NiS (nickel sulfide) crystals contained in glass will undergo a phase change at 379℃, converting α-NiS crystalline form at high temperature into the β-NiS form at low temperature. The phase transition causes an increase in volume by 2%~4%, resulting in rough surface. If these impurities are located in the tensile stress zone, the increase in volume may cause spontaneous breakage of tempered glass.
In order to eliminate the adverse effect of nickel sulfide on tempered glass, the glass processing enterprises often use homogenizing furnaces to homogenize tempered glass after the completion of glass toughening process in the tempering furnace. The homogenizing furnace will heat the tempered glass to the phase change temperature of nickel sulfide and hold the appropriate temperature to promote the rapid phase change of NiS. By triggering the explosion of tempered glass that has the potential hazard of spontaneous breakage in advance, the process will avoid the spontaneous breakage of tempered glass caused by NiS and improve the production yield of tempered glass.