What is Nickel Sulphide Inclusion?

Nickel sulphide inclusion (NSI) is a microscopic particle of nickel sulphide that becomes trapped inside toughened safety glass during manufacture. During tempering this particle is frozen in a high-temperature crystalline state; over months or years it slowly reverts to its low-temperature form and expands, building local stress inside the glass. When the inclusion sits in the tensile core of the pane, that stress can trigger sudden spontaneous breakage, often producing the characteristic “butterfly” fracture pattern.

Nickel sulphide inclusion at a glance

Cause: Microscopic nickel sulphide particles can remain within toughened glass after manufacture.
Effect: As it slowly changes state and expands, it can create stress that leads to spontaneous breakage, sometimes years after installation.
Main mitigation: Specify heat-soaked toughened glass to EN 14179 in critical areas and ensure insurance covers spontaneous glass breakage.

How Does Nickel Sulphide Inclusion Occur?

Nickel sulphide particles have two temperature-dependent stable forms:

High-temperature phase – present during the glass manufacturing and toughening process.
Low-temperature phase – the form the particle naturally adopts at normal ambient temperatures.

During toughening the glass is heated and then rapidly cooled, so the nickel sulphide particle is locked in its high-temperature phase and does not have time to change. Over months or years in service it slowly transforms into the low-temperature phase. That phase change is accompanied by a small increase in volume, which builds local tensile stress inside the glass. If the inclusion sits in the tensile (central) zone of the pane, this stress can trigger spontaneous glass breakage.

How to Identify Nickel Sulphide Inclusion Breakage

One of the most common indicators of NSI-induced glass breakage is the Butterfly Effect pattern. This distinctive breakage pattern features a singular fracture point with cracks fanning out symmetrically, resembling butterfly wings. However, confirming an NSI-related failure requires professional glass analysis.

How to Reduce the Risk of Nickel Sulphide Inclusion Breakages

Because nickel sulphide inclusions cannot be completely prevented or detected in intact glass, risk management focuses on reducing the chance of an affected pane being built into the project. The most effective mitigation is to specify heat-soaked toughened safety glass to EN 14179 for critical or hard-to-replace areas.

Heat Soaking Process

Heat soaking is a secondary process applied after toughening that significantly reduces the risk of NSI-related spontaneous breakage.

What is Heat Soaking?

Heat Soaking (EN 14179) is a controlled thermal treatment process that exposes toughened glass to temperatures of 290°C (+/- 10°C) inside a heat soaking oven. This accelerates the transformation of Nickel Sulphide Inclusions, causing panels with impurities to shatter in a controlled environment rather than after installation.

Effectiveness of Heat Soaking

While no process can completely eliminate the risk, Heat Soaking reduces the likelihood of NSI breakages to 1m² in every 1 million m² of glass, compared to the standard risk of 1m² in every 10,000m² of glass (as estimated by the Glass and Glazing Federation, GGF).

Considerations for Heat Soaking

Heat soaking increases costs and extends lead times.
It is recommended for critical glass installations, such as inaccessible panels or glazing elements that would be costly or difficult to replace.
Architects can specify heat-soaked glass for construction projects where spontaneous breakage poses a concern.

Nickel sulphide inclusion - Belsize Lane - IQ Glass

Warranties & Insurance Coverage for Nickel Sulphide Breakages

Despite the advancements in glass manufacturing, Nickel Sulphide Inclusion remains an unpredictable phenomenon. Glass manufacturers and suppliers do not offer warranties against NSI-related breakages.

What Can You Do?

Check your insurance policy: Many home insurance policies cover spontaneous glass breakage. Confirm with your provider to ensure coverage.
Specify heat-soaked glass: This minimises risk, particularly for projects where glass replacement would be challenging.

Guardian Glass UK Statement on Nickel Sulphide Inclusion

Guardian Industries UK Ltd acknowledges the difficulty of completely eliminating nickel contamination, which can sometimes lead to localized NSI outbreaks. Due to the nature of the phenomenon and the absence of reliable detection methods, Guardian does not provide a warranty against NSI-related breakage.

crane lift for curved glass

Final Recommendations

To minimise the risk of Nickel Sulphide Inclusion breakages:

Always consider Heat Soaking, especially for high-risk applications.
Consult with IQ Glass before specifying materials for your project.
Ensure insurance coverage includes protection against spontaneous breakage.

For further research, you can explore more about the scientific studies on Nickel Sulphide Inclusion and its impact on glass performance.

Need More Information?

If you're working on a project and need expert advice on specifying heat-soaked glass, feel free to reach out to a member of our team. Ensuring your glass is as safe and reliable as possible is essential for both aesthetics and structural integrity.

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See our technical videos for more...

Rock Cove: A Heat Soak Testing Success Story

This stunning project, a cantilevered house in Scotland, is adorned with masses of frameless structural glass. Because of the concerns laid out in this article, each panel underwent heat soak testing to eliminate risks of spontaneous breakage.

You can get the full case study here. Rock Cove coastal home with frameless structural glass panels that were heat soak tested to reduce the risk of nickel sulphide inclusion breakage.

Nickel Sulphide Inclusion FAQs

Is nickel sulphide inclusion dangerous?

When toughened safety glass breaks it fragments into small cubes, but falling glass from height can still cause injury and usually requires urgent replacement. The main concern with nickel sulphide inclusion is the unexpected nature of the breakage and the disruption and cost of changing the glass.

Can nickel sulphide inclusion be completely prevented?

No. Modern glass manufacturing and quality control greatly reduce the likelihood of inclusions, but it is not currently possible to guarantee that glass will be completely free of nickel sulphide. The realistic approach is to reduce the risk with heat-soaked toughened glass in critical areas and to design the glazing so that any breakage is safely contained.

Is nickel sulphide inclusion covered by insurance?

Many building and home insurance policies include cover for “spontaneous glass breakage”, but the exact wording and limits vary between providers. It is always worth checking with your insurer that your policy covers toughened glass in windows, rooflights and structural glazing, particularly where access or replacement would be difficult.

Nickel Sulphide Inclusion