Heat Strengthened Glass is produced from annealed glass using lower air quenching pressure. The sudden cooling causes compression on glass surface and the edges and remaining part develops stress zones.
For a heat strengthened 6 mm thick glass minimum surface compression is 4000-7000 psi.
On breaking the Heat Strengthened Glass breaks in to sharp long pieces. It is twice stronger than the annealed glass. However it can not be used where human safety is concerned until combined with the lamination process of the glass.