What Is Glass Fusing?
Glass fusing is when you heat pieces of glass until they melt and stick together. This usually happens inside a kiln at high heat, somewhere between 700 and 820 degrees Celsius. That’s about 1,290 to 1,510 degrees Fahrenheit. The exact temperature depends on the result you want. At the lower end, tack fusing lets the glass pieces stick together but still keep their shapes. At the higher end, full fusing melts everything into a single smooth piece.
Where Glass Fusing Started
No one knows for sure when glass fusing first began. But there’s evidence that people in Egypt were doing it around 2000 BCE. Most researchers agree on that date. Still, some think the Romans were the first to really develop fusing techniques. The Romans worked with glass more often and were better known for it.
Fusing was the main way to make small glass items for about 2,000 years. Then people invented the blowpipe, which made glassblowing easier and faster. Once that happened, glassblowing took over and became the more common method.
During the Renaissance, glassworking became popular again, but fusing didn’t come back right away. It stayed out of the spotlight until the early 1900s. Then, in the 1960s, it started getting attention again, especially in the United States. Today, people all over the world do glass fusing as a hobby. It’s also starting to show up more in the fine art scene.
Why Glass Compatibility Matters
Not all glass can be fused together. For a clean and lasting bond, the pieces need to be compatible. A lot of people think that if two types of glass have the same COE, which stands for coefficient of expansion — they’ll fuse without issues. But COE is just one part of the equation. It doesn’t guarantee that the glass will work well together.
When you fuse incompatible glass, you’re asking for trouble. It might seem fine at first, but the stress inside the glass will build up. That stress often shows up during the cooling stage. If it doesn’t crack right away, it might crack later. The weak point usually starts where the different pieces meet.
To avoid this, many glass makers stick to one brand. Most kiln-glass companies test their own glass for compatibility. That way, you know which pieces can safely go together.
If you want to check for stress between two fused pieces, you can use two polarizing filters. Put the fused item between them. The tension will show up as dark or bright spots. These are warning signs that the glass might crack later.
Modern Glass Fusing Methods
Most glass fusing today uses a method where you stack or layer thin sheets of glass. These sheets are usually different colors, so when they melt, they form patterns or simple images. The stacked glass goes into a kiln. Most kilns are electric now, but some still run on gas or wood.
Inside the kiln, the glass is heated in stages. It goes through ramps, which are fast heat-ups, and soaks, where the heat is held steady at a certain temperature. As the temperature rises, the glass pieces start to melt and stick together. The longer you hold it at the peak heat, the more the edges smooth out and the glass fuses into one solid shape.
Once the glass looks how you want it, the heat gets dropped quickly. This fast drop from around 815 degrees Celsius to 573 prevents a problem called devitrification, which makes the glass cloudy. After that, the glass needs to cool slowly and evenly. This step is key. It’s called annealing, and it keeps the piece from cracking as it cools.
Cooling and Annealing Stages
The full cooling process usually takes about 10 to 12 hours and goes through three stages. The first stage is a quick drop that brings the glass down into the high end of the annealing zone, around 516 degrees Celsius.
Next is the anneal soak. At this stage, the temperature holds steady at 516 degrees to let the inside and outside of the glass even out. This helps release any built-up stress between layers.
The final step is the cool-down to room temperature. The kiln drops slowly to 371 degrees, holds it there for two hours, then drops again to 260 degrees. That’s where the firing schedule ends. The glass stays in the kiln, untouched, until the built-in pyrometer says it’s cooled all the way to room temp.
Flexibility in Firing Schedules
These numbers aren’t strict. The exact temperatures and times can change based on the kiln, the size of the piece, the number of layers, the look you're going for, and even the glass brand. Smaller projects, like pendants, don’t need much time at all. Some can go from start to finish in under an hour.
What Fused Glass Is Used For
Fused glass is mostly used to make art pieces, jewelry, and tiles. Beads are a common project, too. When you want to make something bigger, like a dish or bowl, you’ll usually need more than one kiln run. The first firing fuses the glass. The second one is a slump, which shapes it into a mold.
Since the 1970s, a lot of people have picked up kiln fusing as a hobby, especially for making beads and jewelry parts. This got easier once companies started making glass designed just for fusing in kilns.
