Why Tempered Glass Self-Explodes and How to Choose Glass

2024 · HIHAUS

Spontaneous breakage of tempered glass is a known phenomenon, not a defect in a particular batch. It happens because tempering puts the glass surface into high compression and the core into tension, which is what gives it strength. Anything that disturbs that balance releases the stored energy at once, and the pane shatters into the small granular pieces the tempering process was designed to produce.

Cause one: thermal stress

A glass surface exposed to direct summer sun while the interior is air conditioned can see a large temperature differential across the pane. The shaded portion and the sunlit portion expand differently, and the resulting stress can exceed what the edge can carry. The risk rises with dark tints, with heavy shadow patterns from adjacent buildings or deep reveals, with internal blinds close to the glass, and with poor edge quality. Thermal breakage usually starts at an edge, which is why edge working matters.

Cause two: nickel sulfide inclusions

Float glass can contain microscopic nickel sulfide particles. When the glass is heated to tempering temperature and rapidly cooled, the inclusion can be trapped in a phase that is unstable at room temperature. Over months or years it slowly transforms, expands, and generates a local stress that eventually cracks the pane. This is the classic self-explosion mechanism, and it is not caused by installation, wind or thermal cycling.

The industry response is heat-soak testing. Panes are held at an elevated temperature, commonly around 290 degrees Celsius, for a defined period so that any unstable inclusions transform in the factory rather than in service. It does not eliminate the risk entirely, but it removes the large majority of it. We heat-soak the glass we supply for high-rise and curtain wall work and can provide the records with the shipment.

How to specify glass so this matters less

• Require certified tempered glass from a traceable source, with the heat-soak records if the application is overhead or high-rise.
• Specify edge working. Polished or ground edges resist thermal breakage far better than a cut edge, and the cost is small relative to the pane.
• Avoid dark tint plus heavy shadowing plus interior blinds on the same elevation. That combination is the highest thermal-stress profile there is.
• In noisy locations, use an insulating unit with a laminated inner pane. The laminate holds the fragments if the tempered pane fails, which converts a falling hazard into a maintenance item.
• On upper floors, design for wind pressure rather than reusing a lower-floor glass thickness. Deflection and edge stress both rise with height.

What to do when a pane fails

Photograph the break pattern before clearing it. A break that radiates from a single point with a butterfly-shaped pair of fragments near the origin is characteristic of an inclusion; a break originating at the edge with no point of origin suggests edge damage or thermal stress. That distinction determines whether the fix is a replacement pane or a change in the assembly.

If you are specifying glass for a tall building or an overhead application, send the elevations and the wind load. We will return the glass build-up, the heat-soak requirement and the edge specification for each area.