Heat Soak Testing is a quality assurance process for toughened glass, to minimise the risk of spontaneous breakage caused by Nickel Sulphide inclusions

Heat Soak Testing is a quality assurance process for toughened glass, to minimise the risk of spontaneous breakage caused by Nickel Sulphide inclusions​

What is Heat Soaking?

Tempered glass is widely used in construction and design because of its strength and safety features. However, it can occasionally suffer from spontaneous breakage due to internal defects. One of the most significant culprits is Nickel Sulphide (NiS) inclusions, which are microscopic impurities that form during the manufacturing process.

Nickel Sulphide inclusions originate when nickel contamination reacts with sulphur, forming NiS crystals inside the glass

NiS inclusions can change phase and expand when exposed to heat (e.g., sunlight, external conditions). This expansion causes internal stresses, leading to the spontaneous breakage of the glass.

How Heat Soak Testing Works

Before HST, the glass undergoes tempering to strengthen it. Tempered glass is heated to about 620°C (1150°F) and then rapidly cooled, inducing compressive stress on the surface and tensile stress within. This makes the glass significantly stronger but also more sensitive to internal defects like NiS.

Heating: The glass is placed in a heat soak oven, where it is gradually heated to 280–300°C (536–572°F). This temperature is chosen because it accelerates the transformation of NiS inclusions from their unstable phase to a stable phase, causing them to expand.

Holding Period: The glass is held at this temperature for 2-4 hours to allow any latent defects to manifest.

Cooling: The glass is then cooled to ambient temperature before further inspection or shipping.

Glass with critical NiS inclusions will typically shatter during this process due to the stress caused by the NiS expansion. Broken panes are discarded, and only the defect-free glass moves forward.

Applications of Heat Soak Testing

Prevents spontaneous breakage of large, installed panes that could endanger pedestrians below.

Ensures glass used in overhead applications won’t fail unexpectedly.

Maintains structural integrity and user safety in glass barriers or railings

Enhances reliability in architectural glass used for external building envelopes

Want to know more about Heat Soaking?

Get in touch by calling us and chat to one of our friendly team,
or email us and one of our team will contact you back.

Custom Complex, Yardley Road, Knowsley Industrial Park North, Merseyside L33 7SS

0151 549 1264

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