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What are Nickel Sulphide Inclusions in Glass?

Posted on 3rd August 2023

Toughened glass can occasionally break while in service with no obvious reason. This can be caused by a variety of things including damage, impact or inclusions within the glass.

One particular type of inclusion, Nickel Sulphide (NiS), has attracted publicity and it is important that specifiers understand its characteristics and the subsequent risk of glass panel breakage while in service.

So, what exactly are Nickel Sulphide Inclusions?

Natural occurrences of Nickel Sulphide Inclusion can be found in glass.

Although it forms naturally throughout the glass-making process, it only becomes an issue when the glass is toughened later on.

The inclusions are too small to be visually detected by the glass manufacturer, and when heated (during the toughening process), the inclusions undergo a phase change that minimises their size

The glass cools down too quickly after toughening to let the inclusion to return to its original state.  This would typically take place over a period of time.

Why are Nickel Sulphide Inclusions a Concern?

If an inclusion is sufficiently large and is located within the centre of the thickness of the glass, at some stage in the future it can cause spontaneous breakage. However, it’s important to appreciate that the risk is quite minute. Glass manufacturers estimate the incidence at less than 1/13,000kg of glass.

Broken Glass due to Nickel Sulphide Inclusions

Can the risk be mitigated?

Yes. There are a few ways to reduce the risk of Nickel Sulphide (NiS) Inclusions causing spontaneous glass breakage:

  • Heat Soak Testing the glass reduces the likelihood of occurrence. Read our article to learn more about ‘What is Heat Soaked Glass?‘.
  • Using laminated glass reduces the impact because if a glass panel does break, it will stay in position rather than fall.

Concluding Remarks

In summary Nickel Sulphide Inclusions are a natural characteristic of toughened glass. They can cause a panel to break independently at any point in it’s lifecycle – with no impact from any one or thing, or any apparent explanation. 

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