Craft · 6 pages · 12 min
Why annealing keeps glass from shattering later
A controlled cool lets thick and thin glass relax internal stress before the piece goes rigid.
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From the opening
Annealing holds a finished glass piece in a warm oven, a lehr, so the whole thickness can pass through the temperature where internal stresses relax, and then cools it slowly enough that new stresses do not form. Glass is a poor conductor, so the outside and the inside do not change temperature together unless you make them. The opening covers outside cools before the inside, the lehr lets stress relax, and thick sections need the slow passage.
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Inside
- Outside cools before the inside
- The lehr lets stress relax
- Thick sections need the slow passage
- Polarized light shows the tension
- Where the idea stops applying
- A distinction worth keeping
Sources
- Annealing glass in a lehr
- Thermal stress and the strain point