This Science-Art Mix

Here’s what keeps me hooked: fused glass is the perfect mix of science and creativity. I get to play with bold colors, cut and layer shapes, and imagine how everything will come together — but then the kiln does its thing, following the same physical laws every time.

And honestly? Opening the kiln after a firing never gets old. Even though I have a better idea of what should happen in the kiln now, it still feels like I'm unwrapping a present, every single time. The science makes it all possible, but the artistry is in how I choose to use it — and in embracing the surprises along the way.