Why You Can't Put Metals in the Microwave
If you’ve ever wondered why there is such a taboo on putting metals in the microwave,
well, wonder no more.
First, it should be noted that it is not unsafe to put all metals in the microwave.
Indeed, you often put metals in the microwave anytime you put a hot pocket in the little
pouch and place it in the microwave.
The pouch has a thin layer of aluminum lining the inside that is designed to absorb the
microwaves and heat up a bit so as to brown the outside of the hot pocket.
On top of that, the inside walls of your microwave oven are made of metal.
If you look closely, you’ll also see that the window you look at the food through also
has metal mesh lining it.
The holes in this mesh are smaller than the wavelengths of the electromagnetic radiation
your microwave is producing.
This makes it so the waves can’t pass through the holes.
Visible light, however, is comprised of much smaller wavelengths, so that form of radiated
energy passed through the holes just fine, allowing you to see inside your microwave
while it’s running without getting cooked yourself.
So if the inside of your microwave is lined with metal and certain food products, such
as hot pockets and pot-pies, have containers that contain metal, why does your microwave
manual say not to put metal in the microwave?
First, let’s talk a little about how a microwave oven actually works.
At its core, a microwave oven is a pretty simple device.
It’s basically just a magnetron hooked up to a high voltage source.
This magnetron directs microwaves into a metal box.
These generated microwaves then bounce around inside the microwave until they are absorbed
via dielectric loss in various molecules resulting in the molecules heating up.
Matter that works well here are things such as water, ceramics, certain polymers, etc.
These all end up converting microwave energy into heat quite effectively by a process known
as “dielectric heating”.
Basically, molecules such as water molecules are electric dipoles.
This means that they have a positive charge and a negative charge on opposite ends.
Thus, they will rotate themselves rapidly when trying to align themselves with the alternating
electric field from the microwaves.
As these molecules rub against each other, they heat up and, as they do so, they themselves
also become part of the cooking process, heating up molecules around them that may not be absorbing
much, or any, of the microwaves.
Metals, on the other hand, are great conductors of electricity, being packed with electrons
that can move freely.
Depending on the shape/type/thickness/distribution/etc. of metal, you may observe some heating of
the metal itself in the microwave or none at all.
You may also observe some arcing of electricity or none at all.
What’s going on here is that when these microwaves hit the metal, free electrons on
the surface of the metal end up moving from side to side very rapidly.
This, in turn, prevents the electric wave from entering the metal; thus, the waves end
up being reflected instead.
However, there is also the potential that this ends up creating a sufficient charge
density that the electrical potential in the metal object exceeds the dielectric breakdown
of air.
When this happens, it will result in arcing inside your microwave, from that metal to
another electrical conductor with lower potential (often the wall of the microwave).
In extreme cases, these electrical sparks can end up damaging the wall by burning small
holes in it.
It can also end up burning out the magnetron in your microwave oven or, in modern microwaves,
can provide a surge that ends up damaging sensitive microelectronics, possibly killing
your microwave or making it unsafe to use, in the case of a hole in the inner metal wall
in your microwave.
Another way it can kill the magnetron of your microwave is when enough of the generated
microwaves don’t get absorbed, such as if the food is wrapped in aluminum foil or mostly
enclosed in a metal container or if you just run the microwave empty for long enough.
On a more mundane level, something like a spoon or a metal plate or the like, positioned
correctly, will simply make your food potentially not cook normally.
On that note, once again, it is actually acceptable to put metal in a microwave under the proper
conditions.
Some microwaves even have metal grates inside for setting food on, such as is often the
case with certain convection ovens.
There are also certain types of metal pots and pans that are microwave safe.
These all, however, are carefully designed to not cause any problems in your microwave
oven.
In general, putting metal in the microwave is unsafe, not because you are at risk of
bodily harm or the like (though in extreme cases a fire might be started in your microwave),
but, more to the point, primarily because it has the potential of damaging your microwave.
Bonus Facts: • Metal powder at room temperature does
a good job of absorbing microwave radiation.
When it does so, it heats up.
It’s not wholly understood what is going on here; but it is known that if the metal
particle size is less than 100 micrometers, the particle will absorb microwaves, instead
of reflecting them.
This is generally how the microwave pouches, such as come with hot pockets or pot pies,
work; though, they sometimes use a form of ceramic instead, with the same effect of generating
heat to brown the outside of the food.
These pouches and containers meant for browning are known as susceptors.
• The type of radiation emitted by microwave ovens is non-ionizing.
This means that it doesn’t contribute to your chance of getting cancer like x-rays,
ultraviolet light, etc do.
Outside of potential burn risks, experiments done with rodents have yet to show any major
adverse effect to prolonged exposure to microwaves at the 2.45 GHz range seen in most microwave
ovens, even with continual low level exposure.
• The particular band of microwaves produced by typical microwave ovens (2.45 GHz) was
chosen primarily due to the fact that it is a frequency set aside for non-communication
uses.
Within the available frequencies that are not set aside for communication, 2.45 GHz
was chosen because 433.92 MHz would require expensive equipment to generate sufficient
power to heat food; 5.8 GHz and 24.125 GHz would require a much higher cost on electricity
used to run the oven; and 915MHz was rejected as it wasn’t a band available world-wide,
as 2.45 GHz was.
915 MHz, though, is occasionally used in industrial microwave ovens.
• Electromagnetic waves were predicted by James Clerk Maxwell in 1864.
It wasn’t until 1888 though that Heinrich Hertz was able to build a device that was
capable of producing and detecting microwaves.
His device used a horse trough; Leyden jars; a zinc gutter (worked as an antenna); and
a wrought iron point spark.
• The first known documented use of the term “microwave” was in 1931 in a Telegraph
& Telephone Journal: “When trials with wavelengths as low as 18 cm were made known, there was
undisguised surprise that the problem of the micro-wave had been solved so soon.”
• Even though most microwave ovens let you choose between power levels, there generally
isn’t any change in the frequency level of the microwaves being generated.
Rather, it simply changes the duty cycle of the magnetron.
In other words, it turns on and off at a different rate.
• A convection oven is basically just a conventional microwave that also has a way
to brown food like a traditional oven.
In order to provide this browning effect, the convection oven may use traditional oven
heating elements or might use something such as a high powered halogen bulb.
• The ability to use microwaves as a heating device for food was originally discovered
by an engineer by the name of Percy Spencer.
Spencer was working on building magnetrons for radar sets.
One day, he was standing in front of an active radar set when he noticed the candy bar he
had in his pocket melted.
Upon noticing this, Spencer made the monumental mistake of telling other people instead of
keeping it to himself and working on it on his own.
He and some other experimenters began trying to heat other food objects, presumably to
get out of actually working while they were at work.
The first one he heated intentionally was popcorn.
The second was an egg, which ended up exploding in the face of one of his co-workers.
Spencer then created what we might call the first true microwave oven by attaching a high
density electromagnetic field generator which would then shoot into a metal box, so that
the electromagnetic waves would have no way to escape and the oven would be much more
efficient and safe.
He then placed various food items in the box and monitored their temperature to observe
the effect.
The company Spencer was working for, Raytheon, then filed a patent on October 8, 1945 for
a microwave cooking oven, eventually named the Radarange.
This first commercial microwave oven was about 6 feet tall and weighed around 750 pounds.
The price tag on these units was about $5000 a piece.
It wasn’t until 1967 that the first microwave oven that was both relatively affordable ($495)
and reasonably sized (counter-top model) was available.
Further, it wasn’t until microwave ovens became extremely popular in the 1970s that
they were commonly known as “microwave ovens”.
Before that, they were typically known as “electronic ovens”.
• Forks are particularly susceptible to sparking in a microwave due to the fact that
their tines are relatively close together and will produce high voltage at the tips.
This voltage will exceed the dielectric breakdown of air, which is about 3 megavolts per meter.
The air then forms a kind of conductive plasma, which is the spark you see.
This, in turn, makes the fork an even more effective antenna for the microwaves, worsening
the problem.
• Microwaves convert Vitamin B12 to an inactive form, which means about 30-40% of the Vitamin
B12 in microwaved foods is not usable by mammals.
On the other hand, spinach loses about 77% of its folate when cooked in a normal stove,
but retains nearly all of it when cooked in a microwave.
In the same way, steamed vegetables, as a rule, tend to retain more of their nutrients
in a microwave than when cooked in a traditional oven.
• At one time, most long distance telephone calls were transmitted via a large network
of microwave radio relay links, such as AT&T’s “Long Lines”.
In the 1950s, about 5400 telephone calls could be run through a single microwave channel
via multiplexing.
The distance between hops was typically around 40-50 miles.
The additional cost of transmitting this way was a large part of why long distance was
so expensive historically.
With the advent and installation of fiber optic lines and advanced satellite systems,
this made the old ground based microwave relay links obsolete for transmitting long distance
calls.
(Although, satellite systems are essentially just a space-based version of this same thing).