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Is glass heat safe?

08 Apr.,2024

When thinking of some of the world’s most dramatic, visually breathtaking buildings, they most likely involve large expanses of glass. Before these architectural masterpieces can be created, the glass may need to be heat-treated for durability and/or safety reasons. The type of processing required—heat-strengthening or tempering—depends on the glass’ specific application.

There are two different types of heat-treated glasses, heat-strengthened and tempered. The similarities between the two include:

Production using the same processing equipment
Heating the glass to approximately 1,200 degrees Fahrenheit, then force-cooling it to create surface and edge compression

The differences between the two glasses are as follows:

With tempered glass, the cooling process is accelerated to create higher surface compression (the dimension of force or energy per unit area) and/or edge compression in the glass. It is the air-quench temperature, volume and other variables that create a surface compression of at least 10,000 pounds per square inch (psi). This is the process that makes the glass four to five times stronger and safer than annealed or untreated glass. As a result, tempered glass is less likely to experience a thermal break.
With heat-strengthened glass, the cooling process is slower, which means the compression strength is lower. In the end, heat-strengthened glass is approximately twice as strong as annealed, or untreated, glass.

The industry standard specification requirements for heat-treated or tempered glasses are ASTM C1048. For heat-strengthened glass, the requirement is a surface compression of 3,500 to 7,500 psi with no requirement for edge compression. Fully tempered glass will have either a minimum surface compression of 69 MPa (10,000 psi), or an edge compression of not less than 67 MPa (9,700 psi).

Safety glazings are typically specified to provide security or to keep occupants safe wherever there is the potential for broken glass to hurt people if it becomes a projectile, such as in a fire, explosion, tornado or hurricane.

Heat-strengthened glass is typically specified when additional strength is needed to resist wind pressure, thermal stress or both, and the additional strength or safety break pattern of fully tempered glass is not required. Another advantage of heat-strengthened glass is that when broken, the glass fragments are more similar in size and shape to annealed glass fragments and thus tend to stay in the opening longer than fully tempered glass particles. Although heat-strengthened glass is NOT a safety glazing by building code, this breakage pattern prevents the glass from falling and injuring someone.

Safety glazing is fully-tempered glass that meets the requirements of the safety glazing standard CPSC 16 CFR 1201 or ANSI Z97.1, or both. Safety glazing, when broken, fractures into relatively small pieces, thereby greatly reducing the likelihood of serious cutting or piercing injuries in comparison to ordinary annealed glass. For some applications, such as glass in doors, tub and shower enclosures and fixed glass in close proximity to a walking surface, fully-tempered safety glass is required by building codes and ordinances. Fully-tempered safety glass is often used for other applications where its strength and/or safety characteristic are desirable, such as table tops, counter tops, showcase enclosures, refrigeration and food service equipment, furniture and similar applications. Fully-tempered glass also is used in applications when significant additional strength is needed to resist wind pressure, thermal stress or both.

However, because of this break pattern, tempered glass is much more likely to fall from the glazing system immediately upon breakage. It’s essential for design professionals to consider this when selecting a safety glass. If it’s important for the glass to stay anchored in the glazing system, another form of safety glass, such as laminated glass, should be considered.

With both heat-strengthening and tempering, distortion is a possibility. Optical image distortion occurs in glass for many reasons, including glazing pressure, wind load, temperature and barometric pressure changes, or even changes in altitude between where a glass is made and where it is installed. Because of its fluidity at higher temperatures, glass also is inherently susceptible to roller wave, bow and warp while it is being heat-treated.

That’s why doing full-scale mock-ups under job-site conditions are recommended to evaluate the optical aesthetics of a specific heat-treating process. In addition, the following tips are suggested in order to minimize the potential impact of glass distortion that is inherent with heat-treating processes:

Produce all heat-treated glass for a given project on the same equipment, using the same processing parameters
Glass distortion may also occur due to strain patterns in heat-treated glass or interference fringe patterns in the fabrication of insulating glass units
Use thicker glass, as it is less prone to distortion
Orient heat-treated glass so that roller wave (the periodic wave imparted to glass during heat-treatment, measured by the peak-to-valley distance) is parallel to the window sill/header

While there is no industry standard to quantify permissible heat-treated glass roller wave, a tolerance of 0.005 inches is often specified. Vitro Architectural Glass (formerly PPG glass) recommends using a millidiopter specification instead, if it is available.

Safety is a prime consideration when determining the type of heat-treating process necessary for a project. Deciding whether to use heat-strengthened or tempered glass depends on the specific application. For example, heat-strengthened glass can be selected for applications that do not specifically require a safety glass product; and tempered glass should be used wherever safety glass is a requirement.

For complete technical information about using heat-treated vs. heat-strengthened glass, read Vitro Architecutral Glass Technical Document TD-138. For any other glass questions, please contact Vitro glass or call 1-855-VTRO-GLS (1-855-887-6457).

Is Tempered Glass Heat-Resistant?

January 18, 2024

The question of whether tempered glass is genuinely heat-resistant has been a subject of importance, given the widespread use of this specialized glass in various applications. Tempered glass is recognized for its enhanced strength and safety features, making it a preferred choice for architectural, automotive, and consumer electronic purposes.

In this short guide, we will review the thermal properties and stress resistance of tempered glass to provide a comprehensive understanding of its ability to withstand high temperatures and thermal shock.

An Overview of the Tempering Process

Tempered glass undergoes a specialized tempering process, involving rapid heating and cooling. During this process, the glass is heated to high temperatures, close to its softening point, and then rapidly cooled using high-pressure air jets. This quenching induces a state of high surface compression and edge tension, giving tempered glass its remarkable strength. While this process enhances its resistance to mechanical stress, it also imparts a degree of heat resistance.

Thermal Strength and Stress

Thermal strength and stress play important roles in determining the heat-resistant qualities of tempered glass. The tempering process provides unique characteristics to the annealed glass, influencing its ability to withstand thermal changes. The rapid heating and subsequent rapid cooling create a state of high surface compression and edge tension within the heat-resistant glass structure.

The hardened surface layer and compressed core contribute to the glass’s enhanced resistance to mechanical stress and its ability to handle thermal stress. Unlike regular glass, tempered glass is less prone to breakage when exposed to sudden temperature changes. The strengthened structure allows tempered glass to absorb and distribute thermal stress more effectively, reducing the risk of breakage due to expansion or contraction caused by heat.

Understanding the interplay between thermal strength and stress provides insight into why tempered safety glass is heat-resistant, making it a reliable choice in applications where exposure to varying temperatures is common, such as architectural glass installations and automotive windows.

High Temperature Tolerance

One of the notable attributes of tempered glass is its high temperature tolerance. The tempering process provides a strengthened structure to the glass, enabling it to endure elevated temperatures more effectively than untreated glass.

Tempered glass can withstand temperatures well beyond those encountered in everyday scenarios. This makes it particularly suitable for applications where exposure to high temperatures is inherent, such as in architectural elements exposed to direct sunlight or automotive windows subjected to intense sunlight and engine heat.

The enhanced thermal resilience of tempered flat glass ensures that it remains structurally intact and aesthetically sound even in environments with elevated temperatures, underscoring its reliability and versatility in various practical applications.

Thermal Shock Resistance

The thermal shock resistance of tempered glass is a testament to its ability to endure sudden and extreme temperature changes without compromising structural integrity. Thermal shock occurs when a material undergoes rapid temperature variations, and untreated glass is particularly susceptible to this phenomenon. However, the tempering process imparts unique properties to the glass, creating a robust structure that can withstand thermal shock.

The high surface compression and edge tension within tempered laminated glass allow it to dissipate the effects of sudden temperature changes more effectively, reducing the risk of breakage or shattering. This quality makes tempered glass well-suited for environments where rapid temperature fluctuations are common, such as in kitchen appliances, architectural facades exposed to varying weather conditions, and automotive applications.

The thermal shock resistance of tempered glass enhances its safety profile, making it a reliable choice in scenarios where exposure to abrupt temperature changes is unavoidable.

Application in Cooking Environments

The exceptional heat-resistant properties of scratch-resistant tempered glass find practical application in cooking environments, making it a preferred material for various kitchen appliances and cookware. Tempered glass is commonly used in oven doors, providing a transparent and durable barrier that allows users to monitor the cooking process without compromising safety. Its ability to withstand high temperatures makes it well-suited for this demanding application.

Heat-strengthened tempered glass is employed in cookware such as bakeware and casserole dishes, where exposure to extreme heat is inevitable. The tempered glass ensures that these cooking vessels can endure the rigors of oven use, offering a versatile and resilient option for home chefs. The transparency of tempered glass also enhances the visual appeal of cookware, allowing users to witness the cooking process.

In all types of cooking environments, tempered borosilicate glass is a reliable and heat-resistant material, contributing to both safety and functionality in the kitchen.

Up to What Temperature Can Tempered Glass Withstand Heat?

Tempered glass is heat-resistant compared to regular glass. While the specific temperature threshold can vary based on the manufacturing process and the quality of the glass, tempered glass typically has a heat resistance range of about 878 to 968 degrees Fahrenheit.

Beyond this range, there is a risk of the glass losing its structural integrity, undergoing thermal stress, or even breaking. It’s important to note that the actual heat resistance can vary, and extreme temperature differentials, such as exposing the glass to intense heat followed by rapid cooling, should be avoided to prevent thermal stress.

In practical applications, such as in oven doors, tempered glass is well-suited to handle the temperatures encountered in everyday scenarios. Always refer to specific product guidelines and consult manufacturers to ensure the safe use of tempered glass in various applications.

Trust the Leader for High-Quality Custom Tempered Glass You Need

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