The use of fully tempered glass is common in the building and car industries, and for good reason n. When tempering is included in the glass manufacturing process, the result is glass that is four to five times stronger than standard annealed glass – giving it the benefits of both durability and safety.
At Haringey Glass & Glazing, we typically install this type of “safety glass,” (as it’s sometimes referred to), in windows and doors, shower enclosures, shopfronts, and other applications. It’s considered safer than other glass because when it breaks, shatters into small granular chunks instead of splintering into jagged shards as ordinary annealed glass does. The granular chunks are less likely to cause injury. And although this glass is certainly safer, sometimes safety glass still shatters spontaneously and unexpectedly, with no visible cause.
CAUSES OF NICKEL SULPHIDE INCLUSION
What causes this to happen? Nickel sulphide inclusions can occur during the process of manufacturing toughened glass. They result from sulphide that reacts with the presence of minute amounts of nickel contaminants that are contained in the glass. Although they are invisible due to their minuscule size—they range from .05 mm up to .1 mm—they may still be present in manufactured glass. These particles remain benign until they are aggravated.
This composite photo shows the relative sizes of two nickel sulphide inclusions (on the left) next to an average-sized grain of table salt (on the right). The upper NSI inclusion measured .076 mm and the lower NSI inclusion measured .113 mm.
If NSI is present in the glass, there is a risk that the glass will spontaneously break, if the particle should change its size or shape.
NSI has two main states; stable at high temperatures and stable at lower temperatures. When the glass cools quickly, the NSI particle is unable to change back to its original form. Over a while, NSI will slowly convert to the original low-temperature structure. This causes the NSI particles to increase slowly in size. As a result, stress can cause the pane of glass to shatter for no visible reason. The length of time it can take for this to happen is unpredictable. It can occur within weeks of manufacture, or years — or never.
Glass manufacturers have implemented changes to eliminate NSI from the production of toughened glass. In their attempts to prevent this issue, glass producers have modified their choices of raw materials, manufacturing methods, and processing techniques. Although they’ve succeeded in making some improvements, it’s impossible to eradicate the risk. The fact remains that all glass comes with a very rare risk of spontaneous breakage due to NSI.
WHAT IS HEAT SOAKING AND HOW CAN IT HELP?
Heat soak testing is a destructive test for the presence of nickel sulphide inclusions. It involves placing the toughened glass in an oven and heating it for a period of time (see BS EN 14179–1 2016 for more details). The glass is kept at this temperature long enough to fracture a large proportion of any panes that might otherwise fail in service.
Heat-soak testing is recommended for safety-critical work such as atrium glazing and balustrades as well as for load-bearing glazing, overhead glazing and highly trafficked areas such as airports and other high-profile areas.
There is currently no requirement under building regulations that states that glass must be heat soak tested. While the risk of glass failure due to NSI may be small, it is vital that full and proper consideration is given to the building use, the purpose of the glass and the risk it presents.
NSI Breakages and Warranties
Nickel Sulphide Inclusion is a rare yet accepted anomaly within glass manufacturing and no glazier processors or suppliers are able to provide a warranty against breakages from this phenomena. This then means that Haringey Glass & Glazing is not able to pass any warranty onto our customers. Most home insurance policies cover spontaneous breakage, please check with your insurance providers to ensure this is the case.