for toughened glass
Heat Soak Test – HST
Heat Soak Test (HST) is a process that increases the durability of toughened glass by reducing the risk of spontaneous glass breakage due to nickel sulfide inclusion.
Why can toughened glass
pane fracture spontaneously
when heated?
Toughened glass manufactured by PRESS GLASS meets the requirements for mechanical strength, fracture pattern, dimensions and tolerances as well as edge finish, which are indicated in the European standard PN-EN 12150-1. The results obtained show that the product has strength even exceeding the requirements of the standard.
However, toughened glass reveals a weakness 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.
Then, as a result of the rapid cooling that hardens the glass sheet, the nickel sulfide particle, which needs a certain amount of time to return to its original volume, is prevented from returning. It is "frozen" in this state, which creates additional stresses in the glass.
In a toughened glass pane with an inclusion of a nickel sulfide particle, which is heated by solar radiation, there is an increase in the volume of the particle, resulting in an additional increase in internal stress. If such a particle is placed in the zone of tensile stress, there is a very high probability that the level of allowable stresses will be exceeded and spontaneous fracture of the glass sheet will occur.
What is
the Heat Soak Test?
Understanding the mechanism of spontaneous nickel sulfide failures helped develop a method to eliminate panes likely to suffer breakage. Such a method is the Heat Soak Test (HST). It involves heating toughened glass to a temperature of about 290°C for a period of time specified in the applicable standard. Close to 99 percent glass panes with inclusions will be identified and suffer fracture. As a result, defective glass is eliminated from further production.
Why is it worth to check
toughened glass in HST test?
The requirement for an HST process for glass is increasingly common on large structures. Regulatory requirements are increasing, and investors are aware of the consequences that can result from the spontaneous breakage of a glass panel placed in the facade after completion. Fractured glass panes must be promptly replaced, a very costly procedure often using a lifting crane or other specialized equipment. It also poses a direct threat to safety and may involve legal consequences.
Benefits and applications
of HST test
Benefits
- Reduced risk of spontaneous glass breakage due to nickel sulfide inclusion
- Increased safety of building users and the surrounding area
- Confirmation of the highest quality of the glass panes
Examples of applications
- Large-scale glazing for facades and structural systems
- Facilities with increased design requirements
- Glazing at high elevations
- Skylights and other horizontal glazing
Specifications
| Glass thickness [mm] | Minimum panel dimensions [mm] | Maximum panel dimensions [mm] | |
| HST | 4-19 | Minimum dimension of longer side 400 mm | 3200 × 7000 |
PRESS GLASS has applied HST to the full range of our toughened 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 toughened glass. Customers who order HST testing get high quality toughened glass checked for spontaneous fracturing caused by nickel sulfide inclusions.