Spontaneous breakage of toughened glass in facades is under control when the Heat Soak Test following EN 14179-1 is rigorously carried out. However, NiS breakages are not only observed in toughened glass but also in heat strengthened glass.
Heat strengthened glass has a lower central tensile zone compared to toughened glass. Consequently, bigger NiS inclusions are required to initiate a crack, as crack initiation is determined by the inclusion’s size in combination with the surrounding stress field.
The critical diameter to cause spontaneous fracture is determined. A good correlation is found with the diameter of reported NiS inclusions causing breakages on a building façade. Knowing the distribution and the diameter range of NiS inclusions in float glass a breakage probability of 1 breakage in 1,100 ± 200 tonnes can be estimated.
More data of broken heat strengthened glass, due to NiS inclusions, is required to confirm the breakage probability, which in reality might be much lower. Due to this low breakage probability it is not industry practice to carry out the HST. Moreover, heat strengthened glass does not satisfy code requirements for safety glazing. Therefore, heat strengthened glass is most often used in combination with laminated safety glass. If a NiS inclusion would cause a breakage of a laminated heat strengthened glass, it would stay in place due to its specific fragmentation as required by EN 1863.