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ANSWERED on Tue 21 Aug 2007 - 10:58 pm UTC by byrd

Question: Mugs and Microwaves

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 21 Aug 2007 15:18 UTCTue 21 Aug 2007 - 3:18 pm UTC 

When you put a mug in the microwave to heat up some coffee or something, some mugs will stay farily cool to the touch, and others get really hot.

Why is that?




 21 Aug 2007 15:20 UTCTue 21 Aug 2007 - 3:20 pm UTC 

Farily cool!!!

As opposed to nearily cool, I suppose.





 21 Aug 2007 22:58 UTCTue 21 Aug 2007 - 10:58 pm UTC 

Hi needhelp,

The reason some mugs or other containers get hot in a microwave oven while others remain cool is due to the material they are made of. To understand why this happens, it helps to understand a little about how microwaves work.

Basically, microwaves are a form of energy, similar to radio waves. Various materials can respond to these waves in one of three ways:

     1) they can absorb the energy,
     2) they can reflect the energy, or
     3) they can simply allow the energy to pass through.

Materials with a high moisture and/or fat content such as food or are examples of materials that absorb microwave energy. The microwaves enter the food and agitate, or "excite" the atoms, which makes them hot.

"Microwave transparent" materials such as glass, paper, parchment, and most pottery and plastics are examples of materials that allow microwaves to pass through.

Metals, on the other hand, reflect the energy, which is why users are cautioned not to use metal dishes in a microwave. The waves simply bounce off the metal instead of going through or being absorbed by it.

So you can see then that certain mugs or dishes that get hot in a microwave might be made of a material that absorbs the waves to some degree. Stoneware is one, especially if there are an chips or cracks in it that might allow moisture to creep in. The water will then absorb the microwaves and allow the cup or dish to become very hot. Lead crystal might also absorb the waves to some extent, as well as some ceramics.

Another reason why some microwave containers get hot is if they are made of very thin and or especially heat conductive material. The heat from the food inside can then be absorbed by the container, which can become very hot itself. For instance, soft plastic cups and dishes like tupperware containers, and things like plastic wrap can easily absorb the heat from the food inside, and may even melt.

It's best to use "microwave safe" dishes when microwaving food or liquids, but that's not a very well defined term. It generally means a container that will not absorb nor reflect microwaves, i.e. not get hot from the microwaves themselves, but also not conduct heat well enough to get extremely hot from the hot food/liquid itself.

A good way to test a dish to see if it is microwave safe is to fill it with one cup of water, microwave it on high for one minute, and then remove it. If it is hot, it is not microwave safe and probably shouldn't be used in the microwave. If it is still cool, then it is safe to use.

I hope you will find this information answers your question, but if you need further assistance, please ask and I'll be at your service. References used in preparing this answer are listed below.


EBSciences: " A Little About Microwaves"

How Stuff Works: "How Microwave Cooking Works"

How Things Work: "Microwave Ovens"

Milestone: Microwave Synthesis - Technical Details

"What Einstein Told His Cook: Kitchen Science Explained" by Robert L. Wolke




 21 Aug 2007 23:53 UTCTue 21 Aug 2007 - 11:53 pm UTC 

Hi byrd,

Thanks for the overview on microwaves.  It was helpful.

But I don't feel I really have the answer to my question yet.  Why does ceramic cup A heat up in the microwave, while ceramic cup B doesn't?  I dont' think cracks and chips in the cups have much to do with it, at least not from my own observations.  It's more likely to be something very fundamental to the materials themselves, I would guess.

That's what I'm looking for.  What is it about some ceramics that cause them to heat up, while others don't?

If you can find some materials that go directly to that question, it would be much appreciated.






 22 Aug 2007 01:07 UTCWed 22 Aug 2007 - 1:07 am UTC 

Hi NH,
Is there any difference in the masses of cups a and b?
If cup a is thin and light, but cup b is thick and heavy, the latter will take longer to absorb the heat from the liquid (just as when you pour hot liquid in it).

Stoneware or other soft ceramics that may have cracks or are partially unglazed, and thus can absorb water, not only will be likely to get hot themselves but are likely to spring apart if the absorbed water vaporizes.

Seemingly identical mugs that are sold in large series  - e.g., fan souvenirs for a professional sports team -  might not be made of the same ceramic material (sloppy production quality, different suppliers).

You could test the cups  - empty -  with a short exposure to microwaves.

(I take no responsibility for any damages.)

Is that any help?

Cheers, Myoarin




 22 Aug 2007 01:32 UTCWed 22 Aug 2007 - 1:32 am UTC 

Hi needhelp,

You're right. The scientific explanation lies with the fundamental qualities of the materials themselves.  However, without having the actual Ceramic A and B cups in hand in a laboratory, it is impossible to give an exact answer to your question as to why one particular mug heats up while another doesn't, as the answer would require a chemical analysis of the composition of each individual cup. Again, the basic (non)answer is, it depends on the material of which each mug is made. They may appear similar on superficial inspection, but their chemical composition may be completely different. 

As mentioned, microwaves are a form of energy, specifically high frequency electromagnetic energy, in the same band as radar waves. The frequency of 2.45 GHz has been reserved for use in home microwave ovens. The interaction of these electromagnetic waves at this frequency with any given material has to do with the fundamental properties of matter, specifically, the two major mechanisms of dipolar polarization and ionic conduction, which are at the root of and explain the absorption, reflection or transparency of any given material or object with regard to microwave energy.

A paper produced by Biotage AB, a company engaged in manufacture and research in this and related areas, says, "A substance possessing a dipole moment when irradiated with microwaves will generate heat." It goes on to further state, "Essentially, the ability of a substance to heat in a microwave field is dependent upon two factors: (1) the efficiency with which the substance adsorbs the microwave energy, normally described by its dielectric properties, and (2) the efficiency with which the adsorbed energy can be converted to heat, described by the loss factor. A convenient way to evaluate the ability of two closely related substances to convert microwave energy into heat is to compare their respective “loss tangent” values, where the loss tangent is defined as the tangent of the ratio of the loss factor and the dielectric properties." 

With regard to ionic conduction, it says, "A solution [or other material] containing ions or even a single isolated ion with a hydrogen bonded cluster in the sample will move through the solution under the influence of an electric field, resulting in an increased collision rate. The conduction mechanism is a much stronger interaction than the dipolar mechanism with regards to heat generating capacity." (brackets mine)

Therefore, basically it is the amount of concentration of dipole molecules and/or ions within a given material that will directly influence how much it will or won't absorb microwave energy. Higher concentrations = more absorption. Lower concentrations = lower absorption. Without chemical analysis, it is not possible to tell from cursory inspection which dishes are and are not "microwave safe," that is, will not heat up when place in a microwave oven. That's why makers encourage the use of the water test to tell if any given dish is not labeled "microwave safe," which presumably means the chemical makeup is known and is not conducive to microwave absorption.

So with respect to your mugs, again, it depends what they are made of. A confusion arises with regard to nomenclature, as china, porcelain, stoneware and/or ceramic are often used somewhat interchangeably. But there is no specific definition for any of these terms that denotes their exact chemical makeup, and different materials may differ considerably with respect to dipolar and/or ion concentrations. Some mugs may be made of material containing no or little dipolar material or dissolved salts, while others might contain higher concentrations. The first would be the "microwave transparent" mugs, or dishes, while the latter would tend to absorb the microwave energy at a greater rate, and be the cups more likely to become hot when exposed to microwaves.

The chemistry of microwaves is actually very complex, much more so than this brief summary can cover. Following are some additional resources that go into much greater detail and depth in explaining the electrical properties of microwaves, the physical properties of materials, and how the two interact.

Tan-Delta, a manufacturer of microwave components of various kinds, has an entire website on the microwave chemistry, which starts out at the basics, and proceeds to discuss the subject at advanced levels. Depending on how much you want to know about the chemistry of all this, you may find this site very helpful.

The Anton Paar company, a manufacturer of high-quality measuring and analysis instruments for research and industry, has produced this very clear but detailed paper on Microwave Technology, explaining the principles in scientific and mathematical terms.

From the Hong Kong Centre for Food Safety, this detailed paper on "Microwave Cooking and Food Safety," explains clearly but in good detail about the scientific principles of microwave heating. Scroll down to the first section, entitled, "Principles of Microwave Cooking."

I hope this further information and these additional resources will provide you with a better understanding of the differential heating of your cups and the more scientific explanation you were seeking. If not or you still need further assistance, please feel free to ask.

Best wishes,




 22 Aug 2007 18:48 UTCWed 22 Aug 2007 - 6:48 pm UTC 


Thanks a lot for that additional information.  It will take me a while to explore those links, but I'm sure there's a lot of (reheated) food for thought there.





 22 Aug 2007 22:57 UTCWed 22 Aug 2007 - 10:57 pm UTC 

Hi needhelp,

You're very welcome, and thank you for the kind words and rating! I'm glad you were pleased with the "reheating" and hope you enjoy investigating the links.



Roger Browne 


 1 Sep 2007 18:11 UTCSat 1 Sep 2007 - 6:11 pm UTC 

As byrd says, nomenclature is often used inconsistently when describing the materials from which mugs are made.

Nevertheless, "porcelain" and "bone china" should be impermeable even if unglazed. As such, they absorb very little microwave energy and don't get hot.

On the other hand, "clay pottery" mugs readily absorb moisture. After a while, even the best glaze tends to develop tiny cracks when used as a mug. These mugs will become much warmer in the microwave.

In my own experience, mugs that get hot in the microwave have brown inside when they chip (e.g. clay pottery), and mugs that don't get hot are white inside when they chip (e.g. bone china).


la phantom 

Former Researcher

 19 Oct 2007 04:34 UTCFri 19 Oct 2007 - 4:34 am UTC 

The high porosity of earthenware makes it necessary for manufacturers to glaze them and this glaze frequently has lead.  If not treated at a high temperature during the glazing process, the leaden glaze will leach over the life of the product..

Which is why I stop family members from microwaving mugs of java or any liquid unless I'm sure that they were not made in countries where toxicity is not a concern! In fact, I'm never sure as I don't know about the toxicity tests required by importing states or countries.




 31 Oct 2007 17:21 UTCWed 31 Oct 2007 - 5:21 pm UTC 

I too am concerned about possible lead contamination in my new coffee cup.  I just bought a cheapo mug with brightly colored paint and a glaze on it.  This particular mug gets VERY hot when coffee is heated up in a microwave.  I didn't know the glazing process uses lead, but I know a lot of paints contain lead.  The cup was made in China and the brand is Emerald.  There is one small chip, but the interior is not brown, it's white.

Could slapped on paint with lead make the cup very hot when microwaved?  Would a let test kit detect any lead?

Thanks.  This is great info on a very obscure subject.  I'm glad I found it.


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