For tempered glass manufacturers, unexpected spontaneous breakage after product delivery is one of the most challenging and reputation-damaging issues. The key to preventing this lies in one essential process — Heat Soak Testing (HST).

The root cause of spontaneous breakage is often the presence of nickel sulfide (NiS) inclusions formed during the glass manufacturing process. When sulfur and nickel combine at high temperatures, they create a high-temperature α-phase nickel sulfide. During the rapid cooling of the tempering process, this α-phase is trapped in a metastable state, unable to transform into the low-temperature stable β-phase. Over time, as the glass is exposed to environmental temperature changes, the α-phase gradually transforms into the β-phase — a change accompanied by a roughly 4% volume expansion. This microscopic transformation disrupts the internal stress balance of the glass, resulting in sudden self-fracture without any warning.

Heat Soak Testing offers a precise and proactive solution. In accordance with the European Standard EN 14179, tempered glass is reheated to approximately 290°C and maintained at that temperature for at least two hours after the glass load reaches full temperature. This controlled reheating accelerates the α-to-β phase transformation of nickel sulfide, causing any glass containing unstable inclusions to break inside the Heat Soak oven rather than in service.

Through this process, potentially defective glass is identified and eliminated within the factory. As a result, the spontaneous breakage rate of tempered glass can drop dramatically. For glass producers, this means not only a significant reduction in costly replacements and warranty claims, but also a stronger market reputation built on reliability and safety.

In today’s quality-driven market, Heat Soak Testing is no longer an optional step. It is a critical safeguard that ensures every piece of tempered glass leaving the factory delivers long-term performance, safety, and peace of mind — reinforcing the foundation of trust behind every successful brand.