Home Features Technical Spontaneous breakages in toughened glass

Everybody knows that glass can break, but what about when there is no obvious explanation? Joe Cilia, technical manager at FIS, considers growing incidents of glass spontaneously breaking.

The Building Regulations cover the use of glass in the 2013 edition of ‘Approved Document (AD) K, Protection from falling, collision and impact’ and ‘AD K4, Protection against impact with glazing’. They state: “Glazing, with which people are likely to come into contact whilst moving in or about a building shall, (a) if broken on impact break in a way which is unlikely to cause injury; or (b) resist impact without breaking.”

Safety glass in critical areas needs to comply with at least the minimum ‘BS EN 12600:2002 Glass in building. Pendulum test. Impact test method and classification for flat glass’ as defined by the relevant Regulations and/or Standards. This can be achieved in the case of glazed partitions by using laminated glass or toughened safety glass.

Spontaneous breakages of toughened glass can occur for a variety of reasons:

  • Impact from an object: often sharp, but the edge of a chair is not unusual
  • Glass in contact with an exposed fixing or metal glazing bead: this could be a structural bolt or even direct glazing without a gasket
  • Damaged or shelled edge: often caused after toughening either in the factory or in transport, on-site storage and handling or installation
  • Misaligned fixings which cause the glass to twist
  • Incorrect processing of the glass
  • Nickel sulfide (NiS) and other inclusions or contaminates

Breakages caused by NiS inclusions are sometimes hard to differentiate from breakages caused by other factors because the glass shatters and often falls into a pile of small cube shaped pieces on the floor.

Occasionally it is possible to identify the cause as NiS inclusions because although the inclusion might be microscopic, it can occasionally be seen as a small ‘dot’ or ‘dark stain’ situated at the centre of a characteristic ‘figure of eight’ or ‘butterfly’ pattern at the origin of the break.

These breakages can occur months and even years after the glass has been installed. However, a figure of eight or butterfly shape is not, by itself, automatic proof of an inclusion. Only when the butterfly and a dot or a dark stain occur together is it likely that the failure mechanism was due to some type of particle inclusion. Only through laboratory analysis can an inclusion be accurately identified as NiS.

It is possible to reduce the risk of NiS causing spontaneous breakages through a secondary process called Heat Soaked (HST). Here, toughened glass is put through a heating cycle where the glass is heated to >280°C then held at a temperature within 10°C of 290°C for two hours before being allowed to cool to an ambient temperature. It is a destructive process because a large percentage of glass where NiS inclusions are present and the glass is at risk of breaking will break. However, this is not a reliable process and some glass may still spontaneously break. As it increases cost and lead time it is often not taken up by clients who may not be aware of the risks.

The Glass and Glazing Federation (GGF) will shortly publish data sheet 4.4.2 titled Thermally Treated Soda Lime Silicate Glass Products – Spontaneous Breakage, which FIS members helped to review. The guide is available on the GGF website http://shop.ggfmembers.com/datasheets

So, who carries the risk for spontaneous breakages?

Geraldine Fleming, of JR Knowles, said: “To avoid potential claims relating to spontaneous breakage of toughened glass, it is vital that companies supplying glass products include an appropriate clause in their terms of business. The clause should clearly state the problem of NiS inclusions, confirm that HST can reduce (but not eliminate) the risk, and that you take no responsibility for any costs incurred by their client if such spontaneous breakage occurs.”

Such a clause would be known as an exclusion clause and you need to draw your client’s attention to it, otherwise it may be ineffective. We would advise a note at the start of your terms that states “the client’s attention is particularly drawn to clause __” and potentially also a similar note in your quotation. Of course you should also always attach your terms to any quotation, don’t rely on the phrase “terms available on request” as again this would render the term ineffective.

If you receive an order from a client before starting work, it is highly likely that this issue won’t be covered, and you will need to issue an order acknowledgement before starting work.

Tudor Pop, a facade engineer at property giant CBRE, said: “We have always recommended HST and actually do not accept toughened glass which hasn’t been.”

Technical manager at Optima Products Peter Long said: “The use of HST or laminating to minimise the risk to users, as advocated by the GGF, should be a prerequisite and encouraged.”

It is apparent that spontaneous breakages are occurring to toughened glass panels used in our sector. What is unclear is the cause in each instance. Is it due to an inclusion or some other form of spontaneous breakage as a result of prior damage or an installation issue? Or was the glass damaged by an impact and inclusions are used as a convenient explanation?

In any event, we recommend that you check your T&Cs and make it clear in your quotations that there is always a risk, and include the cost and lead time options for HST as a standard option.



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