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Microwave Oven Problems & Solutions: Interior Reflective Materials

Q. What material would make a good microwave reflective or partially reflective surface that could be used inside a standard microwave oven without causing arcing or damage to the oven? The same material as the walls of a microwave oven? If so, what is that?

Rick Achatz
- Torrance, California, USA

A. In most microwave ovens that have been built in the last 15 years, no damage will occur to the magnetron if you use aluminum foil to protect portions of your food from overheating. The first ovens built had that problem, but that was solved in the first two years. To prevent arcing, make sure that the foil does not come into contact (or really close proximity)with the walls of the microwave oven. Put your food on a ceramic plate if your 'turntable' (should your oven have one) is metal (most are glass or plastic). Also, do not use plates with 'rings' of metal or form a ring with your foil. Most specifically, do not use plates that have a decorative metallic band around the perimeter. These will put on quite a spark show! That is...until they burn through metallic ring. They won't hurt your oven, but they aren't good for the decoration on the dishes.

I have to say that I was sort of nervous when I read your request for information. I am not sure what you are trying to do. I only caution you that the microwave power used in a microwave oven is at one of the most dangerous frequencies to human beings. That is because it primarily cooks things by exciting the water molecules in the food. PLEASE NOTE: YOU are made up of mostly water too! If your microwave is damaged in some way, I urge you to get a new one rather than fixing it. This is not something for Mr. Fixit to undertake. 2.45 GHz will cook you as well as your food if you do not have the intact layer of shielding that came with the oven. If you were only looking for a way to prevent local overheating of your food (that noise was me, breathing a big sigh of relief), then the best write up I have seen about cooking food products with a microwave can be found at the following site: www.foodproductdesign.com/archive/1993/0493DE.html This explains a lot about microwave oven technology.

To understand what the walls of the oven are made of or what material one could use to partially keep food from cooking, you have to have some knowledge of 'dielectric constant' and 'loss tangent'. The materials used for protectors can be plastics, ceramics.... all depends on the dielectric constant and the RF properties at 2.45 GHz.

For my own cooking, if I want to keep the thinner ends of some piece of fish or meat from cooking while I am defrosting it, I just wrap the ends in foil after they have defrosted. Then they are protected (for the most part) from more cooking. A microwave with a turntable will allow the food to move through the RF field so that hot or cold spots are less likely to occur. If you don't have a turntable, then rotate your item after a few minutes of cooking. By doing this, you will move it from the 'null' spots in the field.

Oh... Don't think that being familiar with all of this keeps accidents from happening. In my very own oven, I was thawing something wrapped in the paper from the store, and unknown to me, the store had changed wraps. This new wrap had a hidden layer of METAL foil in it! What happened? There was a loud SNAP! when the partially unwrapped wrap/foil came near the wall. I stopped the oven and noticed that there was a light brown spot on the wall of the oven. Just the paint was discolored. No harm to the oven. If that has happened to you, there is probably no need for concern. The walls are usually a light sheet metal (or plastic with a layer of metal foil) with a dissipative coating. A little discoloration in the dissipative coating is not really a problem.

Ronna Erickson
Radio Astronomy, Univ. of Mass. - Amherst, Massachusetts

Q. Ronna: I was reading with interest the answer to the question about the coating for the interior of a microwave oven. So I thought you might be the right person to answer this: Being the micro-waves used in conventional ovens of the non-ionizing type. Why, when you heat water to its boiling point and you pour sugar into it, the water effervesces? I have observed this in different ovens. This phenomenon does not happen when you heat water using your stove. Is there something in the water? Ionization probably? If so, I think I don't want to drink micro-waved coffee again.

Alfonso Hernandez
- Aurora, Colorado

Q. Ronna,
Tell us that it is superheating of the water that only needs some seed, like the sugar, to get the water to vaporize. I don't want to stop heating the baby's milk in the microwave! Even that simple task is almost impossible at 2 in the morning.

tom pullizzi animated    tomPullizziSignature
Tom Pullizzi
Falls Township, Pennsylvania

A. Tom,

Lets see if I get it. If you heat up water in your micro-wave oven till it almost reaches its boiling point, then you add something that lowers the boiling point, the water starts boiling or evaporating, right? Now, is there a difference between boiling (you know, big bubbles) and effervescing (as if you put alka-seltzer in it)?. Again, why it only happens when you boil water in the M-W oven? By the way, my kids were raised on micro-waved milk also. Maybe that is the reason they glow at night!

Alfonso Hernandez
- Aurora, Colorado

Q. No, I am saying that you went past the boiling point. And adding something just provided the seed to start the gassing.

I searched for "super heated water" in google and there was a good explanation, "supersaturation" is another term you can search. It has to do with the solubility of one chemical into another, like sugar in water, and unstable systems.

tom pullizzi animated    tomPullizziSignature
Tom Pullizzi
Falls Township, Pennsylvania

A. Tom has the right idea. The behavior you see is due to superheating of the water. When you heat water in the microwave, it differs from the way you heat it on the stove. On the stove, you are heating from the bottom. The pot has imperfections on the sides and that allows convective currents to form. This helps to distribute the heat to the water on top until the whole pot of water heats to a boil and you make your coffee or whatever. In a microwave, you are exciting water molecules (exciting a vibrational state). The field is not uniform throughout the water so some of the molecules are more excited than others. These "hot spots" can happen anywhere in your fluid. Sure, convection can occur, but it is more turbulent and chaotic due to the random position in the fluid and the lack of nucleation sites in the middle of the fluid. This is why the instructions for microwave items often say to "let food stand for 3 minutes before opening/eating". This allows the hot spots to heat surrounding material and make the temperature more uniform. Microwave oven manufacturers try to achieve more uniform heating by either using a rotating platter to move the water/food through the field, or they use what is called a "mode mixer" to change the distribution of the field in the microwave oven. Neither of these work perfectly, so you may find that your liquid has some areas that have been superheated. Unable to boil, these areas lack a nucleation site and any perturbation can cause the effect you have seen. With fluids, this effect happens less in a "well-used" mug with many imperfections than in a brand new, very smooth mug/container. I would guess that the used mug's imperfections provide enough nucleation sites to allow boiling to occur as a constant process while the water heats rather than having it happen all at once. Sounds like you have nice new mugs with few nucleation sites and when you add sugar you "add" the imperfections needed for boiling to occur. If you added nothing, the water could be superheated and might catastrophically boil when perturbed. Manufacturers of microwave ovens recommend that you stir liquids several times during heating to prevent this type of catastrophic boiling. If you melt butter in a microwave, there is a chance of it "bumping", and spreading itself all over the inside of the oven. (Been there, done that...) That is still the same problem of hot spots. Some of the butter melts and some is still in solid form. Heat it up differentially, trapping pockets of hot liquefied butter and ...BLAM! If you trap water inside something (think potato, chestnut, egg) and then heat it up, you will have the experience of coating the inside of your oven. You have confined the vapor trying to escape from the "container" and will succeed until the container fails.

And for those of you with DAWGS... (or cats for that matter), I don't believe Fido or Kitty will explode if put in the oven. They will just cook. Their fluids are pretty well distributed throughout the body. Don't try this at home! To read more about this topic, there are some nice references at urbanlegends.about.com/library/weekly/aa011900b.htm

Ronna Erickson
Radio Astronomy, Univ. of Mass. - Amherst, Massachusetts

thumbs up signNow it is clearer than micro-waved water. Thanks Ronna and Tom. I can now start drinking my morning coffee with confidence again.

Alfonso Hernandez
- Aurora, Colorado

Q. Thanks, Ronna for the info on foil to keep food from overcooking. Yes, my interest was to find something to keep the vegies from boiling while the meat is lukewarm. Something to put over the vegies was my intent. I have a newer microwave with a rotating shelf and this always happens. Looks like the answer has already been invented. Foil. But it would still be nice to find something that eliminates the sparking problem altogether. Hold up! I just put a piece of aluminum foil on a plastic kids dish and nuked it. Arrgh! It lit on fire! So much for aluminum foil! I guess the search continues. Maybe the foil needs a dissipative coating.

Rick Achatz
- Torrance, California

A. Wait a minute! You don't mean that the FOIL caught fire, do you? (I would think NOT) I suspect that you microwaved your plastic dish too long (many plastics are very lousy and not suitable for microwaving food because they get too hot). They cannot take the heat of the certain foods. If your dish was fully covered and had no way for the steam to escape, the contents would get very hot. Especially if the contents contain a lot of fat. Have you ever considered microwaving your veggies and steak SEPARATELY? They shouldn't be cooked for the same length of time, so why not splurge and use two dishes and do it two operations? I would recommend that you use pyrex dishes with glass covers for your heating safety! If you have meltdowns in your microwave, perhaps you are using too high a power setting or leaving the food in WAY too long! Or you are using plastics that are not designed for high temperatures.

Good luck! Why not read the information at that site I recommended? It takes some of the mystery out of using a microwave.

Ronna Erickson
Radio Astronomy, Univ. of Mass. - Amherst, Massachusetts

Q. When taking out different containers from a microwave- which materials that the container is made from would stay hottest longest, and which materials would not heat up so I could hold it with out burning my self?

Purdey F.
- London, England

Q. Interior finish on my microwave is chipped. Recently my microwave sparked when the rack provided with the microwave touched the interior finish. My husband later inspected the interior and noticed that the finish was chipped. We have not used the microwave since and we are debating whether to throw it out, or to "repaint or tape the chipped spot". What do you advise? My microwave is about 25 years old.

Janis E. Ferrell
consumer - Miles City, Montana, USA

Ed. note: Listen to the nice (rocket scientist) lady and throw it out.

Q. Greetings.

After fixing one of those microwaveable soups, and having the container the soup was in explode soup all over the oven, I noticed 2 "tears" in that metallic plastic-like coating on the inner walls of the oven (near where the door would come close to contact with when closed).

I tried to feel/scrape at it to see if it was JUST STUCK-ON FOOD from the explosion, but it seemed too "deep" to be a simple marked patch of coating (my thumb's nail was able to scrape off" a bit of the coating).

My question is: Is it still safe to use this oven or should it be thrown out and replaced? I know the usual answer would be "Yes, dump it and buy a new one!", but there are financial issues that have to be considered (we are on a fixed disability income)

Any hints? Can the "pock marks" be covered with a new coating? Is that stuff even buyable in stores like Home Depot?

Any/all help will be greatly appreciated, I assure you.

Many thanks,

Raymond Christopher
consumer - Flushing, New York

A. This may be a bit late, but if anyone else has a similar situation, I would recommend that you throw the microwave out. If there are breaches in the protective foil coating on the inside and ESPECIALLY in the area where the door seals, microwave power can leak out. This is bad for your health. Do not *ever* operate a microwave oven where the door seal has been compromised in any way. Read my essay above about the physical danger posed by power at this frequency.

Ronna Erickson
- Amherst, Massachusetts

? I'm a bit confused. It sounded like you're suggesting that the paint, inside the microwave is what prevents microwave leakage.

My understanding of microwaves is that they are contained by continuous metal shielding ie like a faraday cage.

Chips in the surface will rust, but are not penetrated by microwave radiation.

Am I missing something in your post?

Doug Gourlay
- Hamilton, Ontario
March 28, 2023

Ed. note: The posting from Ronna was from a long time ago, so we'll see if she's around and willing to respond, but better not count on it.

Q. We recently had our microwave go out. We were steaming something on the stove below it and operated the microwave and there was a loud "pop". The appliance repair man came today. While inspecting, he noticed a scratch on the interior of the microwave. It goes down to the metal. He also said we have an "arching" going on. My wife can't remember if he said something else is broken, too. That scratch has been there probably about a year. He said that since the scratch is down to the metal, everything we have been microwaving since that happened has been subject to microwave poisoning. We have two children and are naturally concerned that this might be true and if it is, is there anything we need to do?

John Andersen
- Olathe, Kansas
February 14, 2008

A. I don't believe a microwave can poison food; I think someone misunderstood something. But damaged microwave ovens are very dangerous to people as you've read here. What you need to do is throw the microwave out. Next time you go to the pediatrician, tell him/her; but after looking at the kids I think s/he will only reassure you that they're fine.

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Q. My 2 year old climbed on the counter when I stepped out of the room to use the bathroom. When I came out I heard a strange noise so I went to the kitchen and my microwave was on and he had put a wrought iron/metal like trivet in there. There was a lot of color and popping and a tiny piece of the rubber of the trivet bottom had a little tic tac size fire that cracked my glass turn table. Sometimes when I run it now I hear a popping sound sort of like popcorn popping, but it doesn't do it every time. What is that noise and I read that sparks won't damage your microwave in the above message, so what is arching and how do I know if my microwave is ok to use?

Sherri Hitchens
- St.Johns, Michigan USA
March 8, 2008

A. He's a man by now, but my son put his Hot Wheels in the microwave as a kid, and it was quite a show. These kinds of stories should be a reminder to the manufacturers to be really careful in designing the interlock switch that prevents the oven from running if the door is ajar.

If you see no damage to the microwave my guess is that there is none, although I am just another user, not a microwave expert. I think popping sounds like popcorn would most likely be tiny pockets of steam -- the thing that splatters the tomato sauce all over the inside of the oven if you don't cover it :-)

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Q. The plastic coating over the mesh on the inside of the door of our microwave at work has a quarter size melted hole in it. The hole in the plastic coating is allowing steam to escape between the mesh and the interior of the glass door(that is how I noticed the hole). Is the plastic coating there for the ease of cleaning the inside of the door or is it a radioactive barrier? I am a little freaked out because it is still being used and no one seems to know (or care about) if it is dangerous or not.

Sally Smith
- Louisville, Colorado, USA
January 10, 2012

Q. I have a fundamental question regarding microwave oven coatings/shielding for internal components of the oven. The "plastic" turntable cog in my home oven is cracked and exposes the metal frame which causes sparking when the oven is turned on. I would like to know a coating I can apply to "seal" the crack in the cog's coating to alleviate this problem. This would seem a simple fix when I know what compound to use. Any suggestions are appreciated.

Paul abadie
- galveston, texas, USA
May 14, 2012

Q. During cleaning the little door on the inside wall of the micro came off and will not stay on. Is it safe to use?

Clif Martin
- Muskegon, Michigan
August 5, 2012

A. Hi Clif. I'm not a microwave expert so I can't answer authoritatively, but I certainly wouldn't use it myself. Our favorite rocket scientist hasn't been around here for a while, but the way I read her postings, this sounds very unsafe.


Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Q. The coating on the inside of my microwave oven has started flaking off. Do I need to chuck it and get a new one?

Anne Stoger
Housewife - Johannesburg, South Africa
May 22, 2013

A. Hi Anne. Yes, get a new one.


Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Q. I took interest in an earlier thread about using a reflective coating material on a plastic that could be used in a microwave oven safely. The topic fell closely to something I was thinking, mainly looking for advice for some sort of reflective mirror, that I could mount on the internal ceiling of the oven, so as my wife can look up and see if anything has spilled over onto the rotation plate. She is short, and the microwave oven is an over the stove unit that she cannot see into it without using a step stool.
Does anyone have any thoughts that could make a busy homemaker's life a little easier? I know some manufacturers have plastic reflective mirrors for concave mirrors, and feel I could use the same technology, but in a smaller form. I just don't know if it could survive a microwave environment. Hope this makes sense, and someone can comfortably chime in.

Terry Wilson
- Coeur Dalene Idaho
May 21, 2021

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