A Conclusion on the Inclusion - Risk of Spontaneous Glass Failures.
Glass is one of the best building materials available. Highly regarded by architects, it’s hard to imagine a modern building in which glass does not have an important part to play, providing natural light into buildings whilst capable of managing solar heat gain (SHG), heat loss (R/U), glare, noise reduction (STC) and provide for human impact and safety from falling to name only a few of the technical properties aside of the obvious aesthetic contribution.
For these and many other reasons there has been an increase in the use of glass in all aspects of new buildings and refurbishments.
Float glass is a man-made material, manufactured in bulk all over the world, crafted from raw materials sand (silica), soda and lime. When you think that these materials are largely sourced straight from the earth, it’s not surprising that some extraneous materials find their way in the mix.
During the melting process when heated to around 1500°C, many of the extraneous materials will be burnt off or dissolve in the glass, but some may remain as included particles within the glass thickness. The more infamous of these inclusions, especially in recent times, is Nickel Sulphide (NiS).
Typically, during float glass production, quality control measures put in place by the manufacturer ensure that particles in the order of >1mm in diameter can easily be picked up in stock sheets as a potential defect and rejected, but inclusions can be smaller than this and pass undetected. That glass is then packaged, shipped, and processed into product that is installed into buildings.
Usually NiS inclusions have no consequence to the strength and stability of the glass and are often small enough that they are not visibly obtrusive and have little to no effect on annealed glass. However, when the glass is toughened, be it for benefit of its safety or strength, the potential effect of NiS inclusions needs consideration.
The benefits of toughened glass are gained from the tempering process, which creates an equilibrium of induced compressive and tensile stress within the body of the glass through a process of gradual heating and rapid cooling.
NiS can be one of the causes of failure in toughened glass, if an inclusion is present. The tempering process causes a phase change in the material and it reduces in volume (on a microscopic level) at high temperature. The inclusion is frozen in this state as the hot glass is quenched. Over a period of time there is a gradual phase change as the inclusion wants to increase back to its original volume. If the inclusion is sufficient in size and within the tensile stress zone of the glass the volume, change can cause sufficient stress to break the glass.
Unfortunately, there are numerous factors that affect the speed of phase change that are near to impossible to measure making it hard to predict if the glass might fail. Heat strengthened glass can also be effected by NiS inclusions as well, but they are much less common.
So how can the problem be controlled?
- A reputable glass supplier should be sourced who understands the issue and have made processing improvements to reduce the likelihood of NiS inclusions, although it’s important to remember that the inclusions can still exist to some degree. A good supplier will be happy to discuss and advise on any issues.
- Alternatives to toughened glass can be used. Annealed laminate can be used for safety application’s in lieu of toughened; a toughened laminate combination - so if a failure does occur the glass largely stays in place; heat strengthened glass for spandrels and areas at risk of thermal fracture.
- An additional process of ‘heat soak testing’ of the toughened glass can be done as an additional process. This is a worthwhile test to induce the NiS phase change if an inclusion exists – causing the glass to fail. If glass survives the test it is much less likely to break from a NiS inclusion. However, heat soaking is not a failsafe but a precautionary measure.
The above 3 points should also be an important consideration in hard to reach areas such as glazed roof lights, bridges and high rise applications which can also be a risk of falling glass particles.
So, what is the conclusion on the inclusion? NiS inclusions are something to keep in mind when using toughened glass as it can be a cause of failure, but is by far not the only cause. Just as important as designing glass structures to be structurally adequate under design loads, it is equally important to design in the consequence of glass breakage, how the breakage is managed, and can be repaired safely. There is no real way to totally remove the issue if heat treated glass is being used, but when it can be better understood it can therefore be better managed.
To remove the consequence of NiS is to omit any fully tempered glass being used, but that does not need to be the case, talk to your glass supplier and request high quality glass from processors with experience in managing this inherent risk or discuss the many other safety glass alternatives available.
Steve Sollberger Technical Glass Engineer, Woods Glass (NZ) Limited
John Colvin, Glass Consultant, Technical Note 02, 8th March 2006
Glass in Building – Principles, Applications, Examples. Bernhard Weller, Kristina Harth, Silke Tasche, Stefan Unnewehr. 2009 Institut fur internationale Architektur-Dokumentation
NOTE: NZS 4223.1: 2008, - Glazing In Buildings - Glass Selection And Glazing (Amend 1), Appendix E - Guidance On Selecting Glass To Minimise The Risk Of Spontaneous Glass Fracture, recommends the use of heat soaking to reduce the risk of inclusion failure, particularly for nickel sulphide inclusions. Related Masterspec sections now recommend heat soaking of toughened and heat strengthened glass in critical locations, like barriers to falling, to reduce the risks.