How To Tell If Glass Is Tempered (Quickly & Easily!)

Ryan Womeldorf
by Ryan Womeldorf

All glass may seem the same but there are distinct differences that can make a major difference. Being tempered, for instance, glass can become much stronger and more reliable. It can mean improved protection in eyewear all the way up to shatter-resistant glass on your car.

But how do you really know if the glass in question has been tempered? Surely, there has to be a way to tell. You can start by looking at the edges for smoothness, check for any imperfections, and keep an eye out for the manufacturer “bug”. These marks will tell you whether or not the glass you are looking at has been toughened or tempered.

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What is Tempered Glass and What is it Used for?

Even if you have heard of tempered glass, do you really know what it is? If you don’t, that’s okay, but knowing more about it can help you better identify this protective glass when you see it.

Plate glass is typically annealed by heating it and cooling it slowly, which makes it less breakable. Manufacturers make tempered glass by reheating this annealed glass to a high temperature of roughly 1,200 degrees Fahrenheit. Then, the glass is cooled quickly to a temperature between 400 and 600 degrees Fahrenheit by using air currents to quench the heat.

This process hardens the glass, giving it a unique structure and making it more durable in the process. It allows the glass to stand up to more tension and punishment than untampered glass. Now that we know what it does, where do we typically find tempered glass?

  • Gadgets. One of the most common uses for tempered glass is in screen protectors. It is not uncommon for phones, tablets, and other small screens to come fitted with tempered glass. When you hear a gadget proclaiming to be drop or shatter-resistant, part of the reason is tempered glass.
  • Cars. Another major use for tempered glass is in vehicle production. Cars are being built safer than ever and part of the reason for that is tempered glass. In the event of a collision, tempered glass is less likely to shatter, keeping the passengers safer.
  • Microwaves. Some of the doors are tempered glass but you will generally see the microwave plates constructed of tempered glass. Because they are meant to hold a variety of dishes (and dish sizes), they need to be able to hold up to the constant pounding of dishes and microwaves.
  • Shower doors/glass doors. The last thing anyone wants out of the doors in their home or building is to shatter suddenly. That is why it makes sense to have them constructed of tempered glass. They can be held up to wide swings in temperate as well.

How To Tell If Glass Is Tempered

Step 1: Check the Edges

One of the quickest and easiest ways to determine if the glass has been tempered is to look for the smooth edges. Plate glass, for instance, is generally harder since it goes through an intense heating and gradual cooling procedure in order to make it stronger and sturdier.

When glass goes through the tempering process, it is treated with extreme heat, yielding a smooth finish. Tempered glass is also sandblasted, so the edges become even and smoother from all of the additional processing.

Glass that hasn’t been tempered will have edges that are quite rough to the touch. If you aren’t sure about the glass in question, simply run your fingers along the edge of a sheet.

Step 2: Look for Imperfections

Tempered glass undergoes quite a bit of work. That said, the changes made to the glass can leave imperfections here and there. It could be slight bending. It could also be some distortions somewhere along the edges of the glass.

The imperfections that you see are because of the intense heating process that tempered glass has to endure. As it gets tempered, the glass becomes more mendable as it gets super-hot. When tongs are used to pick it up, that is what leaves those small imperfections in the glass.

Oftentimes, these imperfections are tiny and can be found in the corners of the tempered glass sheet. Just because you find small imperfections does not indicate that the glass is fake. In fact, they are probably most likely a strong indication that you are dealing with tempered glass, instead of traditional annealed glass.

Step 3: Use Polarized Glasses

Another great way to tell whether or not glass has been tempered is to make use of polarized glasses. Hold the glass up to the sunlight while also using polarized glasses and you should see a substantial difference.

While holding the glass up, look for spots or dark, shady lines across the surface of the glass. If you see those things, the glass is tempered. The lines are part of the tempering process when machine rollers are moving over the surface of the glass. To view these lines, you’ll need to look extra close, as they can be very subtle.

Step 4: Look for the Stamp Identifier

Each glass sheet that gets tempered will have a stamp on it as an identifier. The stamp will show both the CPSC (Consumer Product Safety Commission) standards as well as the manufacturer name. So, if you ever have doubts about the condition of the glass, check for that stamp. It serves as proof for whether or not glass is tempered.

When buying directly from a manufacturer, you can easily check for their logo somewhere on the glass. This is important, especially when buying through a less than reputable vendor or manufacturer. If you can’t find the stamp, then you have likely purchased glass that has not been tempered.

Step 5: Try Scoring a Line

One final way that you can identify whether or not glass has been tempered is by scoring a line using a window-cutting tool. It is important that you only do this if you had planned on cutting the glass anyways, as you wouldn’t want to intentionally damage the glass otherwise.

Additionally, if you are planning on cutting glass yourself, you should only be cutting annealed glass. For tempered glass, it’s best to leave the cutting to the professionals. However, for scoring purposes, you can tell the difference between annealed glass and tempered glass by using a window-cutting tool.

When scored, tempered glass yields a flaky and/or bumpy line. Whereas, annealed glass will create a clean line when scored. Though, if you do not have another glass to compare the tempered glass to, it’s best to have a professional do this for you. Again, cutting scored glass is a bad idea; leave it to the professionals since it can crack and chip.

Maintaining Tempered Glass

Tempered Glass Scratches

Tempered glass is super tough, so it may be confusing to notice scratches here and there. But the scratches are actually apart of the manufacturing process. Generally speaking, those scratches are the result of some fabricating debris.

Dust and debris. During the production process of tempering glass, dust and debris can be present. As it sits there during the tempering, it basically fuses itself to the glass surface. So, when you go to clean the glass later on, it can dislodge some of the debris, creating visible scratches across the surface of the glass.

Poor maintenance. The most common reason for the remaining debris and dust is that the glasswasher used during production has not been properly maintained. When it doesn’t get cleaned regularly, debris can build up and any panels of glass that go through the washer could wind up with small dirt particles trapped to the surface.

How To Avoid Scratches to Tempered Glass

The best way to keep tempered glass scratch-free is to make sure it is free of dust, debris, and dirt during the manufacturing process. Of course, most of us can’t control that since we don’t handle the manufacturing ourselves. So, what are the alternatives?

A good scraper. You can use a quality scraper in tandem with a razor blade to get off even the toughest of particles. That should cover paint, adhesives, and any other sort of sticky substance that can cling to glass windows.

No blade window cleaner. The best way to keep tempered glass scratch-free is to make sure that the window cleaner doesn’t use a blade on the surface of the glass. When scraping a blade over the window’s surface, the loose debris that got trapped during the fabrication process can come loose, scratching the surface as it is moved.

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Related Questions

Can You Drill Tempered Glass?

If you are thinking about drilling into your tempered glass, stop right there. Despite the fact that tempered glass is harder than annealed glass, it is not meant to be drilled into. Doing so will shatter the glass completely. At best, you’re out a piece of glass. At worst, you could be staring down serious cuts.Any holes that need to be created in the glass should be done prior to the tempering process. The heating will temper the glass, making it shatter if you drill it. It is also a good idea to leave cutting tempered glass to the pros since it can chip and shatter, too.

Does Tempered Glass Have to be Stamped?

We know that one of the main identifying factors of tempered glass is in the stamp or logo of the manufacturer. Failing to find that stamp is generally an indication that the glass in question is not tempered. But is it mandatory for the glass to be stamped by the manufacturer?Yes, every piece of glass has to have a logo or permanent stamp on it to indicate who made it. There is one caveat, though. There are no formal standards that regulate where the stamp has to go. It could be on any surface or one of the edges, so make sure that you look carefully. It can be easy to miss that stamp, especially if you don’t know what to look for.

Can You Break Tempered Glass with Your Fist?

Ever knock on your car window and wonder why it feels so much stronger than some of the other glass you’ve come across? As mentioned above, that is because car windows are tempered to make them stronger and safer.If you find yourself locked out and needing to break a window, don’t use your hand. The glass probably won’t break, but your hand will. Tempered glass is meant to stand up to almost 24,000 psi, so your hand isn’t going to be up to the task.

Is Tempered Glass Expensive?

Since tempered glass is stronger, lasts longer, and is safer upon shattering, it is usually more expensive than regular glass. In fact, you’re looking at spending between $12 and $14 more per square foot for tempered glass compared to standard glass. A single sheet of tempered glass usually starts at around $5 per square foot, but customizations and thickness can cause this price to increase.

Ryan Womeldorf
Ryan Womeldorf

Ryan Womeldorf has more than a decade of experience writing. He loves to blog about construction, plumbing, and other home topics. Ryan also loves hockey and a lifelong Buffalo sports fan.

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