AGG Products - Heat Soaking

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Heat Soaking

The heat soak test was developed to detect the presence of nickel sulphide inclusions in toughened glass. Nickel Sulphide inclusions are formed when nickel and sulphur contaminants find their way into the glass batch. The nickel and sulphur combine during the manufacturing process to form “stones” in the body of the glass. These “stones” are often very small and not able to be seen. Whilst the glass remains in its annealed state these nickel sulphide inclusions do not cause any problems because the glass stresses are located on the glass surfaces, not in the centre of the glass where the stones are located.

During the glass toughening process the nickel sulphide inclusions are changed into a different form. This new form continues to increase in volume. This increase in volume puts stress on the glass surrounding the nickel sulphide inclusion that can lead to breakage of the panel, ie. Spontaneous Breakage. These breakages may not occur immediately, and it may take several years (solar heat cycles) for any problems to become apparent.

The heat soak test is based on reheating the toughened glass to an elevated temperature that accelerates the expansion in the nickel sulphide inclusions. This increases the probability that, if inclusions exist in the glass, the glass will break in the heat soak test rather than insitu. The heat soak process can reduce the risk of the potential breakage due to Nickel Sulphide inclusions but it does not guarantee the elimination of Nickel Sulphide impurities.

The Australian Glass Group recommends heat soaking all toughened glass when the glass is to be glazed in structural applications, or when safety is paramount, such as balustrades and overhead glazing.

Click here to view our Technical Bulletin on Heat Soaking.