My window just shattered in the middle of the day while I was reading. Is that normal? Authored Brian Lowe of Chicago Glass Company of Illinois

“My Window Just Shattered in the Middle of the Day While I Was Reading — Is That Normal?!”

If your glass just exploded with no impact, weather event, or obvious cause… you're not alone — and yes, this happens more often than most people realize.

Chicago, IL • Toughened Glass • Nickel Sulphide Inclusion (NSI) • Heat Soak Prevention

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Chicago Glass Company of Illinois is Chicago’s go-to contractor for handling these complex glass failures — managing everything from precision ordering and heat-soaked replacements to seamless scheduling and expert installation.

Why Does Glass Shatter for “No Reason”?

When a window, door lite, or glass façade suddenly explodes in the middle of the day — no wind, no impact, no stress — the culprit is often a microscopic impurity called nickel sulphide inclusion (NSI).

These hidden particles expand over time and can cause spontaneous breakage months or even years after installation. It's one of the most misunderstood — yet common — glass failures in modern buildings.

Key Takeaways:

• NSI is a microscopic metallic impurity that causes random, explosive shattering in toughened glass.


• Heat soaking reduces NSI breakage risk from 1 in 10,000 m² to 1 in 1,000,000 m².


• NSI failures produce a distinctive “butterfly” fracture pattern.


• No manufacturer warranties cover NSI — it's considered an inherent risk.


• Heat soaking at 290°C for 2 hours is the best way to screen out risky panels.

What Exactly Is Nickel Sulphide Inclusion?

Nickel sulphide inclusion (NSI) refers to microscopic metallic sulphide particles — 0.05–0.5mm — trapped inside toughened glass during manufacturing. You can't see them, you can't detect them on-site, and they don’t always cause immediate failure.

Over time, these particles slowly change shape and expand 2–4%, creating immense internal pressure until the panel violently shatters.

The sudden “bang” or “pop” people describe is the exact moment the internal stress exceeds the glass’s tensile strength.

Why Does This Happen in Chicago So Often?

Chicago’s weather is a perfect storm for NSI transformation:

• Extreme temperature swings


• Midday sun heating glass quickly


• Thermal cycling across seasons

These factors speed up the crystalline phase change that triggers spontaneous breakage — especially in:

• Storefronts


• Balconies


• Curtain wall systems


• Interior doors


• Railings & balustrades

How to Recognize NSI Breakage

Nickel sulphide inclusion failures have a signature look:

• A “butterfly” or figure-eight fracture pattern


• Origination from the middle of the glass, not the edges


• Shattering into thousands of dice-sized pieces


• No signs of impact or chips

Definitive confirmation requires electron microscope analysis of the fracture origin — commonly used in forensic glass investigations.

Heat Soaking: The #1 Way to Prevent Future Breakage

Heat soaking is a controlled process where toughened glass is brought to 290°C for two hours. This forces any unstable nickel sulphide inclusions to expand and break BEFORE the glass is installed.

Effectiveness:

• 95% reduction in NSI-related failures


• Makes spontaneous breakage extremely rare


• Required for safety-critical applications

While heat soaking adds time and cost, it eliminates most defective panels and prevents dangerous failures in buildings throughout Chicago.

Which Types of Glass Are Most at Risk?

Glass Type	Risk Level	Cooling Method	Internal Stress
Annealed Glass	Very Low	Slow cooling	Low internal stress
Heat-Strengthened Glass	Low	Moderate cooling	Medium stress
Fully Toughened Glass	High	Rapid cooling	High stress

Laminated glass remains intact even when one layer breaks, making it ideal for balconies, overhead glazing, and high-rise facades.

Insurance, Liability & Replacement

Because NSI is considered a natural phenomenon, no manufacturer warranties cover it. However, most commercial and residential insurance policies DO cover spontaneous glass breakage.

Building owners in Chicago should:

• Review policy language for “spontaneous breakage” coverage


• Document fracturing patterns


• Collect fragments if possible


• Request emergency replacement

Frequently Asked Questions

Is it normal for windows to explode randomly?

Yes — in toughened glass, NSI makes this a known phenomenon.

Is my home or building unsafe?

Not necessarily, but safety-critical areas should be inspected.

How do I prevent future breakage?

Specify heat soaked toughened glass or laminated glass.

Should I call a professional?

Yes — especially for curtain walls, storefronts, or high-rise glass.

Need Emergency Replacement or NSI Evaluation?

Chicago Glass Company of Illinois is the go-to contractor in Chicago for these complex jobs — handling everything from engineering-grade ordering to fast scheduling and flawless installation.

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

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Author: Brian Lowe | Published October 10, 2025