Nickel Sulfide (NiS) inclusions are present in all flat glass and are out with the responsibility and liability of the architectural glass process. Invisible to the naked eye, Nickel Sulfide is stable at high or low temperatures, however during the heating and quenching process for glass tempering can be “frozen” in an unstable form, and if present between the stressed surface and the glass mid plate, when exposed to increase in temperature such as a sun drenched harsh climate, a return to state in the Nickel Sulfide, can occur, resulting in stress release within the glass and consequential spontaneous glass breakage.

Such a phenomena in tempered glass may occur within hours or years after the glass has been processed.NiS inclusions are minute particles, invisible to the naked eye, which can form if glass is cooled too quickly, after having been heated during the tempering process. The NiS particles do not have time to return to the Low Temperature State and are “frozen” in an unstable form.

The return to that State may occur within minutes or take as long as ten years. If it takes place within the tensile portion of the tempered glass and the particles increase in volume, it causes a fracture, which results in spontaneous breakage.Tempered glass panes, which are Heat Soaked, are not altered in any way: the physical, mechanical and optical properties of the glass do not change. Heat Soak tested panes can, however, be identified by the printed logo “HEAT SOAK TESTED”.

As a responsible leading glass processor, Zamil Glass Industries advocates, advises and recommends the Heat Soak test for all architectural glass products. It warrants all of its own Heat Soak tested glasses against spontaneous breakage, resulting from NiS inclusions for a period of 10 years from the date of manufacture.Heat Soak testing of tempered glass significantly reduces the risk of spontaneous breakage in high-rise buildings, skylights, “point fixing” facades, and frameless glass assemblies.

If spontaneous breakage in untreated tempered glass occurs, it may be not only extremely costly to replace, but could also pose a liability to people and building owners.