When glass is heat treated (tempered) the possibility of nickel sulfide inclusions causing spontaneous breakage exists. Nickel sulfide inclusions which exist in heat treated glass undergo a phase change as a function of time and temperature. If the inclusion growth reaches the central tension core of the glass, the expansion of these inclusions may provide the proper environment (stresses in central core) to cause spontaneous breakage. Heatsoaking accelerates the growth of these inclusions if they exist, and provides greater assurance against nickel sulfide breakage.

Heatsoak testing consists of a three stage process that involves (1) a heating up, (2) a period of soak in which the glass is exposed to temperatures of 260˚ C and (3) a cool down phase. This is in accordance to the European Standard EN:14179-1:2005 which requires that the glass be taken to temperature for 2 hours with a +/- 10° C variance allowed. Thermocouples and process tracking verify the glass has remained at temperature for the required amount of time.