What is Tempered Glass?

Building codes require windows to be tempered glass.
Tempered glass may be featured in skylights.
Tempered glass is tougher than ordinary glass, but still requires care when handling.
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  • Written By: R. Kayne
  • Edited By: Niki Foster
  • Last Modified Date: 22 November 2015
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Tempered glass is one of two kinds of safety glass regularly used in applications in which standard glass could pose a potential danger. Tempered glass is four to five times stronger than standard glass and does not break into sharp shards when it fails. Tempered glass is manufactured through a process of extreme heating and rapid cooling, making it harder than normal glass.

The brittle nature of tempered glass causes it to shatter into small oval-shaped pebbles when broken. This eliminates the danger of sharp edges. Due to this property, along with its strength, tempered glass is often referred to as safety glass.

The thermal process that cures tempered glass also makes it heat resistant. Tempered glass is used to make the carafes in automatic coffee makers and the windows in ovens. Computer screens, skylights, door windows, tub enclosures and shower doors are more examples of places you will find tempered glass. Building codes also require the windows of many public structures to be made of tempered glass.

Automobiles use a different type of safety glass for the windshield and tempered glass for the back and side windows. Windshields are made from laminated glass, which sandwiches a sheet of plastic between two panels of glass. When the windshield breaks, the glass panels stick to the plastic film, rather than falling away to possibly injure the driver or other passengers.


Tempered glass breaks in a unique way. If any part of the glass fails, the entire panel shatters at once. This distinguishes it from normal glass, which might experience a small crack or localized breakage from an isolated impact. Tempered glass might also fail long after the event that caused the failure. Stresses continue to play until the defect erupts, triggering breakage of the entire panel.

In recent years, acrylic has replaced tempered glass in many applications in which heat is not a factor. Acrylic is 20 times more impact resistant than glass and does not shatter like tempered glass. Instead, acrylic dents if the impact is strong enough. If the force is sufficient to cause acrylic to fail, it will crack without shattering.

Acrylic is also half the weight of glass and has many other advantages. However, it is flammable. Therefore, you won’t find an acrylic coffee carafe or oven door.


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Post 74

I recently had the tempered glass in my patio door replaced because of a damaged seal. The new glass that was installed is bowed in the middle. When I called the installer he said that the glass should even out in a few days. It's been five days now. Has anyone heard of this happening? Also, the new glass appears thinner than the glass in our other patio door. Thanks for your help!

Post 70

What a nice article about tempered glass. I have a laminated glass canopy.

Post 69

Is tempered glass used for glass top stoves? My stove top broke and Magic Chef has no replacement available. My stove was bought in 1992. Can I just buy tempered glass to replace it?

Post 68

I bought a new tempered glass dining table. Round, 55" dia, 0.5" thickness. I received it yesterday. After cleaning the dust on both surfaces of the glass, I noticed there are two small scratches (actually 2 "dings") on the lower surface. O also noticed something like cloudy curves inside the glass. It is not bumping us, but you can see the cloud from certain angle. I wonder if these are defects of the tempered glass. Should I return it to the shop?

Post 67

Tempered glass is a great safety improvement for protecting a mobile phone's screen.

Post 66

I have a tempered round table that has two sides that can expand. I put one side down and now I can't put it back up again. I don't know if it is due to the fact that it is 85 degrees and the heat has make it expand, but it looks as if there is not space between the two parts. What can I do to get the two parts back together?

Post 64

Tempered glass is a great safety improvement in automotive applications for side car windows. When vehicles were first built, the glass could literally cut your head off and was very dangerous.

Post 63

Can you temper a non-flat cast glass piece? I want to know if tempering an organic sculpted cast glass piece is possible? If not, is there a way to produce a tempering/lamination effect/strengthen the cast glass?

Post 61

@kathyp and anon4980: It is fine to leave your tempered glass out all winter. Think of all the tempered glass used on buildings - it must be able to withstand extreme temperature variation, especially when it's warm inside and freezing on the outside. The only thing I'd worry about is the wind blowing off the table.

Post 60

So you're saying acrylic glass is better? I had acrylic in my basketball goal and the glass broke from shooting a 3. So I got it replaced with acrylic again and it broke from shooting a 3 again. So I moved to tempered and it has never broken, not from my dunks or a 3.

Post 59

I have a three form tempered glass that needs to be installed like a sandwich on a reception area. How much pressure or torque do I need to put on it without it breaking?

Post 57

I'm confused about the the properties of the glass. Can you please give me the properties of a glass?

Post 56

I had a 6foot x 2foot tempered glass which was on top of a dark pine drawer cabinet. The other night at 3 a.m. it suddenly exploded next to us, while we in bed asleep!

It was not chipped, scratched or damaged in any way and had been there for seven years! I had a lot of jewelery boxes on top of it (10 in all). Can't understand why it happened and am now worried about the glass top I have on my dining table which is also tempered, seven years old and very large. Does anyone know if this is normal?

Post 55

what is the difference between as-1 and as-4 auto glass types?

Post 54

I have/had a glass top round patio table, five feet across. last week this table (it was in an outside screen room with a metal roof) "blew up." It shattered and left a pile of little ice cubes. Can anyone tell me why this would happen. Table is about four years old.

Post 52

I am an owner builder with a plan for a curved shower enclosure, having approximately a 44' diameter. Can someone explain the relative advantages and disadvantages of tempered vs. laminated vs cast glass for this application? Thank you.

Post 50

answer to question 19: you can buy fire rated ceramic glass from Technical Glass Products(TGP) in Washington.

Post 49

question 8: Gary, likely you are looking at two different applications for the glass required in your estimate. 1/4" clear tempered is used for all of your interior glazing(vestibules, hallow metal framing etc.)

the low e annealed refers to the makeup of the insulated glass unit on the exterior of the building, in the curtain wall, or windows. this will need to be fully tempered if it is adjacent, or in a door; or 18" from the finish floor.

Post 48

Answer to question 22: tempered glass (1/4" thick) clear, with a rough edge costs less than $2 per square foot from major glass fabricators(Oldcastle, PDC, Arch, etc.) polish edge costs normally 10-15 cents per inch. As you increase thickness, or change the tint, or shape, the cost increases dramatically.

Post 47

Answer to question 25: Don't replace it until it breaks. crazing the edge of a tempered lite does not automatically equal spontaneous breakage. edge stresses are at maximum to about 1/2" and in from the edge of the glass. I have polished ground as much as 1/4" from tempered lites with no breakage. You may get away with it.

Post 46

answer to question 26: If you are looking for resistance to heat/cooling the glass of choice is Heat Strengthened. Many specifications call for all glass to be HS, and fully tempered glass to be heat soaked.

Post 45

Your friend's fireplace glass may be firelite glass which can (and does) run, not shatter.

Post 44

You cannot cut tempered glass, but you can wet-polish grind and wet hole saw tempered glass, very carefully and risk of breakage is extremely high and not recommended.

Post 43

answer to question 30: All overhead glazing must be laminated, many insulated units are laminated glass on the inboard lite, and tempered only on the outboard lite.

The theory is that the tempered glass on the outboard lite has the strength to resist impact from birds, roof gravel in storms, as well as solar heat. The inboard lite will be laminated to confine any breakage from falling onto occupants below.

Wire glass is acceptable as overhead glazing. Wire glass is acceptable in fire rated applications, however in doors, or adjacent to doors, or 18" above finished floor must be impact rated for safety. specifically Wirelite NT.

Post 42

answer to number 38: Many times tempered glass is prone to spontaneous breakage. This occurs when Nickel Sulfide inclusions re-expand during the heat cycling process after installation.

Nickel Sulfide particles shrink during the tempering process and the float/melting process equally, but in the tempering process they are not given time to re-expand during the quenching process, thus are confined to their shrunken state.

When the tempered glass is heated slowly from solar heat gain, the Nickel Sulfide particle grows. It can apply as much as 500,000 psi in a very confined area, thus causing spontaneous breakage. spontaneous breakage does not occur in heat strengthened glass because the nickel sulfide is allowed to re-grow duning the slow cooling process

after the tempering oven, and is also not under the same internal stresses as tempered glass.

It is impossible to eliminate Nickel Sulfide inclusions from glass, however the percentage of tempered lites subject to risk of breakage can be reduced by a process called "heat soaking" Essentially this is just a -"break it now"- theory, but is not 100% effective, it is costly(roughly $1.50/lite/sq ft.) and can weaken tempered glass.

Post 39

No, this glass simply shattered on its own. No one near it. We were inside and heard the noise. Same thing with my neighbor.

Other people who were thinking about having this type of enclosed balcony are now having second thoughts. Also, the balcony glass is seamless, no posts. The glass is on tracks.

Post 38

I have a glass enclosed balcony on the 24th floor of a condo building. After several years with no problems a piece of my glass shattered or exploded for no apparent reason.

What makes this even more interesting is that a neighbor of mine, in the same building has recently had the same thing happen on his balcony, (purchased from same company!).

What would cause this to happen? Keep in mind that there are many other units in this building with the same type of enclosures and this has never happened in the past, as I have checked with our superintendent. Defect in the glass? Bird? Temperature? Thanks.

Post 37

why would a tempered glass table top break without any hit on the glass or edges.

Post 36

I do windshield replacement Atlanta and tempered auto glass parts are used for doors, vents and back glass. It is heated up and cooled quickly so that the outside surface of the glass becomes harder than the middle. This causes the glass to break into small squares rather than shatter into shards during an accident. It is sometimes referred to as safety glass for this reason.

Post 35

There is a crack in our office window about one meter long. It appeared a week ago. there was no impact that we were aware of. What could have caused this? thanks.

Post 34

If toughened/tempered glass shatters when damaged,

why has a friend of mine got a cracked glass in his wood burner? Is this a different type?

Post 33

What is the difference between tempered glass and toughened glass?

Post 32

Can a tempered glass fail after a long while? it had been hardly hit on its thickness?

Post 31

To anon 25940: Every glass panel of my glassed-in off-kitchen balcony has the manufacturer's brand name and the words "Tempered Glass" etched in a corner of each panel.

Surprisingly, here in South Korea, the info is in English only (very unusual).

Post 30

One company tells me that tempered glass is all you need for overhead panels in a sunroom and another company tells me that it must be tempered on the outside and laminated on the inside. Which is correct?

Post 26

Is tempered glass more resistant to heating/cooling damage than normal glass?

Post 25

I have a dual tempered slider that the outer crazed a couple of weeks ago. Will have it repaired, have estimates - but money is tight Duh! How long might I expect the glass in this door to stay put as long as I don't touch it? Yes, I understand there are many factors - but a guesstimate will do. It poses no danger to the inside of the dwelling and we hope the inner panel doesn't duplicate, and no pieces have come away - just crazy. M

Post 24

Can tempered glass be used safely in wood burning stoves and fireplace screens? Will extreme temperatures cause it to "explode?"

Post 22

is tempered glass expensive?

Post 21

What is the u factor and shading coefficient of 10 mm clear tempered glass and grey tint/bronze tint tempered glass? Thanks.

Post 20

you cannot cut tempered glass.

Post 19

where can I purchase two pieces of 18" x 25" x 95" (5mm) pyroceram glass?

Post 18

Would it be a bad idea to clean my tempered glass refrigerator shelves in the dishwasher? Will the temperature change from fridge to hot water possibly break them or should they be allowed to warm to room temp before putting them in the dishwasher? I just hate the job of hand washing them.

Post 17

Can tempered glass be cut without breaking it.

Post 16

Outside table tops that are tempered shoud be fine during the winter. The only concerns would be thermal shock (putting a pot fresh off the stove on to the glass), or if the unit is a metal frame the metal may contract and pop the table top.

J - wholesale glass distributor

Post 15

Anon28649, the glass should pass the safety standards. Should be tempered, heat soaked and laminated. -Pinoy

Post 14

Is there any difference between tempered glass and toughened glass? If yes, tell me what.

Post 13

I was at a shopping center that had a glass balustrade all around the food court on the second floor that just cracked, and fell down onto the lower levels. It had BS class A written on it, this appears very dangerous. If it is so brittle children could run straight through it and fall to their death. Is it normal practice to have a glass balustrade for protection at such height and what is the correct glass to use? I will be forwarding a complaint to the building control officials in the local council authority before some kid dies from the stylish, but dangerous glass.

Post 12

can tempered glass and acrylic filters ultra violet radiation?

Post 11

To: anon25940 - Tempered glass will reveal stress waves when you view it through a polarizer. Besides breaking the glass, that's the only way to determine if it is tempered.

To: smaltzl - Once a glass is tempered, that it, you cannot cut it.

Post 10

How can you tell if it is tempered glass or not?

Post 9

I have tempered glass panel from a sliding glass door. If the frame is removed, can that panel be cut to another size?

Post 8

what is the difference between tempered glass & low-e temp annealed glass, argon filled?

Post 7

With respect to the unexpectedly broken glass, my guess is that the problem probably had to do with a structural defect in the window frame that exacerbated a previous defect in the glass itself.

It is possible that the glass broke because it was not properly fitted to its frame. Without proper fitting, repeated cycles of expansion of the frame (which might be wood or metal) has built up stress in the glass. A slight disturbance, such as your opening the window shade, might put the final "nail in the coffin" for this window.

Also, sometimes older casement (hinged windows that can open like a door) windows are not properly fitted into the window sash. Thus, you might close the window and flex the whole frame, thus putting unexpected stress on the glass.

Post 6

I had a strange thing happen to me. I lower the shade to my bedroom window each nite, a window which gets direct sunlight each morning. One morning as I raised my window shade, the window cracked.

The window was never stuck by anything from either inside or outside. Please explain the cause of the glass break.

Post 5

what is the thermal conductivity of tempered glass with 6mm of thickness?(tempered glass = side windows of cars and rear window)

Post 4

Glass balcony person...if anyone has cleaned with "Dupont 409", I heard, but cannot verify easily, it has an acid in it that etches glass...

If glass is etched...maybe it can be repolished or frame treated back to shiny? or possibly laminated with a polymer...

I do know the olde University of Chicago library had glass blocks for flooring, which was fun...and helped distribute light but over the decades of weary scholarship had all gotten scuffed beyond shiny. Last stop for long-wearing glass issues: Dark Mission--The Secret History of Nasa by Richard Hoagland. Apparently there are very durable types of glass (anhydrous glass).

best wishes, Sparky-Foam

Post 3

I have a tempered glass balcony and it seems as though I can't get them as cleaned as they first were put in. Lots of film on them and tried cleaning with just about everything...

Please help with different options..

Post 2

I have tempered patio furniture, can it stay out during winter?

Post 1

i have an outdoor patio table made of tempered etched glass and i'm wondering if it's safe to leave it out all winter, will the glass break if it freezes?

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