Glass Annealing Schedule Calculator

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

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 soda-lime the base time is 30 minutes at 6 mm (¼ inch); borosilicate uses 45 minutes at the same thickness. Doubling the thickness quadruples the hold, because heat has to conduct from the surface to the centre and back.

Those figures are the formula run straight, with no rounding of the thicknesses. Older tables often show 270, 480 and 1,920 for the last three rows, which comes from treating ¾ in as 18 mm, 1 in as 24 mm and 2 in as 48 mm rather than their true 19.05, 25.4 and 50.8. On a thick piece that rounding costs you hours of soak, which is exactly where you can least afford to be short.

The piece-type adjustment

The calculator applies one more factor the bare formula does not. A solid sculptural mass holds heat in its middle far longer than a blown vessel of the same wall thickness, so:

The worked example below is a blown piece, so its multiplier is 1.0 and the numbers match the plain formula.

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 Blown Bowl, 15 mm Thick

1. Annealing soak at 480°C: Hold = 30 × (15/6)² = 30 × 6.25 = 188 min, or 3 hours 8 min
2. Critical cool (480°C to 430°C, a 50° range): Rate = 15 / 6.25 = 2.4°C/hr, so 20.8 hours
3. Moderate cool (430°C to 315°C, a 115° range): Rate = 30 / 6.25 = 4.8°C/hr, so 24.0 hours
4. Final cool (315°C to room): kiln off, vent below 200°C, roughly 5.9 hours

Total cycle: 53.8 hours, near enough two and a quarter days.

Thicker sculptural work runs into days. Take that same 15 mm piece as sculptural rather than blown and the soak alone grows by 30%. A 50 mm borosilicate sculpture is a week-long cycle. There is no way to hurry it: thermal stress is invisible until the glass fails, and it fails without warning, often after it has left your hands.

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.