Re: Why does glass shatter in the microwave?

Greetings Chris:

Reference:

You can find an animation of the microwave cooking process on
discusses in this answer at the following web site:
How do microwaves cook?

http://www.gallawa.com /microtech/howcook.html

Answer to why some glass containers break when used in a mirorwave oven.

Most cooking containers, including glassware, are composed of
molecules containing many different atoms. Many of these materials,
including glasses, plastics and ceramics also contain molecules that
are impurities. Often it is these impurities that cause excess
heating in a microwave oven. There are hundreds of types of glasses
and not all of them are suitable for use in microwave heating because
they contain inpurities within the glass that absorb microwave energy
which may overheat and damage the glass.

When you purchase cookingware for use in microwave ovens it should
state that it is suitable for microwave heating. Pyrex is one of the
glass types that is suitable for use in microwave ovens. However, I
have seen low cost imitations of Pyrex measuring cups for sale that
are not microwave compatable even though they appear to be identical
to the Pyrex product. It is advisable to test your glass containers
with water in a microwave oven before you plan to cook food in them.

Background Discussion

Although the total electrical charge in molecules is neutral, the
positive and negative charges do not occur at the same location
within the molecule. The amount of separation between the positive
and negative charges is called the dipole moment of the molecule.
Some molecules have small dipole moments and some molecules such as
water have large dipole moments. This is why thr water content in a
food speed up it's heating. The microwave energy passing
through a material placed in a microwave oven rapidly alternates
between positive and negative voltage 2450 million times per second
(2450 Megahertz). The alternating electric field causes the charges
of the dipoles to try to align the molecules with the microwave
field, plus to minus and minus to plus. This causes the molecules to
rapidly rotate back and forth generating heat by friction like
forces.

There are two reasons why plastics heat more then ceramics and glass
in a microwave oven. First, plastics can trap water within the
material during the manufacturing process. The very high temperatures
required to manufacture ceramics and glasses drives all the water
out of the material. The trapped and possible absorbed water
molecules cause the plastics to heat more when exposed to a microwave
field.

The second reason that some plastics heat more than others in
microwave ovens is that plastics are formed from long chains of
hydrocarbon molecules that have there own dipole moments which can
also cause heating of the plastic. Some ceramic glazes used for
coloring also can also cause extra dipole heating in ceramics; these
ceramics are usually not suitable for microwave cooking purposes.
Many glass formulas called crystal contain lead atoms to enhance
their visual characteristics and they can also be are very expensive.
However, the lead atoms in them also make these glasses not suitable
for microwave heating.

Best regards, Your Mad Scientist
Adrian Popa