All float glass contains some level of imperfection. One type of imperfection is nickel sulfide (NiS) inclusion. Most NiS inclusions are stable and cause no problems. There is, however, the potential for NiS inclusions that may cause spontaneous breakage in tempered glass without any load or thermal stress being applied.
Heat soaking is a process that may expose NiS inclusions in tempered glass thereby reducing the risk of potential field cracking. The process involves placing the tempered glass inside a chamber and raising the temperature to approximately 290ºC to accelerate nickel sulfide expansion. This causes glass containing nickel sulfide inclusions to break in the heat soak chamber.