thermal tempering

Glass breaks because a surface crack, pulled open by tension, races through it. Thermal tempering defeats that by giving the glass a permanent squeeze on its skin, so that a crack must first fight through a layer of built-in compression before any tension can reach it. In effect, the surface is pre-stressed shut, like a stone arch held together by the weight it carries. It is the same idea as prestressed concrete, borrowed for glass.

The trick is to freeze the surface and interior at different times on purpose. Heat a glass sheet uniformly above its glass transition, then blast both faces with jets of cold air. The surfaces chill and stiffen first, setting while the inside is still hot and expanded. As the trapped interior finally cools and tries to shrink, it can no longer pull the already-rigid skin inward, so it drags the surface into permanent compression and settles into balancing tension itself. A typical tempered pane carries roughly 100 to 150 MPa of surface compression, and because a crack cannot grow until an applied load overcomes that compression, the glass tests four to five times stronger than the same annealed sheet.

Tempering also changes how glass fails, which is why it is called safety glass. All that stored elastic energy means that once a crack does penetrate the compression layer, the pane shatters instantly into a shower of small, blunt-edged cubes rather than long dagger-like shards, far safer for car side windows, shower doors, and glass tables. The catch is that everything must be done before tempering: you cannot cut, drill, or grind a tempered sheet, because breaching the compression skin releases the stored energy and the whole thing dices. A famous nuisance case is nickel-sulfide inclusions, tiny flaws deep in the tension zone that can trigger spontaneous shattering years later.