My vintage CorningWare is not allowed in the microwave. I saw what happened to Mom's set back when microwaves were new, and I didn't care to repeat that experience with my own estate sale finds. The FAQ at Pyrex Love says the microwave is fine, but not the dishwasher...maybe that was the culprit.
Whatever the true rules are, I didn't want my dishes in the microwave. When Hubby showed me that the bottom said "for range or microwave" a couple of months ago, I was baffled. I bought that set in its original boxes, so I know it's old, too old to be labeled microwave safe...
I solved the mystery when I was cleaning out some kitchen cabinets. Apparently I have two identical casseroles, with different markings on the bottoms.
I have no memory at all of where the newer one came from. And now I've got to re-train the family to read the bottoms of the dishes they're using.