What's the Difference Between Tempered Glass and Regular Glass?

Section 1 of 3:

Differences Between Tempered and Regular Glass

Tempered glass is about 4 times stronger than regular glass, meaning that it can withstand a lot more pressure before shattering. Regular glass, or “annealed” glass, will break at around 6,000 pounds per square inch (psi). Meanwhile, tempered glass, according to federal regulations, must have a surface compression of 10,000 psi or more and usually breaks around 24,000 psi.^{[1] X Research source}

Tempered glass is made by heating up a piece of regular glass to around 620ºC (1,148ºF) and followed up with abrupt cooling using blasts of cold air in a process called “quenching.” This creates tension on the inside and results in an incredibly strong glass. On the other hand, regular glass goes through a slow cooling process that frees it from this internal stress.^{[2] X Research source}

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Regular annealed glass will break into large, jagged shards, but tempered glass will shatter into tiny pieces. This is due to the high compressive stress created in the center of tempered glass during the manufacturing process. The higher the stress, the smaller the pieces will be when broken.^{[3] X Research source}

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Section 2 of 3:

How to Identify Tempered Glass

If you can see the edges, check to see if they are smooth. The extreme heat with which tempered glass is treated gives it a very smooth finish. If the edges are rough, you are likely dealing with just regular annealed glass.

“Bug” here refers to an etched or sandblasted mark directly on the glass. These are usually found on the corner of the glass piece and will often be the symbol of the manufacturer to prove that the piece is legitimate tempered glass. Annealed glass does not have to adhere to the same strict standards as tempered glass, and thus will not likely have any special mark.

In this case, imperfections are not a bad thing. During the tempering process, small particles can be melted into the glass or leave scratches along the surface, so imperfections are common. If you do not have access to view the edges or look for a bug, this is the next best method to check whether the glass is tempered or not. Annealed glass does not go through such a rigorous heating process, so imperfections most likely not be as prevalent.

Hold the glass up so you can view the sun through it. If it's tempered, you will likely see spots or lines going across the glass, which are a result of the heat rollers used in the manufacturing process. They may be very faint at times, so look closely. Annealed glass does not receive such treatment, and thus no marks should be visible.

Scoring a line through regular glass with a window-cutting tool will leave behind a clean cut, whereas scoring tempered glass will result in a bumpy and rough line.
Don't use this method on any glass you wish to leave intact!

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Section 3 of 3:

Pros and Cons of Each Type of Glass

Due to its strength and the way it breaks into small, relatively harmless pieces if shattered, tempered glass is often installed in places where increased safety standards are required.^{[4] X Research source} This includes:

Car windows

Shower doors

Glass tables

Microwave ovens

Skylights

Entrance doors

Patio furniture

Due to the process used to strengthen the glass, tempered glass will shatter if you try to cut it after it has set. Therefore, it is critical that the correct measurements are taken before the tempering process.
If you need to have your tempered glass cut, take it to a professional. However, your request may not be possible depending on the amount of glass needing to be cut.

EXPERT TIP

Douglass Brown
Glassblowing Expert
Douglass Brown is a Glassblowing Expert based in Half Moon Bay, California. He has over 25 years of experience as a glass artist, and is the owner and operator of 2 glass blowing studios in the San Francisco Bay Area, Half Moon Bay Art Glass and Mare Island Art Glass. Douglass believes in sharing hot glass experiences with anyone who wants to learn about the processes of blowing and sculpting glass. When not teaching, Douglass creates glass art that is a mix of his takes on nature and functional glassware. He has also created the Glass Troubadours, which is a mobile glass blowing studio that hosts parties, events, and visits craft shows.

Douglass Brown
Glassblowing Expert

Glass annealing is the process of heating and cooling tempered glass to a point where you can cut it. It will effectively undo the tempering process, weakening the glass to the point where you can cut it. Once it cools back down, it will be a regular sheet of glass again.

Unlike tempered glass, regular glass can be easily and cleanly cut even after the manufacturing process. From a design perspective, it is a desirable material to work with since it can be cut, drilled into, and carved with relative ease.

Regular glass does not possess the high heat resistance and strength of tempered glass, and therefore cannot be used in as wide a range of environments as tempered glass. The large, sharp pieces it shatters into when broken are a danger to people, so it cannot be used in many common objects.

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