1. Glass contains crystalline Nickel Sulphide (NiS). This NiS undergoes a crystalline phase change at 379℃. When it changes from the crystal structure of the α-NiS in high-temperature state to the structure of β-NiS in low-temperature state, its volume will expand by 2%~4%, creating a rough surface. With these impurities stay in the tensile stress area of tempered glass, the volume expansion may cause spontaneous explosion of the tempered glass.

2. In order to reduce the risk of spontaneous explosion of the tempered glass caused by the nickel sulfide in the glass, it is important to have the glass homogenized using a homogenizing furnace after the tempering process. The homogenizing furnace will heat the tempered glass to the phase change temperature of nickel sulfide and hold the temperature to expedite the phase change of nickel sulfide. The process will cause the explosion of the tempered glass in advance, avoid the potential spontaneous explosion in the future, and thus improve the production yield of tempered glass.