What is the Difference Between Toughened and Tempered Glass?

What’s the Difference between Toughened and Tempered Glass?

Let’s settle the difference between toughened and tempered glass, once and for all. Glass is the cherry on top of architecture, but unless it’s toughened or tempered, it shatters. The terms are often used interchangeably, so what’s the difference between toughened and tempered glass? Toughened glass encompasses the broad category of strong glass, while tempered glass is toughened glass with a uniform cooling process that creates a consistent stress profile across the entire pane. All tempered glass is toughened, but not all toughened glass is tempered. Understand the nuanced differences between these two types of glass, to find your sweet spot between safety and practicality.

What Is Toughened Glass?

Toughened glass is safety glass. It undergoes a heat treatment process that significantly increases its strength compared to regular, annealed glass. With a thin layer of surface compression (0.1-0.2 mm) and internal core tension (40-70 MPa), it absorbs impact energy and breaks into small, relatively harmless chunks rather than dangerously sharp shards. That’s why it’s so safe and effective for doors, windows and other structural elements.

Manufacturing Toughened and Tempered Glass

The difference between toughened and tempered glass comes from how they’re made. Both are initially softened through heating in a furnace to temperatures between 600-700°C (1112-1292°F). Cooling methods set them apart:

Toughened Glass – Quenching

After heating, the glass is rapidly cooled using high-pressure air jets. This creates high surface compression and internal core tension, making it highly resistant to scratches and breakage.

Tempered Glass – Uniform Cooling

Instead of rapid cooling, tempered glass undergoes a more uniform cooling process which enhances its thermal resistance. Tempered glass is therefore suited to environments with significant temperature fluctuations.

Mitigating Risks: Nickel Sulphide Inclusions and Heat Soaking

Toughened glass can sometimes contain tiny impurities known as Nickel Sulphide (NiS) inclusions, which may cause the glass to break unexpectedly. Understanding how they work is essential to the safety and reliability of your glass installation.

Understanding the NiS Risk

Behaviour of NiS Inclusions

NiS particles undergo a slow phase change over time, causing them to expand. This expansion creates internal stresses within the glass, which can lead to spontaneous breakage. The phase change process is most significant around 380°C, but even at lower temperatures, the expansion can occur gradually.

Influencing Factors

The likelihood of spontaneous breakage depends on:

Size of the NiS particles - Larger particles pose a greater risk
Location within the glass - Particles located in tensile stress zones are more likely to cause fractures.

Heat Soaking Process

To reduce the risk of in-service failures caused by NiS inclusions, the glass undergoes a process called heat soaking:

How It Works:

The glass is heated to approximately 280°C and maintained at this temperature for several hours
This accelerated heating encourages any glass containing NiS inclusions to break within a controlled environment, before installation.

Why 280°C?

Heating to 280°C is sufficient to accelerate the expansion process of NiS inclusions, without compromising the overall integrity and properties of the glass
This temperature is a practical balance that effectively identifies and eliminates defective glass, while preventing damage to sound glass panes.

Benefits of Heat Soaking:

Enhanced Safety - Significantly reduces the chances of spontaneous breakage after installation
Quality Assurance - Ensures that only reliable and defect-free glass is used in projects
Cost-Effective - Prevents future replacement costs and potential injury or property damage.

Concerned about your toughened glass? Contact Us Now, for technical insights into our safe and reliable glass installations!