Heat Soaked Glass
Reduce the risk from NiS inclusions
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 but there is the potential that NiS inclusions could cause spontaneous breakage in fully tempered glass. When glass breaks, it is typically due to thermal stress, edge quality, impact, wind or mechanical load. While breakage due to NiS is rare, it can happen without the presence of any other factors.
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), crystalline structure. 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 2-4%. That increase in size can cause breakage. Float glass manufacturers, including Guardian, work extremely hard to avoid any nickel-based elements in the batch mix. Instances of NiS inclusions are very unusual.
Heat soaking 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 290°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.
Guardian offers SunGuard coated glass products that can be safely heat soaked. Please note: The heat-soaking process is not 100% effective, adds cost, and carries the risk of reducing the compressive stress in tempered glass.
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.