Glass Fusing Firing Basics: Full Fuse, Tack Fuse, and Slumping

Glass fusing uses a kiln to melt pieces of glass together. How hot you go and how long you hold decides whether you get a smooth, flat piece, a textured one, or a curved bowl. If you're new to glass, start with an introduction to fusible glass.

First, match your glass

Glass pieces fused together must be compatible, meaning they expand and shrink at the same rate (COE). Mixing incompatible glass can cause cracks, sometimes days later. Use glass from the same line, and check the label before combining.

The main firing types

Tack fuse (lower): Glass is heated just enough to stick pieces together while still keeping their shapes and texture. Temperatures are typically around 1,350 to 1,400°F. Good for layered designs where you want a dimensional look.

Contour (rounded) fuse: A bit hotter, the edges soften and round off but the piece still has some relief.

Full fuse (higher): The glass melts into one smooth, flat surface. Temperatures are typically around 1,450 to 1,500°F, depending on the glass and thickness.

Slumping: A cooler firing, typically around 1,200 to 1,300°F, where finished glass sags into a mold to form bowls, plates, and dishes. Browse our glass molds.

These ranges are general. Always follow the recommended schedule for your specific glass and kiln.

Annealing: don't skip it

After the top temperature, glass must cool slowly through a hold called the annealing soak, typically around 900 to 960°F, so internal stress can relax. Skipping or shortening it can cause the piece to crack later. Thicker glass needs a longer anneal.

Plan your schedule

1. Heat up slowly to avoid thermal shock, especially for thick or large pieces.
2. Hold at the top temperature for the time your project needs.
3. Cool quickly through the zone where glass can devitrify (cloudy surface), then
4. Anneal slowly, then
5. Cool gradually to room temperature before opening the kiln.

Protect your shelf

Glass sticks to bare kiln shelves. Use kiln wash or shelf paper (see kiln furniture basics) and let it dry fully before firing.

Safety

• Fire in a ventilated space and use a kiln vent if needed.
• Wear safety glasses and protective gloves when cutting glass.
• Never open a hot kiln.

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