In case of NiS presence, most NiS are stable and their presence is usually negligibly small. It is however that in certain rare cases, NiS when cooled in manufacturing process down to room temperature, the NiS would undergo a change in crystalline structure such that the volume of NiS is increased.

In case of float glass, as the expansion of NiS takes place when glass is at annealing temperatures, the additional stress due to expansion is eliminated by relaxation of stress in annealing process.

The manufacturing process of tempered glass requires quenching, and the rapid quenching process traps the NiS when its volume is small. Over time, the NiS may expand in size. When the NiS is located in the center of tension area, the expansion of NiS is capable to provide stress that is sufficient to break the tempered glass.

The NiS presence can cause spontaneous breakage in tempered glass even when no load is being applied, even after a long period have passed since the fabrication of the tempered glass. The NiS presence is typically small and are impossible to locate and identify in unbroken glass.

In order to deal with such occurrence, Glass manufacturers implemented batch quality control programs as well as eliminating of nickel containing materials in the manufacturing process. The measures implemented are effective to reduce but not completely eliminate spontaneous breakage caused by nickel sulfide.

It is noted that heat soaking may uncover NiS presence in glass, however, it is not totally effective as it is unable to detect all of NiS presence, and can reduce the strength of tempered glass by around 7%.

Though little, the occurrence of spontaneous breakage of tempered glass cannot be overlooked. For safety reasons, designers and engineers only uses tempered glass in occasions where spontaneous breakage will not cause harm or major problems. In areas where added strength of glass is required and tempered glass is unsuitable to be used, heat-strengthened glass or laminated glass are used instead.