1. If there is NiS in tempered glass, there is a risk that the glass will spontaneously break. In order to eliminate the adverse effects of nickel sulfide in tempered glass, heat soaking furnace should be used to homogenize the tempered glass after the tempering is completed in the glass tempering furnace. 

2. The heat soak furnace will heat the tempered glass to the phase change temperature of nickel sulfide to induce a rapid phase change of nickel sulfide. The volume of nickel sulfide will expand by 2% to 4%. 

3. If the location of nickel sulfide is in the tempered glass tensile stress area, the volume expansion will cause the glass with Nickel Sulphide inclusions to break and the potential spontaneous breakage in the future can be avoided.