Fear of Pyrex shrapnel
September 22, 2008 10:36 AM   Subscribe

Am I going to be killed in a lasagna-related accident?

The Pyrex FAQ on this matter is down and the rest of the info I've found seems anecdotal. But not quality anecdotal like Metafilter-anecdotal. Anecdotal like Yahoo Answers anecdotal.

I have lasagna frozen in a Pyrex casserole dish. Can I just throw it directly into the preheated oven? This is what I've heard, but it just seems so wrong. Has anyone actually done this?
posted by A Terrible Llama to Food & Drink (37 answers total)
I made brownies in a Pyrex dish once, took it out of the oven and set it on the stovetop and it exploded. The stovetop was off, so the metal was cool and I assume this was from the rapid transition from heat to cooling. Don't know if it would be different if the dish was placed in the oven while it was warming, but all I can say is beware.
posted by mattholomew at 10:45 AM on September 22, 2008

Best answer: The Pyrex website says:

# Avoid Sudden Temperature Changes to your Glassware. To avoid the risk of breakage due to a sudden temperature change to your Pyrex glassware, DO NOT add liquid to hot glassware, place hot glassware on a wet or cool surface, or handle hot glassware with a wet towel, wet potholder or other wet cloth. DO NOT place hot glassware directly on a countertop or any metal surface, or in the sink. Allow hot glassware to cool on a cooling rack, dry potholder or dry cloth. Be sure to allow hot glassware to cool as provided above before washing, refrigerating or freezing.
# Oven Must be Preheated Before Inserting Glassware. DO NOT insert glassware into oven for cooking or reheating until the oven has been preheated to the desired temperature.

...and more.

This article has anecdata about exploding Pyrex.

As with most things, YPyrexMV in explosiveness.
posted by rtha at 10:49 AM on September 22, 2008 [3 favorites]

I have experience with the Hot to Cold explosion, but not the other way around. When I worked at a restaurant sometimes the waitstaff would grab a glass that had just came out of our Very Hot dishwasher and go to put iced tea or something in the glass. As soon as they did it would shatter in their hand. However (obviously) nothing no matter what the temperature it was that we put in the dishwasher exploded because of the change of heat. So, anecdotally my non-science guess is that your lasagna will be fine, especially since the heating of the dish will be gradual.
posted by genial at 10:52 AM on September 22, 2008

I think the shattering danger comes from suddenly cooling hot Pyrex, rather than suddenly heating cold Pyrex, so go right ahead and throw it in the preheated oven - just be careful when you take it out.
posted by muddgirl at 10:52 AM on September 22, 2008

As far as Metafilter-anecdotal evidence is concerned:

I've moved Pyrex from the freezer to the preheated oven without incident more than once.

As far as Pyrex warnings are concerned:

According to what rtha quotes above, it looks like Pyrex warns against not
As far as other pieces of advice:

No, I do not think you should throw it. I think that would cross over into the realm of "unsafe."
posted by koeselitz at 11:14 AM on September 22, 2008

Best answer: From what I understand, the chemical composition of Pyrex has changed, following the company's acquisition by World Kitchen. It used to be made from thermal shock resistant borosilicate glass and is now made from tempered soda lime glass (as of about 1998). It's not nearly as resilient to temperature changes as it once was.
posted by otherwordlyglow at 11:16 AM on September 22, 2008 [5 favorites]

Response by poster: It seems like there's conflicting information -- on one hand, as rtha notes, the website (I swear it was down before!) says 'no drastic temperature changes', yet as others mention, it seems like it's for some reason okay to do this.

Dasein, the reason for concern is because it's oven safe but like all glass not immune to shattering due to major temperature fluctuations, and since it's frozen it seems like sticking it into a 350 degree oven would be pretty startling to it. For example, as in this cheerful tableau from rtha's link above:

"My left hand index finger was starting to have a bad burning sensation. I continued on washing, not realizing that there was about an 3/4-inch-long piece of Pyrex glass stuck in my left index finger," Dave wrote, adding: "I mean stuck, stuck."

"I am not a pro with pulling sharp objects out of myself. I finally after watching the blood spurt out, told myself I was going to have to pull the glass out of my finger and soon," Dave recalled. "I gave it a real good tug and got it out, then the bleeding started, and continued for at least 30 minutes."

This does not describe the evening I'm hoping to have.

The Pyrex response in that article includes this gem: "When glass breaks, it may appear instantaneous, and may be described with violent words such as 'exploded' or 'disintegrated.' Instead of disintegrating, however, a glass failure generates from one or more fractures, each of which begins at a particular site and grows from there."


Anyway, there's a quote in the same article from, Cook's Illustrated says: "Downshock would result from adding cold liquid to a hot Pyrex dish or from placing a hot dish directly on a cold or wet surface. It is considered safe, however, to transfer a Pyrex dish directly from the refrigerator or freezer to a hot oven, provided it has been properly preheated -- some ovens use the broiler element to heat up to the desired temperature."

So I'll do it.

But I won't like it.

Thanks all.
posted by A Terrible Llama at 11:24 AM on September 22, 2008

From this Cook's Illustrated article on baking (warning PDF)
...the tempered glass bakeware is also vulnerable to sudden drops in temperature, known in the industry as downshock. Downshock would result from adding cold liquid to a hot Pyrex dish or from placing a hot dish directly on a cold or wet surface. It is considered safe, however, to transfer a Pyrex dish directly from the refrigerator or freezer to a hot oven, provided it has been properly preheated...
posted by FreezBoy at 11:27 AM on September 22, 2008

oops, or what you just said.
posted by FreezBoy at 11:28 AM on September 22, 2008

Why is it important to have it preheated?
posted by konolia at 11:39 AM on September 22, 2008

In fact, it seems like it would be better if it wasn't preheated, since the temperature change would be more gradual.
posted by smackfu at 11:42 AM on September 22, 2008

That's what I'd think too.
posted by bink at 11:44 AM on September 22, 2008

Response by poster: Why is it important to have it preheated?

Maybe someone here really understands how Pyrex glass is structured. In addition to not understanding why the oven needs to be preheated, I also don't understand why 'down shock' is a danger but 'up shock' (I guess you'd call it--going from freezer to oven) isn't.

I'd like to understand this, but if I explore what's really near and dear to my heart, not picking glass splinters out of my eye is right up there.
posted by A Terrible Llama at 11:46 AM on September 22, 2008

Can't you just let it thaw out on the countertop and then heat it?
posted by softsantear at 11:48 AM on September 22, 2008

One interpretation: the pyrex will be heated mostly by convective heat, namely hot air blowing over it. At least some of the direct radiant heat will go right through the glass into the food, so no matter how hot the oven is you're limited to air-transfer heating which isn't all that efficient.

Putting a hot pyrex onto a cold countertop or cooktop gives you direct surface-to-surface contact which can move a whole lot more heat leading to stress fractures.

IANAPhysicist or Chemist but I've broken my share of labware.
posted by whuuuu at 11:50 AM on September 22, 2008

Best answer: The issue is not the speed of the temperature change but the evenness of the temperature change. The shattering comes from part of the glass being hot and part being cold, which exerts a stress on the dish. The time aspect only comes into play because heating the glass slowly will give it time to distribute heat evenly throughout the glass. I'm guessing that they warn you to preheat the oven to avoid hot and cold spots.

I'm disappointed to learn that pyrex is no longer borosillicate. Oh well, maybe it's time to dump them and start buying up fused silica bakeware.
posted by kiltedtaco at 11:51 AM on September 22, 2008

whuu is right on the difference between cooling and heating: none. It's just more common for people to suddenly cool a dish than to suddenly heat it. For instance, if you put a glass dish on a burner, I would be very concerned and probably run away. People don't do that in a kitchen as often as they put a hot dish on a cold surface, so it's less of a concern.
posted by kiltedtaco at 11:55 AM on September 22, 2008

Best answer: The question about preheating is answered by the part I left out of my quote, "some ovens use the broiler element to heat up to the desired temperature". That would cause uneven heating or cooling which is the downfall of any glass, really.

My shop teacher once took a paper cup filled with water and applied a blowtorch to the outside. The paper cup did not get burned because the temperature of the cup was being regulated by the water temperature.

Since glass is an excellent conductor, the frozen lasagna should act as a temperature regulator and won't allow the pyrex to heat up too fast.
posted by FreezBoy at 11:56 AM on September 22, 2008

Response by poster: So how come placing the frozen casserole on the hot oven rack doesn't cause the same effect? Is it just the surface area in contact with the pan is smaller?

Can't you just let it thaw out on the countertop and then heat it?
It would take forever, and there's eggs in the ricotta, so I would probably thaw it in the refrigerator--so it would be essentially the same circumstance; cold pan/hot oven.
posted by A Terrible Llama at 12:01 PM on September 22, 2008

I did put a pyrex-type dish on a burner once (when I was young and stupider) and yes, it cracked. No explosion though.
posted by Kiwi at 12:04 PM on September 22, 2008


If I were you, I'd put the frozen dish in a bowl of water while the oven is heating, to 'thaw' the Pyrex.
posted by Kiwi at 12:08 PM on September 22, 2008

The oven is not quick heating.

You can stick your hands into a hot oven, right? You don't put things in the oven and come out with hands burnt to peeling, or even hurting. Think about the difference of putting your hand into boiling water and putting your hand into a 450 degree oven--where are you going to get the worse burn?
posted by that girl at 12:09 PM on September 22, 2008

Best answer: This PDF from Corning, which gives specs for their (genuine) Pyrex lab bottles, cautions against both up shock and down shock, and says not to put cold bottles on hot surfaces and vice versa. (Note: in labs, "cold" and "hot" are often more extreme than at home.) I guess this confuses things more than it enlightens, but at least it suggests that both directions of thermal stress are risky.

For home use, I'm guessing that up shock (freezer to oven) is not such a problem because the heat surrounds the glass pretty evenly so it expands pretty evenly, like kiltedtaco said. (Also, air is not a very efficient conductor of heat.) Down shock usually takes the form of "cold liquid/food in hot glass" which is more localized and sudden. If you put a hot glass dish into a freezer with wire racks, just like your oven except cold, it would probably be fine too. (Again, because it would chill fairly evenly and slowly.)

I've never understood the whole oven preheating thing, unless it's to reduce variability in baking. Most recipes specify time as the end point (e.g., bake 30 minutes) rather than the internal temperature of the food. Different ovens probably heat at varying rates, so if your oven heats slowly, your 30 minutes won't expose the food to as much heat energy as it would get in a fast-heating oven and it might be undercooked. If the oven is already at temperature when you put the food in, it should cook the same as any other oven at the same temperature.

Just for the sake of completeness, here are the thermal expansion coefficients for Pyrex and regular glass. Pyrex expands less than half as much as ordinary glass.

(And I'm also bummed that household Pyrex isn't borosilicate any more! I wonder if Mom would give me her old pie plates?)
posted by Quietgal at 12:18 PM on September 22, 2008

I did this. Bad idea. Broken glass, ruined Lasagna. I'm not sure what would have happened if I HADN'T preheated, but I did, and it broke. Fairly instantly.

It didn't explode...I still have my oven AND my eyeballs, but it ruined dinner 4 surely.
posted by TomMelee at 12:25 PM on September 22, 2008

Response by poster: See, I'm just the kind of person who delights in food that may or may not kill me. Every day, I look for the burger with the handgun, or the slice of pizza that might knife me in the back. So this is really working out great.
posted by A Terrible Llama at 12:42 PM on September 22, 2008 [2 favorites]

I also did this. Added some liquid to a pan of something or other. Made a hell of a bang, but shattered into little tiny, nondangerous pieces like a windshield.
posted by electroboy at 12:43 PM on September 22, 2008

I do what you want to do all the time, except that I usually turn the oven on at the same time as I put the lasagna in. I have never had a problem. Seeing as TomMelee has caused breakage by putting the lasagna into a preheated oven, it would seem that not preheating might be safer.
posted by ssg at 1:03 PM on September 22, 2008

Another explosion survivor here.

The shattering comes from part of the glass being hot and part being cold, which exerts a stress on the dish.

That's what got me--relatively cold stuff in the dish that shifted to one side. The empty side heated up at a different rate from the full-of-frozen-stuff side, and kaboom. It was pretty dramatic; from the next room, it sounded like the entire contents of the refrigerator had collapsed into a big heap.

Made a hell of a bang, but shattered into little tiny, nondangerous pieces like a windshield.

My experience exactly, except that I remember a few sharp pieces. Two years later, I was still finding "diamonds" of the exploded dish down in crevices.

This was a few years ago, and I'm not willing to swear as to the exact make/model of the dish. ("Pyrex" being one of those trademarks people have a habit of using generically, like "coke" or "kleenex".)
posted by gimonca at 2:35 PM on September 22, 2008

kiltedtaco is correct in that the even-ness of the heat is important. So I'd preheat the oven but not the oven racks. Then put a rack in a the same time as the lasagne. That way the dish isn't touching anything hot at all and both will heat evenly together. Having the rack cold will slow down cooking a tiny smidgen but not enough to notice, and having the oven preheated will avoid any cold spots and let your lasagne cook nicely rather than go soggy while the oven heats. The frozen lasagne is going to act as a nice buffering zone preventing your dish from heating up too fast, so there won't be a drastic change in temperature to worry about.
posted by shelleycat at 2:58 PM on September 22, 2008 [2 favorites]

I'm going with: don't pre-heat the oven first.

Put the lasagna in and turn the oven on at the same time. If you have a pre-heat option, don't use it, just use bake, because ovens go to higher-than-the-desired temperature to reach the desired temperature quickly when they are in pre-heat mode.

So, re-capping: put lasagna in oven, turn on bake, let it slowly heat up and then, when you judge it has reached the right temp (lots of ovens have lights that go on/off at this point), start your cooking time from that point.
posted by misha at 3:01 PM on September 22, 2008

They tell you to preheat before putting in the dish because, as noted above, many ovens use the broiler element at the top of the oven, as well as the baking element underneath to preheat. Using the broiler element heats the edges of the dish, but the food shields the bottom of the dish, causing differentials that can shatter the dish. Heating from below is much more even across the dish.

I also think the asymmetry between downshock and upshock is because downshock more strongly decompresses the surface (tempered glass depends on surface compression for its heat resistance), but my argument was too complicated and shaky to post.
posted by jamjam at 3:59 PM on September 22, 2008

I think the preheating rule is because if you are heating the oven to, say, 350, the oven gets hotter than 350 for a bit while it's preheating. It doesn't quite make sense to me why it would do this, but apparently the oven preheats with cooling and heating cycles, so the temperature change must be more radical before it reaches the proper temperature. My sister told me this - those Glad OvenWare plastic pans say to preheat the oven for that reason. I can't find their explanation online, but I found this regarding a plastic oven dish melting:

The issue of preheating may be one reason your product melted and flamed. "Ovens don't heat themselves gradually," says John Drengenberg, consumer affairs manager at Underwriters Laboratory. Instead, he says, when you set your oven at a certain temperature, the oven goes through varying heating and cooling cycles, with a sensor turning the heating element on and off as needed until your desired temperature is reached. Since these containers can withstand a maximum temperature of 400 degrees, says Tucker, there's the possibility they might melt during one of these first heating cycles, which, he adds, "is the worst time that you can get drastic temperature spikes."

Or what jamjam said
posted by artychoke at 4:16 PM on September 22, 2008

Why not let the frozen dish thaw in the fridge while you're at work? I've put cold lasagna in a hot oven many a time with no disastrous results.
posted by computech_apolloniajames at 5:19 PM on September 22, 2008

jamjam, wouldn't downshock compress the glass rather than decompress it? It would shrink when it cools, no?

Interesting to hear that preheating may be required due to the way electric ovens heat up - I've always had gas ovens and never noticed any problems due to crazy temperature excursions if I forgot to preheat.

I love these questions that turn into discussions of the details of oven temperature control and the materials science of cookware!

posted by Quietgal at 8:09 PM on September 22, 2008

Response by poster: Okay. I did it. I lived. Method as follows: preheated the oven for a long time to even out any warm/cooler pockets and potential leftover heat from the broiler maybe running during preheating (I don't think it does anyway, but whatever). I did not deal with the racks separately, I just left them in. Put the frozen lasagna brick in, waited for explosion. None came. An hour later I took the lasagna out and put it on a dry wooden cutting board. Again, no drama.

Let me tell you though: that thing was gross. Let me head some future AskMe off at the pass when someone would like to know if eight months is too long to keep a foil wrapped pan of lasagna in the freezer: yes, it is. It is too long. Should you eat it? You should not.

I had ziti instead. But hey, what a ride.

Thanks again all.
posted by A Terrible Llama at 2:12 AM on September 23, 2008 [1 favorite]

Response by poster: Oh--also meant to add, it's a recently-bought piece of Pyrex, so it's not constructed of borosillicate.
posted by A Terrible Llama at 5:37 AM on September 23, 2008

Response by poster: All right, well, this just showed up on Reddit, which makes it absolutely sound like if not having cookware exploding in your face is a must-have, you may want to avoid Pyrex.

I mean, if you're a coward or something.
posted by A Terrible Llama at 9:37 AM on September 24, 2008

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