Schneier on Security

65535 • June 2, 2018 4:48 PM

@ All microwave experts

Can microwaves be focused or narrowed by common concave glass objects?

As a kitchen person I and others have noticed that normal plastic microwave oven compatible food bowls will suddenly melt or burn the bottom completely out when a concave glass object such as a coffee cup saucer is used and as a lid to cover the food in the plastic bowl.

Other times the microwave oven will perform as normal and just produce warm food no matter the power setting.

A neighbor also mention this phenomenon when trying to heat soup in a plastic soup dish which a glass lid only to find the plastic bowl’s bottom to be completely melted and the soup spread over the rotating glass tray.

This seem to only happen in microwave ovens that use a rotating glass plate at the bottom to evenly heat the food and only in certain conditions [I know that a lot of microwave ovens use a rotating beam to cook food evenly with on rotating platter].

The melting of thick plastic bowls seems to only happen when a concave glass lid is on the plastic bowl with the lower part pointed downward toward the food somewhat like a focusing magnifying glass on the bottom of the bowl. Could the combination of a concave glass lid and relatively flat glass rotating design cause some magnification of power high enough to burn plastic bowls?

The only thing on I could find was this entry in Wikipedia:

“Water and other homogeneous liquids can superheat when heated in a microwave oven in a container with a smooth surface. That is, the liquid reaches a temperature slightly above its normal boiling point without bubbles of vapour forming inside the liquid. The boiling process can start explosively when the liquid is disturbed, such as when the user takes hold of the container to remove it from the oven or while adding solid ingredients…” –Wikipedia

https://en.wikipedia.org/wiki/Microwave_oven#High_temperatures

or

https://en.wikipedia.org/wiki/Microwave_oven

The above doesn’t indicate the melting of proper thick plastic bowls and the like.

On the IT side of things I know microwave oven can melt a credit card chips, disrupt cell phones and could possibly be used to destroy the magnetic surface of a hard drive containing a spindle. But, those objects contain metal.

Another thing I discovered is that microwave ovens make poor faraday cages for cell phones. When I called my cell phone while it was placed in a microwave over it rang and saw no blocking of the signal – so much for securing cell phones in a microwave oven.

Again, how can glass and plastic somehow superheat water or even the plastic container which hold said water in a normal 1000 watt microwave oven?

65535 • June 2, 2018 8:43 PM

@ Clive Robinson

“thus a concave objects surface will focus any EM signal it reflects…Likewise glass has a refractive index and if made into a limited section of a spherical surface it will behave as a lense to any EM signal it can refract…Thus visable light can be both reflected and refracted by a glass lid…However the ability of a glass lid to do that to microwave signals depends on what type of glass it is. Some glass types do have an effect on microwaves but by far the majority do not.”-Clive R.

So, it is possible to focus microwaves via certain glass. Would glass that is alloyed with metal be of that type or just pure clear glass?

I have see those old valves or tubes in radios and TVs with domes of chromium or other reflective material and just thought metal must be only thing used to reflect or direct microwaves away from sensitive equipment. I assume this is not entirely true.

Can you give me an example of glass need to focus microwaves?

Other possibilities such as superheating is not clear to me.

“Micronucleation is not a well understood process. Overly simply –and possibly wrong– for a bubble to form you need some form of seed in the liquid or discontinuity at the liquid boundry…”-Clive R

That is what I understood. I did not see in Wikipedia why a curved container would induce superheating of liquid.

“If you look at the “Champaign effect” where a gas is held in a liquid a bubble starts to form from either the holding vessel side wall or from invisibly small specs of material in the liquid.”- Clive R

Yes, I have the same thought. The carbonic acid is very well mixed in drinks with “Fizz” but small particles from the process do remain in the Champaign or other Fizz drinks to start the fizz effect. I thought proper Champaign is only naturally made and the cheap stuff boosted with carbonic acid.

“…classic example of a flawed surface and gas saturated liquid disassociating on contact with it is the mentos and cola fountain (there are also ill understood secondary effects that make it more rapid, thus almost explosive in action).”- Clive R

I have heard of that factor also and maybe it is an chemical to electrical or chemical to thermo reaction that can boost the Fizz in the soda to almost explosive levels. Maybe some tamper action or the like but, no good explanation is there.

‘“…plastics are known to suffer quite badly in some cases to “dielectric heating” and many are “RF Welded”. Even if a plastic cup/plate etc says “microwave safe” don’t belive it… There are a lot more assumptions than testing in such statments. Further plastics age and absorbe other chemicals. Most of us have seen plastics that have been stained by some food stuff. Plastics also change their properties as they approach their phase change points some of which are below the boiling point of water. Thus a super heated liquid could easily cause a plastic to start absorbing microwave energy…”’-Clive R

That sound fairly realistic. Say an old plastic bowl that has be microwave’d many times might decay down to a level where it could melt faster than boiling water or the superheated water boosts the melting process of old plastic to the point where the bottom liquefies and leaks.

Still I am not sure why Wikipedia says the curve part of the plastic contributes to superheating of water in microwave ovens… unless the glass or plastic helps increase the heating by microwaves.

‘…simple rules of thumb are “No plastics in the microwave” and “no foodstuffs hoter than 60C should be put in plastics”.’- Clive R

Sound reasonable. To be safe use all glass.

Now, I and others believe we have see microwave oven melt glass of certain types. This could be explained by the superheating of water above the melting of said glass. Or possibly the oven contained a curved glass lid that did indeed focus the microwaves to melt the glass plate below. But, that is speculation.

“…just to make you feel realy uncomfortable, those expanded foam fast food containers when hot chips/french fries are added melt… There is some scientific evidence that the chip becomes contaminated with some of the chemical compounds. There is further evidence they can act like artificial estrogens and cause similar problems in humans as they do in fish…” –Clive R

That is an unhappy thought that will probably give me nightmares. Thanks for your observation of microwaves being focused or refracted by glass. I had those question for a while. The superheating or Micronucleation is quite interesting. It looks like the Kitchen can be a dangerous place at times.

65535 • June 3, 2018 9:34 PM

@ Clive Robinson

Thanks for those two papers. One centered on dielectric effects and the dielectric constant which is of interest in semiconductors and gates and so on. The other link to a paper “The Interaction of Radio-Frequency Fields With Dielectric Materials at Macroscopic to Mesoscopic Scales” goes into modeling macroscopic fields and has some photon effects or quantum physics/wave mechanical models of various hypothetical types that are interesting but not exactly explaining superheating of liquids or dielectric heating in a microwave oven. I am not into quantum physics or wave models at this point.

But, I did see this explanation about dielectric heating and the “skin” effect:

“Microwave frequencies penetrate conductive materials, including semi-solid substances like meat and living tissue, to a distance defined by the skin effect. The penetration essentially stops where all the penetrating microwave energy has been converted to heat in the tissue. Microwave ovens used to heat food are not set to the frequency for optimal absorption by water. If that was so, then the piece of food or liquid in question would absorb all microwave radiation in its outer layer, leading to a cool, unheated centre and a superheated surface.”- Wikipedia

[Next]

“Instead, the frequency selected [For microwave ovens -ed] allows energy to penetrate deeper into the heated food. The frequency of a household microwave oven is 2.45 GHz, while the frequency for optimal absorbency by water is around 10 GHz.” -Wikipedia

https://en.wikipedia.org/wiki/Dielectric_heating#Penetration

The “Skin” effect then directs to the use of essentially long tubes of metal to carry extremely high voltage AC electricity over long distances. Apparently, the core of a electric cable generates a magnetic field which forces most of the AC current to the surface of the cable and doesn’t use much of the center of the cable – as DC current would use the entire thickness of the cable.

“Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor, and decreases with greater depths in the conductor.”- Wikipedia

https://en.wikipedia.org/wiki/Skin_effect

I am guessing the reason microwave ovens can melt plastic bowls and even some type of glass is maybe due to partly the “skin” effect of oscillating microwaves in the GHz range and possible some odd focusing of those microwaves on liquid within smooth curved shaped bowls to produce the superheating of said liquids. Or, direct electrical conductivity of liquids below visible spark range and the 4000 volts the magnetron in those ovens use. And, possibly any combination of the above. This superheating may be high enough to melt glass of low quality at say at 550 degrees C.

This is all just guess work. But, it does make me wonder about those diplomatic guys in Cuba with headaches or worse in Cuba. Maybe some form of microwave or related energy was directed at their skulls and a little frying took place. This also is just supposition.

65535 • June 7, 2018 3:34 AM

@ Mark

“My short answer would be: 1. You’ve demonstrated a phenomenon that I absolutely wouldn’t have expected, and which I find interesting. 2. I have no explanation. If we scale a disc that would fit in a microwave oven to the wavelength of visible light, it would be less than 0.002 mm in diameter…Another reason why dinner plates aren’t likely to make effective lenses, is that they aren’t thick enough. The greater the variation between the thickest and thinnest parts of a lens, the greater the focusing power (but the shape has to be right, also). Typical glass plates found in the kitchen don’t have much variation in thickness, and lack the profile of varying smoothly from thickest in the center to thinnest at the edge.”

I see what you mean about the thickness of the dinner plate or a glass coffee cup saucer which are not really that thick. They also are not really shaped like a lens of a magnifying glass.

Thus, the superheating of water or other objects is probably not caused by focusing of microwaves. I would then guess Clive Robinson’s guess about the plastic bowl being old and pours to the point where small amounts of water enter the bottom and are superheated via possibly the “skin” effect of microwaves.

Or, maybe there is some actual direct electrical spark jumping from the 4000 volt cavity magnetron which passing through the glass plate and plastic yet is not visible by the eye melting both the plastic and glass. Who knows.

Thank you for your reply.

@ tyr

“Microwave ovens are fairly simple gadgets. Once powered up the magnetron will keep pouring power into the cooking area. The continuous input can melt almost anything if other conditions are right. Manufacturers cannot test every possibility so only will deliver a unit that functions under “normal” usage. Given the wide variety of cookware around. I’m surprised this melting hasn’t been seen before.”

That is an interesting observation. The melting of both glass and plastic could have happened and everyone just assumed the items was not made to operate and a microwave oven and forgot about it.

@ JG4

“…oven can be viewed as being a) in the near-field, b) not involving much diffraction, although the comments on refractive index of glass, plastic and water by Clive and others are spot on, c) being dominated by what amounts to a standing wave in the oven and d) including some bi-directional coupling between the oven and the magnetron. It would be pretty clever if something like the NE-2 bulbs (but having less metal to avoid lightning shows) could be used to visualize the field intensity. …Getting back to the topic at hand, putting things in the oven cavity alters the standing wave pattern in a way that can’t be explained by simple optical considerations. It’s a coupled-cavity resonator, albeit with weak coupling. The reason that it is important to put a minimum load in the oven (usually 1 cup of water?) is to provide some absorption to keep the standing wave intensity from producing destructive power back at the magnetron. I hope that this helps.”

That is a reasonable observation.

In the YouTube video linked previously a Russian guy using current a 2000 volt [DC?] connector to the step-up transformer to burn 300 W incandescent bulbs to the conductor stem downward and to power vapor lamps of various types. There seems to be one vapor lamp that exploded due to the high power. Also, there are what look like to plasma arcs developing from tinfoil and other metal items about 3:00 min into the video.

https://www.youtube.com/watch?v=ebxtqXcCrf4

The melting of glass maybe possible due so some odd direct electrical field or the microwaves from the cavity magnetron. But, I have never seen any visible spark in the microwave oven.

@ Clive Robinson

…”various antenna designs people have tried and got working, such as the “backfire”, but again few people realise that resonance effects are effected by dialectric effects, such as the insulation on wire. But also you don’t need a conductor… You can in the microwave regions use PTFE and Polystyrene to make what looks like a broad band TV antenna that like the antenna focuses the output from a waveguide into a considerably less divergent beam. As we go “up-n-up” in our data bandwidth requirments we need to similarly go up in our frequency usage. The mechanics of making antennas above 5GHz in metal are becoming cost prohibitive as the consumer price dropps. Therefore I suspect that dialetric antennas will get a lot more investigation over the next few years as there are limits on what etched PCB fractal and similar antennas will give us… people think it’s the quater wave length forgetting it is actually half of a halfwave dipole, the other half supplied by a ground or counterpoise of some sort. If you do the experiment correctly a monopole in free space does not radiate, a lesson the makers of “bugging devices” have to often learn the hard way. Likewise users of twoway radios, hence the reason “tiger tails” and “trailing grounds” are becoming seen more often.”

Your interesting quarter wave length v. halfwave diapole and bugging devices is interesting. That is limited radio waves? I don’t see how superheating occurs of the type I have seen. But, who knows.

I will say that the high voltage of a microwave oven probably will be used for various purposes including destruction on SSDs, radio chips and other hard to erase storage media in some quick way.