Mark Meshulam is a glass consultant & expert witness involved with glass breakage and glass injuries. This article is available as a PDF download for your library.

Diagnosing Glass Breakage book cover

Sometimes it’s easy to know why glass breaks


Photo of broken glass.Glass breakage from impact
Diagnosing the cause of glass breakage can be a no-brainer

For example, a ball on the floor combined with neighborhood kids running away generally can be considered symptoms of impact-related glass breakage.
Today we will look at four types of glass breakage:

  • Impact breakage
  • Stress cracks
  • Edge damage
  • Spontaneous breakage

Impact breakage
If you know anything about glass, you know that it can break, and when it breaks, it’s not a good thing.
Back to the ball-through-the-window example, the glass breakage pattern will vary depending on the speed and mass of the ball, and the size, thickness and post-annealing treatments that were performed on the glass prior to the ballgame.
A very well-hit hardball, or a well thrown rock squarely hitting a piece of annealed glass will produce glass breakage with a circular puncture with cracks emanating outward from the point of impact.

Picture of broken glass.Typical impact glass breakage without puncture
Typical impact glass breakage without puncture

The resulting shards between these cracks are dangerous! Broken glass injuries can be serious, even deadly. If broken glass shards fall out on your arm (as often happens during clean-up) you will soon be in the emergency room.

Experienced glaziers often tape the shards together with duct tape, then remove the entire panel. If you must remove these shards, remove the upper ones first, then the lower ones. Use heavy rubber gloves, protect your arms, head, eyes and feet, and place the shards in a cardboard box, not a garbage bag.

Photo of broken glass. Danger of glass breakage injury. Glass shards from broken annealed or heat strengthened glass are dangerous and must be handled with care.
Danger of broken glass injury. Glass shards from broken annealed or heat strengthened glass are dangerous and must be handled with care.
Blunt or Distributed Impact on Long, Narrow Lite of Annealed Glass

In this example, we see a horizontal crack at the center of the blunt glass impact, with cracks radiating away from the impact. Due to the aspect ratio (relationship between width and height), shards are long and narrow.

Photo of glass breakage. Blunt glass impact on long lite of annealed glass

If you are dealing with a glass breakage epidemic email me at

Small Rock High Velocity
Congratulations to this vandal who was able to select just the right sized small rock, and hurl it at a speed righteous enough to completely puncture this glass. The combination of size and speed resulted in a localized pattern of glass breakage.

Photo of broken glass. This vandal chose quality over quantity. He selected a smaller stone, which he was able to accelerate to a greater velocity, sufficient to completely puncture the glass.
This vandal chose quality over quantity. He selected a smaller stone, which he was able to accelerate to a greater velocity, sufficient to completely puncture the glass.
Larger Rock Less Velocity

This vandal’s eyes were bigger than his throwing arm. Although he scored a large area of glass damage, he failed to achieve the goal of full glass penetration. The impact was large enough, however, to break the interior lite of the insulating glass unit. In the picture below you can see two sets of impact breakage patterns. You can also see the rich source of projectiles: railroad tracks. One week after we finished installing windows in a new high school, local kids had a field day with the rocks and our new windows.

Download this article for free

Photo of glass breakage. Vandal with big ideas hurled a sizeable rock and achieved a big breakage pattern, but failed to breach the castle's defenses.
Vandal with big ideas hurled a sizable rock and achieved a big glass breakage pattern, but failed to breach the fortress


Tempered glass breakage pattern
Tempered glass breakage pattern


Tempered Glass Breakage

When tempered glass breaks, the energy retained in the glass due to internal tension/compression releases explosively and produces a glass breakage pattern sometimes called “cubes”.

Seeing a cubic breakage pattern does not tell you why the glass broke, it only tells you that the glass was tempered. Generally, there are three reasons tempered glass will break: impact, edge damage or inclusions. Inclusions are tiny impurities in the glass. The most well known are nickel sulfide, however there are also ferrous, silica and gaseous inclusions which look like tiny bubbles.

Normally, when tempered glass breaks, it falls down into a pile of little cubes. Only the most patient glass consultants with the most generous client would ever consider piecing the cubes together to determine the cause of breakage. That being said, I have personally spent many hours picking through broken glass looking for an important clue: a pair of adjacent hexagons, known as a butterfly pattern, that borders a nickel sulfide inclusion.

However, occasionally the pieces of broken tempered glass will stay in the opening, locked to each other like blocks in a masonry arch. And just like in a masonry arch, if you remove the keystone, the arch – or glass in this case – comes tumbling down.


Photo of broken glass. If you look closely, you can see the point of impact on this tempered glass. A laminated interlayer holds the cubes in place.
If you look closely, you can see the crack origin on this tempered glass. A This glass is tempered and laminated. The PVB interlayer holds the cubes in place.

This picture shows broken glass which was a part of a laminated unit. The PVB (polyvinyl butyral) interlayer held the pieces in place, giving us an opportunity to observe that impact related glass breakage can be visible, even in tempered glass.

Spontaneous Breakage in Tempered Glass

Glass, and especially tempered glass, sometimes breaks all by itself. This can be quite disconcerting when, as has happened in a public place which will go unnamed here in our great city, large, thick panes of tempered glass basically blew up fairly frequently. The unusual cause in this rare instance: the glazing contractor attempted to grind the edges of the glass after it was tempered, creating a series of time bombs. It is a very bad idea to modify glass after it is tempered!

A more well-known, but also quite rare cause of spontaneous glass breakage is nickel sulfide inclusions. If you read the previous post, you will already be down with the fact that glass is made from melted powders. A nickel sulfide inclusion is a tiny rock of material that remains in the glass. Below see an artists graphic representation of a nickel sulfide inclusion.

Artists representation of nickel sulfide inclusion.

Artists representation of nickel sulfide inclusion.

You can well imagine that a little rock embedded in a slab of glass which is under high tension/compression forces, could weaken the glass and eventually cause glass breakage. But the story gets worse. Nickel sulfide grows an additional 4% of its size over time. If it is located in the strata in the glass between tension and compression, and it grows, kaboom!

If you have a glass breakage injury email me at

Exploding Glass Shower Doors

One of the scariest, yet somewhat common type of tempered glass spontaneous breakage is in glass shower doors. They are exposed to banging against bumpers, heat from the shower, wrenching action of through-mounted towel bars and of course, nickel sulfide inclusions. In hotels, multiply the risk factors by the number of rooms and the lack of care typically taken by a hotel patron.


Photo of broken glass in tub after shower door shattered
Broken glass in tub after shower door shattered

There can be a delay between impact and crack propagation in tempered glass, just like any other glass, and sometimes the time the glass finally explodes seems ironic and and Machiavellian. Quite often the perverted glass will explode while the unsuspecting victim is naked and in the shower.

Let’s count the problems:

  1. The victim is naked.
  2. There are sharp cubes of glass projectiles flying around.
  3. The victim is bare footed.
  4. The victim must walk barefooted over a field of freshly shattered glass shards.

So here’s a tip for readers who actually do take showers: If your shower door shatters, stand still for a moment and take stock of the situation. Hopefully you are not cut too badly. Without moving your feet too much, look for a towel. Try to grab it and lay it down on the glass so you can walk out. Then get the hell out and never shower again.

Stress Cracks

A “stress crack” will usually only happen in annealed or heat strengthened glass. Stress cracks emanate from the edge of the glass and meander about apparently without purpose. But there is a purpose: to relieve stress in the glass. However, the term “stress crack” can be misleading.

If annealed glass is subjected to thermal fluctuations that create glass stress beyond its capabilities, the glass will break in a way that will relieve the stresses induced by thermal changes. This type of failure is often a design issue. Heat strengthened glass may have been a better choice for the application.
However, there can be a near-identical breakage pattern which emanates from damage in the glass edge that fails as normal stresses, such as thermal, are applied. In this case the edge damage, not the thermal stress is the culprit.

Photo of broken glass. Classic meandering pattern of heat-related stress crack, accompanied by evidence of edge damage. In this case, the building had both: edge damage and underspecified glass. Reflective blinds and a South exposure combined to create a high frequency of this type of breakage.

Photo of broken glass. Classic meandering pattern of heat-related stress crack, accompanied by evidence of edge damage. In this case, the building had both: edge damage and underspecified glass. Reflective blinds and a South exposure combined to create a high frequency of this type of breakage.
To tell the difference between a true glass stress crack and glass breakage due to edge damage, look at the edge of the glass for a chip, which we window linguists sometimes call an “oyster”. You might have to look hard because the oyster could be buried in the primary seal on the #2 or #3 surface.

Another clue would be the distribution of glass breakage in the building. It would be normal to find stress-like cracks on elevations with greater temperature swings. But does the breakage also coincide with the use of reflective interior blinds, especially in a partially opened position? That would be indicative of a true stress crack, rather than a crack induced by edge damage. Also, look outside. Is there something that shades the glass partially? That could be a factor.

Download this article for free

New photos: Nickel sulfide inclusions that can spontaneously shatter tempered glass

Want to learn more about nickel sulfide inclusions that can spontaneously shatter temper glass? See this photo album: Nickel Sulfide Inclusion: A tiny speck that destroys tempered glass


Mark Meshulam glass consultant is involved with glass breakage injury
Mark Meshulam,glass consultant, observing that glass is not broken

Need a glass consultant to diagnose glass breakage or investigate a broken glass injury?No matter where you are,
contact me, Mark Meshulam,
the Chicago Window Expert
For the expert attention you deserve
Download Brochure
Download Mark Meshulam’s CV
Download Field Testing Credentials

Current client locations:
Alabama, Alberta, Arizona, Arkansas, British Columbia, California, Colorado, Connecticut, Florida, Georgia, Hawaii, Illinois, Indiana, Iowa, Japan, Louisiana, Maryland, Massachusetts, Michigan, Minnesota, Missouri, Montana, Nebraska, New Jersey, New York, North Carolina, Nova Scotia, Ohio, Oklahoma, Ontario, Oregon, Pennsylvania, Russia, Singapore, Tennessee, Texas, Utah, Virginia, Washington State, Washington DC and Wisconsin.
Coming soon to your area!

81 thoughts on “Diagnosing Glass Breakage”

  1. Pretty good post. I just found your site and wanted to say
    that I have really liked reading your posts. Any way
    I’ll be subscribing to your feed and I hope you post again soon!

  2. I know the family that ownes Florida Crush Stone. About 80% of the aggregate for cement for the whole US comes from them. Do you want me to help forward this site to people in related industries? Do you work with companies around the country? How small is too small?


  3. I would be grateful for readers forwarding this site to other interested parties. Yes, we do work with companies around the country, despite the ChicagoWindowExpert name. In the consulting part of our business, there is no job too large or small. Thanks for writing!

  4. Nice post by smart M and I was enjoyed. Hope new posts would be on the site and serving Chicago.

  5. Thanks for the nice comment from my friend Victor Wei of Yuanda. Yuanda is the Chinese curtainwall company responsible for two of the most memorable buildings of the Beijing 2008 Olympic Games, the Bird’s Nest and the Water Cube. Congratulations to Yuanda for those two wonders!

  6. Hi Mark,

    I came across your website and found it very helpful. I was curious if you had any photos of nickel-sulfide inclusions? My window spontaneously shattered one night and the window supplier is claiming it broke because of impact — I know I didn’t hit it! Based on the tempered glass breakage photo on your website, it would appear that my living room window has a point of impact. But would spontaneous breakage also have the same radial, “point of impact” pattern? I’ve read on other websites about butterfly patterns that appear because of nickel-sulfide inclusions — I think mine shows that, but I would like to get a professional opinion.

    I would be very interested in having you diagnose, but I’m an American expat living all the way in China, so I’m not sure if you can help or not. I’ve attached photos of the broken window taken by the window supplier (it’s a curtain wall, floor-to-ceiling residential window). The outside window pane is intact, according to the supplier.



  7. Hi Kim,
    According to my glass experts, a nickel sulfide inclusion should be visible to the naked eye.
    Attached please find an interesting photo I recently came across.

  8. Hi, Kim,
    I am working in a window glass production plant. Recently I am getting more complaint from my customers regarding temperness. But I found very good cutting in size before packing. will you please be able to help me in this regard. what are the general breakages styles of different reasons in related to stress, strain vice versa.if you have any pictures related to that subject please enclose with details. your co-operation is highly obliged.

    with regards


  9. Kumar,

    The pattern you see on the background of this page (scroll all the way down) is a clasic breakage pattern of tempered glass. I recently learned that it is possible for a lite of glass to be incompletely tempered, i.e. tempered in some areas and not fully tempered in other areas.

    Although I have never seen it, this condition could result in a hybrid breakage pattern that transitioned from the tempered cubes to larger shards. Has anyone out there ever seen such a thing?

    Since tempering occurs when glass is heated then rapidly cooled, I would imagine an uneven temper could result from uneven heating or cooling.

    What is the breakage pattern your customers are complaining about, Kumar?

  10. I have experienced the incomplete tempering process. It is a laminated glass, the breakage happened at surface no.3 however surface no. 4 is intact. The breakage pattern is as shown in the background, just like a tempered.

  11. Hi Mark. I stumbled on your article while researching a problem I have. Perhaps you can help me? I recently broke a peice of safety glass in one of my homes french doors. There are 15 seperate glass panels and they are double paned. Only one panel/section broke and only 1 panel. I’ve been told the door can not be reglazed and they only option is to replace the entire door! It appears to me that there is about 1/8th to 1/4 inch between the broken pane and the second pane behind it. The glass that shattered remained in place entirely – just completely cracked. Now my question – I’m considering just trying to remove the broken pane and leaving it as a single pane of glass in this one area. I have no idea how to carefully get the broken piece out though without possibly breaking the pane behind it too. Should I just carefully tap on it with something like a screwdriver until it punctures it? Any thoughts/tips/advice would be greatly appreicated!

  12. Mark,
    I cleaned the outsides of the exterior windows on a newly constructed large downtown building. The windows have the new external aluminum window louvers attached above in strategic locations on the east south and west sides of the building. This is a very time intensive job and after a month we are finally wrapping this up. (due to many days of heavy wind delays and carefully working around those louvers). at the outset we were instructed to not put any weight on these louvers whatsover as in the past there had been some cracking problems with the same set up but at a different office building (using the same glass installers) I noticed today that one of the windows that we had cleaned at the beginning has developed a large stress crack that goes from upper right hand corner side down to right corner and then up and across entire window to the left side. This just happened over the weekend. What are your thought on this?

  13. Hi David,

    There can be many reasons for this type of breakage. First, there will always be a few lites that seem to break spontaneously during the first few years of building operation – usually this means that there was a hidden weakness, such as edge damage or an inclusion, that finally got around to breaking. Temperature fluctuations help bring out hidden defects.

    If it is not edge damage or an inclusion, then the glass may not be designed for the conditions. In other words, if the louvers or other projections cause a pattern of triangular shading on the glass, if the glass is annealed only, it may break.

    If they want to blame you for the crack, get pictures and ask them to show you the point of impact. If glass is broken by impact, there will be a very noticeable point from which many cracks emanate. If it’s not there, it is probably not your fault.

    Good luck!

  14. Hi Ed,

    Unfortunately too many manufacturers make their products difficult or impossible to reglaze. Shame on you, window manufacturers who do this. You are screwing the public!
    The glass will be held in by two things: stops and sealant. If it is a wood door, the stops will be little strips of wood nailed just to the interior side of the glass to hold it in place, although with some of the high strength sealants out there, it is not really necessary, except for appearance.

    Gently and carefully remove the stop by sliding a sharp razor knife along the seam between the stop and the frame (and also between glass and stop) to separate the stop from the frame. You should now see the glass edge. Measure the actual glass size. Go order a piece of replacement insulated glass. When it arrives, remove the old glass by any means possible: cutting along the glass where it is adhered to the frame on the exterior, cutting away any sealant around the glass edges visible from the interior, prying and prying every witcha-way. If the glass is stubborn, and it is tempered, nick away at the face of the glass with the claw end of a hammer until the glass breaks, then peel and clean it away. Bed the new glass in 100% silicone or follow the manufacturer’s glazing instructions, then install the glass and the stops.

    I am not a fan of removing the broken lite and leaving the intact lite. It will look strange and who needs that?

  15. we put some mirrors in a work out room several months ago and 3 or 4 months later they crack at the corner and than went straight across the mirror. The owner said it was out fault , Now he wants new mirrors. He is a good customer . In front of these mirrors on the floor is a weight rack where they are picked up and put down. I told them that the floor was bouncing up and down. I think I am right.

  16. I have recently visited your website ( and have looked at the glass patterns that you have described. The pattern of my broken window glass is exactly as shown in the “Blunt or Distributed Impact on Long, Narrow Lite of Annealed Glass”. Can this pattern also be a result of thermal or temperature related stress? If not, can you tell me what kind of object or impact would cause such a pattern? Thanks a lot!

    TJ Chana

    Full width of lite

    Closeup of center

  17. Hi TJ,
    It looks like the glass is receiving some loading from above. Remove the interior glass stop at the top of the glass lite and see if the frame is touching the top of the glass (this would be bad).

    Then remove the bottom glazing stops and see if the setting blocks are at quarterpoints. I am going to guess that the setting blocks are at the transition between the cracked area and the middle bottom crescent.

    Let me know what happens.

  18. Hi Mark,

    I finally had a chance to take a look at the glass (weather is finally warm enough!). The frame was touching exactly where you thought it was, I suppose it is due to the settling of the frame.

    The settling blocks were at quarterpoints and between the cracked area and the middle-bottom crescent as you said!

    Thanks for your help!

  19. Flaws discovered in exploding glass balconies
    Published On Thu Aug 18 2011–flaws-discovered-in-exploding-glass-balconies

    Liam Casey Staff Reporter

    Microscopic imperfections buried within the pane are behind the spate of exploding glass balconies on downtown condo towers, an engineer says.

    Mark Brook is a partner at BVDA Façade Engineering which is analyzing glass from the north tower of the Murano building, where at least nine panes have exploded since last September.

    Brook recently discovered nickel sulfide crystals in two shattered panes. These crystals grow over time, which stresses the glass. If the imperfection is in the right place, the pressure builds until the pane explodes.

    “It’s either bullets or inclusions,” Brook said. “And there’s no evidence someone’s shooting at the balconies.”

    To get a definitive answer, Brook needed at least one of two specific bits of glass where the explosion originated — the portion containing the nickel sulfide crystal. That’s nearly impossible when a pane explodes into hundreds of pieces strewn across a busy street.

    But the engineering firm was lucky. It had been testing intact panels from the north tower of the Murano building as the developer took them down. One night, a pane resting against a wall in the testing facility exploded. They found their specific glass bit.

    “It’s not poor installation,” Brook said. “It’s not wind, it’s not the weather, it’s these imperfections.”

    Brook first suspected the faulty glass was from a bad batch. It was actually several contaminated batches from manufacturing facilities in the United States that ended up in Toronto.

    “Exploding glass isn’t that abnormal,” Brook said. “It’s just been making the news because it’s been exploding onto busy streets like Bay St.”

    Brook’s firm has been contracted by Lanterra Developments to examine the glass on three of its condos where balcony railings have shattered: the two Murano towers as well as One Bedford Rd. The developer has said it will replace the tempered panes with the safer laminated variety — an inner layer of tempered glass and an outer layer of heat-strengthened glass sandwiching a sheet of plastic — on these and all future projects.

    Glass has also shattered on three Toronto buildings built by other developers.

  20. I have a 5/8″ OA Guardian RLE 70/36/Clear insulated unit 65x48x5″ (roughly) Rake Top glass pane that I was told has an oyster extending from behind the aluminum frame. When the weather got extremely cold this past winter, a crack started from the oyster and ran down the window about 18 inches. The house was just a few months old, and the Builder is refusing to replace it.

    Can you provide me with a layman’s explanation of an ‘oyster’, how they happen, and why the crack happened during extreme cold. If you have any pictures, that would be very helpful.

    Thanks for your blog. There’s almost no place for a layperson to get the real truth.

  21. Hi Shar,

    An oyster is a clamshell-shaped chip in the surface of the glass, usually emanating from an edge. It is the result of damage to the glass edge, such as might occur if the glass were pried in place with a metal prybar during installation.

    Glass edges are sensitive and brittle. They should never be allowed to contact metal. Wood is used frequently for standing a lite of glass onto prior to installation. Once installed, the glass should be sitting on hard rubber (neporene is often used) setting blocks at the bottom quarter points.

    If your glass was already installed into the window when it arrived to your jobsite, the edge damage probably occurred at the factory. Bring this to the manufacturer’s attention and try to get them to provide material and labor to replace the glass. If they refuse, let me know. Maybe I will be able to persuade them to do the right thing.

    The photo below is at the bottom of the glass just next to the setting block. There are a pair of oysters at the bottom of the glass. They are spaced apart about the width of a pry-bar used often by glaziers.

    The glass cracked about a year after it was first installed. I was asked to become involved because a number of such breaks occurred about 90 feet up the side of an NFL football stadium and a serious safety hazard was of concern.

    I asked to join the glaziers on the swing stage so I could inspect the glass edge as soon as it was uncovered. Edge damage from the original construction was evident on most of the broken lites.

    pair of oyster chips in glass edge

  22. Mark – My sincerest thanks for your detailed response to my ‘oyster’ question and the attached picture. We have already been deposed over the issue(s), and this issue along with other defects is scheduled for trial in a month. The refusal of the builder to come out and look at the problems has necesssitated this action. The window has already been replaced because we are selling the house, but we would like to recoup sizeable expenses.

    Again, I truly appreciate your time and efforts to help those of us who have little knowledge about these subjects.

    Best wishes…

  23. Hi, Mark,

    About 30 minutes ago, while I was enjoying the relative peace and quiet of around midnight, I heard a fairly deep-seated ‘thunk’ emmanate from my kitchen. My garden window in my kitchen – the sloped top portion – spontaneously cracked. At this juncture, it’s fairly stable – most of the noise from the cracks creeping across have ceased – but I’m still a little freaked out by it.

    It is the interior layer of a double-pane. We installed the window about 7 or 8 years ago when we replaced all the other windows in the house. This was the only one that required no retro-fitting or adaptation of the frame; it slipped in flawlessly, and has been my pride and joy every since.

    We’re experiencing a first series of storms, with chilly winds and rain, but nothing unseasonable or out of the ordinary, so this is not the window’s first prom. I think the heater was running at the time, but there again, nothing unusual. So all I can reasonably explain is it must be the Ghost in the Glass.

    The break pattern begins from just off (by an inch or two) of the lower left-hand corner, and extends across the pane like the silouette of a leafless tree branch (sounds romantic, doesn’t it?) or a feather. The shards are almost uniformly an inch or so wide, but some are well over a foot long. It’s gonna be a mess when clean up time rolls around.

    I wish I could submit a picture, but that’s a little outside of my capabilities right now.

    I’m hoping you might present me a palatable explanation, and if an incident like this could be covered under warranty (the theoretical life of the window, free from defects or owner abuse). I’m feeling a little cursed at the moment; who has a window that just spontaneously snaps?

    Actually, any advice or counsel you could offer would be greatly appreciated. Thanks you.

  24. I just had windows installed. Today, the company put in a replacement for a window that had cracked during installation. I noticed something on the left side of the window that they were putting in and asked if that was a crack. They told me it was a clamshell (defect) and that as long as that was inside the frame it wouldn’t be a problem and if the window did crack, it was under warrantee. Should I ask them to replace it again?

  25. We have built a new house using some used parts. Yesterday we came in to find one slider door, a glass header over the front door and the side glass panel of the front door all broken on the inside panes only. It seems odd that they are only broken on the inside panes, in different places in the house and all happened on the same day. No weather, no sign of impact. Hate to think vandals did this but really trying to figure it out. The slider looks like your background of your site, the door ones have a small place where it broke and then travelled.


  26. A small rock was thrown at my window from outside but only the inside pane broke. There are no marks on the outside window except a dirty mark on the frame (PVC). The police have told me it is not possible to break an inside window by throwing something at it from outside and suggested I did this myself from inside. I am shocked at this suggestion and would like to ask how is this possible that only the inside pane would break. The breakage looks like small stone high velocity but on a larger scale.

  27. Mark,

    This is great information, thanks!

    On a large (8′ x 13′) window made of 2 layers of tempered glass laminated with a PVB interlayer, we have one layer of glass broken and one intact. Is there any chance the direction of impact force was on the unbroken side, or would it necessarily be on the fractured side?

    Thank you,


  28. Yes, you can have an impact on the opposite side from the broken side in a laminated unit. As some glass scientists say, glass only breaks in tension, not in compression. When you push on one side of the glass, then tension is on the other side. If there is a weakness on that side, breakage is more likely.

  29. Mark,
    Great site, very informative. I live in a high rise in Chicago and have 2 small creacks in lower corner of window – one along vertical edge and one along horizontal edge each about 2 1/2 inches long. They look like the stress cracks picture and I contend window washer put too much pressure on it with a foot. He says it is because of hail or a small stone. I live on 5th floor so stone is a joke and there are no impact marks or damage to screens. Glass is double paned and only outer glass is cracked. Is it possible that I am wrong?
    Thanks, Eric

  30. Hi Marc,

    We have a glazing company based in Coventry England. In one single day last week we had 6 different customers ringing with broken double glazed units. After surveying each job I had noticed they all had broken in a similar way to your example of blunt or disturbed impact. On the night of the breakage we had temperatures of -2. Would this play any factor in the breakages ? Also I noted that all the broken units were of similar size ( 700 x 250mm approx ). Would it be just coincidence in your opinion .

    Dean Jeff’s
    City Glass

  31. I aquired some double pane storefront type glass approx 5′ x5′ that i am going to use for a sunroom on my house. There are some letters etched the middle side of one of the panes legible looking through the opposite pane. Should this pane be on the outside looking form in or the inside looking from out.

  32. Hi Mark,
    I recently noticed a crack in the interior pane of the doubled glazed window in my bedroom. I didn’t recall any impact and on viewing your info I think it’s probably a thermal crack. I live in a rented house so I told my landlord about the crack and my opinion of the cause.
    A window contractor came to the house today. My landlord has advised that the contractor was of the opinion that the crack was caused by an impact and so I should cover the cost of the replacement. He advised that the contractor based his diagnosis on the presence of shards of glass between the two panes.
    I’m wondering if that make sense to you? Can the presence of shards of glass between the two panes give reason to diagnose the crack as the result of an impact and not if it’s reason to rule out other causes?
    I’ll email you some photos. The crack has grown quite a bit. It started at the bottom right.
    I’d really appreciate any advice you could give.


  33. Hey Mark –

    I’ve got an interesting one for you, and some pics you MIGHT want as exemplars…

    Essentially an A-frame house, metal framed on a concrete pad (settling unlikely…) but with a “clear story” facing due east. Double pane windows with yet another glass slider for essentially triple-glazed.

    When I moved in one was broken, a single curved line from top to bottom. Now I’ve got two more; was sitting at my desk one summer day when one just went “POP!”

    I did have the exterior stucco’d and thought perhaps CONTRACTION of the stucco around the windows was responsible, but the one was broken before I got here in VERY similar fashion.

    The almost-vertical break lines mostly originate in corners, curve inwards, then back out.

    I don’t see how this could NOT be stress caused, but….curious about how to improve the mounting of the replacements. One contractor suggested using cut pieces of rubber for shims rather than wood when mounting the replacements. Though perhaps I’d order the replacements a tad smaller & use larger shims than I normally would — leaving 3/4″ to the rough opening and lotsa foam.

    As the house is steel framed, set in concrete, interior tongue & groove / exterior stucco, I”ve got no stress cracks…hmm, maybe better get up there and double check the stucco for crackin’ in the corners.

    Alas, the windows are at least 20, if not 30 years old, they’ve had a good run, I’m just wanting to make sure the replacements don’t do likewise. No partial shadows on the glass, fwiw.


    Thanks for a great website, btw. Very helpful.

    Wade Nelson, Arboles CO to waste a little time…

  34. We are restoring a storefront and want to use argon filled (what mix of air to argon they are unable to say {?!}) but the only people nearby who will get just the glass for us have a negative impression of argon filled double panes. They believe that argon ruins windows because of leakage causing implosion.
    We’ve had them at our house for ~17 years. They work wonderfully and only one ever had a seal leak, it was 4 times longer than high and on the west side of the house. I understand that can increase stress. It didn’t implode, just fogged over.
    How common is it for argon filled windows to spontaneously break?

    Thank you, Kathleen

  35. Hi Mark,

    Fascinating website. Glass is so much more than one thinks. We have tempered double-glazing. We have been trying to work out if someone threw a stone at the interior pane of our DG’ed patio doors (when they were wide open) or our son engaged the spider lock (lifting the handle up) before closing the door properly and the wind slammed it. My son is determined to convince me that some perp chucked a stone. We could find none. But I spotted those telltale butterfly wing polygons around a tiny hole that looks like an inclusion. I have some pics I could send. Very interesting stuff – I could get hooked.

  36. Hi, a very interesting site! Wonder if you can help me in England? My elderly parents (both 95) heard a bang out of nowhere to discover the patio window inside pane had completely cracked looking like impact damage but they assured me no one or thing was anywhere near the windows when this happened.
    I rang the company who installed it as it is still under warranty but afterv taking a look they stated it was impact which had caused it. I know my parents are telling the truth so I was wondering if there is a way of prooving this either way.

  37. Hi Mark,

    I’m a writer currently working on a crime novel. My character is breaking into a flat by hitting a frosted glass window twice with a hammer in order to make a hole big enough to get his arm through and release the Yale lock from the inside.
    I just wanted to check that it would be possible to break frosted glass in this manner?

    Very many thanks,

  38. Hello mark.

    I dont think you can help but my Window cracked today. and i needed help to figer out why it happend. so it was hot today and there was a pilow infront of the window on the inside. i dont know what glass it is. but to problem is i live in norway so you cant come to help me that way and i dont know how much you can help me by looking at a photo. but im gonna ask any way ill send a photo if you need it but if you cant help ill understant dat to

  39. If the glass is in a door or sidelite it is probably tempered. Tempered glass can be broken with a hammer if hit hard enough. It will break easier if hit with the sharp claw side of the hammer.

  40. If the broken glass is still in the opening, the breakage pattern can be analysed. If it is on the floor, that is near impossible. So you have to argue based upon circumstances (elderly occupants) and show the installing company my article on spontaneous glass breakage.

  41. Hi, very helpful and interesting page. I woke up to the glass dining table in my house broken and on the floor. Comparing it to pictures, I think it was tempered glass. As I was the only person in the house when it happens, I am having a hard time trying to convince people that I didn’t smash it. Is there any way to prove spontaneous breakage?

  42. It is difficult without being able to see the breakage pattern. If, somehow, the glass was held together even after it broke, as it is with laminated or filmed glass, it is relatively easy to diagnose the problem.

  43. Very useful and clearly written post. Could I send you some pictures, even a video, of a broken window to have your ” blind” diagnosis? By “blind” I mean that I don’t give you any other infos than the pictures/video
    Many thanks,

  44. Mark,

    Just how strong is quarter-inch tempered glass? We are thinking about installing it for our windows in a new sport-court area in our house and are wondering if it is strong enough to withstand balls possibly hitting it. The windows are high enough that we do not anticipate basketballs or footballs hitting them, but want to be sure that, if they do, the windows won’t break.



  45. 1/4″ tempered is strong and should be able to handle ball impacts but you can hedge your bets by making sure: 1. The glass edge is polished, 2 The glass is well cushioned within the frame and 3) better yet, have the glass be two pieces of tempered laminated to one another, in other words a laminated unit. If it breaks, it will stay in the opening rather than crashing down.

  46. Mark ,
    I had a 96″x 66″ bathroom mirror installed Monday, that same day before arriving the company cut 2 holes for my 2 light fixtures to be places through the mirror.I know it was that day they made the cuts because they had to come by and re measure to cut the size in, these holes are about 3 1/2″ in diameter. Mind you its been rainy and cold out side 40 degrees and getting colder.
    They install the mirror about 2:30 pm, I get home and I am not real happy of the bat, one the mirror is filthy, they took my light fixtures down but then told my husband the can’t put them back because I need to get new screws.
    So I spent 30 mins cleaning this mirror when I got home that night. After closely examining it I noticed a few light scratches under the left hole. Tuesday goes by and when I get home from work about 7 pm, there IT was a HUGE crack from the bottom of the left side hole that was made, all the way down the mirror.
    | |
    The mirror is sitting on 1×4 wood plank that sits on my bathroom counter as backs plash and the mirror has 2 clear fasteners with little screws on each side in the corners.

    Installer said they used self leveling mastic and complained to my husband that the wall was bowed.
    Our master bath was remodeled with new walls and installation 2 years ago we are on brand new pier and beam, we also had the foundation corrected at the time of the build out.

    Could you possibly give me any opinion as why this could have happened?

  47. The installer probably made a slightly rough cut and had a chip in the edge of the hole cutout. He also (this is normal) had to press on the glass to get it to seat into the mastic and be flat. That pressure, coupled with the chip, caused a crack to propagate. The mirror company owes you a new mirror.

  48. Hi Mark,

    I wish you had a branch here in Austria! My son, aged 7, was playing bowls with a stone next to an glass floor architectural feature, which forms part of the school play area. He threw the stone, missed the snow and hit this glass floor with the stone and it has cracked. The architect has already told us that this glass should be able to withstand 200Kg. Would you expect a slow-moving stone to shatter the glass in this way? It has cracked into may small parts, but has not shattered and gone through to the floor beneath the playground. From a safety point of view there are apparently 2 layers beneath this. Is this normal behaviour an outdoor glass floor?

    This is an interested question, as we are insured and the insurance companies will probably fight it out. My husband and I were just interested as to what one should reasonably expect from an outdoor architectural feature that forms part of a walkway and adjoins a school play area.

    Your opinion would be appreciated.

  49. Hi Karen,
    Can you send a few pics to
    The glass may be able to resist 200kg of evenly distributed load, but that is very different from a rock. First, this is impact not a uniform load. Second, the rock may have a hardness similar to the glass. Hardness is huge. I can break a big piece of tempered glass with a little ceramic chip.

  50. Hi Mark,

    I am bidding on a church addition job for stained glass inserts. The contractor showed me 5 IG units that had the outer panes broken. They showed no impact pattern, rather multiple perimeter fractures that indicated stress. The adjacent units showed extreme inward bowing due to possibly Argon loss??
    My question is, is it common for the bowing force to great enough to break the glass. The units are Pella , 23 years old. The Pella rep blamed the breakage on the stained glass that was installed on the inside of the unit. These are aluminum clad wood framed sashes in central Illinois.


  51. I was on holiday and I have come home to a broken window, maybe 45 year old 3mm glass about 55cm square. The approx 3 pieces went outwards. The breaks are fairly clean but there are some chips at the edge of the broken out pieces and they are on what was the inside. The largest broken out piece was lying on the wooden decking below, I am not sure why it has not further broken as it fell. Another piece has gone further away. The bolt on the door was undone and the movement sensor alarm had been activated at some stage. Could some suction tool have been used?

  52. Fantastic analysis . I am thankful for the facts . Does someone know if I might grab a blank NY MV-82 version to complete ?

  53. Hello there. I recently ran through a double-glazed glass door, headfirst. It appeared open because of the light. Of course I had injuries, lacerations – but my head seems OK & the doctor confirms that. However, I’m scared that the force must have been huge. (I didn’t lose consciousness though.) The glass had been there for at least 20years, experiencing some extremes of temperature between summer & winter. Is it possible that it was fragile/brittle and that I broke it without too much force? Thank you so much if you get round to replying. Cathy.

  54. I have a small house in Santa Fe, NM; I am not there right now, and have been having a neighbor check on it for me. Today she found that the fixed pane of my 6’8″ sliding glass door is shattered. This is double-paned tempered glass, and both the interior and exterior panes are broken (with the myriad of tiny pieces all still hanging in there tenuously).

    I had someone mow the grass a few days ago (again, I wasn’t there for this), and I am wondering if a flung stone could have caused both panes to shatter. It is plausible to me that a stone could shatter the outer pane, but could it possibly break both panes?? Other possible causes I am thinking about include the great temperature variations in northern NM from day to night – could that somehow do this? Or, if a vandal hit the outer pane with a hammer or rock, could that break both panes? One other possibility: I had a number of issues with the contractor/poor construction. Is it possible that the header/ceiling settling could put pressure on the window and break both panes at once?

    My main question is really about whether a stone thrown by a mower could possibly break both panes — but any/all insight you may have into how this could have happened would be greatly appreciated. Thank you!

  55. Can sunroof glass just suddenly explode without a bullet or rock hitting it?

  56. I ran across your website while researching an issue I have. I came home from a trip to find a disturbing issue with a window. There is a small almost perfect circle in my bedroom window. The edges almost look melted. It is only on the outside pane. There is another puncture/crack in the top corner of this same window. My bedroom is on the second floor of my house. There is a small chip/piece of my siding missing on my house right under the particular window. The next day i noticed a piece of glass broken out of the kitchen window downstairs and the window cracked. Around this is a half circle. The police think its a glass cutter but i have also heard about 100 different opinions including that it could be natural. As a single mom with 2 small boys this has caused me to not sleep since. I am perplexed as to why a second floor widow that requires a ladder to get to and why a small hole if someone wanted to break in. Then the kitchen one also makes zero sense to me because it is at the top of the window. What good would a small hole do someone if it isnt positioned above a lock? I can find nothing of the web reguarding glass cutting to break in a house (which is good, but also bad bc i need to know so i can return to normal) i would be extremely grateful for an opinion on the photos. Perhaps there is a scientific explanation or you have seen this before and would know a reason why someone would go to this trouble.

  57. What is the pattern that glass breaks from vibration? Like from blasting? What ppv?

  58. I meant at how many inches/second does it take to break it?

  59. Hey Mark, I have a mystery I hope you can solve. I came home yesterday to find the front picture window had a large hole in it. I live in a 40 yr old house and the glass is non-tempered single paned. After noticing the broke window I went inside to investigate hoping to find the object that caused the break although I didn’t find anything and the shards of glass was mostly on the window sill and very little on the living room floor directly under the hole in glass. When I went back outside to investigate more I found a 3 inch kobalt razor knife laying on the ground about 6 feet away from the broken window. The knife is made of hard plastic and metal. Is it possible that this knife was the object that caused the break? Could it have made the hole but bounced back upon impact? Thank you for your time.

  60. Hi Mark,
    I faced today the same problem (Same as shown in back ground photo) here in one unit as tenant complained. it is a tempered glass of 6mm think (internal side)+Space/gap 10 or 12mm + 6mm thick glass (external side).

    can you please advise that what can be the reason of shattered these two window glasses, one in the bedroom and other in living room. we didn’t find any impact. tenant informed that when he came to the flat after many days I found shattered glass in the bedroom and after few days in the living room.

    your reply in above regards will be highly appreciated.


    Iftikhar Ahmad

  61. Hi, great site and forum!
    Quick query, a family member has blamed our son for breaking the glass on their wood stove as they had cleaned it after last use and my son pointed out a four inch vertical crack which curves right at the top on a Dovre branded stove. He is truthful and promises he has never touched it and it has caused a huge row. The crack is directly above the lever for raddling in the middle of a 550×375 piece of glass. I have been told that an impact would cube this glass and it is therefore a thermal stress crack, sorry no photos. It enlarged by 5mm from 10pm and 10am too. I would really appreciate a professinal pointer on this to avoid further family issues, many thanks!!

  62. Hi Mark, sorry have no photos and no chance to get any as property now 1000 miles away! Glass is 550×375 crack runs from dead centre upwards in straight line then curves slightly to right, end in a narrow point 3/4 the way up. It grew a little from 10pm to the following morning. Your time is much appreciated!

  63. Thanks Mark, it is not possible, very useful site and thanks for your replies

  64. Dear Mark,

    Thanks for your website, its educative.

    What can i look for to conclude that the building window glasses were shattered by lightening & thunder.

  65. Wondering if vandalism. Have not noticed a crack. Went into a building for a few hours and came out to the parking lot to notice a long almost “L” shaped cut/crack starting from near the edge of the windshield. Does that sound like vandalism type damage?

  66. We recently had 75 mph winds at my home on a lake. I have a wall of windows ending in a cathedral. The wind made 5 of my 100′ Beech trees lean to just about at my house. If they fall (which is highly likely as I cannot afford $8000 to have them removed) will the top 5′ x 7′ to a peak shatter, or break in little pieces. My living room is right there – and I have an 832 sf house,above a garage. As the tree is now about 4′ from the house on a (if it were a clock it would be 10 after 2) angle …. 5 of them. Any help would be appreciated? Judy

  67. We built a new home in 2004. We purchased 10,000 worth of Marvin window. We also purchased 2 sliding glass doors. We had to have 1 sliding door replace. It bowed terribly and let snow in. approx 4 years ago we noticed 1 small window in the kitchen was cracked. We replaced it for about $100.00. Around the same time the next year another small upper window in the bedroom cracked. the 3 rear the same upper window in our alcove cracked. and this summer we noticed another small upper window in our garage cracked. last week we found a larger window in our kitchen cove has cracked. The company in Kokomo In informed us they were all stress cracks. I have contacted Marvin and our warranty has expired. Do you have any advice? I feel like maybe the company ran a bad run and now we are seeing the defects.

    Lisa Bunting

  68. After vandalism a glass door showed cracks like a feather: so long cracks that over tens of centimeters showed fine secondairy cracks, on both sides of the main crack, each few mm, each some 10-20mm long. The door was double glass.
    I know the theory that an impact with large energy causes many cracks, but I never saw these feathers.
    What causes these?

  69. Hi Lisa,
    The term “stress crack” has little meaning. The question is not whether there is stress, but where is it coming from?
    Sometimes the crack pattern can tell a story. So please send me, via email at pictures of the glass with the cracks. Also place a straight edge on the glass and let me know if the glass is laying flat or if it is bowing. Thanks!

  70. Most of the times, a tempered glass will disintegrate and fall out of the fixing, but the cause of breakage can be assessed with reasonable fairness, if it stays in fixing. Breakages due to impact or wind loading are less likely to stay in frame.

Leave a Reply

Your email address will not be published. Required fields are marked *

Click to schedule a free consultation

Discuss Your Project