Heat Soaked Glass

Nickel Sulfide (NiS) has been known to be the cause of spontaneous burst in tempered glass. During tempering the nickel sulfide is transformed to the high temperature a-phase (T>380oC) and has no time to return to the ß-phase on quenching. With the passage of time NiS slowly inverts to the low temperature ß-phase with an increase in volume of about 4%. This volume expansion may cause glass breakage. Heat Soaked (HS) glasses are also knows as heat absorbing glasses.

The Heat Soak is a destructive treatment which heats the glass for several hours at about 280 °C to speed up the a-NiS to ß-NiS transformation and consequently forces the contaminated glasses broken within the oven. Heat Soaked (HS) glasses are also knows as heat absorbing glasses.

Standard HST requires more than ten hours thermal treatment including 1 to 6 hours to heat large quantities of glass up to 290°C followed by a temperature plateau at these temperatures for 3 to 12 hours before cooling down slowly to ambient temperature. The time necessary for 99.9 % a to ß-NiS transformation is less than 1 hour at 280 °C. For comparison, more than one hundred hours become necessary to complete the same transformations at 250 °C, i.e. only 30 degrees below! The average temperature difference is about 30 degrees between the hottest and the coldest glass during Heat Soaking within most ovens in use.

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. 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.