Tempered glass is made from normal annealed glass via a thermal tempering process in which the glass is heated to near about melting point, 750C and air quenched at a particular pressure. The sudden temperature difference puts the glass into a stage of compression with the centre core in tension. Tempered glass gains its added strength from these compressed surfaces. The molecule concentration towards the centre increases thereby increasing the impact strength by 5 times. A fully tempered glass is 4 to 5 times stronger than an annealed glass of similar thickness. Beyond a certain impact the glass disintegrates into small blunt pieces which greatly reduce the chances of injuries. The number of fragments per square inch is specified in the standards. Greater the number better the toughening. Normally the process of toughening induces a certain amount of surface distortion in the glass. Today's technology provides for radiation and convection heating in the furnace. This provides homogenous heating and reduces the surface undulation viewed by the human eye. Due to the internal stresses toughened glass cannot be cut to size after the toughening process, hence all glass fabrication like cutting, drilling, and slot cutting, etching polishing and edge preparation should be performed before the tempering process. The clarity, physical properties like heat transmission, solar radiant heat properties etc. of the annealed glass is not altered while tempering.