The heat soaking process is one method of reducing the incidence of NiS induced failure in tempered glass by causing the phase change to occur before the glass panel is installed in the building. Numerous heat soaking criteria have been developed with different levels of success. In any event the heat soaking process is not 100% effective and failures have known to occur with heat soaked tempered glass.

The time to fracture due to NiS induction depends on various factors like: -

• Purity of the inclusion.
• Location of the inclusion within the tensile zone of the tempered glass.
• Magnitude of the tensile stress within the body of the glass.
• Size of vent associated with the inclusion.
• Environmental conditions – temperature and lateral wind pressure.
• Panel size – membrane stress developed in the window.

The presence of nickel sulphide is adventitious in most glass. It has been estimated that in normal float glass production inclusions occur at a frequency of about one per 8 tonnes of raw glass It has been estimated that 1 gram of nickel sulphide can produce approximately 1000 inclusions of 0.15 mm diameter. Consequently, it does not require too much contamination to produce a serious problem.

The heat soak process system consists of:

• The oven
• Glass support
• Separation system

As the standard is based upon extensive research combined with years of experience it was decided to limit the standard to ovens in which a batch of glass is heated up by convection. Other methods of bringing the glass up to the required temperature e.g. micro-waves, immersion, etc... will be subject to further development and will require evidence before it can be accepted as a reliable alternative for the actual HST. Moreover the oven shall be designed and constructed in such a way that it shall allow an unhindered air circulation around each glass pane. Even in the event of glass breakage the airflow shall not be hindered. This will require a careful design of the openings for the air ingress/egress so that fragments of broken glass will not cause blockages.

It is very important to lay down exactly the way the glass is put up inside the HST oven. The glass stack shall be aligned with the heating airflow direction.

The distance between the single glass panes in the stack combined with the alignment of the glass stack are the most important parameters decisively determining the quality of the test. The separators shall assure optimized airflow between the glass panes without hindering the airflow themselves. A minimum separation of 20mm is recommended. An example of a vertical glass support is shown. It goes without saying that glass-to-glass contact is not allowed!

Distances between the glass panes of less than 20mm will not only badly influence the heat exchange but also increase the risk of breakage. The latter will certainly be the case when the oven is fully loaded as huge temperature differences can occur between 2 points in the same glass pane.