Breaking glass shower doors in the news…

There has been news coverage lately about breaking glass shower doors and enclosures. Here are two, both of which included yours truly as a quoted resource:

CBS news report on shattering shower doors
ABC news item about shattering shower doors

Recent news stories on the subject of breaking glass shower doors from CBS (left) and ABC (right) quoted Chicago Window Expert. Thank you CBS and ABC!

Whereas TV news is obliged to hurry up and hit the high points of a story, your Chicago Window Expert can take his sweet time and discuss breaking glass shower doors at length. There is nobody telling him to talk fast because the commercial is fast approaching. Now you can get the full story.

With that introduction, let’s get in the shower together and look around.

Spontaneous glass breakage

I have been involved with many matters in which glass shower doors or enclosures have “spontaneously” broken, often while surprised, defenseless, naked people have been inside. Many of these breaking glass shower door matters involved hotels where multiples of such events occurred.

Breaking glass shower doors in hotels

You can imagine the shock experienced by a bathing hotel guest who suddenly finds herself standing barefooted in a sea of shattered glass. While you are busy imagining that, try imagining the chagrin of a hotel manager who works hard to create a positive guest experience, only to find that his nude guest was just severely cut by a breaking glass shower door.

Bathtub after shower door shattered
View looking down into a bathtub after the glass shower door broke. In this case a bather was in the shower at the time the glass shattered. Bloody footprints are indicated by arrows

All in all, this is a bad situation. A bit of knowledge can reduce the chances of this occurring.

Why do glass shower doors break?

So what causes breaking glass shower door? It’s a number of things, and these things can interact with one another. So if you are looking for a sound bite to explain this phenomenon, take your ADHD tuchas to the top of this page and see what the networks have to say.

Tempered glass shower and tub doors and enclosures

Glass shower doors and enclosures are made from tempered glass. This is because in 1977, due to many horrific* glass injuries, the Consumer Products Safety Commission (CPSC – an agency of the Federal Government) mandated that their Safety Standard for Architectural Glazing Materials, 16 CFR Part 1201 become the law of the land.

In the 16 CFR 1201 standard, the highest standard for strength (Category II) was applied to;

Tempered glass breakage pattern
Tempered glass breakage pattern
  • Shower doors and enclosures
  • Bathtub doors and enclosures
  • Sliding glass doors
  • Storm doors that contain any piece of glazing material greater than 9 sf
  • Doors that contain any piece of glazing material greater than 9 sf

Tempered glass is “safety glass”

Tempered glass is defined by the CPSC as, “a piece of specially heat treated or chemically treated glass that cannot be cut, drilled, ground, or polished after treatment without fracture. When fractured at any point, if highly tempered, the entire piece breaks into small particles.” This renders the particles far safer than those of annealed or heat strengthened glass.

So if your glass shower door breaks and you are nearby, be thankful to the CPSC for enacting 16 CFR 1201. Without this or similar regulations**, your injuries could have been much more serious because the glass shards would have been much more large and dangerous.

Why does tempered glass break in small pieces?

I’m glad you asked. Tempered glass is glass that has been subjected to the tempering process after it was originally formed at a float glass plant.

In the float glass plant, the glass first becomes glass by melting a series of powders including silica sand at high temperatures and “floating” it out on a bed of molten tin to the desired thickness. Farther down the line, the ribbon of glass is slowly cooled, or annealed, to reduce and unify internal stresses in the glass. The glass is then cut into large sheets, crated and shipped to other factories.

Float glass being made from melting silica sand and other powders
Float glass being made from melting powders, including silica sand. Here the mixed powders are being dropped into the furnace

Secondary factories perform additional fabrications to the glass such as cutting, drilling, edge polishing, tempering or heat strengthening, coating and insulating. So when the CPSC and other agencies require tempering, this is done at secondary facilities that specialize in tempering. These are known as “tempering plants” or factories with “tempering lines”.

You can’t cut tempered glass or it will explode

Tempered glass for shower doors and enclosures must be cut to size, edge polished, drilled where holes are necessary before the glass goes into the tempering oven because once it is tempered, there can be no further fabrications. Any further fabrication will bring breakage, as the CPSC definition states.

Rapid cooling of heated glass creates tempering

During tempering, the glass is heated to about 1300 degrees F, so that it is nearly in a plastic state, then quickly cooled (quenched) with the use of strong blowers which blow simultaneously at both side of the glass. It is this rapid cooling that creates the unique properties of tempered glass. One property is that the tempered glass is 4-5 times as resistant to breakage due to uniform load (think: steady wind) as annealed glass. The other property is that when it breaks, tempered glass breaks into thousands of small cubes instead of large shards, which is desirable for safety.

During quenching, the outer layers of the glass, which are about 20% of the glass thickness on each sides, cool more quickly than does the core. As the outer layers cool, they shrink and solidify while the core is still nearly plastic. Then when the core cools, it also shrinks, but the outer layers are already solid, so they resist this shrinkage.

Strong internal tension/compression is fundamental to tempered glass

The surface of tempered glass is being permanently compressed by a core that is permanently
The surface of tempered glass is being permanently compressed by a core that is permanently “trying” to shrink

By the end of the tempering process, and forever thereafter, a permanent tug-of-war is present inside the glass. The core is trying to shrink while the surface layers are unable to allow this. Thus it can be said that the core is permanently in a state of tension, being pulled apart by the outer layers. The outer layers, in turn are said to be in a state of compression, because the core is constantly pulling on them as it tries to shrink. The tensile zone, where this fight occurs, is at a depth of 21% of the glass thickness from each face.

In order for glass to be considered fully tempered (FT), the residual (after tempering) surface compression must be at least 10,000 pounds per square inch (6.9 e+7 pa) per ASTM C1048. Some glass can be even more pressurized because there is no maximum in the standard. This is a lot of compression, and is the reason tempered glass breaks into thousands of pieces when it breaks. The release of energy that occurs when tempered glass breaks is almost unimaginable.

Tempered glass breaks at blinding speed

The speed at which the breakage occurs is also mind-boggling. Cracks in tempered glass propagate at speeds of approx. 4,900 feet per second. That’s 3,354 miles per hour, or 4.4 times the speed of sound. If you have a glass shower door that measures 3′ x 6′ and it is chipped at the bottom corner, the time that it takes for the glass shower door to be 100% broken will be a touch over 1/1000 of a second! That breakage could have happened 100 times during the time you blinked your eye once!

Chain of events when the glass shower door breaks

Let’s think about this. The glass shatters from a solid object to a flexible aggregation of thousands of cubic puzzle pieces in just over an instant. Then the glass has a leisurely second or two to decide what it is going to do next. If the glass was evenly supported at all edges like in a fully framed shower door, it might just stand there, suddenly festooned with an attractive cracked-glass pattern after first letting out a quick cracking noise. But more often, there are forces on the door that cause the glass to seem to explode. A towel bar with a towel hanging on it will impart a load on the glass that will encourage it to fall out of its standing position and possibly shatter in the direction of a naked bather.

Frameless shower doors

That most common type of high-end shower enclosures available today are called “frameless”. These beautiful bath enclosures – I have them in my house – utilize minimal framing. At corners, for example, the only hardware present might be a couple of small patch fittings. The rest of the structure is the glass itself. Hinges and door handles are frequently attached to the shower door through the use of drilled holes. All of these attachments impart loads to the glass that influence its breakage pattern and particle distribution when a breaking glass shower door occurs.

Since frameless shower enclosures rely on the strength of the glass for support, thicker glass is used. Many frameless shower enclosures here in the U.S. utilize 3/8″ thick tempered glass. This gives a hefty feel, but more weight might fall on you in an event involving a breaking glass shower door.

What causes the glass to break?

Another good question! Here is Chicago Window Expert’s top ten causes of tempered glass shower door and tub enclosure breakage. They are not in any particular order because I simply don’t have enough data.

1. Edge damage under normal operation

Edges are by far the most vulnerable part of glass. Our frameless bath enclosures leave these exposed. The glass edge can not be allowed to hit anything other than a soft bumper when going through its travel. I have seen frameless glass doors be allowed to hit a ceramic tile baseboard edge. Bad idea. Ceramic is a hard material that can equal the hardness of the glass. In a collision, both will be damaged.

Cracks at top and bottom of a frameless shower door
Cracks at top and bottom of a frameless shower door usually result in complete breakage
2. Edge damage under stressed operation
Glass shower door guide allows too much in-out movement
Glass shower door guide is too flexible. It allows too much in-out movement which can result in the opposite glass edge hitting the frame

It’s not enough to make sure a sliding door hits the bumper when it travels normally. What happens if someone leans or pushes on the door while sliding it? Try to push the door into and also out of the enclosure while opening and closing it. Does it hit the frame or anything else?
A child or senior might inadvertently push the glass door leaf out of alignment during closing so that it hits the frame instead of the bumper, breaking the glass. By the way, don’t put towel bars inside the shower enclosure. If a bather falls, it is better that they go down by themselves without pulling a shower door down on top of them.

Shower door out of alignment can hit the frame and break
This shower door was easily pushed out of alignment so that it was capable of hitting the frame while being closed.
3. Stressed operation without impact

If your sliding glass shower door allows too much in and out movement as discussed above, there may be an opportunity to overstress the door by twisting it, bringing about breakage. Much leverage can be developed in such movements, and strong forces can be transmitted to weaker parts of the glass door system, such as drilled holes. A chip near a drilled hole could be encouraged to creep toward the tensile zone under such stress, then crrraaaccck! Treat the doors gently. If they have too much slop, get thicker doors, better hardware or tighten the hardware.

4. Edge damage under forceful operation

What happens when you slam the shower door? Does it hit anything other than a bumper? When there are overhead rolling hardware systems that support the glass from overhead, sometimes they don’t effectively restrict the glass from upward movement. If you close the door with force, and it hits a bumper that is not positioned at the glass’ enter of mass, the glass door can hop upward and hit something other than a bumper. A few hops like this and you will be hopping out of the shower over a pile of shattered glass.

5. Edge damage at hardware penetrations
Shower door grommet is deformed, creating a potential for glass damage
Shower door grommet is deformed, indicating the hole is too large for the penetration, creating a potential for a breaking glass shower door

It is obvious that any hardware penetrating a hole in the glass should be separated from the glass with a rubber grommet and washers. Metal should never touch glass! If the hole drilled in the glass is too big for the grommet, the grommet can become deformed during usage and eventually allow metal screw threads to contact the glass in the drilled hole. Likewise, if the hardware is loose, a catastrophic contact between metal and glass can occur. Grommets must fit the holes tightly and be maintained.

6. Bad hole fabrication

Studies have shown that the cleaner the hole, the stronger the glass. This is why ASTM, American Society of Testing and Materials (of which I am a proud member) has included in ASTM C1048 Standard Specification for Heat-Strengthened and Fully Tempered Flat Glass, provisions for drilled holes in glass:

  • ASTM C1048, 7.9.5 Chips and flakes at hole edges must not exceed 1.6 mm (1/16 in.).
  • ASTM C1048, Inner surfaces of notches and cutouts must be smooth seamed or polished.

If you ask your shower door supplier if they adhere to these standards, I assure you they will view you with surprise and a new level of respect. And some will tap dance and sing the “We’ve never had a problem” song.

7. Hole misalignment

Holes drilled in glass show alignment and chipping problems
Holes drilled in glass show misalignment of holes drilled from each side but not meeting well (arrow at left) and chipping problems around the drilled hole (right). The right hole shows an additional problem: soap scum shows eccentric position of towel bar. Clearly the hole was too large for the hardware.

In order to get a clean hole in the glass, fabricators often drill a hole into the glass from each side. These holes must meet smoothly in the middle, but sometimes they don’t. If there is too much of a misalignment between the holes, only the “higher” side of the hole receives the impact when, for instance, a door mounted bumper is used. Does glass like loads that are not evenly distributed? No! Now position that cusp at a depth of 21% of the glass thickness, where the tempered glass tensile zone is positioned. Does anyone besides me see a problem here?

8. Nickel sulfide inclusions

With thousands of tons of glass being produced 24/7 in a float plant, it’s not hard to imagine that impurities might get into the glass batch. The most famous of these impurities is the nickel sulfide inclusion, which is a tiny little sphere as small, or smaller than a grain of salt. In annealed or heat strengthened glass it sits benignly and unnoticed for years, even though it expands up to 4% in size after some time when exposed to moderate heat. In tempered glass, however, if positioned in the tensile zone, it can suddenly break the glass shower door and with a true spontaneous break. These types of breaks are puzzling because there may have been nobody present for days, the glass just shatters by itself.

Nickel sulfide inclusion at edge of tempered glass
This nickel sulfide inclusion in tempered glass was the cause of a spontaneous breakage in a sliding glass door

The other breakage categories may also show shattering at a later time than when the impact occurred. Cracks in glass can be small (fissures) or even smaller (microfissures), and they may take some time to creep over to the tensile zone. Vibration or lesser impacts help it along. This is why tempered glass with edge damage can be compared to a time bomb.

9. Doors come off the track

In sliding shower doors, the glass door panels are suspended from above by a pair of brackets attached to the glass, each of which has a wheel that rolls on an overhead track. Often these tracks have shallow indentations for the wheels, so the wheels can sometimes rise up out of the track, and the door can actually fall down. When this happens, breaking glass can quickly follow. Make sure your doors have anti-jump devices that prevent the door from falling off its track.

10. Excessive tempering

As mentioned earlier, in order for glass to be considered fully tempered, it must have a minimum of 10,000 psi surface compression per ASTM C1048. If a glass lite is tempered to a state that is far in excess of that, the glass may be less tolerant of imperfections and more prone to breakage. Think of a skittish thoroughbred horse. It is so tightly wound that it takes off like a rocket with little provocation.

You can estimate the surface compression by the cube size. Industry sources state that 60-80 cubes in the 5cm x 5cm area would indicate that the desired compressive stress of 10,000 psi is present in the glass. In order to count the cubes, you need to find a piece where the cubes are still engaged with one another.

Counting the cubes in tempered glass
The chart at left indicates the number of cubes one might expect to find in a 5cm x 5 cm piece of broken tempered glass, by thickness and compressive stress. The glass lite at right, 1/4″ thick, had 130 cubes in that area, far above the minimum.

What can I do to prevent breakage of my shower doors and enclosures?

You are full of good questions. Here are some tips:

Shower door bumper that prevents glass from touching frame, even when pushed perpendicular to the direction of travel
Shower door bumper that prevents glass from touching frame, even when pushed perpendicular to the direction of travel
  1. Treat the shower doors and enclosure gently. It’s glass!
  2. Don’t let kids hang on towel bars. Don’t slam the door.
  3. Tighten loose hardware. Replace any deformed or broken grommets or plastic washers.
  4. If there are more than one bumper that the shower door hits, adjust the door so they are impacted simultaneously. If only one, position it at the center of the door edge so that there is no “hop” when the door hits the bumper.
  5. Buy glass made in the U.S. Standards of cleanliness for float glass and cutting/drilling are higher here than some other counties and this reduces the chance of nickel sulfide inclusions and spontaneous breakage.
  6. If you have sliding by-pass shower doors, make sure the overhead rollers are equipped with anti-jump mechanisms to prevent the rollers from inadvertently coming off the track.
  7. If you are managing a facility that has many glass shower doors, such as a hotel, hospital or apartment building, consider using a film on the glass that holds the glass together in the event of breakage. If you go down this path, it is imperative that the film covers the entire glass and is not cut around hardware attachments such as overhead rollers. If you neglect this step, when the glass breaks, a large, very heavy “wet blanket” of glass may fall on the bather and definitely ruin their day.

What should I do if I am showering and the glass shower door shatters?

The first thing you should do is stand still. If you move your feet, you might step on broken glass. Look around and assess the situation. If there is a clear path where you can escape, do so. If not, look for a towel or bathrobe that you can drape over the broken glass and walk above the fray. Brush a path with your loofa, squeegee or back-scrubbing brush. If you have a shower cap, put it on a foot.

If you have no choice but to step on the glass, make no sudden movements but rather apply weight to your foot gradually to give the glass time to lay down beneath it. Hold faucets, shower heads or soap dishes to steady yourself. As soon as you are past the danger, sit down and carefully brush all glass from your feet.

You may need the information to make a claim of some sort, so take pictures before cleaning up the broken glass shower door. If it might be a big claim, whether because of multiple occurrences or serious injury, leave the glass as is until an expert or claims agent can view the scene.

If you are a manager of a hotel or other multi-unit facility, get an expert involved sooner rather than later. Many injuries from breaking glass shower doors are minor, but the next one can be big and it is your duty to expend reasonable efforts to avoid it. Call me, I will be happy to help make your hotel a safer place to bathe.


* From the CPSC 16 CFR Part 1201:
“The Commission estimates that 73,000 injuries associated with architectural glazing materials in the architectural products within the scope of this standard were treated in hospital emergency rooms during 1975, and that about 2,400 of these injuries required the patients to be hospitalized. Extrapolating to total injuries in the United States the Commission further estimates that approximately 190,000 injuries were associated with architectural glazing products covered by this standard.” (in just 1975)

Laceration from broken glass shower door
Laceration from broken glass shower door. This occurred in an upscale hotel

“Although injuries occur at any age, children aged 14 and under appear to be at particular risk of injury since as a group they represent approximately half the injuries while comprising less than 30 percent of the population. Lacerations are the most common injuries associated with architectural glazing materials and account for 72 percent to 93 percent of the injuries associated with the architectural products identified in paragraph (a) of this section. These lacerative injuries span a broad spectrum of severity and extent of body part affected. During 1975, an estimated 200 injuries were treated in emergency rooms for lacerations over 25 to 50 percent of the victims’ bodies and over 7,000 persons were treated for lacerations to the head or face.”

** In the private sector, ANSI (American National Standards Institute) has a similar standard, ANSI Z97.1 upon which the CPSC 16 CFR Part 1201 was based. For years, building codes such as the International Building Code, have referenced one or both of these standards. And as tends to happen in the world of standards, they continue to evolve.

Mark Meshulam inspecting broken shower door glass
Mark Meshulam inspecting shattered shower door glass for clues to the shower door breakage. Elementary, dear Watson.

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63 thoughts on “Breaking Glass Shower Doors & Enclosures”

  1. Mark,

    I really liked your article on tempered glass in shower doors. The “reputation” that tempered glass is obtaining, not just for shower doors but glass balustrades, glass handrails, canopies, etc. is both changing and expanding at a rapid rate.

    Attached is a recent article on glass balustrades that was sent to me from one of our engineers in Milwaukee. I wonder what you think about the impact of widespread fabrication techniques in glass and how not all suppliers follow the rules laid out in the ASTM and AGMA guidelines.

    Click here to see the article – A Tale of Three Cities and Lots of Broken Glass

    Again, great article. You should find someone better looking to do the video on the shower door explanation for more “hits” ( hahaha)

    Jim Zorn

  2. Hi Jim,

    Thanks for reading and writing!

    When it comes to railings, no matter how good the glass, the fabrication and the hardware, the glass can still break by
    impact and leave a fall hazard. With a filmed or laminated product, this risk is minimized or eliminated.

    As for finding someone better looking for the video, you said you were busy!

  3. Thank you for spreading the word about the dangers of glass shower doors, and guiding people to the safest practices.

    My mother has drilled this thinking into my head all my life, ever since her friend lost a loved one in a shower door accident.

    This friend’s father was taking a shower as usual, but this one time he slipped in the tub. As he fell, he somehow hit the shower door. It shattered, and by the time his wife got to the bathroom, he had bled to death. A horrible loss for his family.

    So grateful that you are helping to prevent this from happening to anyone else!
    — S.S. in Skokie, Illinois

  4. Thanks Mark,
    Too bad I didn’t have this info 20 years ago when my shower door and its replacement shattered! Lol.

  5. Spontaneous failure of toughened glass is common in all sorts of glazing configurations. Break characteristics of this glass type mean that once broken there is a totally integrity failure where all the glass leaves the frame. As a window film company we often work in care homes and hotels fitting anti shatter safety film to shower screens because of this very problem. Although this film will not necessary stop the glass from breaking, it will help to contain the glass and hold it together so that is does not fall on the occupant of the shower.

  6. Fiona, I believe we will be seeing more and more acceptance of retrofitted films used to hold broken glass together. Have we arrived at any acceptance of this strategy by code/trade organizations?

  7. n the UK the safety film we use modifies glass to safe breakage in conformity with EN12600 2b.

  8. Heat soaking for frameless shower enclosures is not highly recommended considering the cost of heatsoaked glass. You could reduce only the occurrence of spontaneous breakage but how about the other factors that will result for glass breakage. (i.e: glass processing defects & installation defects). Safety film could be used but must be applied first prior for the installation of hinges/hardwares & to ensure that the film clamped also by the hinges.

  9. Glass in shower enclosures is governed by product standard EN14428 here in Europe. There is however a loop hole in the EN12150 standard for thermally toughened soda lime silicate glass in that it is only applicable for flat glass and not bent glass. So by making shower enclosures with bent glass you can effectivly bypass the EN12500 – not ideal.

  10. I very much appreciate your information on European standards.
    Here in the US I don’t think we have a code-accepted way to add film to annealed glass so that it qualifies as safety glass. Anyone with experience on this, please join in.

    Fernando. I agree that heat soaking only filters our NiS inclusions and does nothing for impacts and damages that occur post-tempering and your comment about applying the film to the whole piece of glass is right-on.

    Niels, thanks for bringing to our attention the curved glass exclusion. We in the glass industry need to be vigilant for situations where public safety is threatened especially when not addressed by codes and standards.

  11. Dear all, with respect to all mentioned above, first heat soaking is to minimize the percentage of glass breakage due to availability of Ni sulfide in glass but not prevent it which means we can’t guarantee 100 % that this case will not happen. Second, safety film is one way to hold glass during breakage but this depends on glass thickness. Take in your consideration also that you installed that film via self glue with water. Even the detergent used in bath may start to remove the film from the glass surface itself. Client should replace it from time to time which means big cost with time.

    Best way is to laminated safety glass 2 X 5mm and professional hardware that can carry the load for that panel so in case of breakage it will stay in its position and will not harm the person who uses it, even kids. In case the client didn’t like the look of 2 glass pane laminated together the weather sealing can be applied as water proofing.

  12. I would add that using Heat Strengthened or fully tempered laminated glass would achieve the optimal results of increased psi strength and anti-shatter properties. Heat Strengthened or even annealed lami would be preferable as the panels will remain more rigid in the event of failure. The added plus is the ability to include decorative inter layers to enhance the appearance of the enclosure.

  13. Try to look in to safety film that is used to hold glass together if it breaks.

    Breakage is probably due to toughed glass and that is quite a common problem.

  14. It must be the spontaneous breaking of tempered glass, which is a common sense to many glass manufacturers. To minimize this breakage, the tempered glass must be heat soak tested before release from the factory. But this requires extra cost to do so!

  15. Its got to be the edge of the glass making contact with the fasteners. Perhaps rubber spacer blocks were not installed to maintain clearance between the screws and the glass.

  16. Nice article. You covered almost all of the issues I was contemplating.
    One you didn’t discuss, thermal stress, can contribute to the potential for breakage, especially if there was already stress in the fabricated areas.

    The old manual or semi-automatic hole drilling can stress the glass by using the wrong type tool, the wrong feed rate and the wrong speed on the drill. The microscopic fissures can cause the glass to break in the quench. Viewing the tempered glass through a polarized lens and you’ll see some of the stressed areas.

    The advent of CNC high speed drilling allows controlled to the speeds and feeds. The tools are matched to the fabrication process, and cooled with automatic flooding of the area. The alignment issue also goes away with the more advanced equipment.

    For managers that are trying to lessen the probability of breakage, maintaining what they have in place by doing exactly what you advised. A reasonable and prudent manager, that had multiple incidents, would call an expert.

    These days, it seems that there are far fewer tempered glass door fabricators that fabricate manually. There may be those facilities that allow the glazier to fabricate his own glass, for tempered only orders, but many are pricing the service higher to discourage the practice.

  17. Spontaneous breaking of tempered glass, is quit common put tension it into any thing releasing it can be quit interesting. I agree to minimize this breakage, the tempered glass can be heat soak and tested at the factory before installation. This brings the risk of breakage way down. As heat soaking become more popular the cost will come down but all quality has a cost!

  18. I disagree with the assertion that spontaneous breakage is quite common. The term “spontaneous breakage” is misleading. Many of the instances of “spontaneous breakage” are in actuality delayed breakage from stress. Those stresses can be created by any of the issues Mark outlined in his article. Although nickel sulphide contamination does occur, the float facilities are very conscientious of the issues and take precautions to eliminate the contamination.

  19. Well, I found the articles at last and it was a nice job, Mark!! I ‘d like to add one more comment on spontaneous breaking: not only can NiS stone cause breaking but other stones, like Silicon or Sapphire, do the same. I collected many butterfly particles over last decade and only 1/3 of them were identified NiS, the rest were Silicon or Al2O3 stuff. My conclusion is any impurity formed in hard stones in tension zones with diameter over 200 microns are most likely to cause glass to spontaneously break during service, in addition to mechanical edge damages. It is not difficult to tell the difference between spontaneous breakage caused by embedded stones and mechanical breaking caused by imapct, before disposal of broken glass

  20. Todd we process 30,000 units a week and for what ever reason mostly material impurities some processing batch problems “spontaneous breakage” is quite common 5 % to 6% is common to us! Some manufacturers are having a lot more problems with new product than is commonly known. Heat soaking reduces breakage in the field,

  21. Mike 5-6% is a lot of breakage. Are you catching it in the heat soaking? The additional cost to heat soak test would be even higher if it caused a loss of 5-6% of production. That is interesting. Spontaneous implies there is no cause for the breakage, where as the proximate cause of the breakage is seemingly defined by impurities. Which float producer is the worst offender?

  22. This is a very interesting article, and definitely helps us on the installation end to make sure we are doing everything we can to limit breakage. Knock on wood, but we have very little, and I agree with Todd in that we see very little “spontaneous breakage”. Usually somebody did something they weren’t supposed to! Mark, I’d like to add a page to my website referencing this information if that would be okay with you?

  23. Todd yes it is seemingly defined by impurities. Which float producer I will let you know privately as we still have to use them we do get 80% credit. And 30-20 higher end heat soak.

  24. Hi everyone,

    Was out of town on assignment and too busy to check this thread but now I am back and gratified to see so much interest and activity on this topic. Thank you for sharing your experiences!

    Some responses to individuals:

    Denise, thanks for your comments and feel free to link to the article from your website. This goes for anyone else who wants to link.

    Mike and Todd, being in the UK and Bermuda respectively, are you seeing variations in float glass purity based on country of origin? Any general comments on which countries are better and worse? (I hope this question is not considered offensive to some readers, and I apologize in advance if it is.)

    Jianjun, thanks very much for pointing out the other types of glass impurities. I had to dig deep to find a study on silicon inclusions and this is the first time I have heard about sapphire.If you have pictures of breakage patterns of these, I would be very happy to post them at the article with credit to you.

  25. I have been in the glass industry since 1967 and have never seen spontaneous breakage of toughened glass. I have seen a very few breakages from inclusions but most breakages I have seen have been caused by ‘external’ factors. Bad handling, poor edgework, glass to metal contact etc etc.
    Good quality float glass has a low incidence of inclusions and I would worry if I was getting 5% ‘spontaneous’ breakage from my production.

  26. Mark I am a project manager in Bermuda. I have seen no “spontaneous breakage” since I’ve been in Bermuda. My previous experience, we were losing glass in the quench, but not all because of float impurities. We tempered float from almost all of the melters.

    I have purchased tempered glass from Columbia, China and other countries over the years. There is no discernible difference in the quality of the shower door products. Last year I did ESPN’s Superbowl portable soundstage. The 1/2″ tempered lites were roughly 60″ x 132″. I chose to buy the glass from Columbia, SA. The quality was excellent.

    I had a conversation with Rischer Hall about “spontaneous breakage” some years ago. He felt the occurrence was very rare. His idea “something was the proximate cause of the stress that eventually made the glass break”. He sited the damage done by a bird in flight, the bird hits the glass, falls to the ground and a cat takes the bird. The glass breaks and falls to the ground. There is no apparent cause – must be spontaneous breakage.

  27. Mark, I read your article and find it an excellent description of the glass and it’s properties and issues. I have one question. You mention that the glass is 1300ºF. In my business I kilncast 10mm glass to give it a decorative texture. I then take it to a tempering factory for tempering. I have had the good fortune to observe the tempering line at work. When the glass exits the heating chamber, there is an overhead optical pyrometer that reads the temperature of the traveling glass. It reads 600ºC (1112ºF). From my testing, I know that float glass begins to change shape at 1100ºF. So this temperature makes a great deal of sense to me. It is just soft enough to begin to change. The temperature nuances are critical to my understanding of how the glass works. I get it that that temperature is not critical to understanding the process.

    I have been experimenting re-fusing broken tempered glass, to make art objects. This glass behaves strangely to me. There seem to be quite a bit more internal stresses in it than when working with sheet glasses.

  28. I disagree with Chris Hogg, it’s know by nickel sulphide inclusion leads to spontaneous breakage of toughened glass and glass industry calculate with 7%. I have seen it in façades, sport centres (during the night without any reason of stress). That’s the reason Europe has in its EN norms the heat soak test as standard, which is a semblance security, because 3% breaks spontaneous still. Balcony and roof glazing I would always go for laminated and depending of stress load chosen for heat strengthened of full tempered glass use. why not for showers, most for the obvious reason is money, single tempered is much more cheaper the laminated-tempered, but more save, also by any breakage for personal injury.

  29. What is a reasonable amount of protection from injury? The end user, the developer/hotel owner has to decide what the acceptable risk should be. The use of monolithic tempered in showers seems to meet the expectations of the general public. If the injuries sustained became problematic, the economic impact on the hotels would cause the issue to be addressed.

    The seemingly “spontaneous” failures “at night”, “no-one around”, “un-occupied rooms” may all be delayed reactions to earlier mishandling of the glass. see the article linked below:

  30. How exactly is a heat soak test conducted?

    We know that microstarts on the edge, from wheel cutting, or caused by drilling or sawing defects can cause the glass to fail in the quench. Do these flaws show up as possible causes for spontaneous destruction, or only at the quench?

  31. Bert I think that some of the stresses that the cutting, drilling, sawing, etc. create can survive the quench process. Overtime the glass may be subjected to additional stress that literally pushes it to the breaking point. It may seem “spontaneous”, but the failure might really the result of the initial stress.

    Here is a link to Guardian’s Heat Soak Testing description:

    Interesting to note, both the article form USGlass and Guardian’s site state the HST is not 100%.

  32. You might think of heat soaking as an accelerated aging test where destruction indicates failure. It will do a reasonable job of filtering out initial flaws, including inclusions but it is powerless to address events that occur in the future.

    With products, such as shower doors and balcony railings, that function in a dynamic environment, especially with edges exposed, I don’t think we will see a time where a form of lamination will not be needed to hold the pieces together for safety.

    If any of the variations of lamination are used, it would then probably be the most cost effective solution to forego the heat soaking in favor of the lamination.

  33. Mark, if I am reading your comment above correctly, I think the popularity of exposed edge designs will limit the use of laminated products. The exposure of the interlayers to the environment set-up potential delimitation issues. The fabrication of the component tempered lites can create alignment problems during the laminating process. Not to mention the price difference.

  34. Its a question of semantics. If Nickel Sulphide or any other inclusion creates the breakage
    then it is not spontaneous.

    I have also seen toughened glass broken in strange circumstances but not always found the

    Whilst Heat Soaking has an EN Standard its is not mandatory. It is also worth mentioning that the thicker the glass the greater the post installation problem risk. Inclusions in thinner glass more often than not cause breakage in the toughening process.

    I agree with Paul on using laminated Heat Strengthened to give a slightly stronger composite glass but I believe that impact loads, preventing water ingress, weight and overall thickness are just as problematical as cost.

  35. Hi Todd and Chris,

    Our industry has a known problem with edge delamination when laminated glass is exposed to moisture and weather. Yet we see a trend toward more exposed edge designs.

    This says to me that we need to develop laminated processes that are more weather resistant.

    Does anyone out there have experience with any laminated or applied-film processes that are weather resistant?

    What about edge deleting the PVB and filling the space at the edge with silicone?

  36. Mark the lay-up of the sandwich doesn’t really allow the edge deletion of the p.v.b. If the p.v.b. doesn’t get good adhesion as it pass throughout he pinch rollers there can be delimitation issues. Also, silicone is not a good sealant for stopping water vapor. I recall there being a compatibility issue with p.v.b. and silicone as well. There are different types of interlayers, even different p.v.b’s. There are also different types of silicone. Serious compatibility issues could be avoided by some compatibility testing.

  37. Hi Mark

    I have never been involved with sealing the edges of a laminated product with PVB, EVA or lonoplast interlayers but know it is being done.

    However, Todd is correct in that there are definitely compatibility problems with some silicones and PVB as have personal experience of that in glazing situations.

    Other materials are involved in the sealing and I seem to recall mention of a butyl edge sealant being used, perhaps in Germany, but I’m not sure.

    I assume that the interlayer manufacturers must have some knowledge as I understand that edge seal is not uncommon in specialist automotive applications.

  38. Mark,
    The Sentry Glas laminates seem to perform well in the exposed edge aspect, maybe worth looking at?

    We have found them to perform quite well in the testing environment and I have not witnessed any silicone migration issues that tend to plaque the PVB products.

  39. Hey Bill. The SGP product may be an alternative with less drawbacks, except for the cost differential between it and monolithic tempered. Again, the reasonable and prudent level of protection, will guide the decision maker. (Limiting the discussion to interior bath enclosures with high humidity and hotel, or multi-family housing.)

  40. Mark -the use of glass shower doors in hotels has been around for years. The installation is all about price, many contractors have adopted out sourcing from regions that have less demands on regulations. This is causing an influx of Nickel sulfide inclusions in glass leading to spontaneous breakage.

    Heat soaking is the test of choice to find such issues.

    In the US and Canada companies are also adopting a manufacturing process where they fabricate glass after it is tempered. We are seeing a lot of this and it is an issue. This is an issue that is also causing spontaneous breakage.

    Hardware, there is a lot of hardware on the market. That is not well designed, and when the hinge is not installed properly the glass will release from its mounting.

    SGP is a solution however the look and expense will cause people to move from this solution. Safety films in a shower application will not last . Silicone is not recomend to be used with PVB .

  41. Thank you so much for this article. A panel in my children’s bath/shower did this yesterday and I am so thankful that I didn’t get around to giving them a bath yesterday morning!! We had not even touched the door for at least 2 days as we hadn’t been home. Nobody was even on the same floor as that bathroom when it happened. I believe that a bus may have driven by just prior, we get quite a bit of vibration from traffic behind the house.

    My concerns are now that the others in our home may shatter as well; they are all from the same mfr and presumably all installed by the same person. Can you recommend an aftermarket film to help minimize this? I will likely tear out the whole enclosure in the kids bathroom, I would much prefer a shower curtain for them knowing this now. I never noticed and cracks, no looseness on its track, nothing before it happened.

    Thank you for bringing attention to this issue.

  42. As a result of its safety and strength, tempered glass is used in a variety of demanding applications, including passenger vehicle windows, shower doors, architectural glass doors and tables, refrigerator trays, as a component of bulletproof glass , for diving masks , and various types of plates and cookware. In the United States, federal safety laws require that window glass be tempered if each of the following criteria are met: sill height within 18 in (0.457 m) of the floor, top edge greater than 36 in (0.914 m) from the floor, area greater than 9 ft² (0.836 m²), and horizontal distance to nearest walking surface of less than 36 in (0.914 m).

  43. This is a tempered glass problem, so it involves various door manufacturers and the ages of glass. If this happens to you, call the Consumer Product Safety Commission and the door manufacturer.

  44. Due to the recession, I finally have time to tackle things around my own house, so after three years, I’m finally finishing the upstairs bathroom. To save money, I’ve opted to install my own shower door. I bought a frameless, 3/8″ shower door that’s pivot mounted. I have to drill three holes in the bottom of the door in order to accept the baseplate. I’ve always thought you couldn’t drill through tempered glass without it breaking into a million pieces or shattering like car glass. What’s the best technique for this task?

  45. Hi Mark,

    Thank you for this blog. My wife and I live in PA and built our house 4 years ago. We had hanging tempered glass shower doors installed. 4 weeks ago, one of my son’s shower doors burst. The cleaner stated he sprayed windex to clean the outside of handprints and the minute he sprayed the door it exploded. His hands and legs were cut up from the breakage. We called the installer and person who sold us the doors. He swore that the cleaner must have banged the doors or slammed the doors. The cost to replace $350. Today, the same cleaner was spraying the glass doors of my other sons bathroom and this bathroom door shattered as well. This time, however, were bigger and heavier doors. He called us and we came home, his hands were bleeding profusely and he was clearly shaken. He told us that he sprayed the outside and the door burst again.

    I called the installer again. Of course, what did I get as an explanation? The house must have settled? What??? Then it was the cleaner must have slammed the doors? We have the remaining glass doors in those bathrooms as well as in all of our other bathrooms. I do not know what to do. Should we replace the broken doors? Do remove all of the glass completely? Are there other alternatives?

    I have 4 young sons ( 10,9, 6, and 3) The thought of any of them being in the shower or around the shower doors when they explode is very scary to us.

    Any help of advice you could offer would be great.


  46. I have a custom 3 piece glass shower enclosure in the corner of my bathroom, 3/8″ glass. Two of the pieces have chips at the bottom corner where they meet over the bathtub surround. I’m having the smaller of the two pieces replaced as it has some chips at the top and bottom corners. The larger panel that forms the wall of the shower (5′ x 6′) has a chip 1/2″ long on the vertical side of the corner, 1/8″ deep over the beveled edge of the glass. Do you think the panel needs to be replaced also?

    What would cause the edge of the glass to be chipped like that? No one has ever used the shower and that portion of the shower is stationary. Is it likely to be caused by installation or maybe someone kicked it while standing on the tub surround to install things on the wall nearby?

    I’m concerned about the danger this corner chip might pose to the integrity of the glass structure.


  47. Hi Mark, I really learned a lot from your article. I am going to install a glass door or glass screen on my bathtub as part of a remodel. I want to apply a film like you suggested. I have a few questions.
    1. What brand of film is best, and where can a homeowner buy it ?
    2. Would it help to film both sides or is it best to do the dry side only ?
    3. If I doubled the film would that be better ?
    4. How do I know when to replace the film.
    5. What can I do to increase the life of the film and it’s adhesion.
    Thanks for your info in advance.

  48. Hi Archie,
    3M has a safety film that should work. Just apply it to the dry side, but make sure to remove all hardware, cover the glass, then re-apply the hardware.
    Replace the film when it looks bad. Don’t use abrasives when cleaning.

  49. Hi David,
    Without seeing the chip, the 1/2″ one sounds bad and worthy of replacement. The tensile zone of the glass is just 21% in from the surface so if chips are deep, they are just waiting for the right moment to fail. Hopefully nobody will be nearby when that happens. Replace chipped glass or coat it with a safety film.

  50. I have never heard of Windex harming glass and I don’t think this situation fits that description.

    The glass probably had some concealed flaws. If you don’t want to involve lawyers, have a safety film applied to the dry side of the glass.

  51. I just had a shower door explode in my hands yesterday and it was frightening to say the least. I had just told my son to finish up his shower when I noticed that the inner door wasn’t gliding so smoothly. After he finished, toweled off and left the room, I decided to remove the outer door so I could figure out what trouble the inner door was experiencing. As I removed the outdoor I very gently placed it on top of the foam-like bathmat and every so slightly leaned it towards the outer molding of the bathroom window. The instant that the panel made contact with the molding it shattered in both of my hands sending a cloud of dusty at me in the process. It scared the living daylights out of me and everyone else in my house. Fortunately I was uninjured and my eyes closed in just enough time to avoid the dust. This was not the first time I had removed these doors to make adjustments but it was the first time I did it while the glass was still warm when it made contact with a cooler surface (the window molding). I question whether the temperature differential caused this breakage. Had I known about the risk of explosion (under any circumstances) I would have definitely not considered removing the door until the temperature of the door matched the temperature of the room.

    Until my kids get older I will not be putting glass doors back on. I will stick with a shower curtain. The only questions I have now are how to remove the track from the tub without damaging it (I’m assuming it was glued on by the previous homeowners’ contractor) and how to safely dispose of the remaining glass door (which I removed after a 90 minute cleanup job. Should I cover the door completely in duct tape (front and back) and then just strike it with a hammer on my garage floor? I’m envisioning rolling it up and tossing it in a heavy duty can liner.

  52. Hi Chris,
    Thanks for sharing that harrowing story.
    To dispose of the remaining door. lay it down onto a strong, disposable dropcloth. Wrap it in the dropcloth, then whack the glass edge with the claw of a hammer. Then put the entire package in the dumpster.

  53. Hi Mark,

    Because of these safety issues with glass, do you know of or see any future trend to using other materials for shower enclosures? Specifically polystyrene? I know that material is often used in the European shower/tub enclosure market, but I can’t find any information on whether PS is/has ever been used in the US for shower doors. Just wondering if you had any experience/knowledge of PS/other materials being used in the US marketplace, any info would be much appreciated!


  54. That seems like a good idea to try to make sure that your doors have anti-jump devices to make sure that they don’t fall down. That would be bad. My sliding glass door has gotten some spider cracks going throughout it now. I’d like to get them fixed before it actually shatters. I’ll have to find someone who does glass repair before that happens and look into the anti-jump devices.

  55. Hi Mark,

    This was a very useful article. I’ve been looking for a protective film for our floating glass shower enclosure, because my kids tend to whack the glass enclosure with the metal ends of our squeegee. Can you recommend a particular adhesive film we could use? The ones I’ve found say they are inappropriate for showers because of the constant exposure to water.

    Thank you!

  56. Hi Mark, I’m in New Zealand. I’ve had a new glass sliding door installed in the shower – using it for the first time yesterday, finished my shower and started sliding the door open, it exploded and showered me with glass. I have many minor cuts but one bad one on my left hand (which would have been holding the handle) which has required stitching.
    The installer and the glass manufacturer are discussing what may have happened, but it does look as though each is going to blame the other, or at least go “it’s a mystery”. I’ve checked all of the fittings and they seem to have rubber intact, and there doesn’t seem to be any way the door could have hit something solid.
    Do you think there is any possibility that the glass wasn’t toughened at time of manufacture? This is a large publicly listed company, but mistakes must happen from time to time.
    My problem with “it’s a mystery” is that I feel unsafe just having the same product installed again.
    Any thoughts?
    Cheers, Kerry

  57. Hello Mark. Is it possible for a shower glass door to break due to construction vibration?

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