All float glass contains some level of imperfection. One type of imperfection is nickel sulfide (NiS) inclusion. Most NiS inclusions are stable and don’t cause any 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. The process involves placing the tempered glass inside a chamber and raising the temperature to approximately 550°F or 287° to accelerate NiS expansion. This causes glass containing NiS inclusions to break in the heat soak chamber, thus reducing the risk of potential field breakage.

The heat-soaking process is not 100 percent effective, adds cost and carries the risk of reducing the compressive stress in tempered glass.