The glass used for building facades are normally toughened to enhance their strength. So why would this glass explode spontaneously, where normal float glass does not? The answer lies in the toughening process and the ingredients for glass. The sand mixture used in glass manufacture sometimes contain micro particles of Nickel Sulphide. In normal float glass these particles do not pose any risk, but it is the heating process of toughened glass that creates the potential problem. When heated NiS (Nickel Sulphide) contracts, but expands again when ambient temperature is reached. The whole reason behind heating the glass is to create higher elastic strain energy in the glass, thereby strengthening it. When there are NiS particles present this may cause exactly the reverse. If the NiS particle has a stronger expansion force to the stored elastic strain energy the glass may explode.
As these micro particles are not easy to detect, the glass could be installed with the flaw, creating a ticking time bomb. Fortunately, through correct structure design or the use of heat-soaking* this problem can be eliminated.
*Reheating glass after toughening to induce NiS breaks for quality check.