Nickel sulphide and heat soaking in toughened glass

Very rarely, a pane of toughened glass can break with no impact, sometimes months or years after it was fitted. The usual cause is a tiny nickel sulphide stone trapped in the glass when it was made. Heat soak testing is an extra step that breaks most affected panes in the factory instead of on site. It is not an option in our online configurator.

What is a nickel sulphide inclusion?

Float glass is made from raw materials melted at very high temperatures. Occasionally a microscopic particle of nickel sulphide survives the melt and stays in the glass. It is far too small to see.

In ordinary annealed glass it does no harm. Toughening changes that. The glass is heated and cooled fast, which locks the surface in compression and the centre in tension. If a particle sits in the tension zone, it can slowly grow in volume as it changes form at room temperature. When it grows enough, the pane can shatter.

What does a spontaneous break look like?

Toughened glass breaks into small, blunt pieces across the whole pane. In a break caused by an inclusion, the cracks often radiate from one point, sometimes with two small fragments shaped like a butterfly at the centre. That point is where the particle was.

A break like this is not caused by fitting or use, and it is very rare. A chip in an edge, a knock from a hard object, or a hinge or clamp pressing on bare glass are far more common causes of a toughened pane breaking.

What is heat soak testing?

After toughening, the panes go into an oven and are held at a raised temperature for a set time. That speeds up the change in any nickel sulphide particle, so a pane carrying one is far more likely to break in the oven than in your building. The test reduces the risk; it does not remove it completely.

Heat soaked toughened glass is usually asked for on facades, overhead glazing and other places where a breaking pane would be hard to replace or dangerous. Where the glass must stay in place after breaking, laminated glass is the answer, not heat soaking alone.

What if my specification asks for heat soaked glass?

1. 1

   Step 1: Check the specification

   Look at your architect's or engineer's glazing specification. Heat soaking is often written into it for commercial and overhead work, sometimes with a requirement for evidence of the test.

2. 2

   Step 2: Do not order it as a standard product

   Heat soaking is not an option in the online configurator, so a pane ordered online is not heat soaked.

3. 3

   Step 3: Talk to us before you order

   Contact us with the sizes, the thickness, where the glass is going and the wording of the specification, before anything is made.

Does this apply to my shower screen?

Shower screens, bath screens and splashbacks are made from toughened glass, so the same very small risk applies to them as to any toughened glass. Our online products are not heat soaked. If heat soaking matters to you, talk to us before you order rather than after.

What you can control is how the glass is fitted and cared for. Keep hardware gaskets in place, never let metal press on bare glass, and check edges for chips. Our care and cleaning guide covers this, and visual quality and tolerances explains how to record a concern.

What happens if my toughened glass breaks on its own?

Keep everyone clear, do not try to pick up the pieces by hand, and photograph the break pattern before anything is moved. Then tell us with your order reference. Your statutory rights apply to faulty glass, and our returns and damage guide explains how a claim works.

Where to go next

• Ask about your job: contact our team
• Toughened, laminated and annealed glass
• Visual quality and tolerances
• Where the rules ask for safety glass

Questions we get asked