Excerpt from the book The Complete Microwave Oven Service Handbook -- Available on CD-ROM (CLICK HERE)
Copyright ©, 1989-2003 by J. Carlton Gallawa

An essential part of all microwave ovens is the cooking shelf, which, depending on the model, is a clear glass tray, a carousel or turntable, or a ceramic shelf. The purpose of the cooking shelf is to hold the food at about 1 to 1¼ inches above the metal cavity floor.

Whether sealed in or removable, the shelf is virtually transparent to microwaves. Therefore, the energy passes through the shelf and reflects off the metal bottom then back up into the food. It is as though the food is suspended in mid-air as far as the microwaves are concerned. Thus, they can penetrate the food from all sides with uniform intensity.

The clear glass trays used in microwave ovens are made from specially processed safety glass, which will absorb certain amounts of RF energy and are tempered for greater impact strength. This borosilicate glass has special properties making it highly heat resistant and enabling it to absorb RF energy, in the event of prolonged noload operation (sometimes to the point of glowing red), without breaking. Also, this glass is much more pure than ordinary glass; a flaw in glass that is exposed to microwave energy could cause the glass to explode.

Ceramic shelves, usually sealed in with an RTV sealant, are made of a special ceramic called Pyroceram (range cook-tops are made from the same type of material). These shelves also absorb certain amounts of RF energy, so in the event of no-load operation they too act as a dummy load and absorb enough of the energy to minimize arcing, leakage, or reflected energy.