Glass Kiln Casting Schedule Calculator

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

1. Initial Ramp: Slow heat from room temp to burn-out temperature (to remove moisture and burn organic mold materials)
2. Burn-Out Hold: Hold at burn-out temp to fully remove binder/wax
3. Casting Ramp: Heat to the glass casting/fusing temperature
4. Casting Hold: Hold at peak temperature for the glass to fully fill the mold
5. Annealing: Controlled slow cooling through the annealing range, then natural cool to room temperature

Temperature Reference Points (Bullseye COE 90 glass)

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

1. Room temp to 120°C at 55°C/hour (2.2 hours). Hold 1 hour.
2. 120°C to 370°C at 83°C/hour (3 hours). Hold 2 hours (burn-out).
3. 370°C to 816°C at 165°C/hour (2.7 hours). Hold 30 minutes (casting).
4. Crash cool to 516°C (open kiln briefly or set fast ramp, glass stays viscous).
5. Hold at 516°C for 4 hours (annealing soak).
6. Cool 516°C to 371°C at 83°C/hour (1.7 hours).
7. 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.