WindowTreat has been involved in the specification of glazing during the design stage of large and complex build projects in order to significantly reduce the risks from shattered glass. In addition, WindowTreat has retrospectively secured thousands of toughened glass in place with 3M External Safety Films. 3M Internal Safety Films and edge retention systems to secure the filmed glass in place.
This does seem like a silly question, but I am asking it in all seriousness. Toughened glass is considered a safety glass because, when broken, its fragments are smooth and bean-sized, practically eliminating the danger of human injury from cuts and lacerations.
If toughened glass rained down in lightweight pieces and avoided our eyes then it might be a manageable risk. The problem is that the fragments of toughened glass remain in large, heavy clumps. A heavy clump of glass landing on your head is a hospital visit if you are lucky. When it hits the ground it will disperse into the thousands of bean-sized pieces that you may be more familiar with.
For businesses and retail centres this usually involves a loss of several days' trade until the remaining toughened glass is made safe.
Toughened glass is made by the heating annealed glass to about 650 C and then cooling it rapidly through a process called ‘quenching. The outer surfaces cool more quickly than the centre resulting in a unit of glass that is 4 to 5 times stronger (and probably the need for another knowledge centre article to explain why how this phenomenon occurs).
Nickel Sulphide particles are occasionally present in glass and can present a problem. Nickel in the manufacturing plants and sulphurs in the air combine to create minuscule nickel sulphide particles that are virtually undetectable to the human eye.
Water expands when cooled sufficiently and becomes ice. This phase change is what we commonly know as freezing. Similarly, nickel sulphide also goes through a phase change, but at a much higher temperature. However, during the glass cooling process, the nickel sulphide particles remain confined to their shrunken, pre-transformation size.
There is a very real risk that the Nickel Sulphide inclusions expand in size and if the expansion is large enough then the resulting stress will shatter the toughened glass.
If the glass has gone through a heat soaking process then it will usually have a kitemark including a reference to BS EN 14179.
What this means is that the manufacturer has tried to cause the glazing or sample of glazing to break within the facility instead of months or years later on the roof or side of a building. The process involves heating the toughened glass back to temperatures of 288 C to 316 C for 2 to 4 hours. The idea is to encourage any nickel sulphide inclusions to go through the phase change.
Statistically, toughened glazing which has been heating soaked is significantly less risk of spontaneous glass breakage.
Toughened glazing which has not been heating soaked will often have the kitemark BS EN 12150.
The immediate action is to avoid access to areas close to or underneath toughened glazing. Unfortunately, in a shopping centre this may involve closing shops.
