Encyclopedia Brown and the Case of the Exploding Oven Door
December 21, 2005 8:13 AM   Subscribe

On Thursday night, my stove's glass door exploded for no discernable reason whatsoever. Since MeFi's population encompass many trades, I'm hoping there's someone with a background in physics, engineering, or an appropriate tradesman who either knows or can venture a fairly educated guess as to why it happened.

I had just finished cooking two small pizzas for 450 degrees for 14 minutes. I had taken them out of the oven and was cutting the pizza on the tray with a pizza cutter. [The stove is a GE 30" Free-Standing Gas Range (Model No. JGBS07PEHWW). It is not a self-cleaning oven, nor is it a glasstop oven. I believe it was brand-new, as the landlord is in the process of renovating the building and is putting new appliances in each unit as they renovate them.] I heard a large bang/shatter and saw a lot of glass crumbs fall on the kitchen floor. The glass was safety glass, like the kind you see in windshields — in all the cleanup that followed, I never came across a single glass "crumb" from that door that had a sharp edge to it. Indeed, my cat's crystal litter was more uncomfortable to trod upon than the one or two crumbs I missed during clean-up.

I opened the oven door and immediately noted that the inside glass had shattered, and that there was glass inside the stove, inside the door's metal frame, around the edges, etc. Thankfully, it was safety glass, so none of it appeared to have any hard edges.

It bears repeating that I never slammed the oven door (nor could I slam it hard enough to shatter it; I’m simply not that strong). I also do not believe that the glass slid or fell out of its holder, due to the presence of glass around the edges and in the oven (suggesting it burst inwards towards the oven). Some of the glass was warm to the touch, but then again, I had just finished cooking the pizzas.

After I cleaned it up, I of course looked on the Internet, and found others whose G.E. oven ranges' glass doors had exploded — specifically, links here, here, here, here. (I don't know which model the complainants had, except in the latter.)

On the bright side, it doesn't appear as if it's happening on a national basis, if, with such a popular brand as G.E., I can only find a handful of complaints of it happening spanning over six or seven years — plus, were it exploding regularly and nationally, a recall would have undoubtedly followed. So I think that the odds of it happening to me again are low, but not out of the question. Hopefully, it would again result in no injury, since this model's glass door is solely on the inside of the oven, with a sort of heat shield front that is in no danger of breaking similarly.

My landlord replaced the oven with another one of the same model. I know that Ask MeFi encompasses a large crowd of professionals, and I am frankly wondering if anyone (perhaps with a background in physics, or engineering, or the appropriate trade?) has an educated guess, or knows, why the glass broke. I would feel a bit more comfortable having this mystery explained (or at least having a possible few rational explanations under my belt). And, given the less-than-fruitful discussions the above-linked individuals had with G.E., I don’t think I’m going to get much of a straight answer out of them.

posted by WCityMike to Home & Garden (25 answers total)
Once safety glass develops a crack it is pretty easy for the whole piece to fracture like you saw. Perhaps it had a defect and then the high heat induced a stress sufficient to let the crack propagate whereupon the whole pane shattered. That the oven is new lends credence to the defect theory. (pure conjecture of course)
posted by caddis at 8:22 AM on December 21, 2005

Gas or electric?
posted by Pollomacho at 8:26 AM on December 21, 2005

My guess:

The glass is tightly set in the metal door, and the two materials have different rates of absorbing heat, especially at the relatively high temp of 450.

The heat differential causes a size differential and one squeezed the other.

Maybe you should preheat to a lower temp first.
posted by StickyCarpet at 8:26 AM on December 21, 2005

Perhaps an accumulation of heat-conducive grime (oil splashes, etc.) at a stress point?
Design flaw?
Poltergeist? :-)
posted by Chunder at 8:27 AM on December 21, 2005

Bakers warn that you should place a towel over the oven window when pulling anything with moisture out of a hot oven. A little bit of moisture on very hot glass can cause the window to shatter.

It could be that the hot pizza oven and some run-off from the pizza caused the initial crack.
posted by precipice at 8:41 AM on December 21, 2005

It's a gas range.
posted by WCityMike at 8:41 AM on December 21, 2005

If the explanation were some given, e.g., design flaw or the heat differential between two panes of glass, there would be a higher rate of failure, which he said he had not seen in his search. But for that reason, it would be good to report this to the company.
posted by artifarce at 8:42 AM on December 21, 2005

How old was the oven? The newer it is, the more I would think that caddis' theory is the right one — heating and cooling, especially from one side only, can produce a fair amount of stress in solids. It seems likely that these stresses could, over the course of weeks or months, exacerbate a microscopic defect to the point where the glass would break. In other words, the old oven probably just had a bad piece of glass in it — you were unlucky and got that one-in-a-(large number) piece that wasn't quite up to snuff when it came out of the factory.
posted by Johnny Assay at 8:43 AM on December 21, 2005

I think StickyCarpet might be along the right track, though there are a couple of oversimplifications in his explanation.

First, the issue would be difference coefficient of thermal expansion (CTE) which is not directly related to energy absorption but rather atomic bonding structure within the materials. (You probably don't want the details.)

Second, I would expect that G.E. would accommodate for CTE differences between the metal and the glass because that would be a ridiculously huge design flaw.

What I predict is that the glass broke because it cooled/heated at an unequal rate, though it may have had minor flaws to begin with. If the glass were to cool at different rates (because of oil splashes as Chunder suggests?), some parts might try and expand more than others. And, as we know, glass is rigid, so that doesn't work well. I'd especially believe this might be the case if the oven door was left open or it was cool/cold in the kitchen (more rapid cooling). Finally, I'd imagine G.E. does some sort of testing, so I'd expect you might have had some small flaw in the glass to begin with which created stresses or micocracks, which easily propagate under temperature differentials.

Additional thoughts:
1) If you ever get a small chip in your car windshield and don't get it repaired right away, you might find the crack propagates over the course of a week, especially if there is a large temperature swing between day and night. (I had a 2-3 cm crack grow to about 300 cm in a week while living in Los Alamos, NM, where there was a ~40F temperature difference between night and day.

2) The reason Pyrex is so great in the kitchen or the lab is because it has an extremely small CTE, so it's less likely to break from thermal shock. Try* filling a regular glass with boiling water and then dumping it into a sink full of ice water. It will likely shatter.

*Do not actually try this without proper safety gear or with a glass you like.

On preview: Damn, I'm long-winded.
posted by JMOZ at 8:43 AM on December 21, 2005

heat causes things to expand, typically, and although thermal glass is designed to expand very little it still does so. if one bit gets a lot hotter than another bit, the relative difference in size due to expansion can cause a crack.

also, this kind of glass is built under tension - it's made so that the inner glass is "smaller" than the outside surface, but of course it can't be, so the inside is "stretched" and pulls the outside "in". do you see what i mean? it's achieved by making the outside cool, so it sets, while the centre is still molten. then as the centre sets, it wants to contract slightly.

anyway, this tension means that when the glass expands, it does so without cracking, since it's all held together. that's the theory. but if you have some kind of imperfection then it might not work; in pacrticular, once a crack starts, the internal stress that was holding it all together is likely to splinter it into many small pieces.

in general, such a failure is going to happen the first few times it's used, and some failures are probably normal. they should replace it free of charge.

i'm not sure that's very clear - basically it's made to be "unstable", but in a way that normally helps it survive heat gradients. however, that same instability can make it go "crash" unexpectedly if there's some internal manufacturing flaw (given the way glass is made, it's probably impossible to guarantee that such a flaw is always absent).

disclaimer, i thought i'd explained this kind of thing before, here (found this) and that someone corrected an error. unfortunately i cannot remember what my error was.
posted by andrew cooke at 8:44 AM on December 21, 2005

From here:
Careless handling and improper installation sometimes produce damage, which leads to “spontaneous breakage” or “delayed breakage”. This means that occasionally a sheet will not shatter immediately at the time of damage, but perhaps weeks later.
Don't know if this is applicable to oven doors, but it seems plausible.
posted by Johnny Assay at 8:52 AM on December 21, 2005

Your oven uses tempered glass. This has been specially treated to provide extra strength, up to five times normal window glass. The process creates a very high tensile stress on the thin layer of glass on both surfaces. This pre-stressing is what creates the bending strength -- it acts like an I-beam. The down side is that the tiniest scratch in that surface will eventually lead to catastrophic failure. It is likely that there was a flaw in the glass, probably on the very edge which was concealed by the mounting when the oven was manufactured. It was like a time bomb ready to go off after a certain number of thermal cycles. Since the oven was new, they should have replaced it under warranty. These things happen. There is no one to blame. See this link.
posted by JackFlash at 8:56 AM on December 21, 2005

Re-reading my explanation, I did definitely get one thing wrong- cooling glass would contract at different rates rather than expand.
posted by JMOZ at 8:57 AM on December 21, 2005

I'm with caddis on this. Safety glass is a mess of huge internal stresses -- see Prince Rupert's Drops for details -- and the tiniest flaw can cause early thermal failure.
posted by scruss at 8:59 AM on December 21, 2005

This happened to us, but we weren't cooking at the time. A repairman was pulling on the oven (he was checking something about how it was set) and the same thing happened, bang, shatter, glass everywhere.

It was scary, but no one was hurt, and now we have a great new oven. This thread is very interesting to me!
posted by GaelFC at 9:12 AM on December 21, 2005

Following up on other's explanations about tempered glass, a lot of the glassware used in kitchen applications where it's subjected to heat is pyrex or safety glass. That's why there are warning statements on glass coffee machine carafes not to use them if they get chipped or cracked.
posted by LionIndex at 9:37 AM on December 21, 2005

Safety glass is another name for tempered glass. As andrew cooke describes, the fact that the outside of the glass is under stress causes the edges to be dull when the glass breaks. Being under pressure, when the glass breaks, it completely shatters into small cubes, rather than breaking into dangerous shards. This can be quite dramatic, but being in the path of exploding safety glass isn't that dangerous.

Pyrex is Dow-Corning's trade name for a borosilicate glass. Borosilicate, glass doped with boron as opposed to say lead which makes crystal, makes a very tough glass which doesn't chip easily, and, as mentioned by JMOZ, doesn't change much in size with temperature. Pyrex is also very resistant to chemical reaction, unlike softer "regular" sodium glasses which soften and scratch easily in basic solutions (like dishwasher detergent).

Glass windows in oven doors are often made of borosilicate which has been tempered to give a strong, temperature resistant panel which won't break in a dangerous way. Like everyone else, I suspect your door had a tiny defect which was triggered by the door cooling. This could be a clip that was on too tight, a tiny scratch in the glass, too-tight mounting in the frame---it's impossible to know now. These things usually show up right away, in new stoves. It's hard to damage tempered borosilicate yourself in a way that wouldn't be obvious, like dropping a heavy, sharp weight on it. It is very difficult to scratch tempered Pyrex with a stray metal object---the glass is harder than many metals. The only thing that will damage borosilicates is long exposure to caustic oven cleaners---you shouldn't let the Eazy-off sit for hours on your oven window.
posted by bonehead at 10:47 AM on December 21, 2005

While all this about glass is true, i suspect if there was a scratch or a mounting error (ie metal clip on glass) that just one of the glass panels would shatter not both. Glass doors on stoves (just like gas, wood fireplaces and stoves) are mounted with a gasket so the glass "floats" and can expand and contract with these rapid temature changes.

"I heard a large bang/shatter " If the burner/igniter was malfunctioning or misaligned you could get what we call in the biz "a delayed ignition event" in other words a tiny explosion from unburnt gas finally reaching a spark/heat source. This happens in gas fireplaces when logs or pilot lights are blocked or misinstalled so when gas comes out it does not find the pilot or hot surface ignitor soon enough.

"Consumer complaints and incident investigations of gas ovens in the late 1990’s appear to indicate that unburned fuel gas can accumulate within ovens and ignite, producing hazardous flare-ups or explosions." CPSC pdf 30 pages

A mild case of this sometimes goes "WumP", extreme cases will blow glass out of units or even launch chimney tops up in the air (i have both done this and seen it done by others). This would not necessarily blow glass out all over the room, but could be enough pressure to bust the glass("is a mess of huge internal stresses "). Under the right circumstances this could even happen to a stove that had just been turned off, but was still hot.

The best thing was already done, unit replaced.
posted by blink_left at 12:26 PM on December 21, 2005

If the explanation were some given, e.g., design flaw or the heat differential between two panes of glass, there would be a higher rate of failure, which he said he had not seen in his search. And twelve other equally well-reasoned replies:

Word. Tons of people (myself included) have been using ovens with glass doors for years -- nobody's ever said to me "don't buy an oven with a glass door -- they explode."

You got a bad door, or they banged it in shipping and it resulted in an unnoticed defect that weakened the glass. This is exactly what happens when tempered glass fails. GE should replace it.
posted by Opposite George at 12:57 PM on December 21, 2005

(and just to outline my reasoning -- homeowners love to spread FUD, especially on expensive purchases -- there are so many ill-founded prejudices out there you'd think a well-founded one would be all over. Also, the fact that this happened in a brand new stove suggests you got a bum one.)
posted by Opposite George at 1:21 PM on December 21, 2005

Fud? Did someone say "don't buy an oven with a glass door -- they explode."? not me. No appliance is 110% perfect and sometimes things happen, this is not fud unless you think the CPSC is wasting it's time. With millions of these devices installed a few are bound to have problems, sometimes these events are dangerous thus we have fire departments, smoke detectors and so on.
As the OP stated "My landlord replaced the oven" no Fud there ether. Problem was already solved, this is the after action report and discussion, only the warranty person at the shop will ever know for sure what happened, if at all.
For the record, appliances have fires and sometimes explode every year in america, just not that many thanks to UL, CPSC and other such groups.

Every day i deal with homeowners wanting to do dangerous things saying "it will be ok...", replacing that oven door without knowing why it happened is a perfect example.
posted by blink_left at 2:08 PM on December 21, 2005

Hot Pyrex can shatter explosively, depending on the circumstances.

This happened to my wife when she placed a pumpkin pie in a pyrex pie dish onto an electric stovetop to cool, then turned on the wrong element to heat up a kettle of water for tea. The pie started smoking, so she quickly picked it up, then placed it down on a cool element. And then backed away and screamed as it exploded in all directions.

Fortunately she wasn't hurt, but the pie was ruined :-(.

The glass base of the pie plate was a perfect circle, matching pattern of the cool element it was sitting on. We figured that the angled side of the pie pan had popped off like a spring.

Anyway, the point is that your oven door was under a lot of compressive stress. A small scratch in the surface, or spot-cooling from touching cool metal or a drop of water, could easily have set this off.
posted by Araucaria at 3:07 PM on December 21, 2005

Not suggesting you're spreading FUD, blink_left.

It's easy to appreciate how scary, or at least puzzling it might be if it happens to you, especially if you're used to ovens with metal doors. Listen, if I was sitting at home and my oven exploded I'd be freaking to the point where they neighbors would be calling the cops.

Where the FUD comment comes into play has nothing to do with your post or your experience. I'm just saying the population at large often seems to be less, shall we say, enlightened than the AskMe community; standards of proof are lower and the value placed on anecdotes is higher. This means stories spread quickly, and you'd expect even rare problems to be well known. In other words, FUD is a major factor in the spread of prejudices among homeowners, and I'd think scary events get a priority bump as a result.

I can guarantee you if anybody in my condo complex had an oven do what yours did everybody else would know about it within a month and most of those people wouldn't buy a glass oven regardless of the explanation, until, in their senility, the story was lost. Many homeowners (e.g., my parents) give anecdotes plenty of, maybe too much weight in making a purchase decision. These reporters, not you, are the ones spreading FUD, just saying FUD really gets the spurs in 'em, and again, I'm not hearing screams from the rooftops.

Sorry about the misunderstanding; good luck with the landlord (don't get me started on them...)
posted by Opposite George at 4:12 PM on December 21, 2005

bonehead writes "Safety glass is another name for tempered glass."

When I hear safety glass I think laminated glass but this could be a technical jargon distinction.

Speaking as a former major appliance technician I can almost guarantee this was caused by either a defective mounting clip or a foreign object trapped under a mounting clip. A malformed clip with a sharp edge creates a stress riser which will shatter a tempered piece of glass. Same thing if a grain of sand or tiny sliver of metal gets trapped under a clip. I've, uh, tested this theory on more than a couple occasions. A manufacturing defect in a piece of tempered glass self destructs almost immediately. Note that in 99% of ranges only the inside glass is tempered, the outside glass is just regular glass.

Also you need a pretty substantial scratch to damage a piece of tempered glass as it needs to penetrate the tension layer into the middle. Take a look at the scratches in a older car's side/back window glass which is all tempered to see how much of a scratch a tempered glass will handle without failing.
posted by Mitheral at 4:15 PM on December 21, 2005

My thanks to everyone who responded. A few replies ...

The towel-on-oven window thing was a useful idea, and I just may well try that. Ya never know.

Just to clarify, regarding reporting it to the company; since I am not the owner, I'm not in a position to do so. My landlord is returning it to where they bought it from (Home Depot), and I imagine Home Depot will report the flaw to G.E. But judging from G.E.'s responses in the links I provided, I really don't think they're going to be that proactive or concerned; they seem to approach it from a "shit happens, want a replacement piece of glass?" perspective.

Johnny Assay, it was, as far as I know, brand-new. I got the manual with it when I came into the apartment. But I think the idea of a microscopic defect in the glass has merit - if you notice the links I posted, a lot of people reported these shatters after having had the oven for several years, not brand-new. (Thanks also to Jack Flash and others for this point.)

Andrew, it's very interesting to learn about how this kind of glass is made, and that it's already under a great deal of internal stress. That goes a ways towards explaining it.

Blink_left, there was only one glass panel, the one that was in the stove on the inside. The "outside" was just a solid piece of plastic/heat shield, and I'm not even sure what it was made of. I am almost 100% certain that it was nothing related to an explosion because I was quite literally standing in front of the stove. There was no signs of ignition (no heat, no flash, no `ba-LUMPH!'), and, most importantly, the glass blew *INWARDS* into the stove, not *OUTWARD* into the heat shield (I could tell from the way some of the pieces still connected to the frame were slanted). No flare, no light, no sudden heat around the edges of the stove, no burns, nuttin'. So although I appreciate your theory, I do not think that it is what happened.

Mitheral, thanks for sharing your experience as a former major appliance technician, too.

Thanks again, everyone. Good to be able to wrap my brain around this a bit better now!
posted by WCityMike at 8:45 AM on December 22, 2005

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