Heat Soaking

Why does a tempered glass pane fracture spontaneously when heated?

Tempered glass manufactured by PRESS GLASS ranks among the very top quality products. It complies with official standards 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 tempering 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.

How does the Heat Soaking work with tempered glass?

Understanding the mechanism of spontaneous nickel sulfide failures helped develop a method to eliminate panes likely to suffer breakage: Heat soaked tempered glass is heated up

to approximately 555^oF for a period of time specified in the applicable standard. Close to 99% glass panes with inclusions will be identified and suffer fracture.

Why should you ask for heat soaked tempered glass?

There is a growing tendency to require glass used for large projects be heat soaking tested. Regulations have become more stringent and investors are more aware of the 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. The broken piece can potentially hit people who happen to be close to the building, and that may mean legal problems for the building manager.

Market offering

PRESS GLASS has applied heat soaking to the full range of our tempered glass products. We use an heat soaking oven calibrated by IFT Institute of Rosenheim to perform heat soaking.

The heating process for a heat soak test does not impair the safety class or strength of tempered glass. Customers who order heat soaking get high quality tempered glass checked for spontaneous fracturing caused by nickel sulfide inclusions.

MOVIE

Verification of tempered glass by Heat Soaking

We constantly modify our manufacturing processes and change habits to limit the negative effect on the environment.

In order to facilitate the changes, we have adopted ISO 14001, the worldwide management system for environmental protection.