All float glass contains some level of imperfection, that’s why glass processors carry out additional measures to improve their strength characteristics. 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. In most cases, the glass can be destroyed due to thermal stress, edge quality, impact and wind loads. Despite the fact that destruction due to NiS inclusions is rare, the situation is complicated by the fact that it can occur at any time regardless of externals.

What is a nickel sulfide (NiS) inclusion? A nickel sulfide (NiS) inclusion is a rare imperfection that can occur if nickel-rich elements are present in the glass-manufacturing process. These elements, such as stainless steel, can combine with sulphur to form nickel sulfide inclusions.

When glass is heat-treated to produce fully tempered glass, nickel sulfide inclusions change size from what is known as a low-temperature (LT) structure to a high-temperature (HT). When cooled quickly, the NiS particle is unable to change completely back to its original form (LT). Over time, NiS may slowly convert to its (LT) form but with an increase in volume of about 4%. That increase in size can cause breakage.

Heat soaking test is a process utilized to expose NiS inclusions in tempered glass before the glass reaches the field. It involves placing tempered glass inside a chamber and raising the temperature to approximately 300°C to accelerate nickel sulfide expansion. This is intended to cause glass containing nickel sulfide inclusions to break in the heat soak chamber, thus reducing the risk of potential breakage in the field.

Design professionals can also reduce the risk of breakage due to inclusions by specifying heat-strengthened glass or laminated glass. We can assist you in determining which option is right for you.