An annealing oven, or lehr, is a refractory chamber held at the glass’s annealing point, 510 °C for soda-lime, 560 °C for borosilicate and 430 °C for lead glass, which finished work enters straight from the bench, soaks for 30 minutes and then cools at 1-2 °C per minute. For a 5 mm wall the whole cycle takes about 3-4 hours for soda-lime and 5-6 hours for borosilicate. Without it a piece cracks within hours from stress locked in by uneven cooling.
Part of Glass Blowing and Forming
Building and running the controlled-cooling oven that relieves stress in finished glass.
The Lehr Is The Part Of The Workshop That Decides Whether Pieces Survive
Stress arises because the surface of a hot piece cools and stiffens first while the interior is still soft, then the interior shrinks against a rigid shell. Holding the piece at the annealing point lets the stress relax, and cooling slowly through the range where glass is still stiff but not rigid prevents it from returning. The reasoning is in Viscosity-Temperature Curve, and the corpus’s Annealing article gives its own schedule.
The carrier’s schedule, for a 5 mm wall:
| Glass type | Hold temperature | Cooling rate | Total time |
|---|---|---|---|
| Soda-lime | 510 °C | 2 °C/min | 3-4 hours |
| Borosilicate | 560 °C | 1 °C/min | 5-6 hours |
| Lead glass | 430 °C | 2 °C/min | 3-4 hours |
Hold for 30 minutes, cool at the stated rate to 100 °C below, then the rate can increase.
Other corpus articles disagree, mostly in the slower direction:
| Source | Soda-lime schedule |
|---|---|
| Carrier (this article) | 510 °C, 30 min hold, 2 °C/min |
| Annealing | 530 °C, 45 min for 6 mm, then 0.5-1.5 °C/min through the strain point |
| Glassblowing | 480-520 °C, 30 min per cm of wall, no more than 2-3 °C/min to 370 °C |
| Glassmaking | about 500 °C, 1-2 hours per cm, then 50-100 °C per hour |
These are not reconciled. When in doubt, cool slower, since a slower rate is the safe direction for stress, and confirm with Stress Testing.
What You Need
- A refractory chamber sized for the work, with a door; see Refractory Materials.
- A heat source that can hold 560 °C and be turned down gradually: a small fire in a separate firebox with a damper, or a cooler zone of the main furnace as the glassmaking article describes.
- Heavy insulation, so the chamber cools slowly by itself.
- A way to read the temperature. The corpus gives none for 510 °C. Colour fails, since glass and brick glow only above about 600 °C. The practical instrument is the polariscope: adjust the schedule until pieces show no colour.
Method: Building And Running
Step 1: Build the chamber large enough for a day’s output with space between pieces, and thick enough to hold heat.
Step 2: Pre-heat to the hold temperature before the first piece goes in.
Step 3: Load immediately. Pieces go in straight from the bench.
Step 4: Hold 30 minutes (carrier), longer for thicker walls.
Step 5: Cool at the stated rate by easing the fire back and closing dampers.
Step 6: Do not open the oven until it is at room temperature, per the glassmaking article.
Worked Example
Size the floor for one day of bottles from Bottle and Jar Production: 60 bottles standing upright, each 7 cm across (illustrative size, as in that article). Footprint per bottle is 7 × 7 = 49 cm², so 60 × 49 = 2,940 cm², about 0.29 m², with the bottles touching. A floor of 70 × 70 cm is 4,900 cm², which is 4,900 ÷ 2,940 ≈ 1.7 times the bare footprint and leaves room to space them and to reach in with tongs.
Cooling time for soda-lime from the carrier’s table: 30 minutes hold plus 100 ÷ 2 = 50 minutes to 100 °C below gives 80 minutes, with the remaining time in the 3-4 hour total being the faster tail.
Common Mistakes
- Loading into a cold lehr. The piece cools unevenly before the hold.
- Putting thick and thin pieces on one cycle without lengthening it for the thick one.
- Opening the door early. Cold air causes thermal-shock fracture.
Safety
Hot glass does not look hot
The inside of a lehr at 510 °C is below the 600 °C glow threshold and looks dark, and the glass in it looks cold. Opening the door releases air that burns; the door and walls are too hot to touch. Treat a dark oven as hot until it has stood at room temperature.
Fire and carbon monoxide
A wood or charcoal firebox beside the oven produces carbon monoxide (recalled general fact, not from the corpus). Vent the chimney outdoors, keep timber and thatch away from the chamber and keep the fire attended during the hold.
What’s Next
- Learn next: Stress Testing — proving the schedule works
- Combine with: Annealing — the earlier-phase schedule and the reasoning behind it