Glazing Vision discusses heat soaking and how it can reduce the risk of spontaneous glass breakage in toughened glass.
Spontaneous glass breakage is known to occur in toughened glass, due to the stabilising of nickel sulphide (NiS) present in the material. The risk of breakage is relatively low but very unpredictable.
This can be helped by heat soaking the glass, which filters out about 95% of potential problem units. Toughened glass is heated to 290°C and held at that temperature, accelerating the process of any nickel sulphide inclusions reverting to their ‘beta state’ and causing failure.
Heat soaking does add cost, but it significantly improves product quality and consistency, reducing the use of potentially faulty glass in the manufacture of rooflight products.
Glazing Vision heat soak tests all of the toughened glass it produces as standard, to make sure customers get a reliable and trustworthy solution for their project.
Heat soak testing is not a guarantee that glass will remain failure-free. The reality is that whilst a Nickel Sulphide inclusion is a possible cause, it is the most unlikely one. In particular, if the glass has been heat soak tested.
There is a problem in the industry with any unexplained glass breakage being too readily blamed on Nickel Sulphide inclusions. Nickel Sulphide has had a bad press over the years and it is very easy to blame this as the cause of an otherwise undiagnosed glass failure.
The risk of a spontaneous breakage in thermally toughened glass that has been heat soak tested in accordance with BS EN 14179-1 is 1 in 400 tonnes of glass. Based on a typical size pane of glass in a double-glazed unit this equates to a risk of 0.015% or approximately 1 in 7,000 sealed units or 1 in 14,000 panes of glass.
Other much more likely causes of glass failure are impact, other inclusions such as particles of refractory brick, undissolved silica or the chemical element silicone, surface scratches or damage from such as weld spatter or grinding dust. The surface damage can occur at any time during transport, site handling, storage or after installation.
Only Nickel Sulphide inclusions can cause spontaneous failure without any other influence. When the inclusion is not Nickel Sulphide or there has been surface damage to the glass, the toughened glass is weakened and therefore may fail when additional factors come into play to overstress the glass. This could be because the pane is oversize for the thickness of glass supplied or it has been overloaded by wind, snow or maintenance loads, over and above the design loads or it could be additional stress induced in the glass through having insufficient support by the framing system or structure or through poor installation or handling.
All toughened glass, when broken, will display a typical ‘butterfly’ pattern at the source of the break regardless of the cause, including impact. So, looking for the butterfly pattern in the broken glass is useful to identify the location of the cause, it does not tell us the root cause. The only way to do that is to preserve the pieces of glass around this area and have them analysed under an electron microscope where surface damage or inclusions will be able to be identified.
Glazing Vision Ltd,
01379 658 300
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