When glass needed to be more resistant to heat and mechanical forces it is submitted to a process called tempering. Basically, tempering is heating the glass evenly to a certain point and cooling it down rapidly in specially built furnaces.  The glass is heated up around 700°C homogenously and cooled down . During cooling down the outer surfaces of the glass cools quicker than the inner core of the glass hence outside surfaces will be hardened and inner core adopts  itself to  surface and shrinks. On the ground there will be compressional  forces whereas inside core is tensioned.  Therefore, adversity which makes tempered glass to resist heat shocks untill 300°C that annealed glass can break at 30 to 50°C.