We have seen articles showing shower glass cubicles or glass doors 'exploded' leaving dices of glass fragments behind while some were reported to be injured by the fragments. First of all, not all types of glass will exhibit such kind of breakage which is commonly known as spontaneous shattering. Tempered glass which have gone through the tempering process will run such risk of spontaneous shattering.
Tempered glass is about 4X stronger than 'ordinary glass' or annealed glass which has a different breakage pattern. Annealed glass typically breaks into large fragments/chunks compared to the dices seen to a broken tempered glass. In order to make the tempered glass 4X stronger, it has to go through a tempering process which is to heat up the glass through an oven and immediately it undergoes a cooling process to 'quench' the hot heated glass. The 'quenching' cools the outer surfaces much faster than the center portion of the glass resulting having tension in the center and outer surface in compression thus yielding its strength (long explanation short).
While tempering may sound great in terms of glass strength, there is this notorious culprit called Nickel Sulphide (NiS) typically termed as impurity which cause almost all spontaneous shattering to tempered glass. Why? During the tempering process, NiS under heat will expand in size (alpha phase) however because of latter quenching, the NiS is unable to cool and return to its original size (beta phase) in time. As such the expanded size of NiS is trapped within the quenched tempered glass. Over time, as the NiS revert back to its original size (alpha transit to beta), there is a change in the stress between tension and compression leading to a spontaneous breakage and there is no specific timeline that when it will happen - it is as if a time bomb awaiting to explode!
How glass break also tells us the type of glass used. Annealed glass will break in large fragment which crack lines along the glass. As for tempered glass, it will be the thousands of glass dices spotted.
There can be various reasons of shattering such as stress points due to installation details or simply by impact. However spontaneous shattering can be deduced from the initiation point of breakage - a Figure of 8 or a butterfly shape. Within the Figure of 8 or a butterfly shape, you will be able to discern a "DOT" which is the notorious culprit - Nickel Sulphide.
Below are some photos which we have noted in one of the glass breakage along the curtain wall facade showing spontaneous shattering due to Nickel Sulphide.
Having said that, not all tempered glass has impurities leading to spontaneous shattering. Thus many tempered glass now are installed in a form of laminated glass which is having an adhesive interlayer sandwiched between two panes of glass panel. Should tempered glass break, it will be held on by the interlayer to provide safety.
There are ways on how we can deal with such time bomb which is awaiting to explode (laminated glass is one way). This we will share soon! Hope this bite size information brings you some understanding of this topic.