With regard to the recent reports about glass falling out [of buildings in Hong Kong], there appears to be a lot of debate without properly establishing the nature of the problem.
Even when tempered (or toughened) glass breaks, the small glass cubes generally remain locked together, with the broken sheet of glass staying in place unless a secondary force is applied such as wind suction, physical impact (possibly opening and closing a window) or gravity in the case of glass that is not in a vertical position.
Even if the recommended secondary heat soaking process is correctly carried out on tempered glass, there remains a residual risk of nickel sulphide (spontaneous) breakage under certain conditions, albeit an extremely small percentage of the total production quantity. Correctly carrying out the secondary heat soaking process is paramount to minimising the risk.
Unfortunately, this process is time consuming, requires skill in execution, causes disruption to glass production if nickel sulphide inclusions do cause breakage in the heat soaking oven and, probably more importantly, adds additional cost in the glass production process. It is not difficult to see why this secondary process is either omitted or not subject to the appropriate quality control systems - properly executed as opposed to simply ticking boxes (a fairly common practice in Hong Kong and on the mainland).
Improving the checking procedures and carrying out the secondary heat soak process will reduce the risk of future tempered glass breakage, but, if the risk is to be zero, then do not use tempered glass. If the specification or regulations require a safety glass, then use laminated glass rather than tempered glass; if the requirement is to perform under highly fluctuating solar temperatures, use heat-strengthened glass rather than tempered glass.
John Campbell, chairman, The Society of Facade Engineering (HK Branch)