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How To Tell If Glass Is Tempered

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Maybe you are installing new windows, or you have a keen interest in glass furniture. If you wonder whether the glass you are installing or already own is tempered, we can help! We've researched how to identify tempered glass. You'll find a few key features listed below that can tell you if the glass is tempered.

There are several ways to determine if the glass is tempered, such as: 

  • Smooth edges
  • Visible imperfections
  • Viewable lines or dots through polarized glass

Now that we know how to identify tempered glass, let's take a closer look at where to find these identifiers. Please continue reading as we discuss the tempering process, how tempered glass is used, and how to locate the glass's stamp of approval. We'll also discuss whether tempered glass is breakable, including whether or not it can be cut. 

The modern loft interior with aquarium, How To Tell If Glass Is Tempered

Tell-Tale Signs of Tempered Glass

The odds are that you have tempered glass items throughout your home. But unless you are initially purchasing or installing the glass sheet, it can be challenging to determine whether the glass is annealed or tempered. First, let's take a look at the tempering process to understand better how this glass is formed. And then, we'll delve into how to identify tempered glass by using the previously mentioned features. 

Tempered glass gets its name from the very process used to create it, called tempering. This heating and cooling process strengthens the glass --it can also be achieved chemically but is less commonly done this way because it is far more expensive. Tempering involves heating a sheet of annealed or regular glass to 1112-degrees Fahrenheit (600-degrees Celcius) and then rapidly cooling or quenching the glass with a blast of high-pressure air. This process causes tension in the glass sheet's center, making it significantly stronger than annealed glass and requiring more force or pressure to cause it to break. Tempered glass will have a few notable features to ensure that it has been through this process. 

Smooth Edges

Sheets of Factory manufacturing tempered clear float glass panels cut to size

Possibly the most distinguishing feature of tempered glass is the smooth-to-the-touch edging. You'll notice rounded, smoothed edges as compared to annealed glass's rigid, rough edges. Of course, you'll only be able to check by feeling the exposed edges of a sheet of glass that has not been framed or mounted in some way pre-installation. 

Visible Imperfections

You might notice slight imperfections in a sheet of tempered glass, particularly near the edges. This does not mean the glass is weakened. These scratches, dings, or melded glass bits have occurred as a result of the tempering process. Imperfections result from the handling of extremely hot, moldable glass. Once installed, these areas are typically covered by a frame. 

Viewable Lines or Dots Through Polarized Glass

polarizing filter for camera accessory for camera bright Sunny day nd filter

Every wonder why lines or dots appear on your car's windshield if you tilt your head ever so slightly while wearing polarized sunglasses? This is a tell-tale feature of tempered glass. Look at the sheet of glass through a polarized glass or lens to detect subtle lines that have been machined into the glass by rollers during the tempering process. 

Does Tempered Glass Have to be Stamped?

Every sheet of tempered glass should display a manufacturer's stamp or etched identifier. This stamp may be a symbol, logo, or serial number located on the glass sheets' corner edge. By building code standards, to ensure that tempered glass is approved by the Consumer Products Safety Commission (CPSP) for impact-resistance, it must also display a stamp reading, "CPSC 16 CFR 1201 CAT II."

What is Tempered Glass Used For?

The modern loft interior with aquarium, How To Tell If Glass Is Tempered

Tempered glass is virtually everywhere you look! In fact, you might even be reading this on a tempered glass touchscreen. At home and in commercial buildings, tempered glass is used for windows, doors, furniture, and appliances.

When you're headed downtown in your car, you are likely surrounded by a tempered windshield and windows too. Even while participating in recreational activities like diving, skiing, going to the gym or a hockey game, and playing racquetball, tempered glass goggles, walls, and mirrors keep us safe in potentially hazardous environments.

Applications using tempered glass have reached great heights in construction and technology. Not only is this strengthened, safety glass used to construct skyscrapers but, on a daily basis, it protects while using public transportation and safeguards a casserole in the oven. The next time you look at a screen or grab a snack from the fridge, consider the many uses of tempered glass: 

  • touchscreens
  • automotive windshields and windows
  • entryway doors
  • shower doors
  • stairs and railings
  • tabletops and countertops
  • indoor/outdoor furniture
  • refrigerator shelves
  • cookware and bakeware
  • solar panels
  • skylights 
  • aquariums 
  • protective eyewear

Is Tempered Glass Breakable?

Destruction of bus shelter after a vandalism incident

The ultimate safety glass, even used for bulletproof windows, you'd think tempered glass is unbreakable. But, you might be surprised to learn that tempered glass is absolutely breakable.

Although it is impact-resistant, four to five times stronger than annealed glass, this safety feature does break more willingly than you might expect. Tempered glass does not shatter but instead breaks into tiny fragments. The idea being that small pieces cannot injure someone as readily as sharp shards. 

How Much Force Does It Take to Break Tempered Glass?

Suppose you have a tampered glass furniture or stairs in your home. Is it very durable? The strength of tempered glass is measured based on how much compression it can withstand in pounds-per-square-inch (psi). Typically, tempered glass can withstand a minimum of 10,000 psi to 24,000 psi --compared to just 6,000 psi for annealed glass. At the heavy end, that's roughly 60 lbs of force!

How much weight can your glass tabletop hold? That depends on the tabletop's dimensions and the thickness of the tempered glass. You can use equations to determine this, or simply use a glass weight load calculator --a great tool for determining the weight limit of glass shelves, coffee tables, and desks.

Is There Anyway to Cut Tempered Glass?

Glass sheets are pre-cut to the desired size before undergoing the tempering process. After tempering, only professionals should cut the glass sheets.

If you attempt to cut or drill tempered glass without the know-how or proper tools, it will simply shatter. Remember, tempered glass is also referred to as safety glass; it is intended to break upon impact to prevent you from being injured by sharp edges. Professionals use laser cutters to reshape tempered glass.

You might have heard that you can cut tempered glass if it is heated first. Heating tempered glass will make it more pliable and removes the tension from tempering, essentially returning it to annealed form.

The glass has lost its safety, impact-resistant properties and is more likely to break upon attempted cut even when using a glass cutter --however, rather than minuscule bits, you'll get rough shards. Heating and cutting tempered glass is not recommended, and any cut made should be left to the professionals.

Final Thoughts

Home modern style aluminum rail and tempered glass

With so many applications for tempered glass surrounding us, it helps to know that it not only looks appealing but has safety implications as well. You can tell if the glass in question is tempted by examining the edges, looking for imperfections, and viewing it through polarized glass for identifiable lines. Officially, tempered glass should have a stamp or etching to identify it. Please leave the processing and cutting of this durable glass to the pros! You might also enjoy reading: 

Is the Shower Door Glass Typically Tempered?

How To Keep People From Walking Into A Glass Door