Reliable and Shorter Heat Soak Test to Avoid Spontaneous Fracture of Heat Strenghtened and Tempered Glasses

F. Bordeaux and A. Kasper

The spontaneous fracture of tempered and heat strengthened glasses attributed to nickel sulphide inclusions (NiS) has been a recurrent problem over the last 40 years. At the present, destructive heat treatment called Heat Soak Test (HST) which consists of holding the glass for a long time (few hours) at 260-290 °C appears to be the most effective method of avoiding the later spontaneous cracking.

Established on an empirical basis, Heat Soak Tests do not necessarily take into account the kinetics of spontaneous failures. As a consequ-ence, the Heat Soak Test is not optimum with respect to temperature and time.

Measurements of transformation rates and identification of the dangerous NiS compositions associated with a thermo-mechanical model were used to predict the occurrence of spontaneous failures in Heat Soak Test furnaces. The good agreement between theoretical and experimental data monitored inside HST furnaces allowed us to propose a new optimised Heat Soak Test, shorter and still reliable.

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The Authors

Dr. Andreas Kasper Dr. Saint-Gobain Profession: ChemistOccupation: Head of project group "chemistry of glass melting"Responsibilities:Chemical analysis of glass and raw materials; environmental measurements (emissions of glass tanks); s... Read more