Microwaves, Temperature, and Cup Color
I have noticed over the years that almost
every coffee cup with dark colored glaze on it
heats up to an unbearably hot temperature in a very
short time in a microwave and, furthermore, the
contents fail to heat in an apparent inverse ratio
to the amount of dark glaze on the cup. These cups
I am referring to are often specifically labeled
"microwave safe". One cup I own is white with a
dark blue design on it. After heating in the
microwave, the area of the dark blue design is
significantly hotter than the remaining white
portion of the cup. Is the concentration of heat
due to microwaves being absorbed by the darker
pigments in the glaze instead of heating the water
or is it due to the makeup of the glaze (i.e. lead
and/or other metals in the glaze formulation)? Is
industry aware of these complaints (I did an
Internet search and I'm not alone in these
observations)? For example, I have a whole set of
new rust colored dish ware that is clearly labeled
"microwave safe" yet when food is microwaved on/in
the dish the dish gets VERY hot long before the
food does. My old white and plain glass dishes only
get hot where the heated food is in contact with
You seem alarmed that an article claimed to be microwave safe, can get
hot when placed in a microwave oven. There is nothing unusual about
this. "Microwave Safe" only means that the article will suffer no
damage (e.g. melting, or cracking) in the microwave oven. It has
nothing to do with the utensil getting hot.
Microwave ovens are usually designed to operate at 2.45 GHz, and
produce heat in substances having polar molecules, most notably water.
Exposure to microwaves causes the molecular dipoles of these
substances to oscillate rapidly, resulting in frictional heat. Water,
whose molecules are strongly polar, is especially affected (and
heated) by microwave energy, and many other substances are to a lesser
degree as well.
There are two possible reasons for the heating effect you have
observed in some ceramics, and especially some glazes. One reason is
that the ceramic or glaze molecules are somewhat polar and generate
heat upon exposure to microwaves. The other possibility is that if a
glaze has a metallic content, microwave energy can induce electrical
current flow in the metal particles (much as a radio signal induces
current flow in an antenna). This current, flowing in an electrical
conductor, generates heat (the same way that current flowing in a wire
or a resistor causes heat). Some microwave popcorn bags even have a
strip of metal foil in the bag to help generate heat in this fashion.
I am sure most people have seen firsthand that some plates heat up more than
others in microwaves. As with any radiation, different materials absorb
microwaves with different efficiency, but that is just the start of this
story. With microwaves, not only does the composition of the plate matter,
but its size and shape also play a role (although predicting a plate's
sensitivity based on seeing its size or shape is not a simple task). Even
more strange, coatings on the dish -- perhaps like your colored glaze -- not
only might absorb more microwave radiation that the rest of the dish, but
also could cause more radiation to be absorbed by the dish and its contents.
Microwaves do not interact strongly with everything, but you can design
coatings that will allow the microwaves to be absorbed more readily by the
container. The coating acts like a "funnel" that can guide more microwave
radiation into a vessel or its contents (or act like a "shield" to block
microwaves). For example, here is a link to a patent describing a device
that takes advantage of this effect:
I do not know much about ceramics industry practices, but I would guess that
'microwave safe' means that the plate will not be harmed by the microwave, not
that the plate cannot harm you. I suppose almost anything could be
'overheated' in a microwave and become unsafe to handle.
I want to thank my colleague Dr. Lee Mosbacker for helpful discussions in
preparing this answer.
Hope this helps,
Many glazes use metallic oxides to produce their colors, while darker colors
and black are produced with higher concentrations. Up to 2% manganese should
produce purples and browns, while at 10-15% black is produced. The greater
concentration of metallics should generally give a stronger response to the RF
(radio frequencies) inside your microwave, heating the glaze.
As for food not being heated as well, that is simple distribution of energy.
My own microwave is rated at 700 Watts. If one of my bowls absorbs 300 watts
of that power, then only 400 watts are acting to heat the food. If a cup nearly
surrounds the beverage, and absorbs 500 watts of power, only 200 watts manage to
heat the coffee!
As for being microwave safe, that term generally refers to two things. First,
will the item itself be damaged by a microwave? Ceramic plates with metal
foils probably are not the best choice for a microwave oven. Second, will
the heating cause the dish to 'leak' chemicals into your food? Provided these
two criteria are met, the dish is probably considered as microwave safe,
although it may not be microwave effective.
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Update: June 2012