FAQ – What is Nickel Sulphide Inclusion and how does it cause a spontaneous breakage?

Nickel Sulphide Inclusion (NiS) occurs during the process of manufacturing toughened glass. As a result of nickel contaminants present in the glass melt, reacting with sulphur.

Visually you will not be able to see these tiny particles, but they may still be contained within the glass, after the glass has completed the manufacture and toughing process. The particles remain benign until they are aggravated.

If NiS is present in the glass, there is a risk that the glass will spontaneously break, if the particle changes size or shape.

NiS has two main states; stable at high temperatures and stable at lower temperatures. When the glass is cooled quickly, the NiS particle is unable to change back to its original form. Over an undefined period of time NiS will slowly convert to the original low temperature structure. This means the NiS increases in size. As a result the stress can cause the pane of glass to shatter for no apparent reason. The period of time this takes is unpredictable. It can occur within weeks of manufacture, or years, or not at all.

Over time steps have been taken to eliminate NiS from the production of toughened glass. With changes in raw materials, manufacturing methods and processing techniques. However it is not possible to eliminate it completely. Meaning all glass comes with the very rare risk of spontaneous breakage due to NiS.

How can I tell if a glass breakage is due to Nickel Sulphide Inclusion?

The only way to be certain that the spontaneous breakage is as a result of NiS, is to analyse the glass around the point of the break, under an electron microscope, where surface damage or inclusions can be identified.

The following characteristics also point towards a NiS fracture:

  • The implosion will centre around a singular point on the glass. With two distinctive glass fragments at the fracture point – roughly hexagonal or pentagonal in shape, and are generally larger than the other fragments. This is often referred to as the “cat-eye,” “butterfly,” or “figure eight” pattern.
  • The NiS inclusion is visible at the mid-plane of the fractured interface, between the two cat-eye fragments. The inclusion, or “stone,” is spherical and approximately 0.1 to 0.2 mm in diameter.
  • No apparent sign of localized crushing of the glass surface at the fracture origin (surface crushing indicates potential impact).

What is heat soaking and how can it help?

To reduce the risk of a spontaneous break as a result of NiS, it is possible to put the glass through a finishing process called ‘heat soaking’. The heat soaking process thermally treats the glass after completion. In large heat soaking ovens, the glass is heated to a very high temperature, encouraging any pane with NiS to shatter. The glass that survives the testing has a lower chance of containing NiS, and therefore reduces the risk of spontaneous breakage as a result. The heat soaking process does increase the cost of the glass, as well as the lead time, but will reduce the risk of spontaneous NiS breakage. Reducing the risk from 1 m2 in every 10,000 m2 of glass, to 1 m2 in every 1 million m2 of glass (estimated by the Glass and Glazing Federation).

Any glass unit can be specified as heat soaked glass. If you are worried about NiS, we would recommend ensuring that heat soaked glass panels have been specified.


Due to the impossibility of finding such very small inclusions, all toughened glass manufacturers can’t guarantee the complete absence of NiS inclusions in the glass. As a result, they cannot provide a warranty against the possibility of NiS being present. Consequently Aspect Windows are unable to pass on any warranty to our customers.

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