Heat-treated glass: heat-strengthened or tempered glass?
Heat-treated glass
Heat-strengthened and tempered glass
Heat-treated glass is produced by heating glass panes to over 1100°F or 600°C and then rapidly cooling them, creating a permanent stress profile that enhances their strength and thermal-shock resistance. This type of glass is commonly used in applications requiring increased strength and safety, such as architectural projects and safety glass.
- What is heat-treated glass?
- How is heat-treated glass made?
- What are the differences between tempered and heat-strengthened glass?
- What are the maximum and minimum dimensions for heat-treated glass?
- Spontaneous breakage and heat-soak test
- What are the applications for heat-treated glass?
- Visual quality of heat-treated glass
- What are the applicable standards for heat-treated glass?
- Technical support
What is heat-treated glass?
Glass produced from the float process is known as float glass or annealed glass. In architectural applications, annealed glass often provides limited strength to resist applied loads and is unable to satisfy local building code safety requirements. Therefore, heat-treated glass is often the choice for applications where increased strength and safety are required. There are essentially two types of heat-treated glass:
- Heat-strengthened glass
- Fully tempered glass, also called toughened glass
How is heat-treated glass made?
The process begins with glass panes, already cleaned and pre-processed (cut-to-size, edge processed, potentially drilled), are uniformly heated while moving back and forth in a tempering furnace to over 1100°F or 600°C.
After the glass has reached the required temperature, it is transferred to the cooling zone. There, the glass is quickly cooled, or in other words quenched by blowing cold air through nozzles and high pressure.
This “quenching” causes the glass surface to cool down faster than the core of the glass. This creates a permanent stress profile in the glass with compression in the surface and tension in the core. The intensity of the cooling speed defines the level of surface compressive stress.
The compressive stress at the surfaces gives the heat-treated glass a higher tensile strength than annealed glass, which results in increased mechanical durability and thermal-shock-resistance.
Does Guardian produce heat-treated glass?
We do not produce heat-treated glass, but we work with a network of glass fabricators and processors who can temper or heat-strengthen our glass, including our high-performance and low-E products with heat-treatable coatings. You can find information on the heat-treatability of a product on its dedicated page, under “Product Information – Manufacturing Options”.
What are the differences between tempered and heat-strengthened glass?
Glass strength
Fully tempered glass is cooled down with the maximum cooling pressure while heat-strengthened glass is cooled down more slowly. Higher cooling speed leads to higher surface stress. Therefore, fully tempered glass has higher strength than heat-strengthened glass:
- Heat-strengthened glass has between 1.5 and 2 times the mechanical and thermal stress resistance of annealed glass of the same thickness.
- Fully tempered glass has 3 to 4 times the mechanical strength and thermal stress resistance of annealed glass of equal thickness.
Breakage pattern
Heat-strengthened glass is not considered safety glass because it breaks into large pieces with sharp edges. On the other hand, because of this specific fragmentation, the glass usually remains in the frame when broken, helping to reduce fragment fallout, and providing time for glass replacement.
Fully tempered glass produced according to safety standards and labeled accordingly, is considered safety glass. When broken, it breaks into small pieces and is less likely to cause severe injuries due to sharp piece of glass than annealed or heat-strengthened glass.
Apart from strength and breakage characteristics, heat-treated glass retains the normal properties of annealed glass, including chemical resistance, hardness, expansion, and deflection.
What are the maximum and minimum dimensions for heat-treated glass?
The maximum and minimum dimensions for heat-treated glass depend on various parameters, including the maximum size available from the glass manufacturer (which can be found on our product pages under 'Product Information - Maximum size'), and most importantly, the tempering facilities at the glass fabricator/processor.
Important consideration: the orientation of the glass panes in the tempering line is crucial due to the roller waves effect, making it essential to clearly identify the width of the glass to achieve a consistent horizontal orientation across a glazed façade.
You can contact us to discuss your project’s specific requirements and get guidance on what fabricator/processor can help you achieve your design objectives.
Spontaneous breakage and heat-soak test
Fully tempered glass is more prone to spontaneous breakage due to inclusions than annealed or heat strengthened glass. Certain types of inclusions which can occur during the glass manufacturing process have been found to cause spontaneous breakage. One such inclusion is nickel sulfide, which can be minimized but not completely eliminated.
In situations where spontaneous breakage due to nickel sulfide inclusion is a concern, an alternative such as heat-strengthened glass or heat soaked tempered glass should be strongly considered.
Heat-soak test
To minimize the risks of spontaneous breakage from nickel sulfide (NiS) or other critical inclusions, a heat soak test can be performed. The process involves placing the fully tempered glass inside a chamber and raising the temperature to approximately 290ºC or 550°F for at least two hours to accelerate nickel sulfide expansion. This may cause glass containing critical nickel sulfide inclusions to break in the heat soak chamber, thus reducing the risk of potential field breakage.
The heat soaking process is not 100 percent effective to eliminate the risk of spontaneous breakage.
The current standard available globally about the heat soak test is the European standard EN 14179. In Europe, all glass which has passed the heat soak test should be visibly marked with the “tempered heat-soaked glass” label.
Want to know more about glass and its properties?
Guardian Glass offers you a wealth of technical notes, tools and online learning to enhance your knowledge about glass and help you specify the most appropriate glass for your project. Visit our Resource Hub to learn more!
What are the applications for heat-treated glass?
Heat-strengthened glass is commonly recommended for applications without safety requirements but which are subject to elevated thermal stress and mechanical loads. Besides increased strength, frame retention is a benefit from using heat strengthened glass. Another advantage of heat-strengthened glass is when used in laminated glass, the laminated glass provides much higher residual load capacity meaning that the glass remains in the shape after breakage.
Fully tempered glass is commonly used for applications with extreme structural design loads and where thermal stress is greater than heat-strengthened glass can resist. Many regions use it to comply with safety glass requirements in accordance with applicable codes. These applications include doors, areas with access to public, pool/shower enclosures.
Visual quality of heat-treated glass
While the heat-treatment process can increase the strength of glass, it can impact its visual quality.
Roller waves and optical distortion:
When glass is heated to high temperatures in the furnace, it softens and slumps between the conveyor rollers, leading to what we call roller wave distortion. Roller wave distortion is inherent to the heat-treatment process and cannot be completely eliminated. However, state-of-the-art tempering lines can heat the glass more efficiently and uniformly, helping to reduce the risk of overheating and to minimize distortion.
The level of roller waves can be monitored visually with a “Zebra board” at the end of the line to verify the optical quality in reflection. The deformation can be measured manually with roller wave gauges or with automated online surface scanners.
The maximum allowed tolerances for optical distortion are described in the relevant standards, which vary depending on the region of the world.
Anisotropy, or quench marks:
Anisotropy, also known as quench marks, strain pattern or iridescence in some regions of the world, are typical irisation formations on heat-treated glass due to the internal stress distribution in each pane, typically visible under polarized light conditions as arrays of iridescent spots or lines. Under some lighting conditions, these patterns can be seen in ambient light without the use of polarized lenses. Currently, anisotropy is considered a visual effect, not as defects.
However, state-of-the-art tempering lines can limit the effect of anisotropy significantly by adjusting the furnace and quench zone. Furthermore, measurement technologies are available today for monitoring the light retardation effect associated with anisotropy.
What are the applicable standards for heat-treated glass?
To address manufacturing process, specification and tolerance for heat-treated glass, different standards exist. These standards vary depending on the region of the world.
In Europe for example, the standards defining the heat-treated glass are:
- EN 1863 for Heat- Strengthened glass
- EN 12150 for Thermally Toughened glass
EN 12600, describes the test procedure for safety glass. In the U.S., the standards defining heat-treated glass are:
- ASTM C1048 - for Heat-Strengthened and Fully Tempered Flat Glass.
- CPSC 16 CFR 1201 provides the requirements under the law to produce safety glazing materials.
- ANSI Z97.1 establishes the test method for safety glazing materials.
There are also other standards available in other parts of the world.
👉 Expert talk
Based on your project’s specific needs, you can decide whether to go for annealed or heat-treated glass. Annealed glass is standard and suitable for many applications, but if you need something stronger and safer, heat-treated glass might be recommended. Let’s take some time to assess your project’s specific requirements carefully and make an informed decision:
- Strength: does your project demand a glass that can withstand significant force?
- Optical quality: Does the glass need to be perfectly clear, without any distortions? This is particularly important for projects where visual clarity is paramount.
- Safety: if the glass is going to be used in a public space, safety can become a major concern. You need to help ensure that the glass won't pose a risk if it breaks.
- Bending strength and thermal breakage resistance: how well does the glass hold up under stress and temperature changes?
You need help for thermal stress analysis for your glazing project? Our team can provide recommendations.