Annealing is the step that decides whether a piece survives. It is also the step beginners rush, because the piece already looks finished and the kiln takes hours. Most cracked work is not a shaping failure. It is an annealing failure that showed up late.
What stress actually is
As glass cools it contracts. If the outside of a piece cools faster than the inside, the outside wants to shrink more than the inside will let it, and the two pull against each other. That mismatch is locked into the wall as internal stress, and it stays there until something gives.
Annealing removes that stress by holding the piece at a temperature where the glass can still flow just enough to relax, then cooling it slowly enough that no new stress forms.
The two temperatures that matter
| Point | What happens there | Soda-lime | Borosilicate |
|---|---|---|---|
| Annealing point | Stress relaxes in minutes | Around 516 °C | Around 560 °C |
| Strain point | Stress relaxes in hours, cooling must be slow | Around 470 °C | Around 515 °C |
| Below strain point | No further stress relief possible | Room temperature approach | Room temperature approach |
The critical insight is that the strain point governs the cooling rate, not the annealing point. Above the annealing point stress disappears quickly. Between the annealing point and the strain point it disappears slowly, and below the strain point it never disappears at all. Everything that happens after the strain point is permanent.
Hold the piece above the annealing point long enough to relax, then creep through the strain point. Those are two different jobs and they need two different segments in the schedule.
Thickness drives everything
Heat has to diffuse out from the centre of a piece, and diffusion time scales roughly with the square of the distance. Double the wall thickness and the schedule gets roughly four times longer. This is why a solid paperweight can need a longer anneal than a thin vessel many times its size.
- Thin work under 6 mm can often be annealed in a couple of hours.
- Vessels in the 6 to 12 mm range typically need an overnight schedule.
- Solid work and thick sculpture can need a full day or more.
- Never shorten a schedule because the piece looks fine. Stress cracks arrive days later.
Building a schedule
Set the soak temperature
Slightly above the annealing point of the glass family, around 516 °C for soda-lime and 560 °C for borosilicate.
Hold long enough to relax
Soak time scales with the square of thickness. When in doubt, soak longer. An extra hour costs electricity, not quality.
Cool slowly to the strain point
This is the segment that matters most. Keep the rate well inside the limit for your thickness.
Continue slowly through the strain point
Do not speed up when you reach the strain point. Most failures happen just below it.
Let it fall to room temperature
Below roughly 100 °C the rate no longer matters much, but do not open the door early.
A clean crack that appears days later, often along a colour boundary or a thick-to-thin transition, is the signature of a schedule that was too aggressive.
Common annealing mistakes
- Loading a hot piece into a cold kiln. The kiln should already be at annealing temperature.
- Opening the door mid-program to check on the work. A rush of cold air cracks everything inside.
- Crowding pieces so air cannot circulate. Leave at least 25 mm between them.
- Trusting the controller without logging the actual curve. Drift is the early sign of a failing element.
- Skipping the soak because the piece is small. Small pieces still have a centre.
Annealing is invisible when it works and obvious when it fails. Budget the time, log the run, and the crack rate drops to almost nothing.
This article is educational reference material. Hot glass work involves severe burn, fume and fire hazards. Wear eye protection, heat-resistant gloves and natural-fibre clothing, ventilate every heat source to open air, fit a carbon monoxide alarm, and take hands-on instruction before operating any hot glass equipment.
No guarantee of results. Techniques, temperatures and schedules described here are reference guidance based on common studio practice. Glass behaviour depends on your materials, equipment and technique. Test everything on your own setup and follow the manufacturer's instructions.