Client
Saint Gobain
Date
October 2022
Location
France
Service
HST Oven

Heat Soaking Test oven for Saint Gobain

Heat soak testing involves an accelerated ageing of the glass that indicates the presence of nickel sulphide in its composition. It is the only process certified worldwide to meet European Standard

There is a growing tendency to require glass used for large projects be HST tested. Regulations become more stringent and investors better aware of potential consequences caused by spontaneous fractures of already installed glazing systems. Fractured glass panes must be promptly replaced, a very costly procedure often calling for a lifting crane or other specialized equipment.

A broken pane may leave facilities with restricted access wide open. Fracture of a tempered façade pane occurring at a high elevation may cause injuries when glass shards, despite their small size and blunt edges, hit people who happen to be close to the building, and that may mean legal problems for the building manager.

How does the Heat Soak Test (HST) work with toughened glass?

Understanding the mechanism of spontaneous nickel sulfide failures helped develop a method to eliminate panes likely to suffer breakage: Heat Soak Test, or HST.
Toughened glass is heated up to approximately 290oC for a period of time specified in the applicable standard. Close to 99% glass panes with inclusions will be identified and suffer fracture.
  • Superb temperature uniformity in the glass.
  • Software offering quality control and traceability of the process.
  • Single door lock.
  • Multi-chamber option.
  • Process and quality control: Lamiflex software

WHY DOES TEMPERED GLASS PANE FRACTURE SPONTANEOUSLY WHEN HEATED?

HST prove glass product exceeds strength parameters stipulated in the standard. Nevertheless, this state-of-the-art product shows a certain weakness characteristic of float glass: tiny nickel sulfide crystals (NiS) which may occur in molten glass during float glass production process. When glass is heated during toughening, nickel sulfide impurities in the glass pane increase their volume; when the heated glass is rapidly quenched

in the toughening process, the impurities, which need a certain amount of time to return to their original state, cannot do so and become “frozen”, producing additional local stress concentration. The mechanism resembles a delay-action bomb: when toughened glass containing nickel sulfide impurities is used in a structural glazing system on a building façade, it gets heated by sun rays, allowing frozen crystals to expand, and thus generating even more internal stress. With an inclusion in the core tensile zone, stress is highly likely to exceed the acceptable level, causing spontaneous fracture of the pane.

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