Heat soaked glass
Heat soaked glass

All float glass contains some level of imperfection. One type of imperfection is nickel sulfide  inclusion. Most  inclusions are stable and cause no problems. There is, however, the potential for 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 inclusions in tempered glass. 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, thus reducing the risk of potential field breakage.