Heat Soaking Avoids Spontaneous Cracking of Thermally Toughened Safety Glass

Toughened glass applications on buildings occasionally suffer from so-called spontaneous fractures. If these are not caused by inadequate assembly or vandalism, in most cases a special kind of a glass inclusion, the so-called nickel sulfide stones, have been proved to be at the origin of the break. Nickel sulfide (Millerite) is a mineral coming into the glass in very small quantity during the glass melting process. At temperatures below 350'C it undergoes a slow modification change, a so-called allotropic transformation, accompanied by a volume increase of 4%. Because of this specific mineralogical property it is able to cause sudden failure of toughened glass panes under certain conditions. These are: location of the inclusion in the tensile zone of the glass; minimum diameter of the inclusion (>50 microm); favourable temperature/time regime to allow the allotropic transformation to take place.

Since it is clear and proved by many laboratory results that the allotropic transformation of the dan-gerous inclusions is crucially influenced by temperature, a destructive process was developed called the Heat Soak Test. In every case it consists in heating up the panes to a certain temperature, holding this temperature for some hours, and cooling again to room temperature. Recently, in Europe, a new standard proposal (prEN 14179-1) was drafted. It fixes (290+/-10)°C as the temperature the glass has to be kept on during at least two hours. The prEN also contains the exact prescription of the calibration procedure of the process oven to assure that at every possible load the glass in its entity is subject to the process conditions.

We did statistical calculations, based on more than 1200 breaks, observed in a pair of HST ovens working since a long time in accordance to the prEN, and on overall HST break statistics, taking into account more than 25000 tons of glass processed. This worst case estimation results that on a building with 10000 m2 of heat soak processed glass the one year's break risk is less than 1%, i.e. in 100 years perhaps one glass may break because of NiS. How much less? We cannot tell it at the moment. Perhaps a lot.

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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