- The glass tray had a hairline crack or stress fracture that was not visible
- It got a little too hot in one area
- The expansion from the heat caused the fracture to expand and therefore crack and break
An accidental bumping of the microwave plate on the countertop while going from the microwave to the dishwasher might set up the above situation. The spoon your seven year old left in the mac and cheese might have arced and nicked a small piece of the glass. Whatever the cause, microwave trays break and they break for a lot of reasons.
All foods cooked in the microwave produce heat. Microwaves themselves are not hot, but the foods cooked in a microwave get hot - some much hotter than others.
Even though most microwave glass trays are tempered, they are not designed to hold up under high temperatures. You will see most microwave trays stamped with the words "For microwave oven use only" This is to let us know that it's probably not a good idea to use the trays in a conventional oven.
Any air pockets, cracks or gaps in the glass can enlarge under heat. Since glass isn't very receptive to being stretched, breakage can occur.
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