I purchased a glass bed to use with my still-in-transit Ender 3. Since the bed came in before the printer, I pulled it out and used a flat edge ruler to see how flat the glass surface is.

It appears the glass is slightly "dished" in the center from one side and "raised" in the center on the other side. I am wondering if this is a sign of a defective glass, or if it is likely that the glass may change shape slightly as it is heated?

If it does change shape, should it be put with the dish side up or down?

Logic would say that if it does change shape, the bottom would probably get hotter since it is against the heater while the top is slightly cooler, so in theory the bottom may expand more "pulling" the center down?

I guess the bottom line is, should I send the glass back as defective or wait until the printer arrives and see what happens when I heat it up?