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.