Is preheating plates in microwave dangerous? If so, why? My understanding of microwaves was that due to the high frequency they are easily absorbed into any material. The more "loose" the material, the easier the absorption. 
I know it's dangerous to let microwave on when empty, mostly because there is nothing that will absorb the microwaves and that may cause magnetic "storm" inside :) But I heard today that it's equally dangerous to use microwave to preheat plates. 
I would like to know if and why it's true. I think that the plate is a material like any else. But due to the high density it absorb the microwaves slower than say a glass of milk. And also the vibrations may cause it to break after extended period of time. 
 A: If you have a small fracture near an edge, water may seep in the plate. If this water bubble instantly vaporizes (since it preferentially absorbs microwaves), it may send a shard flying at high speeds. It's unlikely to hurt you - the front window with its mesh will almost certainly stop it - but it will damage the plate and possibly scratch your microwave.
If your plates contain sufficient water, then yes, it's safe to heat them in the microwave. The energy will be distributed well enough through all the water to heat the plate fairly uniformly. But be aware: fine china, glass and plastic plates contain no water.
A: The worst that can happen is a broken plate. It depends on the material the plate is made from, and how porous the plate is. Usually plates do not conduct heat very well, so it is possible that one part of the plate gets heated more than other parts, causing the plate to expand at that part , in turn causing shearing stress in the plate. 
If the plate is made from porous material there may be air bubbles trapped in the material. The air will heat, and the pressure inside the cavity will rise, again causing the plate to break. 
Plastic plates are usually fine. 
A: Many plates are made of dielectric materials (e.g. porcelain, glass, plastic) that won't absorb microwave radiation; so the plate won't be heated and the microwave oven will effectively be running empty, with the same risk of damage.
If the material does absorb the energy (e.g. ironstone, china that has absorbed some moisture) it's likely to heat unevenly and crack from stress due to thermal expansion.
