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This Pyrex glass safety label says "Always preheat oven". Any idea why?

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    Now that's interesting - the fact that it's on a safety label could mean that bringing it up to temp slowly has adverse effects on the glass. I'm now curious enough to dig. – Tim Post Apr 16 '16 at 17:01
  • @TimPost Generally it's fast heating that messes with glass, and it does say right below it to avoid sudden temperature changes. – Cascabel Apr 16 '16 at 17:27
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    It says preheat oven... Not the pan. Preheating the pan empty is not usually recommended – Catija Apr 16 '16 at 17:37
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    Arguably (as @Catija implied) preheating the empty pan is instructed against, at least for cold food. I reckon the "always preheat" instruction is about the time it takes for the food to come up to temperature in a pan with poor thermal conductivity and large thermal mass. Now that modern ovens preheat much faster and some reckon that preheating is only required for some foods, this instruction is probably open to debate. – Chris H Apr 16 '16 at 18:07
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    I suspect that's for the food, not the glass. Placing food in a cold oven has disastrous effects, culinary speaking. – BaffledCook Apr 16 '16 at 19:45
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While an oven preheats, the heating element or gas burner will be running at full output. For an electric oven in particular, this will generate a great deal of radiant heat. Radiant heat increases the temperature of the objects it shines on, without directly changing the air temperature.

So, if you place a pyrex dish in a cold oven and then turn it on, some surfaces of the dish will be exposed to this intense radiant heat for a long period of time as the oven heats up, while the air temperature in the oven the rest of the dish is exposed to will still be much cooler.

If the dish is placed in the oven after it has preheated, it will still be exposed to the radiant heat as the oven cycles on and off to maintain the temperature, but it will be for shorter periods of time and the ambient temperature the rest of the dish is exposed to will not have such a large differential, resulting in less thermal stress on the glass.

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    This is an excellent answer. I'd add that thermal stress, if it creates enough temperature variation, can cause your Pyrex to explode. That's also why they say to put it on a dry, cloth potholder as that will slow the loss of heat from the dish to the countertop and why you don't use it on the stove or broiler (lots of direct heat = lots of thermal stress). Similarly, adding liquid to a dish before cooking meat or vegetables means that the parts of the dish not covered by meat/vegetables won't heat up too fast. – Duncan Apr 18 '16 at 21:47
  • @Duncan — There's a plethora of website extolling the virtues of glass cookware. I can tell you from personal experience that when they unexpectedly break while cooking a meal in the oven, it kills all the fun. – ElmerCat Apr 18 '16 at 22:11
  • I have a number of glass cookware pieces and have been fortunate enough (and careful enough) that I've never had one explode on me. My sister has, though. Turns out that lasagna with chunks of glass is an underappreciated family Thanksgiving delicacy. – Duncan Apr 18 '16 at 22:22
  • @Duncan — I would wager your sister's replacement lasagna pan is not made of glass. Any possible benefit glass bakeware might have is outweighed by the prospect of a total disaster occurring at the worst possible time. – ElmerCat Apr 18 '16 at 22:56
  • Now I want to see an actual test of this. If someone wants to test and video, I'll pay for the pan you break. – FuzzyChef Apr 20 '17 at 18:42
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As Fabby says, it must have something to do with a sudden change in heat. An hypothesis: they misinterpreted the French original written by Pyrex. It says, again thanks to Fabby:

Assurez-vous de toujours préchauffer le four avant d’y mettre votre plat en vitre.

This means, "make sure to always preheat the oven before putting in your pan". It could be interpreted in two ways: "always preheat the oven if you're going to use the pan in it", or: "always preheat the oven before putting in the pan, not after putting in the pan".

The latter would make sense: pre-heating is a special function in many ovens with its own button. This function uses the grill on the "ceiling" to make it heat up extra fast (in addition to the normal heating element located outside the internal oven compartment), and this would result in too much direct heat on the pan. The warning in the picture in the Question also says "do not use under a direct heat source".

Another (weaker) hypothesis: they interpreted the French correctly. A good oven should not do this, but some ovens may turn on the grill even when heating up the oven normally (non-pre-heating), to make it heat up faster from room temperature. If, however, you put it in after the oven has already reached the desired temperature, the grill element won't be on at full power all the time any more, if at all.

My oven only uses the added heat from the grill when I'm pre-heating, so this wouldn't happen—as it shouldn't. But maybe some ovens are crazy.

  • I don't quite understand the distinction between the two meanings here... if you heat the oven after putting the pan in, you're not preheating it. – Cascabel Apr 18 '16 at 1:33
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    @Jefromi: Perhaps I should have been clearer: this is about using the preheat button on the oven, not about preheating in general. Edit: is this better? – Cerberus Apr 18 '16 at 2:05
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    So the short answer here is, it's important to have the oven at a consistent temperature before introducing the pan, something that often wouldn't be true (for various reasons) while the oven is heating. – Shog9 Apr 18 '16 at 2:26
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    @Shog9: Yes, well, if my primary hypothesis is true, it is even more important not to have pan in the oven while the "pre-heating" function is operative, because the pan is not supposed to be under a "direct heat source", i.e. the grill at full power. Which I think is strange, because normally oven pans don't seem to have any problems with the grill function, but that's what it seems to indicate. – Cerberus Apr 18 '16 at 4:06
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    Glass baking pans can handle high temperatures well, but tend to handle temperature differences poorly, often spectacularly so. I suppose if the pan was close enough to the flame / element, the radiant heat could cause it to heat unevenly and fail in a manner less likely to be triggered by the more even convective heat present later on. – Shog9 Apr 18 '16 at 4:14
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That's just a precaution to extend the life of your Pyrex glassware.

Transliteration from the French original:

Some precautions:

Plates in Pyrex glass can be used to heat and reheat food, in the oven or microwave oven, if you take a few precautions:

  1. Avoid sudden changes in the temperature of the glass.
    • Ensure to always preheat the oven before you put your glassware inside.

Pyrex is French so that was the first hit on google.fr for Pyrex préchauffer four...

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    But wouldn't that be contradictory? Putting it into an already hot oven would be a more sudden change in temperature than letting it heat up slowly as the oven heats. – Joe Apr 17 '16 at 19:53
  • @Joe That's what they say and what I've been doing for a very long time too... Just don't pour any cold liquids in them while they're boiling hot and don't drop them and you'll keep them for 25 years (my oldest one) – Fabby Apr 17 '16 at 20:54
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    What originally was named Pyrex was a glass designed for laboratory glassware, and extremely rugged regarding temperature changes, prompting users to take advantage of these properties and using it almost like metal cookware, including subjecting it to strong and rapid temperature changes. Still sold in Europe. American market Pyrex is ordinary, though certainly high quality glass. – rackandboneman Jul 1 '17 at 14:43
  • @rackandboneman: I live in Europe and even the original French stuff doesn't stand cold tap water after coming straight from the oven at 200°C... (I used to own some test tubes too and you had to start with very hot tap water and slowly go down the T° scale after taking them out of the Bunsen-burner) ;-) – Fabby Jul 10 '17 at 19:56
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all pyrex or other glass container cookers, please pay attention:

  1. glass should not be heated suddenly 1.b if placing hot food in glass pot then first put warm water in the pot and then hotter water to mach the temp of the food

    1. place matching hot water in a larger metal pot/pan

    2. empty the now warmed glass pot and place hot food in the glass pot and place the glass pot with food in it in the larger metal pot/pan with hot water in it r. place both metal and glass pot/pan with hot water between them in the preheated oven(this is called cooking in a water bath)

    3. it is ok to let all water evap during cooking, if the glass pot/pan should break while handling the combined metal and glass pots/pans you will be safe from cuts/injuries

    4. the water betwee the metal and glass pots/pans with temper/slow temperature changes in a uniform manner Yours in Christ, Phil

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I think the always-preheat instruction is there so that the cooking time is similar to that of metal pans, rather than for safety.

Glass conducts heat poorly compared to metals; preheating the oven fully is one way to make the cooking time stay similar to that of the same dish, in a metal pan.

protected by Community Feb 21 '18 at 7:13

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