Large fronts of tempered safety glasses are well known design features in modern architecture. However, these tempered glasses are susceptible to spontaneous cracks and shattering. Moreover, these problems can occur many years after installation.  Figure 1 shows an optical image of a spontaneous fracture. To a glass expert, visual inspection already provides important information on the damage mechanism.   The optical image of the crack is characterized by a particulate inclusion. A more detailed investigation with analytical methods such as Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy (SEM/EDX) provides additional insight into the root cause of the crack formation.
Figure 2 shows the backscattered image acquired with a Scanning Electron Microscope. Figure 2 also shows the particle dimension (ca. 200 µm) and morphology. The observed image contrast between the particle and the surrounding safety glass also indicates the particle consists of elements which are heavier than Si. A detailed chemical characterization of the particle composition can be obtained by means of Energy Dispersive X-ray Spectroscopy (EDX). A comparison of Figures 3a and b shows the analyzed particle mainly consists of nickel sulfide.  EDX thus confirms NiS inclusions (Millerite) which are know to cause spontaneous fractures of toughened safety glass.  The fracture is caused by a slow volume increase of nickel sulfide particles due to so-called allotropic transformation.  This NiS transformation process can be forced with a heat soak test at the end glass production.