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Why Tempered Glass Sometimes Breaks on Its Own — and What Window Film Can and Can’t Do

Now and then a tempered glass pane breaks into small pieces with no impact and no obvious cause. Here is what glass makers and practitioners say about why, what heat soak testing can and cannot do, and where window film fits. Film does not prevent the break.

Published October 22, 2026 · Armored Glass Solutions

The Short Answer

Short answer: tempered glass can break without being hit, and film can help hold the pieces, but it cannot remove the cause

Tempered glass is made to be stronger than ordinary glass, and to break into small, roughly cubical pieces when it does let go. Occasionally a pane breaks with no impact and no obvious cause. Glass makers call it “spontaneous breakage.” Their own documents give several explanations, including one that sits inside the glass itself, and they say heat soak testing lowers the odds but does not remove them.

Window film does not change any of that. None of the film maker documents we read says film prevents a break. They describe film as helping to hold the glass together after it breaks, and the ones that address this use ask for the film to be attached to the frame. We install film, so we are telling you that up front. Where we give our own reading, we say so.

Causes

What “spontaneous” breakage usually turns out to be

Vitro, one of the large U.S. glass makers, says glass sometimes breaks “without any obvious cause” and lists what may be behind it: edge or surface damage from handling and glazing that weakens the glass during high winds, building or framing movement, vandalism, or an inclusion inside the glass. In its technical document TD-138 (2022), Vitro says that when tempered glass breaks for no apparent reason, “it is most often determined that the glass broke due to existing surface or edge damage”.

Pilkington’s 2013 technical bulletin adds hard-body impacts, glass-to-glass contact at the edges during installation, weld splatter and deep scratches. It also names slow-building stresses such as building movement that crowds the edge clearance in a frame. Thermal stress is a separate mechanism; Vitro notes that heat-treated glass is more resistant to it than ordinary annealed glass. We cover that side in our post on thermal stress on dual-pane glass.

Nickel sulfide, one cause on the list, gets most of the attention.

Nickel Sulfide

Nickel sulfide: a microscopic stone that can grow

Float glass can contain tiny inclusions. Vitro says there are more than 50 types, and the one most discussed is the nickel sulfide stone. By Vitro's account it can measure from 0.003 to 0.015 of an inch across. Pilkington and AGC (2010) describe the same sequence: nickel contamination forms a stone during melting, and the fast quench used to temper glass traps it in an unstable form. Over time the stone can slowly change and grow. Vitro puts the growth at roughly 2 to 4 percent in volume. If the stone sits in the center tension zone of a tempered pane, that growth can be enough to crack it.

The timing is unpredictable. Pilkington says the change can take months or years, that warmer glass changes faster, and that the break “can occur without any load being applied to the glass, at any time, even five or ten years after the glass has been tempered.” Pilkington also says breakage may not happen at all.

The stones are very hard to find. Vitro says their size rules out the inspection methods used in float glass production, and that there is no known technology to totally eliminate the possibility of them. The Construction Specifier authors say testing for them once the glass is in the building is generally impractical.

Glass Type

Why this is a tempered glass problem, and where heat-strengthened glass stands

Tempering puts the surface in compression and the core in tension, which is why a tempered pane is, per Vitro, four to five times stronger than standard annealed glass. It is also why a stone in that tension zone matters. The Construction Specifier (September 2025) states that this kind of fracture occurs only in fully tempered glass.

Heat-strengthened glass has less built-in tension. Vitro says it is not aware of any occurrence of spontaneous breakage in heat-strengthened glass, and Guardian’s glossary (archived February 2025) says it has a much lower potential incidence than tempered. Pilkington is more careful: it says nickel sulfide breakage seldom occurs in properly heat-strengthened glass, “even though it is possible and has been observed.” Our reading: far less likely, and not reported by some makers, but not a case to rule out.

Heat-strengthened glass is not a drop-in swap, because codes and designs still call for fully tempered glass in many places. Viracon (2024) advises using fully tempered glass only when code or design loads require it.

Diagnosis

The butterfly pattern: how experts spot it, and why they still test it

When a nickel sulfide stone starts a break, the origin often shows two larger, roughly hexagonal or pentagonal fragments that meet where the stone sits. The Construction Specifier says this is often called the “cat-eye,” “butterfly” or “figure eight” pattern.

Sources differ on how much weight the pattern carries. An IIBEC article (Rojas, March 2026) calls the butterfly pattern a “definitive diagnostic indicator.” The Construction Specifier authors say the classic signs are “often insufficient to positively identify a break for a warranty claim,” and recommend that the fragments be tested in a laboratory. Our reading: the pattern is a strong clue, and confirmation comes from a lab. They also say it is crucial to preserve the fracture origin for testing.

Heat Soak Testing

Heat soak testing: it reduces the odds, it does not erase them

Heat soak testing holds finished tempered glass at an elevated temperature for hours so that panes with a harmful stone break in the factory instead of in the building. Vitro calls the objective “break now, not later.” The National Glass Association (technical paper FB56-18, 2018) says a heat soak test “may reduce the risk, but will not eliminate the potential of a spontaneous break due to an inclusion.” It also notes there is no North American standard for the test; some companies follow the European standard EN 14179-1. Temperatures differ by document and edition, so check the specification before relying on a figure.

Others say the same. Vitro says its outcome can only be expressed statistically, and Pilkington notes a slight risk of de-tempering the glass. Guardian says the process “is not 100 percent effective.” The Construction Specifier authors write that when done properly, heat soak testing “significantly reduces, but does not eliminate the risk of NiS-induced fracture in-service.” Viracon recommends it when tempered glass is required, to minimize the risk.

How Often

How often does it happen? The honest answer is that it depends

You will see figures online, and they do not line up: some count stones per ton of glass, some count breaks in factory testing, some count panes in one building. A March 2026 IIBEC article says estimated failure rates “vary widely across different sources and conditions.” Warranty thresholds are commercial terms, and structural design probabilities are different numbers again.

Because the bases differ, we are not giving you a single rate, and you should be wary of anyone who does. Pilkington’s bulletin says good glazing design places tempered glass only “in locations where the unlikely event of spontaneous breakage will not cause significant problems.” The better question is where a particular pane hangs and what is under it.

After the Break

What a broken tempered pane does next

Tempered glass is designed to break into innumerable small, roughly cubical pieces, and that break pattern is what qualifies it as a safety glazing material. Vitro adds a catch: because of that pattern, “tempered glass is much more likely to evacuate the glazing system immediately upon breakage.” Pilkington says clumps of the small particles tend to stay together after breakage unless additional forces are applied, so a broken pane can come down in pieces or in clumps. See is window film safety glass?

Window Film

What window film can and cannot do here

What the film makers say. The film maker documents we read all describe film as holding glass together after a break. Madico says buildings with spontaneous breaks have used safety films for years “to hold a broken sheet of tempered glass in place once NiS failure occurs.” 3M's Safety S80 specification (January 2025) lists spontaneous glass breakage among the applications where its film “helps hold broken glass together.” We did not find a film maker, or any other source, that says film prevents the break, delays it, or changes the stone. That is our reading of the documents, not any one maker's statement.

Edge attachment. This is where the documents agree. 3M's spec says the film “must be installed with the 3M™ Impact Protection Attachment (IPA) Sealant for all break and entry applications and for all spontaneous glass breakage applications on single pane tempered glass.” Madico does not recommend a simple daylight installation, because once the pane breaks, “the entire sheet of filmed glass may potentially fall from the opening.” It recommends an anchoring restraint system. Cardinal IG, an insulating glass maker, warns in its 2022 service bulletin that with tempered glass fractures the glass “could fall out of the opening in one piece adhered to the retrofit film.” Our post on film and earthquakes covers daylight versus attached installs. Film choice and attachment design are decisions for the film maker and a site assessment.

Read the test glass. 3M publishes a Class A (Unlimited) ANSI Z97.1 impact rating for its safety film, but the table in its S140 data sheet lists the test glass as annealed, not tempered. That is a safety-impact result on annealed glass. It is not a test of what happens when a tempered pane breaks on its own, and we did not find an independent test of film on tempered glass for that purpose.

What film does not do. It does not remove the cause, and it does not make tempered glass meet a safety code by itself; Cardinal says any such claim should be reviewed with the film supplier.

Warranty

Who pays when tempered glass breaks on its own?

Nothing here is legal advice. Here is what the documents show. Viracon’s standard warranty (August 2026) lists “glass breakage” among its exclusions and refers heat-soaked tempered glass to a separate warranty, which we did not read. Practitioners writing in The Construction Specifier describe warranties for heat-soaked glass that pay for replacement once breakage passes a threshold, for a set period, and that “often do not cover mitigation or remediation measures,” with adding protective film given as an example.

Film can also affect the glass maker's side of the ledger. Cardinal says it “will not warrant the IG units that have a retrofit film applied.” Our reading: if your recourse for a break depends on a glass maker's warranty, adding film may remove it. Read the actual warranty first; we compared several makers in does window film void your glass warranty?

Before You Spend Money

Questions to ask before you spend money

What glass is it? Annealed, heat-strengthened, tempered or laminated. A permanent mark in the corner usually says.
Was the tempered glass heat soaked, and how old is it? Ask the glazing supplier for what they have on file.
Where does it hang? Overhead, above a walkway, or at a balcony or guardrail is a different risk from a shower door.
What does the glass warranty say about breakage, and about film applied afterward?
If a pane breaks, keep the fragments at the starting point. A lab may need them to identify the cause.
What does the film maker require for attachment on single-pane tempered glass?

If you want a second set of eyes on a specific opening, we will look at it and tell you plainly whether film fits the job.

Questions

Tempered glass breaking on its own: FAQ.

Why did my tempered glass shatter for no reason?

Often there was a cause that was not obvious. Vitro and Pilkington list edge or surface damage, building movement and contact during installation. In some cases a nickel sulfide stone inside the glass is the cause. A laboratory is needed to be sure, and the butterfly fracture pattern is a clue, not proof.

What is nickel sulfide in glass?

A tiny contaminant stone formed during glass manufacturing. Vitro gives its size as roughly 0.003 to 0.015 inch. It can grow a few percent in volume after the glass is tempered, and in tempered glass that growth can crack the pane. Makers say it is too small to find with practical inspection methods.

Does heat soak testing remove the risk of tempered glass breaking?

No. The National Glass Association says a heat soak test may reduce the risk but will not eliminate the potential of a spontaneous break due to an inclusion. Vitro, Pilkington, Guardian and a group of forensic engineers writing in The Construction Specifier describe it the same way: it lowers the odds.

Can window film stop tempered glass from breaking on its own?

No source we read says it can. Film makers describe film as holding the glass together after it breaks. 3M requires its attachment sealant on single-pane tempered glass for this use, Madico recommends anchoring the film to the frame, and Cardinal warns that a filmed tempered pane can fall out in one piece.

If tempered glass breaks by itself, is it covered by warranty?

It depends on the glass maker, whether the glass was heat soaked, and the exact warranty wording. Thresholds, time limits and exclusions vary, and some glass makers will not warrant glass once film is applied. Read the actual warranty before you decide, and ask your glazing supplier or the building owner's counsel about your situation.

Sources

Where these findings came from.

We read these on October 20, 2026. Older documents are labelled by date, because makers’ guidance changes. Where we give our own reading or inference, we say so. Film makers’ and glass makers’ statements are about their own products and documents. This post is general information, not engineering or legal advice; a glazing professional should assess a specific opening.

Vitro Architectural Glass, Nickel Sulfide and Spontaneous Breakage, web article, shown as updated August 21, 2026. Glass maker. Link.
Vitro, Technical Document TD-138, Heat Treated Glass for Architectural Glazing, revision 8, September 7, 2022. Glass maker. Link.
Pilkington North America, Technical Bulletin ATS-165, Spontaneous Breakage of Tempered Glass, January 14, 2013. Glass maker; dated. Link.
AGC, Technical Information 09/2010: Risk of Nickel Sulfide presence in glass. Glass maker; dated. Link.
National Glass Association, Glass Technical Paper FB56-18, Heat Soaking Testing of Tempered Glass for Architectural Glass Applications, 2018 (PDF dated 2021). Trade association. Link.
Viracon, Heat Soak Testing, Tech Talk VTT-010E, 2024. Glass maker. Link.
Viracon, Standard Ten-Year Limited Warranty, August 26, 2026. Glass maker. Link.
Guardian Glass, glossary entry “Heat-Soaking.” Glass maker; read from an Internet Archive copy dated February 15, 2025 because the live page did not load, so it may have changed. Link.
Brandmeier, Jackson and Schwartz, Fully tempered glass and spontaneous fracture, The Construction Specifier, September 24, 2025. Trade magazine; practitioners’ article, not a standard. Pages 2 to 4 cited. Link.
Rojas, Glass Breakage Analysis: The Impact of Nickel Sulfide Inclusions in Building Glazing, Part One, IIBEC, March 27, 2026. Trade journal. Link.
3M, Safety & Security Window Film Safety S80, Technical Specifications, Revision D, January 2025. Film maker. Link.
3M, Safety S140 Technical Data Sheet, Revision C, January 2025. Film maker. Link.
3M, Safety Window Films & Attachment Systems: Spontaneous Glass Breakage Protection, segment card, 2017. Film maker marketing; dated; used only to describe how figures are presented. Link.
Madico, Glazing Protection. Film maker; undated. Link.
Cardinal IG, Technical Service Bulletin IG04, Use of Retrofit Films, August 2022. Insulating glass maker. Link.

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