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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.

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  • $\begingroup$ Obviously microwaves aren't that easily absorped. The walls and front window do not heat up that much; they mostly reflect the microwaves. $\endgroup$ – MSalters Mar 10 '14 at 16:04
  • $\begingroup$ MSalters: The inside casing is cold. But the plate is hot. Which led me to the conclusion that the ceramic material of the plate is more absorbing than the casing => should be no problem as part of the waves is absorbed $\endgroup$ – Tomáš Fejfar Mar 10 '14 at 21:05
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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.

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  • $\begingroup$ And is that very different from the situation when the water bubble vaporizes slowly? The pressure is ~ the same - the maximum that the material around can hold... until it can't. $\endgroup$ – Tomáš Fejfar Mar 10 '14 at 21:09
  • $\begingroup$ The problem is, bubbles don't vaporize slowly. The transition from 372 K to 374 K is not a gradual one. Remember that the microwave puts out enough power to heat significant amounts of water in seconds, a small bubble can boil in milliseconds. $\endgroup$ – MSalters Mar 11 '14 at 7:53
  • $\begingroup$ But if it would, it would be the same result, wouldn't it? The pressure would build up (not milliseconds, but seconds) to the critical pressure when the shard will let go and fly away. Therefore this problem is still present even if the bowl is full of water.... $\endgroup$ – Tomáš Fejfar Mar 11 '14 at 11:39
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    $\begingroup$ @TomášFejfar: If a small bubble slowly evaporated, the small amounts of steam generated will seep away at lower pressures, preventing the catastrophic failure mode. Also, if the bubble heats slowly, the loss of heat to the surrounding ceramic will balance the influx. $\endgroup$ – MSalters Mar 11 '14 at 12:35
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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.

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    $\begingroup$ Air bubbles aren't the risk. Water bubbles are far more dangerous. Air expands by about 25% if you heat it to the boiling point of water. Water turning into steam espands by several thousand%. This is especially dangerous for plates with a hair fracture as water bubbles along the fracture can evaporate all at once. That turns the hair fracture into a real fracture in milliseconds. $\endgroup$ – MSalters Mar 10 '14 at 16:03
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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.

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