Spontaneous Glass Breakage: Why Tempered Glass Sometimes Breaks on Its Own

You may have heard of tempered glass tables or shower doors suddenly shattering without impact. According to FGIA research, while rare, this phenomenon is real—and understood.

The Nickel Sulfide Problem

According to ASTM and GANA technical publications, most spontaneous breakage is caused by nickel sulfide (NiS) inclusions:

What they are: Microscopic contaminants (from raw materials or equipment) that become trapped in glass during manufacturing

Why they matter: NiS undergoes a slow phase transformation at room temperature, expanding slightly over months or years

What happens: If an inclusion is in the tension zone of tempered glass and expands enough, it triggers failure

How Common Is It?

According to FGIA data:

• Spontaneous breakage is rare—estimated at 1 in 8,000 to 1 in 15,000 panels
• Most breakage attributed to "spontaneous" actually has an identifiable cause
• Risk increases with larger glass panels and higher stress

Other Causes of "Spontaneous" Breakage

According to GANA, many breaks blamed on spontaneous failure actually result from:

Edge damage: Chips or nicks from handling/installation that propagate over time

Frame issues: Improper installation allowing glass to contact frame or setting blocks

Thermal stress: Dark coatings or exterior shading creating uneven heating

Impact damage: Unwitnessed impact that creates delayed failure

Heat Soak Testing

According to ASTM standards, heat soak testing can reduce NiS breakage risk:

Process: Tempered glass is held at ~550°F for several hours

Purpose: Accelerates NiS transformation—causing susceptible panels to break during testing rather than later in service

Limitation: Not 100% effective; adds cost; not commonly specified for residential

Minimizing Risk

According to FGIA guidance:

1. Specify quality manufacturers: Better raw material control
2. Proper installation: Avoid edge damage and frame contact
3. Consider laminated for critical locations: Skylights, glass floors, railings
4. Heat soak for high-risk applications: Where breakage would be dangerous

The Bottom Line

Spontaneous breakage of tempered glass is real but rare. According to GANA, the safety benefit of tempered glass (breaking into small pieces) far outweighs the small risk of unexpected failure. For critical applications like skylights, consider laminated glass which stays intact if broken.

*For complete safety glass information, see: [Safety Glass Requirements](/guides/safety-glass-requirements)*