Nickel Sulphide Inclusion (NSI) in Toughened Glass – Chicago, IL

Understanding spontaneous glass breakage, prevention strategies, and heat soaking standards.
What to look for? Butterfly type glass inclusion in middle of glass pane.
Below some helpful info for architects, contractors & building owners across Chicago and the greater Midwest.

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Key Takeaways About Nickel Sulphide Inclusion

• Nickel sulphide inclusion (NSI) is a microscopic impurity that can cause toughened glass to spontaneously shatter.


• Heat soaking reduces NSI risk from 1 in 10,000 m² to 1 in 1,000,000 m² (Glass & Glazing Federation).


• NSI breakages create distinctive “butterfly-pattern” fractures that radiate from a central microscopic particle.


• No glass manufacturer offers warranties against NSI failures — it is considered an inherent risk.


• Heat soaking at 290°C for 2 hours remains the most effective mitigation method.


• What Is Nickel Sulphide Inclusion?


• Nickel sulphide inclusion refers to microscopic metallic sulphide particles (0.05–0.5mm) unintentionally trapped inside toughened glass during manufacturing. Although invisible to the naked eye, these particles can expand over time and create enough internal stress to shatter an entire panel without warning.
















• NSI forms when nickel contamination from stainless steel equipment reacts with sulphur in furnace fuel or fining agents. During the tempering process, the particle becomes “trapped” in a high-temperature crystal structure (β-phase). Over months or years, it transforms to its low-temperature α-phase, expanding by 2–4% and triggering spontaneous breakage.
  
  
  This phenomenon most commonly affects toughened glass used in:

• Structural glazing


• Curtain wall systems


• Balustrades & railings


• Storefronts & commercial facades


• Exterior cladding and high-rise applications


• Chicago’s temperature swings accelerate this transformation, making NSI awareness essential for local architects and building owners.


• How Nickel Sulphide Forms During Glass Manufacturing


• NSI originates during the float glass production process when raw materials are melted at temperatures above 600°C. Nickel contamination enters the molten glass from:


• Stainless steel rollers and machinery
• Fuel sources containing trace nickel
• Raw material impurities
• The real danger begins during the tempering stage. Rapid cooling from 700°C traps the nickel sulphide particle in the unstable β-phase. Over time, this particle expands, creating internal tensile stress that eventually exceeds the strength of toughened glass.
• Recognizing NSI Breakage Patterns
• NSI breakages are unique and recognizable to trained glass professional's


• Distinct “butterfly” or figure-eight fracture pattern


• Polygonal fragments (pentagon & octagon shapes)


• No external impact point or edge-related origin


• Failure begins from the interior of the glass panel


• Often occurs months or years after installation


• Confirmation requires electron microscope analysis, especially when breakage occurs at height or fragments scatter.
• Heat Soaking: The Best Defense Against NSI


• Heat soaking is a controlled process used to trigger failures in defective toughened glass before installation. Under EN 14179 standards, glass is heated to 290°C for at least 2 hours — accelerating the crystalline transformation that causes spontaneous breakage.​

Effectiveness:

• Reduces NSI failure rate from 1 in 10,000 m² to 1 in 1,000,000 m²


• Eliminates approximately 95% of NSI-susceptible panels


• Widely required for high-risk applications such as overhead glazing
• Glass Types and Their NSI Risk
• While heat soaking increases lead time and material costs (typically 15–25%), it dramatically reduces liability and replacement costs in the long run — especially in commercial buildings in Chicago’s urban environment.


• Annealed GlassNegligibleSlow coolingLow tensile stress
• Heat-Strengthened GlassLowModerate coolingMedium tensile stress

Fully Toughened GlassHighRapid coolingHigh tensile stress

Laminated constructions improve safety because the interlayer retains broken glass, preventing dangerous fallout even if an NSI-related failure occurs.

Warranty & Insurance Implications
No major glass manufacturer provides warranties for nickel sulphide inclusion. It is universally classified as an inherent risk of toughened glass production.

Most commercial insurance policies, including those common in Chicago skyscrapers and retail spaces, cover spontaneous breakage events. However, deductibles and policy definitions vary.

Architects and contractors carry a responsibility to inform clients about NSI risks during specification, especially for overhead or structural glazing installations.

Prevention Strategies & Emerging Technologies

• Heat Soaked Toughened Glass – industry standard for risk reduction


• Laminated glass for overhead and structural elements


• Enhanced raw material testing and furnace monitoring


• Emerging non-destructive detection methods (ultrasound, laser scanning)


• Fuel switching to lower-nickel energy sources


• While promising, non-destructive NSI detection technologies are not yet commercially viable — meaning heat soaking remains the most reliable approach today.


• Real-World Case Studies


• London City Hall

Multiple spontaneous breakages occurred two years post-construction, confirmed through electron microscopy as NSI failures. The timing variability demonstrated the unpredictable nature of crystalline transformation.

Rock Cove House, Scotland
100% of glass was heat soaked. Although the project cost increased by ~20%, the building has had zero NSI failures over five years, validating the investment.

Commercial High-Rise Buildings
Many Chicago office buildings now specify heat soaked glass for curtain wall and structural glazing to minimize insurance claims and liability exposure.




Frequently Asked Questions
Can NSI be detected without heat soaking?
No reliable non-destructive method exists today. Only heat soaking or electron microscope analysis can detect NSI.

What happens if NSI failure occurs in structural glazing?
Immediate structural loss may occur. Emergency stabilization and engineered replacement are required.

Are architects required to disclose NSI risks?
While not legally mandated, industry best practices strongly recommend disclosure during the design and specification phase.

Do temperature changes affect NSI?
Yes — thermal cycling accelerates the crystalline shift that leads to spontaneous failure. Chicago’s climate increases this risk.

What new technologies are being researched?
Ultrasound imaging, advanced laser inspection, improved furnace systems, and modified tempering processes.




Need Help With Glass Safety, Heat Soaking, or NSI Prevention?
Chicago Glass Company of Illinois provides expert consultation, heat-soaked glass supply, and replacement services for commercial and residential buildings throughout Chicagoland call/text (708) 800-7120

Serving Chicago, Melrose Park, Oak Brook, Naperville, Schaumburg, Evanston, Hinsdale, Orland Park & more.

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