Of the various causes of ‘spontaneous fracture’, only that associated with the presence of foreign particles in the glass is more likely to cause fracture in toughened glass than in other forms of glass, because they can disturb the very high built-in stresses in toughened glass. While there are several (rare) types of foreign particles that may cause ‘spontaneous fracture’ of toughened glass, one type in particular, nickel sulphide, is directly associated with the toughening process.
In 1962, Ballantyne of the CSIRO (Building Research, Melbourne) published a report indicating that the cause of many spontaneous breakages was nickel sulphide (NiS). NiS is a complex material, which undergoes a phase change (a change in crystalline structure), at 380oC, which is accompanied by a volume change. The 1-NiS, which is stable above 380oC, has a smaller volume than the 2-NiS, which is stable below 380oC. The toughening (tempering) process in glass requires the glass to be heated to around 620oC followed by rapid cooling. Any NiS in the glass is converted to the 1 phase at the high temperature, but the rapid cooling does not allow time for the conversion back to the 2 phase. The NiS is thus ‘frozen’ into the toughened glass in an unstable form. Over a period, the 1 phase slowly converts back to the 2 phase, the conversion being accompanied by an increase in volume of 2%-4%. If a particle (inclusion) of NiS is sufficiently large and is in the central (tensile stress) zone of the toughened glass, then the expansion caused by the conversion can exert sufficient excess stress to cause a crack to propagate, leading to disintegration of the pane.
Source of NiS The NiS is a contaminant in the glass. Sulphur compounds are unavoidable, but the nickel can be reduced by careful control. Sources of nickel contamination have been found in the raw materials, the fuels, and the component parts of the melting tank structures and the steel components of equipment in the melting tanks. Reputable glass manufacturers have taken action to reduce nickel contamination over the 30 years since NiS was shown to be a cause of spontaneous fracture. The occurrence of NiS in glass is now considerably lower (by at least an order of magnitude) than it was in the 1960’s.