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cause of hot mug handles

updated tue 23 jan 01


Gavin Stairs on mon 22 jan 01

Hi all,

I haven't read all of the messages in this thread, but I sense some mystery
around a white clay getting hot in a microwave. I can suggest what may be
happening, but I'd need more information about the clay to be sure.

A microwave heats objects by immersing them in an intense field of
alternating electromagnetic radiation. Radio waves. This will cause any
polar molecule or molecular structure to flip back and forth with the
waves, absorbing energy from them. The polar structure can be either
magnetic or electric. There are both types found in ceramics. Most of the
colored things are iron or other ferro-magnetic substances--things we call
ferrites. The bulges in power cords of computers contain ferrite rings to
act as filters on the wires, so that potential electro-magnetic
interference (emi or rfi) is absorbed before it can cause mischief. These
are often made of iron filings in a ceramic or plastic binder, but some are
made of ferric iron oxide and similar materials.

The electric polar materials are usually white to some shade of grey or
brown. They often contain exotic materials like yttrium, etc., but also
more common ones like barium. They form structures called electrets, which
are ceramic structures very like water, with positive and negative
poles. When the electric field flips rapidly, the electrons in the polar
structure are shoved around, which absorbs energy, heating the
substance. These materials are commonly found in electronic components
called capacitors, and also in some kinds of microphones (electrets, for

Now, what I suspect is that the clay which is heating the handles is a
white earthenware with a bit of barium, or some chance formulation which
has some stuff in it which can form these electret structures.

I suspect that many clays have these properties to some degree, but usually
not enough to become noticeable. Iron in quantity will give this property,
but iron with some casual impurity may do it much better, just because of a
crystalline species which is formed. Similarly, some white clays with just
the right accidental mixture of oxides may make significant structures to
absorb microwaves. If someone would get such a clay analyzed, we might
find out what particular oxides are contributing to the problem. Short of
that, a quick oven test will tell if a clay is going to give
problems. This is probably the best thing to do, since the degree to which
a clay will heat in the oven is not a linear function of what oxides may be
present. It also depends on their relative abundances and details of the
heat process, among other things. The effects manifest when certain
spinels and other crystalline structures appear. These formations will
depend on fluxes and all the usual complexities of the clay body, so
prediction is not easy, unless you are specifically trying to make such a
structure, as in an electronic grade ceramic.


Gavin Stairs
Stairs Small Systems
921 College St., # 1-A
Toronto, Ontario, Canada M6H 1A1
phone: (416)530-0419