In certain occasion, tempered glass or its composite could meet the quest of thickness and air-pressure for building facade, but it also brings the threat, spontaneous breakage, when setting-up in high buildings. But, fortunately, heat soak test make us out of the risk with advanced technology.

There are two sakes of spontaneous breakage: one is natural , and another one is made: the natural one is caused of gaseous inclusions and other inclusions, include nickel sulphide inclusion, and the making one is caused of the deficiencies on corner or edge by neglect in process, or the failed setting-up. Strict quality control could solve problems of gaseous inclusions, impurities of large particles, and deficiencies on corner and edge, and the course of setting-up is control-able as well, but nickel sulphide inclusion is invisible and hardly examined, so after all of that, tempering processing and setting, one could only seek his fortune away from the danger of spontaneous breakage.

There are two types of nickel sulphide molecules, α and β, which would change its structures with temperature to cause spontaneous breakage. But why not in lehr? For slowly cooling process provides sufficient time for nickel sulphide inclusion to transform from α to β, however, through tempering process over 380℃, type β turns into unstable type α "frozen" in tempered glass when no time to turn back by rapid cooling, worse, stress balance will be destroyed as type α is turning back to type β by any chance, then spontaneous breakage will occur. Heat soak test prompts unstable type transforming and evoking spontaneous breaking if any possibility. In this case, heat soak test is for selecting. According German DIN 18516, the process of heat soak test is in 2 hours heating slowly close to 290℃ and keeping in 290℃±10℃ through 8 hours and then to room temperature in 2 hours. So that 99 percents above of nickel sulphide would be transformed into stable type or failed glass would break caused of its blemishes.

Plus, there is tempering recession when heated above 300℃, so rising and keeping control of temperature is very important during whole process. Tempered glass could be examined by specified equipments, but none to examine weather heat soak test or not, so it is only guarantee of reliable company