Nam Heng ( NHG ) Tempered glass may break without warning due to the expansion of nickel sulfide inclusions ( NiS ) present within float glass. When glass is heat treated, these nickel sulphide inclusions undergo a phase change as a function of time and temperature. If located near the central tension core of the glass, the expansion of these inclusions may provide sufficient stresses to produce spontaneous breakage. The inclusion expands at a rate greater than the glass and literally causes the glass to break from within. When tempered glass is heat soak tested, the glass is placed in an oven and subjected to temperatures of 550°F±50°F(290°C±10°C).