Melting glass together requires heating two or more pieces in a kiln until they bond into a single unit, a process called glass fusing. Depending on how much you want the pieces to merge, you’ll fire them to temperatures between 1,300°F and 1,450°F. The process is straightforward once you understand three essentials: using compatible glass, controlling temperature in stages, and cooling the finished piece slowly enough to prevent cracking.
Why Glass Compatibility Matters
Every type of glass expands at a specific rate when heated, measured by a number called the coefficient of expansion (COE). Common art glass comes in COE 90 or COE 96. Borosilicate glass used in labware sits around COE 33. If you try to fuse two pieces with different COE ratings, they’ll expand and contract at different speeds during heating and cooling. The result is internal stress that causes cracking, sometimes immediately, sometimes days or weeks later.
Stick to one COE system for your entire project. Glass sold for fusing is labeled with its COE, and reputable suppliers test for consistency. Mixing a COE 90 piece with a COE 96 piece might look fine coming out of the kiln, but the invisible tension between them will eventually win. Even within the same COE family, different manufacturers can have slight variations, so many fusers stick to a single brand for reliable results.
Choosing Your Equipment
A standard electric kiln with a digital temperature controller is the most reliable tool for fusing glass. These kilns let you program exact temperatures and hold times for each stage of the firing, which is critical for consistent results. They come in sizes ranging from small tabletop models (around 8 inches across) to large units that can fire full shelves of work.
Microwave kilns offer a budget-friendly alternative for very small projects like jewelry or test tiles. These are silicon carbide-lined boxes that sit inside a household microwave and absorb radiation to generate heat. The internal chamber is tiny, roughly 3 to 5 inches across, and there’s no temperature readout. You control the heat by adjusting the number of microwave cycles and timing them manually. They cut firing and cooling time by more than half compared to electric kilns, but the lack of precise temperature control makes them unsuitable for anything beyond simple, small-scale fusing.
Preparing Your Glass and Kiln Shelf
Clean every piece of glass thoroughly before placing it in the kiln. Fingerprints, dust, and residue from cutting oil can cause cloudy patches or bubbles in the finished piece. Wipe each piece with rubbing alcohol and a lint-free cloth, then handle only by the edges.
Glass will bond to a bare kiln shelf just as happily as it bonds to another piece of glass, so you need a separator between your project and the shelf. The two main options are kiln wash and shelf paper.
- Kiln wash is a brush-on coating made from refractory powder mixed with water. It works well for textured molds and deep forms, but it requires scraping and recoating between firings and can transfer a gritty texture to the bottom of your glass.
- Shelf paper (like Bullseye ThinFire) is a thin disposable sheet you cut to size and lay on the shelf. It produces a smoother, glossier bottom surface, requires no mixing or drying, and leaves only a light residue that wipes off. It’s single-use and not suited for casting or heavy texture work, but for standard fusing and slumping it saves considerable cleanup time.
The Four Temperature Stages
Glass fusing isn’t a matter of blasting pieces with heat and hoping for the best. A firing schedule moves through distinct stages, each with a target temperature, a rate of climb, and a hold time. Here’s what happens at each level.
Heating Up
The kiln ramps from room temperature to your target fusing temperature. A typical rate for the initial climb is around 300°F to 500°F per hour. Going too fast through the lower ranges (especially between 100°F and 1,000°F) risks thermal shock, where the glass cracks because the outside heats faster than the inside.
Fusing
This is where the actual melting happens, and you choose your target based on the effect you want:
- Tack fuse (1,300°F to 1,350°F): The pieces stick permanently to each other but keep their original shape and texture. Edges stay distinct. This is the go-to for layered designs where you want dimension.
- Contour fuse (1,350°F to 1,375°F): Edges soften and round over, and the pieces begin settling into a smooth shared surface while retaining some relief.
- Full fuse (1,400°F to 1,450°F): The glass flows together into a single flat layer with no visible seams. Typical full-fuse schedules hold at 1,450°F for about 15 minutes before cooling begins.
These temperatures apply to standard fusible art glass (COE 90 and 96). Borosilicate and other specialty glasses require different ranges.
Rapid Cool
After the fuse stage, you want to drop the temperature relatively quickly to the annealing range (around 900°F) to lock in the shape and prevent the glass from continuing to move. Many kilns accomplish this by simply turning off the elements and venting the lid slightly.
Annealing
This is the most important stage for the long-term survival of your piece. At around 900°F, glass is soft enough internally that stress can equalize. You hold the piece at this temperature for a set time, then cool it through a series of progressively slower ramps.
Annealing Schedules by Thickness
Thicker pieces trap heat in the center longer, creating tension between the hot interior and the cooler surface. The thicker the glass, the longer and slower the annealing needs to be. Based on Bullseye Glass guidelines for uniform-thickness pieces:
- 1/4 inch (6 mm): Hold at 900°F for 1 hour. Total firing cycle is roughly 3 hours.
- 1/2 inch (12 mm): Hold at 900°F for 2 hours. Total cycle is about 5 hours.
- 1 inch (25 mm): Hold at 900°F for 4 hours, then cool at just 27°F per hour through the critical range. Total cycle is around 14 hours.
- 2 inches (50 mm): Hold at 900°F for 8 hours. Total cycle approaches 47 hours.
- 3 inches (75 mm): Hold for 12 hours with a first cooling rate of only 3°F per hour. Total cycle is about 99 hours, or roughly four days.
Most beginner projects are two layers of standard 3mm glass stacked together, producing a fused piece about 1/4 inch thick. At that thickness, the entire firing from start to finish takes about 3 hours, which is manageable even on a weeknight.
Preventing Devitrification
Devitrification is a cloudy, rough-textured surface that forms when glass is held too long at certain temperatures or fired with a dirty surface. It happens because the glass begins reverting from its smooth amorphous state to a crystalline structure. Some colors and formulations are more prone to it than others.
The simplest prevention is thorough cleaning before firing and avoiding unnecessarily long hold times. When a project demands extended heating or uses devit-prone glass, a spray-on solution helps. A common homemade recipe uses 1 quart of distilled water, 1/4 cup of borax (the laundry booster), and 2 tablespoons of a clean-burning glass glue. Apply it in a thin, even coat using an airbrush or a small atomizer bottle. The borax creates a flux layer on the surface that discourages crystal formation during firing.
Safety Precautions
Cutting and grinding glass produces fine silica dust, which is harmful to your lungs with repeated exposure. Wear a dust mask or respirator whenever you’re scoring, breaking, or grinding glass. Use infrared-blocking goggles rated for wavelengths between 600 and 6,000 nanometers when looking into a hot kiln. Prolonged exposure to the infrared glow can contribute to cataracts over time.
Wear protective gloves when handling hot or freshly cut glass, and keep loose clothing and hair secured around the kiln and any grinding equipment. Good ventilation in your workspace helps clear any fumes released during firing, particularly if you’re using shelf paper, adhesives, or inclusion materials that burn off at high temperatures.