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 has to be controlled at both ends. Too fast a ramp cracks the mold, and too fast a cool shatters the glass from thermal stress, sometimes weeks later on a shelf.

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, scaled across the band For thickness > 50 mm: Hold = 12+ hours (add 2 hours per additional 12 mm)

The 25 to 50 mm band scales smoothly from 8 hours at 25 mm to 12 at 50, rather than jumping to 10 the moment you pass an inch. A 26 mm casting is not meaningfully different from a 25 mm one, and its schedule should not be either.

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, which is four times slower for twice the thickness.

Note that 25 mm is not exactly an inch, it is 0.984, so the calculator returns 86°C/hour rather than 83 for a 25 mm piece. Both are the same rule; the round number in this paragraph is the round number, and the tool does the arithmetic properly.

Below the strain point (371°C), cooling can be faster: approximately 2× the annealing cooling rate.

Glass reference points by expansion coefficient

The table further up is Bullseye COE 90. The other two glasses this calculator supports sit a little lower:

Never mix expansion coefficients in one piece. COE 90 and COE 96 look identical cold and will tear each other apart on the way down.

Worked Example: a 25 mm Pate de Verre Bowl in COE 90

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 (3.0 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, roughly 15 minutes with the kiln cracked open. The glass is still viscous, so this part is safe.
5. Hold at 516°C for 4 hours (annealing soak).
6. Cool 516°C to 371°C at 86°C/hour (1.7 hours).
7. Cool 371°C to 50°C at 172°C/hour (1.9 hours).

Total: about 19 hours, which is the sum of the steps above rather than a rounder number. Kiln casting is an overnight-plus-a-morning proposition, and there is no shortening it.

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.