problem in the microwave?
For Pyrex No.
IF there was any lead added when the glass was being made, it's only a small percentage as part of the "recipe" for the glass. For metal to cause problems in the microwave, it needs to be a much larger amount.
You can use small pieces of aluminum foil to cover a portion of whatever you are heating/cooking to prevent it from over cooking, by covering that area with the foil. As long as there's a lot more of what you are heating/cooking, than aluminum foil.
For example (OK amount of foil): You decide to reheat a whole, already cooked chicken, but in doing so (as an example) the wings will overcook and dry out during the reheating process. So you cover the wings with pieces of aluminum foil just large enough to cover them, then proceed to reheat. Leave them covered for 1/2 to 2/3 of the heating time, then remove the foil for the rest of the time. Covering them with foil prevents the microwaves from penetrating, so no cooking/heating occurs while covered.
Look at microwave popcorn, the bags have some sort of metal in them on the one side of the bag, which heats up, heating the oil/corn kernals, which helps them pop more uniformly and completely.
Obviously if you cover a large bowl with aluminum foil and put it in the microwave, yes that will cause problems and it will look like 4th of July in there. Not good!
The link below says some metals are safe (in small amounts) in the microwave.
Kevin
Well, okay, just some kinds of metal. But still...
www.goodhousekeeping.com