Glass Annealing Schedule Calculator
Calculate annealing hold times, ramp rates, and cooling schedules for glass based on thickness, type, and COE.
Annealing is the controlled cooling of glass to relieve internal thermal stresses. Without proper annealing, glass will crack or shatter — sometimes hours, days, or even weeks after it leaves the kiln. The process involves heating the glass to its annealing point (where the molecules can rearrange to release stress), holding it at that temperature, then cooling slowly through the strain point and beyond.
Key Temperature Points
| Glass Type | COE | Annealing Point | Strain Point | Room-Safe Temp |
|---|---|---|---|---|
| Borosilicate (Pyrex) | 32–33 | 565°C (1050°F) | 515°C (960°F) | 370°C (700°F) |
| Soda-lime (soft glass) | 90–96 | 480°C (900°F) | 430°C (806°F) | 315°C (600°F) |
| Lead crystal | 80–85 | 435°C (815°F) | 395°C (743°F) | 290°C (554°F) |
| Fusing glass (Bullseye) | 90 | 516°C (960°F) | 465°C (870°F) | 315°C (600°F) |
Hold Time Formula
The soak time at the annealing point depends primarily on the thickest cross-section of the piece:
Hold Time (minutes) = Base Time × (Thickness / Base Thickness)²
For most glass, the base time is 30 minutes at 6 mm (¼ inch) thickness. Doubling thickness quadruples the hold time because heat must conduct from the surface to the center and back.
| Thickness | Hold Time (soda-lime) | Hold Time (boro) |
|---|---|---|
| 6 mm (¼ in) | 30 min | 45 min |
| 12 mm (½ in) | 120 min | 180 min |
| 19 mm (¾ in) | 270 min | 405 min |
| 25 mm (1 in) | 480 min | 720 min |
| 50 mm (2 in) | 1920 min (~32 hr) | 2880 min (~48 hr) |
Cooling Ramp Rates
After the annealing soak, the critical cooling zone is between the annealing point and the strain point. The ramp rate here must be very slow:
Phase 1 — Critical Cool (Annealing → Strain Point): Rate = 15°C/hr ÷ (Thickness / 6mm)²
Phase 2 — Moderate Cool (Strain Point → Room-Safe): Rate = 30°C/hr ÷ (Thickness / 6mm)²
Phase 3 — Final Cool (Room-Safe → Room Temp): Can usually be kiln-vented or turned off and allowed to cool naturally.
Worked Example — Soda-Lime Bowl, 15 mm Thick
- Annealing soak at 480°C: Hold = 30 × (15/6)² = 30 × 6.25 = 187.5 min → 3 hours 8 min
- Critical cool (480°C → 430°C = 50° range): Rate = 15 / (15/6)² = 15 / 6.25 = 2.4°C/hr → Duration = 50 / 2.4 = 20.8 hours
- Moderate cool (430°C → 315°C = 115° range): Rate = 30 / 6.25 = 4.8°C/hr → Duration = 115 / 4.8 = 24 hours
- Final cool (315°C → 21°C): Kiln off, vent after 200°C. Approximately 6–10 hours.
Total annealing cycle: ~51 hours for a 15 mm soda-lime piece.
Thicker sculptural work can take several days. A 50 mm borosilicate sculpture may require a week-long annealing cycle. Never rush this process — thermal stress is invisible until the glass fails catastrophically.
COE (Coefficient of Expansion)
COE measures how much glass expands per degree of temperature change (× 10⁻⁷ per °C). All glass in a single piece MUST share the same COE, or differential expansion will cause cracking at the joint. Even a COE mismatch of 2–3 points can cause failure.