Heat-Soak Toughened Glass

Why does tempered glass fractures spontaneously?

Test results prove that tempered glass manufactured by Yalodomi complies with European Standards and it ranks among the top quality products. Nevertheless, this 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. In order to understand the Heat Soak Test, it is important to understand how the nickel sulfide inclusions can contribute the breakage.

In fact, nickel sulfide inclusions in glass are quite rare. Though rare, they are very troublesome and potentially dangerous when present in tempered glass. The reason for all this trouble is a delayed phase transformation in nickel sulfide. Nickel sulfide crystals have a high temperature and a low temperature form.

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 glass process, the impurities, which need a certain amount of time to return to their original state, cannot do so. Ultimately, they become “frozen”, producing additional local stress concentration. This mechanism resembles a delay-action “bomb”.

There are three possible sources of nickel; the raw materials, the materials used in storage and handling of raw materials and the contamination coming into the glass melt via fire bricks and the burners. As a result of some early characterization work, it was recognised that fuel oil contains traces of nickel, and due to this glass processing, tanks which originally used fuel oil burners, have been converted to use natural gas. The conversion to natural gas and the modifications in handling of raw materials have improved the situation with nickel sulfide but have not eliminated the problem.

Reduce the potential for spontaneous breakage

Tempered glass is required to meet safety glazing requirements and requirements for added strength. Heat Soak Test is a method used to ensure this safety and to eliminate panes likely to suffer breakage.

In a Heat Soak Test the toughened glass is stacked inside a hot box and is reheated to 290 °C for a few hours.

The Heat-Soak process induces the inclusions to transform to their low temperature form, which causes the problematic glass to fail. The advantage in using this process is that the Heat-Soak removes more than 95% of problematic glasses and does not significantly reduce the temper of the glass. The disadvantage of the process is that it adds extra cost because of the need for extra processing, and because it is destructive.

All shaping, hole-drilling and finishing of the glass must be done before toughening, so the loss of windows in a destructive test represents a loss of time spent in preparing the glass.

This process is in accordance with EN 14179.


With some hours dwell time, we are confident that we have minimized the potential for breakage to its lowest level.

The Heat Soak Test, provided by Yalodomi, is offered to provide greater assurance against nickel sulfide breakage, and when compared to the actual cost of replacing broken tempered glass in the field, there is substantial justification for the additional process charges.

We realize that there are situations where tempered glass simply must be used because of its superior strength and break-safe characteristics. In these situations, we feel compelled to avoid the potentially rare but serious occurrences of spontaneous breakage.


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