In rare instances, toughened and heat-strengthened glass can spontaneously shatter. This can happen for several reasons, including edge damage, vandalism and improper installation.
It can also be caused by a rare phenomenon called nickel sulphide inclusion (NiS). This is where tiny particles of nickel sulphide are present in the glass after manufacture.
NiS is natural. It's not considered a defect and it doesn't affect the appearance or clarity of the glass. However, it can cause spontaneous breakage when the glass is exposed to sudden temperature changes.
Heat soak testing helps identify instances of NiS and dramatically reduces the risk of spontaneous breakage. For this reason, we recommend that all tempered and heat-strengthened glass is heat soak tested – especially if the glass will be installed in a hard-to-reach area.