Microwave Ovens

Might it have something to do with the amount and the type of material in the bowl?

Hi PW

checked the weight of the bowl in question with another one that does not get hot, not a lot of difference between the weights, but the material could be different, I grant you that, but how does ceramic material differ to the point where some bowls get hot, other don't?

...the amount and the type of material in the bowl.

opps, lost the instructions for the Mark 1 eyeball there for a moment, and not using the glasses.

It could be the food, I'll try with a bowl that up to now has not shown any signs of heating up other than from the heat of the food within, I'll let you know

Try 100ml of water at ambient and 100L of, say, butter at ambient in the same type of bowl for the same time. What happens?

Well thanks. I put 100 litres of butter into the microwave as you suggested. What a bloody mess!

If you try to heat plain water and the bowl does not heat up more than the water, then the dish is microwave safe.

Different foods cannot alter the behaviour of the dish itself. However, a rational explanation has to be found for the observed behaviour.

In my opinion the chilli (unfamiliar to me) might be a thick or dryish food. The microwave could have created steam bubble pockets between the food and the walls of the dish. Steam transfers heat rapidly (Condensing steam can have up to15000 btu/(hr.sqft.degF)

This can heat up the dish walls more than with a food that is watery.

I use a small Pyrex round glass bowl to cook my egg in the morning- rain water is added & a egg broken into bowl- on high for 40 sec- then 20 sec on high- inspect- usually another 15 sec- perfect poached egg- water is hot- outer of glass bowl is just as hot. reason for stopping @ 40 sec- if carried on say not 40 but say 75 sec- usually is explosion inside glass bowl- which I put down to steam pocket under egg erupting & blowing egg all over inside of oven!. Pyrex is considered safe for all forms of cooking- whereas plastic is not- heat can leach chemicals from plastic into food. Earthenware I don't know about- would depend on materials in mix I would say. In my case I would say that the water is transferring heat thru the glass bowl- after all we all know that nothing is a perfect insulator- that is why beer in a cold bottle, removed from the fridge, soon heats up to ambient temp( in a warm climate of course!).

I regularly warm ceramic plates by microwaving them for 1 minute at 800W, nothing else in the oven & the plates come out just hot enough to hold without burning my fingers. Obviously the plates are absorbing the microwave energy, whether this is due to iron or moisture I don't know.

The effect brich has noted may be down to the thermal conductivity of the food being heated.

The surface of the bowl may have some contribution to the heating effect.

Some surfaces are smooth and do not hold moisture and if these were exposed to the microwave energy the material may not get hot (assuming the material itself does not respond to the energy supplied). On the other hand if the surface is like unglazed pottery where the moisture is difficult to remove - this will end up getting hot because the water (in form of trapped moisture) is being heated up.

It would be a good experiment to try - use the dish as you normally use it without any food in it and heat it in the microwave and then do the same by drying it in conventional oven and then trying it in the microwave see which gets hotter.

We have some "melamine" bowls and plates that are not recommended for microwave use. Kids have mistakenly used them before, and they do get hotter than the contents, and would probably crack/explode if taken to an extreme. Perhaps the bowl that is heating up is melamine?

Tom

And even the best quality of china-ware having decorations may heat & damage the Oven.

Hi all

I have tried the following with the following results

Bowl One, the one that gets hot, in the microwave with a cup of water in the bowl, bowl and water came out after 2 mins, bowl hotter than the water.

Bowl Two, (same size and volume) the one that does not get hot, same volume of water & same time, result water hot and the bowl is warm, I guess the bowl is warm from the increase temp of the water.

I have also tried it with the chilli and the same result, the bowl is to hot to touch, I have also noticed now I'm studying the effects, that Bowl One boils the chilli where the chilli is in contact with the inner surface of the bowl

I know that both bowls are ceramic as I've broken enough of them over the years.

Thanks to redfred for the detailed explanation of how a microwave works, I do understand the principals, I have tried your suggestion of stirring the chilli in Bowl One half way through the heating time, same result, the bowl get hot, the chili boils at where it contacts the bowl.

Bioramani, chilli is a minced meat dish cooked in tomatos and chilli peppers with other spices, much like a curry and has a thick consistency.

So it would seem that Bowl One is not microwave oven friendly, but why does it get hot?

ps, the butter idea...... makes great gee!

Try heating the bowls with nothing in for, say, 20 or 30s. This may tell you if all the heat comes from the food or if some is from the bowl absorbing RF energy.

tried it, after 15 seconds with Bowl One with nothing in it, way to hot to touch!

Well, there's your answer. Seems that bowl is NOT microwave safe!

Tom

Yep I know that but why does it get hot or have I missed something?

A previous answer stated that it might be something to do with iron content, how do you tell?

Or is it just a fact that after all the tests, boiled chili, singed fingers and enough boiled water to make tea for a football team do we just accept that some bowls you just don't use without a logical reason why you should not? Not forgetting the melted butter!!

Whatever the reason is, thank you to all who did make a contribution... and if anyone wants butter gee... ask RobertOz!

Well, the short answer is that there is some material in the composition of the bowl, that absorbs the microwave energy, whereas a bowl that is "microwave safe" is made up of materials that are microwave transparent, so all the energy goes into the food. As far as the physics of the material goes, you would have to know the complete makeup of the material to figure out who the "offender" is.

Tom

Brich, two things come to mind, either or both may be in play here. Yes the iron/metal content of the ceramic may be one of them, moisture absorption is the other. some ceramics/glazes are more prone to absorb moisture than others. That moisture will preferentially absorb the microwave radiation and get hot, same with the metal content of the ceramic/glaze. Ceramic glazes are often colored by metallic oxides. blue is colored with cobalt for instance, reds are often iron oxide.

The way to tell would require a spectrographic analysis of the two bowls. but of course that may be rather expensive.....

Hi Rorschach and Tom

Thanks for the answers, now I know why.... but I still don't have a possilbe answer or suggestion as to why the result is different when heating difference foods.

By the by, the bowl in question is blue in color!! Bowl Two is white

Spectrographic analysis or new microwave friendly bowls?

Ikea here I come!

Your answer as to why bowl 1 can absorb microwave energy and not bowl 2 is buried in my earlier post. Just as water absorbs microwave energy because it has a dipole moment, some molecule in the structure of bowl 1 must have a dipole moment. Now what that molecule is, who knows.

Regards. In my view and have noticed these effects:

The containers [if Microwave safe] only get hot if the contents are of denser & the material takes time to get hot.

As chillies are not dense so in time you need to heat it absorbs very little heat.

Try by putting come curry & see.

And of course ingredient only absorb heat if are not "Microwave Friendly" Friendly will not heat.

Could the wording of your question contain part of the answer? You merely mention taking the chili out of the microwave; one must assume it was being cooked from room temperature, possible out of tin at room temperature...or, likewise, from fresh "assembled" (where meat has been precooked/seared/sauteed?). OTOH, your porridge (oatmeal) and milk was "heated" (re-heated?)...presumably from fridge temperature...where either the milk, or milk and porridge in the case of leftover reheating, had been kept prior to "heating," [per-se], or cooking.

Consider also, much of the energy generated in the milk or water used to cook the porridge escapes increasingly as vapor; plus, the maximum instantaneously heat the porridge can develop is water (or salted water) boiling point. On the other hand, fatty/oily foods like chili have no such limitation on temperature, and do not give up heat at boiling point of water. Since, in the case of chili, heat is not so readily released (as vapor) into the oven's ventilation system (and, hence, into the surroundings), it is "more available" for release into the bowl...continuing until the time which there is less food heat to be sunk by the bowl. Summarizing, in one case the heat enters the bowl, in the other case, it enters the air.

As to "microwave safe" (bowl) material ... yes, of course. After all, the kiln from which the bowl came imparted heat at orders of magnitude more than cooking chili or porridge. BTW, even microwave speciality dishes get hot...again, comparatively hotter with oily foods (just like ceramics); but microwave dishes are not very suitable for greasy/fatty foods...so ceramics/glass/etc are completely suitable. Just keep the oven mitts handy.