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What are Tempered Glass Windows?

Wednesday February 1st, 2023
When deciding which type of glass you want for your windows—be it for a commercial or residential project—it’s important to understand the glass types available. One popular option is tempered glass, which is perhaps the most common and widely used safety glass on the market today. If you’re considering tempered glass for your windows, it’s important to understand what it is, how it’s made, where it’s applicable (and where it isn’t).

Tempered Glass Windows FAQs

  1. What is Tempered Glass?
  2. How is Tempered Glass Made?
  3. How to Tell if Glass is Tempered?
  4. How Strong is Tempered Glass?
  5. When to Use Tempered Glass Windows
  6. Are Tempered Glass Windows Energy Efficient?
  7. How to Clean Tempered Glass Windows
  8. How to Fix Tempered Glass Windows
When you know the ins and outs of tempered glass, you’ll be better equipped to evaluate if tempered glass windows are right for your situation. In some cases, tempered glass is required, so it’s important to know what areas of the home or business necessitate safety windows. This way, you can avoid potentially dangerous situations and remain compliant with local residential and commercial building codes.

Frequently Asked Questions About Tempered Glass Windows

By addressing each of the frequently asked questions about tempered glass windows, we’ll provide you with everything you need to know about tempered glass. This way, you’ll feel confident and knowledgeable about your options throughout your entire window buying process.

1. What is Tempered Glass

Tempered glass is a type of “safety glass,” which means the glass is safer than regular annealed glass. Tempered glass is much stronger, more heat resistant, and breaks differently than annealed glass. Tempered glass is also referred to as toughened glass—these names are often used interchangeably.

Rather than shattering into jagged shards (as annealed glass does), tempered glass breaks into many small pieces with rounded edges. This type of broken glass is much safer than large shards of sharp glass, preventing injury when the glass is shattered. Because of its strength, resistance to heat, and the way it breaks, it is used in a variety of items, including an electronic screen protector, glass tabletops, glass shelves, glass doors, a shower door, and the side and rearview window glass on cars.

Furthermore, tempered glass or laminated glass (another type of safety glass) is required in areas where human safety is a concern. For example, if there was an explosion or severe storm that can shatter a window, those nearby are less likely to be injured by tempered glass than by annealed glass. Plus, cleaning up the debris is safer with tempered glass than annealed glass.

Because tempered glass is four times stronger than regular annealed glass, it has more impact resistance. This means it’s harder to break, and is less likely to shatter from normal wear and tear. For this reason, glass doors, windows near walkways and doors, and glass table tops are required to be made from tempered glass. As you can imagine, if a glass door were to be slammed shut, you would want a toughened glass that can handle the impact.

That said, tempered glass can still break, so it is by no means a bulletproof glass. Moreover, it is more likely to completely shatter when broken, which is why it isn’t considered a security glass. If you’re interested in glass that is shatter resistant, consider laminated glass, which is the strongest safety glass on the market.

You can still use tempered glass for home windows in areas where tempered glass isn’t required. Perhaps you would like the extra safety or want to make sure your windows are more break resistant. Tempered glass replacement windows are a great option in new or older homes, especially in areas that experience frequent and severe storms.

2. How is Tempered Glass Made?

Tempered glass is made from regular annealed glass that goes through an intense heating and cooling process. It can also be made with a chemical treatment.

With the heat method, the annealed glass is first cut to size and inspected for any imperfections that could affect the tempering process. The glass is then readied to be tempered by removing sharp edges with sandpaper. All of this is done before the tempering process. Once the glass is tempered, it can’t be cut or edged down because it will cause the glass to lose its strength or even shatter. This is a critical aspect of tempered windows to keep in mind, especially when ordering custom replacement windows, as the window glass will need to be the correct size and can’t be cut or resized after ordering.

After the glass is inspected, it is ready for the tempering process. Using a tempering oven, the glass is exposed to extreme heat with temperatures over 1,000 degrees Fahrenheit. Then, the glass is immediately moved to an area for rapid cooling by a machine that blows frigid air at it. This second half of the process, where the glass undergoes rapid cooling, is called quenching.

Through exposure to extreme temperatures, the glass changes its structure. The outside of the glass cools much faster than the internal glass, creating tension internally and causing surface compression on the outer layer. This tension causes the glass to be ‘tempered,’ making it stronger and safer than annealed glass. Because this tension is necessary for tempered glass to function properly, the tension can’t be disrupted by cutting the glass, which would cause the structure of the glass to change as the tension dissipates the area it’s cut.

Heating is the most common way tempered glass is made, but glass can also be tempered with chemicals. This is referred to as chemical tempering, and occurs when various chemicals exchange ions on the glass surface to create surface compression. However, chemical tempering is much more expensive than the heat method, so it is not as widely used.

3. How to Tell if Glass is Tempered?

At first glance, tempered glass windows are indistinguishable from regular annealed glass windows. Because they are nearly identical, it can be hard to determine if the glass is toughened or not. Luckily, there are a few ways to identify tempered glass from annealed glass.

Look for the Stamp

The first and easiest way to determine if you have a tempered glass window is to see if there is a stamped identifier on the corner of the glass pane. This stamp is referred to as a “bug,” and is either an etched or sandblasted marking. The stamp identifies the glass as tempered, and will show the glass manufacturer’s name and the CPSC (Consumer Product Safety Commission) standards code.
As you can imagine, sometimes it’s not possible to see the stamped identifier, like if the glass is sealed into a window frame. Also, due to weather and age, these stamps can fade over time, and may no longer be visible to the naked eye.

Inspect the Surface for Slight Imperfections

The second thing you can do to check if your window glass is tempered is to closely inspect the glass surface for imperfections. Look for slight bends or minuscule dents on the glass surface. These barely perceptible imperfections are caused by the tools used in the heating and quelching process. You may also be able to see small scratches on the glass surface. These occur when the annealed glass is heated at extreme temperatures and small pieces of debris melt on the glass.

Observe Glass Pane Edges

Another way to identify tempered glass is to inspect the edges of the glass pane. In some cases, this might not be possible, especially if you’re dealing with a window that sealed into its window frame. But, if you have a glass pane you’re inspecting, check and feel the edges of it. Due to the heating and quenching process, tempered glass edges have a smooth and shiny finish. If the edges of the glass are sharp and rough, it is most likely regular annealed glass.

Wear Polarized Sunglasses

If you are trying to identify the type of glass you have and it’s sealed into a window, you can try looking at the glass with polarized sunglasses. It’s best to do this when the sun is shining through the glass. With the polarized sunglasses on, the sunlight will illuminate streaks or lines stretching along the glass pane.
The reason to use polarized sunglasses, rather than just any pair of sunglasses, is that polarized sunglasses block out certain types of light. When certain types of light are blocked, you can see “imperfections” or markings that are normally hidden by regular light. Tempered glass has nearly invisible streaks due to machine rollers used in the heating and quenching process. These lines are difficult to see, so you will have to closely investigate the glass surface.

Drill or Score it

Lastly, you can damage the glass by drilling into it or scoring a line down it to see if the glass is tempered or not. Obviously, you don’t want to do this with new glass or window glass that is still installed in your home or business. But, if you’re bound and determined to discover if your glass is tempered or not, this is a sure way to find out.

As mentioned, tempered glass breaks differently than annealed glass. When you drill into it, tempered glass will shatter into small pieces with rounded edges. If you drill into regular glass, it will shatter into large, jagged shards with sharp edges. This isn’t the safest way to damage glass, so be very careful if this is the route you decide to go.

Cut the Glass with a Glass Cutter

Alternatively, you can simply cut the glass with a glass cutter. Regular annealed glass produces a clean, straight line (depending on how steady your hand is). Tempered glass, on the other hand, produces a bumpy and uneven line, no matter how still and straight the cut is. As discussed, the heating process changes the structure of glass, thus causing the surface of tempered glass to be less smooth than regular glass. Again, exercise caution when cutting and working with glass, as it can be extremely dangerous.

4. How Strong is Tempered Glass?

Tempered glass is incredibly strong, and is about four times stronger than regular annealed glass. According to Federal Specification DD-G-1403B, tempered glass must have a minimum surface compression of 10,000 psi (pounds per square inch) and a minimum edge compression of 9,700 psi.

Given tempered glass’s psi, its general breaking point weight is 24,000 psi, whereas annealed glass’s breaking point is 6,000 psi. With its increased impact resistance, tempered glass can withstand more wind pressure—about twice the amount as annealed glass.

When it comes to heat and thermal stress, tempered glass can withstand hotter and more extreme temperature changes when compared to annealed glass. Regular glass will crack or break when exposed to temperatures around 105 degrees Fahrenheit, whereas heat-strengthened glass has a temperature threshold around 500 degrees Fahrenheit. Because of its incredible resistance to heat, thermal breakage is not much of a concern with tempered glass.

However, on rare occasions, temperature fluctuations can cause tempered glass to shatter spontaneously. During the heating and quelching process, the glass can develop tiny particles, including nickel-sulfide, which expand during heat treatment, halt expansion when cooled, and then continue expansion when the glass is exposed to high heat temperatures again. This expansion can cause the glass to shatter, but it is extremely rare. This is why manufacturers take special care to inspect the glass before beginning the heat process.

Because of the incredible strength of tempered glass and its resistance to heat and wind pressure, it is often used for windows in tall buildings, such as skyscrapers as well as automobile windows, windows near doorways, glass doors, and anywhere human safety is a concern.

5. When to Use Tempered Glass Windows

Tempered glass can be used whenever extra strength and heat/impact resistance is needed for windows. In certain areas, safety glazing, which refers to safety glass, is required according to federal, state, and local building codes.

Tempered glass requirements for windows include skylights, any windows near walkways, and any windows within 24 inches of a doorway if the bottom edge of the glass is 60 inches or less above the walking surface. However, if there is a window within 24 inches of a door and it is not on the same plane as the door, or the bottom edge of the glass panes is 60 inches or above the floor, it does not need to be tempered glass.

There are a few more exceptions to window safety glass requirements within 24 inches of a door, including; decorative glass, if there is a permanent barrier (such as a wall) between the window and the door, or if the door leads to a small area less than three feet in depth (closet or storage area).
Any fixed or operable window larger than nine square feet with a bottom less than 18 inches from the floor, a top greater than 36 inches from the floor, and within 36 inches of a walking surface should have tempered glass panes. Any windows near stairways and ramps where the bottom edge of the glass is less than 60 inches above the walking surface are also required to have tempered glass.

These areas are considered “hazardous locations,” which is why they require tempered glass windows. These are just a few of the basic tempered glass requirements for windows. There could be more requirements based on local building codes. Because of this, it’s important to check your area’s local building codes when purchasing a new or replacement window. Furthermore, it’s helpful to know the different parts of a window so you can measure correctly when determining whether your replacement glass needs to be tempered.

6. Are Tempered Glass Windows Energy Efficient?

Yes, tempered glass windows can be exceptionally energy efficient, but they are not any more energy efficient than regular annealed windows.

What makes a window energy efficient isn’t necessarily the type of glass used, but how many panes there are and whether there is a special coating applied to the window glazing. A tempered glass window with a single pane will be less energy efficient than a standard glass window with double panes, and vice versa.

Double pane windows are more energy efficient than single pane windows because the chamber between the two panes is filled with a nontoxic gas to increase the glass’s insulation, which increases its ability to block heat transfer and sound transmission. Furthermore, double pane windows are a sustainable and long-term solution to maintain the home’s internal temperature because it effectively insulates the home and prevents air from seeping in or out.

To further enhance a window's energy efficiency, you can choose Low E glass (low-emissivity glass), a type of glazing created to enhance a window’s insulating performance. A special coating is applied to the glass to minimize the amount of infrared and ultraviolet light that comes through without minimizing the amount of natural light coming into the space. This coating is microscopically thin, and isn’t visible to the naked eye.

Tempered glass windows can have double panes and Low E glazing, which will help increase their energy efficiency.

7. How to Clean Tempered Glass Windows

Cleaning tempered glass windows is relatively easy, but does require a little more care and know-how than regular annealed glass windows. By following these instructions, you’ll avoid damaging or scratching the glass—which is especially important with tempered glass.

With tempered glass, you don’t want to use a regular window cleaner, such as Windex, or an alcohol/chemical-based cleaner. Rather, you want to use water mixed with a mild dish detergent.

You can put your soapy water mix in a spray bottle or bucket and use a microfiber cloth or microfiber sponge. The key is to use a gentle and soft material to clean the glass and a cleaning liquid that is free from harsh chemicals.  

Once you have your soapy water mix ready and a microfiber cloth or sponge, you can gently clean your tempered glass windows. While you’ll want to be gentle for most of the cleaning, but don’t be afraid to use a little elbow grease to scrub tough grime or caked dirt.

Once clean, use a dry, soft cloth to buff and dry the glass. This will give the glass surface a streak-free finish.

Avoid using abrasive cloths or paper towels on tempered glass. Never use steel wool or a scouring pad, as it will scratch the glass. Also avoid using vinegar, as the acid will damage the glass surface.

8. How to Fix Tempered Glass Windows

The answer on how to fix tempered glass windows depends on which part of the window needs repair. We recommend contacting a window repair company to inspect the problem. They’ll be able to provide more information about whether your window can be repaired, or if you need to plan on a full window replacement.

In some cases, such as if the glass panes are broken or if there is fog between the panes, you’ll need a full window replacement. Other scenarios, such as a damaged frame, might not necessitate a replacement. It all depends on the situation and what a professional window repair company recommends.

Custom Tempered Glass Windows for Strong and Safe Windows

Tempered glass is an important type of safety glass when it comes to certain residential or commercial windows. They’re much stronger and safer than regular glass, and they’re required in certain hazardous locations. For these reasons, it’s good to know the “ins and outs” of tempered glass—whether you’re a homeowner, contractor, or business owner.

If you’re ready to purchase a new window for your home or business, contact Rustica today! We’ll offer information on the types of windows available, and give you ideas on how to customize them to meet your unique design needs.
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