What is the difference between tempered glass and heat soaked glass?- Hangzhou Blue-Sky Safety Glass Co., Ltd.

When choosing safety glass for architectural, automotive, or industrial applications, two terms often appear: tempered glass and heat soaked glass. Though both types undergo special heat treatments to improve performance and safety, they are not the same. Understanding the difference between tempered glass and heat soaked glass can help architects, builders, and buyers select the right product for their projects. This article provides a detailed comparison, including definitions, manufacturing processes, benefits, limitations, and key applications.

1. What is Tempered Glass?

Tempered glass, also known as toughened glass, is a type of safety glass that undergoes a thermal treatment to increase its strength. It is produced by heating annealed glass to approximately 620–650°C and then rapidly cooling it with high-pressure air jets. This rapid cooling process compresses the surface while the interior remains in tension, significantly increasing its mechanical and thermal resistance.

Key Characteristics of Tempered Glass:

Strength: 4 to 5 times stronger than standard annealed glass.

Breakage pattern: When broken, it shatters into small, blunt pieces, reducing the risk of injury.

Thermal resistance: Can withstand temperature differences up to 250°C.

Safety compliance: Commonly used in applications requiring safety glass, such as doors, facades, and shower enclosures.

2. What is Heat Soaked Glass?

Heat soaked glass refers to tempered glass that has undergone an additional heat soak test—a quality control process designed to minimize the risk of spontaneous breakage caused by nickel sulfide (NiS) inclusions.

This process involves reheating the tempered glass to about 280–300°C and holding it at that temperature for several hours. If NiS inclusions are present, they may expand and cause the glass to break during the test rather than after installation.

Purpose of Heat Soaking:

To identify and eliminate tempered glass panels that are at risk of spontaneous breakage.

To reduce post-installation failures caused by undetectable internal flaws.

Heat Soaking Does Not Mean Re-Tempering:

The structural and thermal properties of the tempered glass remain unchanged. The heat soak test only serves as a screening process.

3. Differences Between Tempered and Heat Soaked Glass

Aspect	Tempered Glass	Heat Soaked Glass
Definition	Safety glass thermally strengthened	Tempered glass subjected to additional heat soak test
Main Purpose	Improve strength and safety	Detect and eliminate glass prone to spontaneous breakage
Production Process	Heat treatment + rapid cooling	Tempered glass + reheating at ~280°C for 2–4 hours
Risk of Spontaneous Breakage	Moderate (due to NiS inclusions)	Significantly reduced (due to pre-breakage during test)
Applications	Doors, partitions, facades, showers, appliances	High-rise buildings, overhead glazing, curtain walls
Cost	Lower	Higher (due to added testing and energy use)
Compliance	Meets standard safety codes	Often required by international building codes for critical areas

4. Why Spontaneous Breakage Happens in Tempered Glass

The major concern with tempered glass is spontaneous breakage, primarily caused by Nickel Sulfide (NiS) inclusions. These microscopic impurities may form during the float glass manufacturing process. NiS particles can change phase over time and expand, causing internal stress and eventual breakage—especially in tempered glass, where internal tension is already high.

While these inclusions are rare, the consequences—especially in overhead or exterior glazing—can be serious. This is where heat soaked glass becomes important: it is tempered glass that has been "tested for time bombs."

5. When Should You Use Heat Soaked Glass?

While tempered glass is sufficient for most interior and low-risk exterior uses, heat soaked glass is recommended or mandated in critical situations where glass breakage could lead to injury or property damage.

Typical Use Cases for Heat Soaked Glass:

High-rise building facades

Overhead glazing and skylights

Structural glass walls

Glass balconies and balustrades

Railway stations, airports, and public buildings

In such applications, a single spontaneous breakage event could cause injury or require expensive replacements, making the added cost of heat soaking worthwhile.

6. Standards and Regulations

Many international standards address the use of heat soaked glass in safety-critical applications:

EN 14179 (Europe): Specifies requirements for heat soaked thermally toughened soda lime silicate safety glass.

ASTM C1048 (USA): Covers tempered and heat-treated glass requirements but does not mandate heat soaking.

GB15763 (China): Specifies requirements for tempered and heat-soaked safety glass in construction.

Architects and contractors should consult local building codes and project-specific specifications to determine whether heat soaked glass is necessary.

7. Summary: Which One Should You Choose?

Choose Tempered Glass if…	Choose Heat Soaked Glass if…
You need basic safety glass for interior or non-critical installations.	Your project involves overhead, structural, or exterior glazing where spontaneous breakage would be hazardous.
Cost is a major concern and the risk of NiS inclusions is minimal.	The glass must meet stringent safety standards or client demands.

Conclusion

Tempered glass and heat soaked glass both offer strength and safety far beyond ordinary glass. However, they serve different purposes. Tempered glass is a general-purpose safety glass ideal for everyday applications, while heat soaked glass is a premium option for minimizing risks in high-stakes environments.

By understanding the difference, users can make informed decisions that balance performance, cost, and safety—ensuring the long-term durability and reliability of glass installations.