My wife just bought 6 Pyrex dishes with seal proof lids./ No where did I find any reference to the product being suitable for freezing. Our freezers are set to -22 Deg centigrade, so I was apprehensive the product would survive long term freezing.
Now, I know from my school physics that Pyrex was developed as a strong, low expansion glass, so heating and cooling should hardly affect it and the expansion rate of the glass should be minimal. That's ok provided the temperature change occurs gradually (several hours), and that's because glass is an extremely poor conductor of heat, so a rapid change of temperature is not on. Putting a Pyrex plate in a Microwave, then taking it out and putting on a S/S sink drainer is inviting disaster. Believe me, when I did it, the plate exploded after 1 minute into a thousand pieces, same as toughened glass windshields do when hit.
So to test the salesman saying, yes it's ok to freeze food, I would point out several factors you should consider.
1. Make sure the food to be frozen is at room temperature (for several hours) before putting dishes in freezer.
2. Make sure that the ice formed can expand vertically, without causing pressure on the glass. This is achieved by the sides of Pyrex dishes being tapered towards the bottom, so that expansion forces are taken with a vertical component, not sideways. So insure your dishes have tapered sides. This is critical.
3. After several weeks deep freeze, take the dish out of the freezer and place on rubber/plastic/wood base or chopping board. Leave for several hours (At least 3-5 ) before further action, make sure both food and dishes are at room temperatures, especially if same dish is placed in microwave.
I looked closely at the literature. Nowhere did it mention the Pyrex bowls/dishes could be put in deep freeze. On each item we bought (made in the USA) there was stamped - microwave safe, do not broil, do not put on stove top. So I wonder if you can actually put it in an oven or in a deep freeze. We have had Pyrex Casserole/dishes cooking roasts in the oven, so I presume this is OK. My wife assures me she has used Pyrex many times in the oven. Pyrex however, will not admit to this, I think because they cannot prevent people sticking something from the oven into cold water sink to cool it quickly. Nothing will take this temperature shock without severe contraction/expansion forces. Try pouring boiling water onto a S/S sing base, and the explosive "pop" can be alarming. (Men should know what I'm talking about when they splash the walls). Corning Ware is safe and suitable for ovens and I have tested this. I feel it has lower coefficient of expansion than Pyrex.
My verdict is yes, you can freeze, Pyrex, but take the precautions mentioned above.
The whole Pyrex breakage thing is pretty complicated:
As JoJo-Zep said, fast temperature changes are a no-no, particularly when only part of the container experiences a big change.
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