This Pyrex glass safety label says "Always preheat oven". Any idea why?
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.
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.
That's just a precaution to extend the life of your Pyrex glassware.
Transliteration from the French original:
Plates in Pyrex glass can be used to heat and reheat food, in the oven or microwave oven, if you take a few precautions:
- 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...
all pyrex or other glass container cookers, please pay attention:
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
place matching hot water in a larger metal pot/pan
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)
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
the water betwee the metal and glass pots/pans with temper/slow temperature changes in a uniform manner Yours in Christ, Phil
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.