New Service: Heat Soak Test for Tempered Glass

Glass has become one of the most popular cladding materials. At the same time, architects search for a more appealing material than steel and reinforced concrete traditionally used for load-bearing structures. As a result, structural glass gradually becomes the material of choice: glass ribs, glass steps, glass floors to name just a few examples of the new applications made possible thanks to a newly developed method of tempering glass. Now, structural components made of monolithic glass can not only withstand considerable bending and compressive stress but meet all safety requirements as well.

Tempered glass manufactured by Press-Glas ranks among the very top quality products. It complies with PN-EN 12150-1 standard regarding strength, fracture pattern, dimensions and tolerances as well as edge finish. Test results prove our 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 tempering, 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 tempered 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.

Understanding the mechanism of spontaneous nickel sulfide failures helped develop a method to eliminate panes likely to suffer breakage: Heat Soak Test, or HST. Tempered 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.

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.

Press-Glas has applied HST to the full range of our tempered glass products. We use an HST oven calibrated by IFT Institute of Rosenheim to perform HST testing. Test results confirm the heating process conforms to PN-EN 14179-1 standard “Heat-Soaked Thermally Toughened Soda Line Silicate Safety Glass”.
The heating process for an HST test does not impair the safety class or strength of tempered glass. Customers who order HST testing get high quality tempered glass checked for spontaneous fracturing caused by nickel sulfide inclusions.