Glass Kiln Casting Schedule Calculator
Generate a kiln firing schedule for glass casting.
Covers ramp rates, hold times, and annealing for pate de verre and lost wax.
Glass kiln casting involves melting or fusing glass into a mold, then carefully cooling (annealing) it to relieve internal stress. The schedule must be precisely controlled — too fast a ramp can crack the mold, and too fast a cool can shatter the glass from thermal stress.
The Five Phases of a Casting Schedule
- Initial Ramp — Slow heat from room temp to burn-out temperature (to remove moisture and burn organic mold materials)
- Burn-Out Hold — Hold at burn-out temp to fully remove binder/wax
- Casting Ramp — Heat to the glass casting/fusing temperature
- Casting Hold — Hold at peak temperature for the glass to fully fill the mold
- Annealing — Controlled slow cooling through the annealing range, then natural cool to room temperature
Temperature Reference Points (Bullseye COE 90 glass)
| Phase | Temperature | Purpose |
|---|---|---|
| Moisture drive-off | 120°C (250°F) | Removes water from mold |
| Binder burn-out | 370°C (700°F) | Burns out wax/organic binder |
| Tack fuse | 704°C (1300°F) | Pieces stick together |
| Full fuse | 788°C (1450°F) | Pieces merge fully |
| Cast/flow | 816–871°C (1500–1600°F) | Glass flows into mold |
| Annealing point | 516°C (960°F) | Stress relief begins |
| Strain point | 371°C (700°F) | Stress relief complete |
Annealing Hold Time Formula
The annealing hold time depends on the thickest cross-section of the casting:
For thickness ≤ 6 mm (1/4"): Hold = 1 hour at annealing point For thickness 6–13 mm: Hold = 2 hours For thickness 13–25 mm: Hold = 4 hours For thickness 25–50 mm: Hold = 8–12 hours For thickness > 50 mm: Hold = 12+ hours (add 2 hours per additional 12 mm)
Cooling Rate Formula
After the annealing hold, cool at: Rate (°C/hour) = 83 / (thickness in inches)²
For a 1-inch (25 mm) casting: 83 / 1² = 83°C/hour through the annealing zone (516°C to 371°C). For a 2-inch (50 mm) casting: 83 / 4 = 20.75°C/hour — much slower.
Below the strain point (371°C), cooling can be faster: approximately 2× the annealing cooling rate.
Worked Example — 25 mm Thick Pate de Verre Bowl
- Room temp to 120°C at 55°C/hour (2.2 hours). Hold 1 hour.
- 120°C to 370°C at 83°C/hour (3 hours). Hold 2 hours (burn-out).
- 370°C to 816°C at 165°C/hour (2.7 hours). Hold 30 minutes (casting).
- Crash cool to 516°C (open kiln briefly or set fast ramp — glass stays viscous).
- Hold at 516°C for 4 hours (annealing soak).
- Cool 516°C to 371°C at 83°C/hour (1.7 hours).
- Cool 371°C to 50°C at 166°C/hour (1.9 hours).
Total schedule: approximately 18.5 hours.
Mold Materials
Investment/plaster-silica molds require the burn-out phase. Ceramic shell molds can often skip it. Always vent the kiln during burn-out to allow gases to escape.