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. 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.
The heat soaking process is not 100 percent effective, adds cost and carries the risk of reducing the compressive stress in tempered glass.
Guardian offers SunGuard® coated glass products that can be safely heat soaked if it is determined the heat soaking process is necessary.
Heat-strengthened glass has a much lower potential incidence of spontaneous breakage than tempered glass. For applications where additional glass strength is required due to thermal stress, and safety glass is not mandated, Guardian recommends heat-strengthened or laminated glass to reduce the potential for spontaneous breakage.
Nickel Sulfide (NiS) inclusions:
While extremely rare, nickel sulfide inclusions are another potential cause of breakage.
When glass is heat-treated to obtain fully tempered glass, nickel sulfide inclusions change size from what is known as a low-temperature (LT) structure to a high-temperature (HT), crystalline structure. When cooled quickly, the NiS particle is unable to change completely back to its original form (LT).
Over a certain period of time, NiS will slowly convert to its (LT) form but with an increase in volume of about 2 - 4 percent. That increase in size can cause breakage. Float glass manufacturers work extremely hard to avoid any nickel-based contaminants in the batch mix. Instances of NiS inclusions are very unusual.
Design professionals can reduce the risk of breakage due to inclusions by specifying heat-strengthened glass, heat-soaking for fully tempered glass , or laminated glass.