# "Spontaneous" Breaking of Auto Glass? Possible? **URL:** \ **Category:** Factual Questions\ **Created:** [June 12, 2002, 2:12pm UTC](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647 "2002-06-12T14:12:13Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [June 12, 2002, 2:12pm UTC](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647/1 "2002-06-12T14:12:13Z")
Just happened to my bosses Nissam Maxima rear passenger door glass. A co-worker was walking buy and jumped when she was startled to hear his glass pop around 9.30 EDT \>just outside Philly, Pa…temps around 78…going to 95…all his windows were shut. He had never noticed anything wrong with the glass…and the car was positioned such that even a kid w/ a BB-gun couldn’t target it. Glass was inside and outside the car after shattering…about 50/50 in and out. We thought about the heat…but no one saw this happen before and we get summers w/ plus 90 days all the time…and this was morning. Security cameras saw nothing unusual and a friendly co-worker was genuinely scared when it happened. 😕 ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [June 12, 2002, 2:15pm UTC](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647/2 "2002-06-12T14:15:37Z")
;j Oops…that was unintentional. :eek: ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [June 12, 2002, 2:19pm UTC](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647/3 "2002-06-12T14:19:03Z")
I was warming up my old Porsche 924 back around 1985 or so on a very cold winter’s morning in the desert. I’d turned on the defroster to get the ice and frost off of the windscreen (I’d already scraped it) and went inside for a minute. I heard what sounded like a .22 rifle being fired. When I went back outside there was a large crack in the windscreen. Obviously the warm air was heating the frigid glass unevenly and that’s what caused the failure. While something may _seem_ spontaneous, I think there is a logical explanation of the cause. Perhaps there was a flaw in the glass and the heat you mentioned caused it to fracture? ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [June 12, 2002, 2:20pm UTC](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647/4 "2002-06-12T14:20:03Z")
Happened to me with a car sitting in my driveway on a summer day. Most of the glass was outside the car, no one was in sight. Don’t know the reason for it, nor how much of a factor temperature might be, but I know it’s possible. ---
**Author:** ![Amp](https://avatars.discourse-cdn.com/v4/letter/a/6a8cbe/32.png) [@Amp](https://boards.straightdope.com/u/Amp)\ **Post date:** [June 12, 2002, 2:23pm UTC](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647/5 "2002-06-12T14:23:15Z")
Seen it happened once when I was a kid. My mom and I were comming out of a grocery store and heading towards our car. It was a pretty scorching day out. As we passed a van we heard a loud pop or crack and saw a side vent window on the van just shatter. At the time it was weird. ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [June 12, 2002, 2:24pm UTC](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647/6 "2002-06-12T14:24:44Z")
> [@](#): > > \*Originally posted by Johnny L.A. \* > \*\*I > While something may _seem_ spontaneous, I think there is a logical explanation of the cause. Perhaps there was a flaw in the glass and the heat you mentioned caused it to fracture? \*\* JLA, Spontaneous: I had it in quotes. 🆒 Been 'round these boards just long enough to know “spontaneous” could be pounced on. I, too, have seen and heard of the cold/hot glass breaking incidents. Never saw one heat related w/out killer heat yet. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [June 12, 2002, 2:53pm UTC](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647/7 "2002-06-12T14:53:40Z")
> [@](#): > > Been 'round these boards just long enough to know “spontaneous” could be pounced on. Sory about not catching that. It’s June the 12th, and therefore the day I’ve chosen as my personal holiday. I slept late and replied while still half-asleep. I do that a lot. 🙂 ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [June 12, 2002, 2:54pm UTC](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647/8 "2002-06-12T14:54:16Z")
Spontaneous is perfectly appropriate: “having no apparent external cause.” Doesn’t mean there isn’t a cause, just that we had no reason to see it coming. ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [June 12, 2002, 3:08pm UTC](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647/9 "2002-06-12T15:08:18Z")
Maybe a “rod” hit it. 😃 ---
**Author:** ![Moirai](https://avatars.discourse-cdn.com/v4/letter/m/96bed5/32.png) [@Moirai](https://boards.straightdope.com/u/Moirai)\ **Post date:** [June 12, 2002, 3:51pm UTC](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647/10 "2002-06-12T15:51:59Z")
**Johnny** , 7:00 am is sleeping late?!? Damn man, do you have kids or something? I would kill to sleep in until 7 or 8! This happened to my mom and I in our garage, but I think ours was a pressure thing. We got in the car and closed the door and heard a huge, coughing roar. We turn around, and my best friend is sitting there in the back seat, looking totally shocked, with little cubes of safety glass still falling down on her! Freaky. ---
**Author:** ![Lisa\_Ann](https://avatars.discourse-cdn.com/v4/letter/l/bc79bd/32.png) [@Lisa\_Ann](https://boards.straightdope.com/u/Lisa_Ann)\ **Post date:** [June 12, 2002, 5:26pm UTC](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647/11 "2002-06-12T17:26:54Z")
My husband and I were parked in front of a grocery store and having an argument on a mild California day. I got out to go in the store, heard a loud noise, turned around, and there was a huge fist-sized smashed area on the windshield right in front of my husband. I thought he did it and he thought I did. 🙂 I guess something could have fallen off the high store roof - someone thought they saw a ball bouncing but we couldn’t find it or any other explanation. ---
**Author:** ![Kilt-wearin\_man](https://avatars.discourse-cdn.com/v4/letter/k/82dd89/32.png) [@Kilt-wearin\_man](https://boards.straightdope.com/u/Kilt-wearin_man)\ **Post date:** [June 12, 2002, 6:51pm UTC](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647/12 "2002-06-12T18:51:59Z")
I heard it happen and saw the aftermath at a friend’s wedding reception. We heard a LOUD “glass shattering” noise and ran outside, thinking there was some kid breaking car windows in the parking lot. Well, there was a broken window, but no kid and no way a kid could’ve gotten away unnoticed (or hidden, considering how many of us there were in the parking lot almost instantly). The driver-side window of one of the guests had shattered, spraying glass a pretty good distance away from the car. Most of the glass seemed to be outside the car. This was in the middle of summer in Texas, and I seem to recall it being a pretty hot day - temperatures approaching 100 degrees. ---
**Author:** ![Breezy](https://avatars.discourse-cdn.com/v4/letter/b/b3f665/32.png) [@Breezy](https://boards.straightdope.com/u/Breezy)\ **Post date:** [June 12, 2002, 6:54pm UTC](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647/13 "2002-06-12T18:54:26Z")
My husband works at a glass shop and he says this happens a lot. 🙂 ---
**Author:** ![Omnivore](https://avatars.discourse-cdn.com/v4/letter/o/e47774/32.png) [@Omnivore](https://boards.straightdope.com/u/Omnivore)\ **Post date:** [June 12, 2002, 7:11pm UTC](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647/14 "2002-06-12T19:11:24Z")
Things like this used to happen a lot in older cars where they had to be manually vented by a knob under the dash board. Summer heat would bring the inside pressure up real high and would usually blow out the back window. I guess it can still happen if you park the car and do not set the a/c control to normal or vent, where it opens an outside vent or leave a window open a tiny bit. If the frame shifts on the car a little bit, it can crack the front windshield and some cars ‘settle’ over the years. It happened to mine. ---
**Author:** ![malaka](https://avatars.discourse-cdn.com/v4/letter/m/ecc23a/32.png) [@malaka](https://boards.straightdope.com/u/malaka)\ **Post date:** [June 12, 2002, 7:22pm UTC](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647/15 "2002-06-12T19:22:34Z")
Similar to the story from **Johnny L.A.** … My aunt was sitting in her car on a cold winter morning with the winshield defrost on high. She suddenly hear a very high-pitched tone, like a squeak. She watched as a thin crack slowly worked its way across the length of her winshield. The crack originated at a small fisheye previously caused by a piece of gravel that was kicked up at her while driving. The windshield didn’t shatter. Several days later, she took the car in to be serviced before the windshield could break completly. ---
**Author:** ![Walloon](https://avatars.discourse-cdn.com/v4/letter/w/fbc32d/32.png) [@Walloon](https://boards.straightdope.com/u/Walloon)\ **Post date:** [June 12, 2002, 8:03pm UTC](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647/16 "2002-06-12T20:03:12Z")
It happened to the rear window of my car on a summer day. ---
**Author:** ![Sofa\_King](https://avatars.discourse-cdn.com/v4/letter/s/58956e/32.png) [@Sofa\_King](https://boards.straightdope.com/u/Sofa_King)\ **Post date:** [June 12, 2002, 9:05pm UTC](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647/17 "2002-06-12T21:05:30Z")
I’m pretty sure that the theory behind it is a pressure differential between the inside of the car and the outside of the car. Big S-Class Mercedes Benzes from the early 1980s were prone to doing this when the door was slammed. The door seals were big, fat rubber things and by slamming the door you could spike the internal pressure enough that a window could burst. BMWs were purportedly prone to doing the same thing. How did Benz and BMW solve this problem? Next time you’re in one, watch the windows when you open the door. They automatically drop a centimeter or so until the door is closed again. Then, they automatically seal up. It’s one of those, “ain’t it cool” gimmicks that sealed more than one deal when I was in the business. ---
**Author:** ![Lisa\_Ann](https://avatars.discourse-cdn.com/v4/letter/l/bc79bd/32.png) [@Lisa\_Ann](https://boards.straightdope.com/u/Lisa_Ann)\ **Post date:** [June 12, 2002, 9:54pm UTC](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647/18 "2002-06-12T21:54:14Z")
**Sofa King** - are you implying I slammed my truck door in a fit of anger? 😉 Interesting point. The seal on the truck is still great after all these years. Maybe that _was_ it. It _was_ my fault. Damn. ---
**Author:** ![Tennessee\_Ben](https://avatars.discourse-cdn.com/v4/letter/t/f0a364/32.png) [@Tennessee\_Ben](https://boards.straightdope.com/u/Tennessee_Ben)\ **Post date:** [June 12, 2002, 10:10pm UTC](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647/19 "2002-06-12T22:10:20Z")
Here’s my guess: The glass in car windows is tempered. Scientific American [explains the process](http://www.sciam.com/askexpert_question.cfm?articleID=000C1D15-39B6-1C71-84A9809EC588EF21&catID=3&topicID=13): > [@](#): > > Next, the glass begins a heat treatment process in which it travels through a tempering oven, either in a batch or continuous feed. The oven heats the glass to a temperature of more than 600 degrees Celsius. (The industry standard is 620 degrees Celsius.) The glass then undergoes a high-pressure cooling procedure called “quenching.” During this process, which lasts just seconds, high-pressure air blasts the surface of the glass from an array of nozzles in varying positions. Quenching cools the outer surfaces of the glass much more quickly than the center. As the center of the glass cools, it tries to pull back from the outer surfaces. As a result, the center remains in tension, and the outer surfaces go into compression, which gives tempered glass its strength. Not only is tempered glass stronger than annealed glass, this variance in temperature also affects how it breaks. When regular glass breaks, it tends to break into long shards. When tempered glass breaks, it tends to “explode” into a bazillion little pieces. (That’s why it’s good for cars, by the way–long shards will will cut you, while tempered glass shrapnel tends not to.) My guess as to the “spontaneous” window break is this: I suppose that there was a flaw in the tempering process that left the outer layer unusually thin in one spot. As the glass warmed and cooled over a period of days (Months? Years? Who knows?) the flaw in the glass was exacerbated. Finally the tensioned glass in the center overcame the ability of the outer glass to contain it, and the glass failed. That _might_ also explain the loud bang. Note also that this (ending up with a seat full of broken glass) would only happen with side windows. As Johnny L.A.'s experience shows, front and rear windshields are tempered _and_ laminated. While the physics behind the breakage are the same, the lamination probably confines how the glass breaks, and definitely keeps it from “exploding” like regular tempered glass. Ben (whose father makes–you guessed it–tempered glass) ---
**Author:** ![jiHymas](https://avatars.discourse-cdn.com/v4/letter/j/439d5e/32.png) [@jiHymas](https://boards.straightdope.com/u/jiHymas)\ **Post date:** [June 13, 2002, 12:54am UTC](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647/20 "2002-06-13T00:54:30Z")
> [@](#): > > \*Originally posted by Tennessee Ben \* > **My guess as to the “spontaneous” window break is this: I suppose that there was a flaw in the tempering process that left the outer layer unusually thin in one spot. As the glass warmed and cooled over a period of days (Months? Years? Who knows?) the flaw in the glass was exacerbated. Finally the tensioned glass in the center overcame the ability of the outer glass to contain it, and the glass failed. That _might_ also explain the loud bang.** That sounds a lot better than any explanation I’ve ever been able to come up with! I was working in my basement office one fine summer’s day when I heard some glass breaking … checked all around, couldn’t find anything, but eventually discovered that the porthole on my washing machine had shattered. Note that the washing machine was out of direct sunlight and was not in use, I was alone in the house … not even a pet goat hanging around to butt the thing, so it was most mysterious. I must give the manufacturor, Miele, a plug here. They sent me a new porthole by courier with no charge … when I had been expecting hoots of derision at my story. [Next page](https://boards.straightdope.com/t/spontaneous-breaking-of-auto-glass-possible/113647.md?page=2)