Toughened glass can break when nothing has hit it. The usual cause is a nickel sulphide inclusion, a tiny impurity that was in the glass before it was toughened. It is rare. It is also worth understanding before you specify glass at height or overhead.
At The Glass Shop Online we supply toughened and toughened laminated glass, and we can heat soak where the application justifies it.
What an inclusion is
Nickel sulphide inclusions are metallic impurities trapped in the glass while the raw material is being made, often where stainless steel equipment reacts with sulphur. You cannot see them. Once the glass has been toughened they can grow over time, build internal stress, and push past the strength of the pane. The panel then shatters suddenly, sometimes years after it was installed.
People often assume something must have hit the glass, because nobody was near it. The break pattern is the clue. An inclusion usually leaves a distinct butterfly fracture at the origin, rather than damage starting at an edge or a corner.
Which glass is at risk
The products at risk are toughened glass and toughened laminated glass. The practical problem is greatest where a broken pane is difficult to replace, such as glass at height, overhead glazing, or a screen that is hard to reach. Annealed glass and ordinary laminate do not fail in this way. Where the application allows a specification that is not toughened, that removes the issue altogether.
How to reduce the risk
Heat soak testing holds the pane at a high temperature so that most inclusions fail in the oven, before the glass reaches site. It dramatically reduces the chance of a later break. It is not a guarantee that a pane will never break.
Toughened laminated glass does not stop the break, but the interlayer holds the pieces in place, which matters on a balustrade, a screen or anything overhead. If you are ordering for a location where a replacement would be difficult, tell us and we will talk through heat soak, toughened laminated, or both. Browse toughened glass. Browse toughened laminated glass.
If a pane breaks
Photograph it, make it safe, clear it up and replace it as soon as you can. It is highly unlikely that the other panels on the same project are also affected. This is an inherent risk in toughened glass, even though it rarely happens. If it does, the replacement is chargeable.
Height, overhead glazing and how hard a replacement would be decide whether heat soak or toughened laminated is the sensible step.
Questions you might still have
How common is it?
About 1 in 1,100 tonnes of raw material may contain an inclusion. It is something to plan for on difficult replacements, not a reason to avoid toughened glass on a normal job.
Does heat soak remove the risk?
It reduces the risk sharply. It does not guarantee a pane will never break later.
Is a replacement covered?
No. If this rare break occurs, the glass is replaced on a chargeable basis.