Why can tempered glass break into many pieces?

Tempered glass can resist some loads better than comparable ordinary glass yet fragment into many small pieces when it fails. These properties come from stresses deliberately left inside the material during manufacture, not from the glass becoming soft or flexible.

During thermal tempering, the surfaces cool and stiffen before the hotter interior finishes cooling. Later contraction leaves the outer regions compressed and the interior under tension. Compression tends to close a crack, whereas tension tends to open it. The opposing stresses balance within the unloaded pane.

Surface compression makes it harder for many surface flaws to open under an applied load. However, a crack that reaches the tensile interior can grow rapidly. Elastic energy already stored in the stressed glass can drive further growth and branching, dividing the pane into many fragments.

The particular result depends on the stress profile, thickness, defects and circumstances of failure. A drawing of branched cracks is not a prediction of fragment size or proof that pieces are harmless. Ordinary annealed glass has a different residual-stress state, so the same stored-energy explanation does not automatically apply. The schematic separates a protective compressed surface from a tension-driven failure mechanism beneath it.