Glass is one of the most common and useful materials in our daily lives. We use it for windows, doors, mirrors, screens, containers, art, and more. But not all glass is the same. Depending on how it is processed, glass can have different properties and characteristics that affect its performance and suitability for various applications.
One of the most popular and widely used types of glass is tempered glass. In this article, we will explore what is tempered glass, how it is made, what are its benefits and drawbacks, and what are some possible alternatives to it.
What is Tempered Glass?
What is Tempered Glass? Tempered glass is a type of safety glass that has been processed by controlled thermal or chemical treatments to increase its strength and durability compared to normal glass.
Tempering creates compressive stresses on the surface of the glass and tensile stresses in the interior, which make the glass more resistant to breaking, thermal shock and impact. When tempered glass breaks, it shatters into small granular chunks instead of sharp jagged shards, reducing the risk of injury.
It’s widely used in various applications that require strength, safety and thermal resistance, such as vehicle windows, shower doors, aquariums, architectural glass doors and tables, refrigerator trays, mobile phone screen protectors, bulletproof glass components, diving masks, and plates and cookware.
How Tempered Glass is Made
Tempered glass is made by heating annealed glass (ordinary glass that has been slowly cooled after being formed) to a uniform temperature of about 700°C in a special furnace.
Then, the glass is rapidly cooled down by blowing air jets on its surface. This process creates a stress pattern in the glass that puts the outer layer into compression and the inner layer into tension.
The compressive stress on the surface prevents cracks from propagating or expanding, while the tensile stress in the core gives the glass elasticity and toughness. The resulting tempered glass is about four times stronger than annealed glass and can withstand temperature changes of up to 250°C.
The following table summarizes the main differences between annealed and tempered glass:
| Property | Annealed Glass | Tempered Glass |
| Strength | Low | High |
| Thermal Resistance | Low | High |
| Safety | Low | High |
| Breakage Pattern | Sharp shards | Small pebbles |
| Cutability | Yes | No |
Tempered glass can also be made by chemical treatments that involve immersing the glass in a bath of molten salts (usually potassium nitrate) at a temperature of about 400°C. This process exchanges sodium ions on the surface of the glass with larger potassium ions from the salt bath, creating a layer of compressive stress on the surface. The chemical method can produce thinner and more flexible tempered glass than the thermal method, but it is more expensive and time-consuming.
Benefits of Tempered Glass
Tempered glass has many advantages over annealed glass that make it suitable for various applications that require strength, safety and thermal resistance. Some of these benefits are:
1. Strength
Tempered glass can withstand higher loads and pressures than annealed glass without breaking or deforming. It can also resist impacts from objects or projectiles better than annealed glass.
For example, tempered glass can be used for car windows (except for windshields) that need to protect passengers from road debris or accidents.
2. Thermal Resistance
Tempered glass can endure high temperatures and temperature changes without cracking or shattering. It can also prevent heat transfer through the glass, which can save energy and reduce greenhouse gas emissions.
For example, tempered glass can be used for oven doors or cookware that need to withstand high heat without breaking or melting.
3. Safety
Tempered glass is safer than annealed glass because it breaks into small granular chunks instead of sharp jagged shards when broken. This reduces the risk of injury from cuts or punctures for people or animals that come into contact with broken glass.
For example, tempered glass can be used for shower doors or aquariums that need to prevent injuries from broken glass in case of accidents or vandalism.
4. Durability
Tempered glass is more durable than annealed glass because it is less prone to scratches, abrasions or corrosion. It can also resist UV rays and chemicals better than annealed glass.
For example, tempered glass can be used for mobile phone screen protectors or diving masks that need to protect the display or lens from damage.
Disadvantages of Tempered Glass
Despite its many benefits, tempered glass also has some drawbacks and limitations that need to be considered before choosing it for a specific purpose. Some of these disadvantages are:
1. Non-Cutability
Tempered glass cannot be cut, etched, carved, polished or altered after it is tempered because any change in its shape or surface will disturb the stress pattern and cause it to break.
Therefore, any cutting or shaping must be done before tempering. This limits the design flexibility and customization options for tempered glass products.
2. Spontaneous Breakage
Tempered glass can sometimes break spontaneously without any apparent cause due to nickel sulfide inclusions. These are tiny impurities in the glass that can expand and contract due to repeated heating and cooling during tempering.
This can create internal stresses that can eventually cause the glass to shatter. Although this phenomenon is rare and unpredictable, it can pose a safety hazard and a quality issue for tempered glass products.
3. Optical Distortion
Tempered glass can have optical distortion or waviness due to the uneven cooling of the glass during tempering. This can affect the clarity and quality of the glass, especially for large or curved panels. For example, tempered glass can cause visual distortion or reflection for architectural glass doors or tables that need to have a clear and smooth appearance.
Alternatives to Tempered Glass
Tempered glass is not the only type of safety glass available in the market. There are other types of glass that can offer similar or better performance and characteristics than tempered glass for certain applications. Some of these alternatives are:
1. Laminated Glass
Laminated glass is made by bonding two or more layers of glass with a plastic interlayer (usually polyvinyl butyral or PVB) in between. The interlayer holds the glass together even when it is broken, preventing it from shattering into pieces.
Laminated glass can offer better safety, sound insulation, UV protection and impact resistance than tempered glass. It can also be cut, drilled or shaped after lamination. However, laminated glass is more expensive and heavier than tempered glass.
2. Heat-Strengthened Glass
Heat-strengthened glass is made by heating annealed glass to a lower temperature (about 650°C) than tempered glass and then cooling it slowly. This process creates a moderate stress pattern in the glass that makes it about twice as strong as annealed glass but less than tempered glass.
Heat-strengthened glass does not break into small pebbles like tempered glass, but into large shards like annealed glass.
However, it has less optical distortion and less risk of spontaneous breakage than tempered glass. It can also be laminated for additional safety and performance
3. Chemically Strengthened Glass
Chemically strengthened glass is made by immersing annealed glass in a bath of molten potassium nitrate at a temperature of about 400°C for several hours.
This process exchanges sodium ions on the surface of the glass with larger potassium ions from the salt bath, creating a layer of compressive stress on the surface.
It strengthened glass can offer higher strength and scratch resistance than tempered or heat-strengthened glass without affecting its optical properties. It can also be cut, drilled or shaped after strengthening.
However, chemically strengthened glass is more expensive and time-consuming to produce than tempered or heat-strengthened glass.
FAQ’s About Tempered Glass
Is Tempered Glass Unbreakable?
Tempered glass is not unbreakable. It is stronger than regular glass and can withstand more force before breaking, but it can still be broken with a sharp blow or a lot of pressure. When tempered glass does break, it shatters into small, relatively harmless pieces. This is in contrast to regular glass, which breaks into large, jagged shards that can be dangerous.
What is the Weakness of Tempered Glass?
The weakness of tempered glass is its susceptibility to damage along the edges. If the edges suffer any impact or chips, it can cause the whole glass to shatter into small pieces. Furthermore, once tempered glass is broken, it cannot be repaired, and any attempts to modify it, such as cutting or drilling, will lead to fragmentation.
Can tempered glass survive heat?
Yes, tempered glass can withstand high levels of heat. The tempering process makes the glass more heat-resistant compared to regular glass. Tempered glass can typically handle temperatures up to 470°C (878°F) before it begins to soften, making it suitable for use in applications where exposure to heat is a concern, such as oven doors, fireplace screens, and automotive windows. However, sudden extreme temperature changes can still cause tempered glass to break.
Conclusion
Tempered glass is a durable and safety-enhancing material widely used in various applications. Its benefits include resistance to shattering, increased strength, and heat resistance. Despite its advantages, tempered glass can be difficult to repair and may still break under extreme force. Laminated glass and heat-strengthened glass are suitable alternatives, offering different features based on specific needs. Understanding What is Tempered Glass? and the properties of each type allows users to make informed decisions for their intended use.
