During the float architectural glass manufacturing process, undissolved particles of the batch material, called inclusions, are formed within the glass. Nickel sulfide (NiS) inclusion is a type of inclusion that has no effect on the performance of annealed glass, and its presence cannot be detected in advance.
When glass is thermally toughened, during the rapid cooling process which the glass undergoes, the nickel sulfide inclusion retains crystal structure. At a future point in time, when the toughened glass is exposed to high temperatures, the nickel sulfide crystals can expand at a rate greater than the architectural glass itself, providing sufficient stresses to produce spontaneous breakage, without warning or any indication of a reason.
Even though the frequency of occurrence of this phenomenon might be relatively low, it is evident that when human life depends on the integrity of the glass pane, it is of outmost importance to reduce the potential for spontaneous breakage to a rate that approaches zero percent (0%). This is accomplished by submitting the toughened glass to a Heat Soak Test. This method of accelerated ageing comprises of a cyclical, controlled heating/cooling phases, with temperatures reaching 290°C.
During the test, any thermally toughened safety glass pane that contains nickel sulfide inclusions will collapse, while contrary to that, all glass panes that retain their integrity cease to pose a risk for spontaneous breakage in the future.
At VASGLASS, the Heat Soak Test for thermally toughened glass is offered certified (CE marked) according to the European Norm EN 14179 (HST), under the brand-name VAS-HST.
Nickel sulfate (Nis) inclusions found in a glass pane, in photographic magnification.