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Why does toughened glass shatter and how can I prevent it?

Monday 29th May, 2017

heat_soak_test

The development of toughened glass has to be one of the most significant architectural advancements of the modern age.

Used in a vast array of applications, from windows and doors, to furniture, flooring and cookware, in many ways toughened glass has helped revolutionise the way we build and the way we live.

Toughened, or tempered, glass is up to five times stronger than regular plate glass, can withstand surface compression of more than 10,000psi and is highly resistant to thermal breakage.

But, of course, it isn’t indestructible. And when toughened glass breaks, it does it in style, shattering in its entirety into small pebbles. In many respects these pebbles are far safer than the razor-sharp shards of regular annealed glass, which is why toughened glass is classed as a safety glass and is specified in so many areas where safety is a concern.

However, in certain circumstances the small broken chips can still pose a danger. For example, a glass balustrade, spandrel or overhead glazing that shatters and falls from its frame could cause significant injury if anyone happened to be unfortunate enough to be standing underneath it.

Even without the issue of safety, replacing toughened glass in inaccessible areas can be a major (not to mention expensive) inconvenience.

What causes toughened glass to shatter?

From time to time, toughened glass has been known to shatter, seemingly without reason.

This is known as spontaneous glass breakage and is, in actual fact, generally triggered by one of four factors:

  • Microscopic internal defects within the glass, such as nickel sulphide inclusions

  • Minor damage during installation such as nicked or chipped edges that later develop into larger breaks.

  • Overly tight binding of the glass in the frame, which causes stresses to develop as the glass expands and contracts due to thermal changes or deflects due to wind

  • Inadequate glass thickness to resist wind load

While factors such as installation damage and incorrect fitting can be avoided to some degree by experienced glass technicians, avoiding glass that contains nickel sulphide is somewhat more difficult.

These inclusions are invisible to the naked eye and are almost impossible to completely avoid in the plate glass manufacturing process.

During the tempering process, plate glass is heated up to its softening point, then rapidly cooled. The surface cools quicker than the internal glass, which creates the centre tension that gives toughened glass its increased strength.

However, any nickel sulphide inclusions that are present are frozen in an unstable state by the rapid cooling. Over time these inclusions will revert to their stable, expanded state. It is this expansion that can create a weak spot in the toughened glass and which can, sometimes at a much later date, cause spontaneous glass breakage.

How can spontaneous glass breakage be avoided?

There is no way to identify 100% of nickel sulphide-contaminated toughened glass. However, the process of heat soaking glass has been shown to weed out up to 95% of affected panes.

During the heat soak test, panes of toughened glass are heated to 290°C over a number of hours. This heat treatment will accelerate the expansion of any particles of nickel sulphide which, depending on where they are in the glass, will cause the pane to shatter.

The heat soak test is therefore a destructive process, that aims to eliminate any contaminated panes before they make it onto site.

Tufwell Glass has its own heat soak facility and can advise when and where this optional process should be considered.

For further information or to request a quote, get in touch with Tufwell today. 

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