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It is because people use heuristics to navigate their lives. Instead of doing complicated engineering reasoning all the time to ensure their safety they learn s

It is because people use heuristics to navigate their lives. Instead of doing complicated engineering reasoning all the time to ensure their safety they learn simple “rules of thumb”. These rules are incorrect in as much as they have both false positives (something which the rule prohibits, but is safe) and false negatives (something which the rule lets you do, but is not safe). I’m a hobby jeweler and I use an ordinary household microwave to melt copper and silver to cast my projects.[1] At the same time “everybody knows” that you can’t put metal in a microwave. What gives? Is everybody wrong? Of course not. It is just that the statement “don’t put metal in a microwave” is a simplified form of the true statement which would go something like this “don’t put metal in a microwave, unless you follow these safety precautions, and wear these safety gear, and your crucible is made of the appropriate materials, and your moulds are bone dry and …”. Aint nobody has the time to think about all the caveats and dangers when all they want is to warm their meals. So it is simpler, easier, and safer to tell people the abbreviated form of the rule. 1: here is a video https://youtu.be/Oim2QsDp0rY?si=CZrhg6ux1GfiDWqd https://youtu.be/Oim2QsDp0rY?si=CZrhg6ux1GfiDWqd

valtism 2y ago
You can also just have a spoon in the microwave too, right? I think It's only from things like crinkles in aluminium foil and the prongs of a fork that produce sparks and arcing inside.
- femto 2y ago
  My understanding is that if you want to put metal in a microwave keep it away from the walls. If not, you can get an arc jumping from the metal to the wall and it can burn a hole in the wall of the oven. You should also put something (eg. a cup of water) in the oven to absorb some of the microwave energy. The metal doesn't absorb much energy so without an additional absorber the microwaves can get reflected back into the magnetron and overheat the oven.
  - obloid 2y ago
    A while back on a whim I put my stainless steel coffee cup in the microwave and it just warmed the coffee. No sparks or drama. The metal didn't even get hot.
    - nine_k 2y ago
      If your piece of metal is sufficiently thick, the eddy currents will not overheat it to the point of failure. If its surface is sufficiently smooth, there will be no arches jumping off the sharp edges. Crumpled foil is usually very much unlike that, and it gets violently destroyed. A steel coffee cup with a smooth top would likely do just fine. My microwave oven comes with a metal stand that you're supposed to put into it when doing certain kinds of baking. Likely it's engineered to be massive enough, it has rounded edges everywhere, and is supposedly sized so that there are no pieces that would resonate at 2.4 GHz and thus consume a lot of energy and overheat.
- nomel 2y ago
  Nice! As a perfect example of what he's saying, you've been downvoted for stating something that is absolutely true, outside of certain circumstances, driven by the downvotes not having an understanding of those circumstances.
- hayley-patton 2y ago
  ElectroBOOM did a video testing that, and he came to a similar conclusion: https://www.youtube.com/watch?v=OyTmJX_TC84 https://www.youtube.com/watch?v=OyTmJX_TC84
  - robertclaus 2y ago
    This was really interesting!
- niccl 2y ago
  CDs are fun in a microwave, arcing and spitting like fury for a while. So not just crinkly things
  - Gormo 2y ago
    CDs are extremely crinkly at a microscopic level. They're effectively thin sheets of foil that use embossed pits to encode data.
- adastra22 2y ago
  Yes, spoons are perfectly fine in the microwave. Mehdi Sadaghdar has a great video on this: https://www.youtube.com/watch?v=OyTmJX_TC84 https://www.youtube.com/watch?v=OyTmJX_TC84 Kudos to anyone who does actual experiments to test “common knowledge.”
nomel 2y ago
I think the social friction from getting all of this right/wrong really damages inquisitive minds. The visceral negative response you get from, "Wait, I don't think that's right, it's more complicated than that", helps prevent speaking up the next time, unless you have an inherently disagreeable personality (which I cherish working with). I see it all the time with kids. They'll see some fundamental truth that takes a sideways perspective to see, and be told they're wrong, hacking off an interesting way of perceiving the world. It's unfortunate, but it's also something most adults are bad at, including myself. And, I very often microwave my spoons, forks, even metal bowls, without them heating up at all. But I also have a basic understanding of resonance, absorption, insulation, apertures, and the wavelengths involved. I don't do it at work, because it's not worth the effort responding to "you're going to break the microwave", even if you show them that the food is hot and the fork is cool. That said, I wouldn't melt metal in an expensive microwave. The metal has poor absorption relative to the magnetron antenna itself, so both will heat. I wouldn't be surprised if the antenna is also glowing a bit (at least early on)!
- Dylan16807 2y ago
  > That said, I wouldn't melt metal in an expensive microwave. The metal has poor absorption relative to the magnetron antenna itself, so both will heat. I wouldn't be surprised if the antenna is also glowing a bit (at least early on)! You don't make the metal absorb the microwaves. Either your crucible or the inner lining of your insulator will be designed to do the absorbing.
  - krisoft 2y ago
    > the inner lining of your insulator will be designed to do the absorbing. That is exactly the case with our setup.
  - nomel 2y ago
    Ah, well I'm guilty! The videos I saw, in the past, were just a ball of fairly fine wire on a plate. This was probably 20 years ago, when I was looking into melting black beach sand in the microwave. For a rant about that, it turned out pretty neat. I put the beach sand in a thin layer on a dinner plate that was a half wavelength above the bottom of the microwave, turntable removed. The beach sand would start to melt where the standing waves were, but then quickly grow into a wire structure, with the growth of the tips driven by where absorption would be maximized, allowing the thing to heat more, and grow more. I tried to model the resulting "antenna", but none of the open source tools at the time could handle the complexity.
fwip 2y ago
Exactly. All models are wrong, some are useful. And a simpler model is easier to teach and remember.
LoganDark 2y ago
I suppose what you're doing is insulating the metal and also obscuring the reflective surface, right? Or does it not reflect microwaves? Curious.
- krisoft 2y ago
  The way I understand it. (and here I'm just telling you what I have heard from other people) is that the brick we use has two layers. So going from the outside to the inside the structure is: layer1, layer2, graphite crucible, metal to be melted. Layer1 is thermally insulating. Keeps the heat in the brick. It is still 300ish celsius at the end of the melt, and it increases further as the whole structure "cools". Layer2 "captures" the microwave radiation and is the thing the microwave oven heats. All that energy is captured inside the brick and layer1 doesn't let it out so it heats up very quickly. And then layer2 heats up the crucible via mainly heat conduction (so the interface has to be neat between them) and only to a lesser extent via heat radiation. The main dangers of the operation is the same as with any other process which results in tens of grams of molten metal. Those would be not new to anyone who casts metal. (But entirely outside of the experience of someone who never worked with molten metal before.) Everyone with normal household experience would know that molten metal is hot, what might surprise someone without specific training or very good intuitive grasp of physics is that if it contacts water the water immediately turns into steam and that will make the molten metal fly everywhere. [1] So that would be the first unexpected turn of events. Hopefully you have a bucket of sand ready to put out the resulting fire, because water is not your friend in this situation. It is also important as you pack the crucible to not form a "bridge" between the walls with the metal. Because before the metal melts it expands. And if you pack the to-be-melted metal too tight it will crack the graphite crucible. Which would ruin the brick. Then the other surprise is that the layer1 keeps heating up even after you have removed the brick from the microwave. (because layer2 is very hot, and that heat is slowly dissipating through layer1) And if you use the brick too much without letting it cool down between melts it might crack, or it might crack the glass plate of the microwave. 1: this is how that looks on a much bigger scale https://youtu.be/oWSHEC1N770?si=lc1ZKXxQ9kOsuhz0 https://youtu.be/oWSHEC1N770?si=lc1ZKXxQ9kOsuhz0
foobarian 2y ago
This whole thread is the most significant thing I've learned in the last 5 years. Thank you.
dekhn 2y ago
There'a really funny chapter in "Good Eats" by Alton Brown where he has you make your own microwave popcorn (basically put unpopped kernels in a paper bag). It contains a long section on whether it's safe to use a staple to keep the popcorn bag shut in the microwave; he structures it as a dialog with his father, who is a physicist. He concludes that this specific instance is not unsafe (if you have a turntable, the mass of the staple is low, etc). But then, I've put grapes in a microwave and produced plasma.
Dylan16807 2y ago
"Don't put aluminum foil in the microwave" works pretty well as a safety rule, and it's plenty simple. It's not exhaustive but neither is a rule against metal.
hoosieree 2y ago
"No metal in the microwave" is good general-purpose advice but a reasonable person might notice that the microwave interior is basically all metal so there must be more to that advice than meets the eye.[0] [0] Microwaves also contain transformers.
leobg 2y ago
Just had this exact discussion with the kids at dinner yesterday. In our case, we talked about the simplified traffic light rule “go on green, stop on red“. This to come up is wrong, of course. Because if the light turns red while you are crossing, stopping would be the worst thing you could do.
- vel0city 2y ago
  Also, just because the light is green does not mean it is safe to go. You should still look and make sure the other traffic is actually coming to a stop.
goodrubyist 2y ago
If you're using a heuristic, it can be fine, but only if you also know that it's only a heuristic, instead of thinking that it's truth and lambasting anyone who suggests otherwise.

valtism 2y ago

You can also just have a spoon in the microwave too, right? I think It's only from things like crinkles in aluminium foil and the prongs of a fork that produce sparks and arcing inside.

femto 2y ago
My understanding is that if you want to put metal in a microwave keep it away from the walls. If not, you can get an arc jumping from the metal to the wall and it can burn a hole in the wall of the oven. You should also put something (eg. a cup of water) in the oven to absorb some of the microwave energy. The metal doesn't absorb much energy so without an additional absorber the microwaves can get reflected back into the magnetron and overheat the oven.
- obloid 2y ago
  A while back on a whim I put my stainless steel coffee cup in the microwave and it just warmed the coffee. No sparks or drama. The metal didn't even get hot.
  - nine_k 2y ago
    If your piece of metal is sufficiently thick, the eddy currents will not overheat it to the point of failure. If its surface is sufficiently smooth, there will be no arches jumping off the sharp edges. Crumpled foil is usually very much unlike that, and it gets violently destroyed. A steel coffee cup with a smooth top would likely do just fine. My microwave oven comes with a metal stand that you're supposed to put into it when doing certain kinds of baking. Likely it's engineered to be massive enough, it has rounded edges everywhere, and is supposedly sized so that there are no pieces that would resonate at 2.4 GHz and thus consume a lot of energy and overheat.
nomel 2y ago
Nice! As a perfect example of what he's saying, you've been downvoted for stating something that is absolutely true, outside of certain circumstances, driven by the downvotes not having an understanding of those circumstances.
hayley-patton 2y ago
ElectroBOOM did a video testing that, and he came to a similar conclusion: https://www.youtube.com/watch?v=OyTmJX_TC84 https://www.youtube.com/watch?v=OyTmJX_TC84
- robertclaus 2y ago
  This was really interesting!
niccl 2y ago
CDs are fun in a microwave, arcing and spitting like fury for a while. So not just crinkly things
- Gormo 2y ago
  CDs are extremely crinkly at a microscopic level. They're effectively thin sheets of foil that use embossed pits to encode data.
adastra22 2y ago
Yes, spoons are perfectly fine in the microwave. Mehdi Sadaghdar has a great video on this: https://www.youtube.com/watch?v=OyTmJX_TC84 https://www.youtube.com/watch?v=OyTmJX_TC84 Kudos to anyone who does actual experiments to test “common knowledge.”

nomel 2y ago

I think the social friction from getting all of this right/wrong really damages inquisitive minds. The visceral negative response you get from, "Wait, I don't think that's right, it's more complicated than that", helps prevent speaking up the next time, unless you have an inherently disagreeable personality (which I cherish working with). I see it all the time with kids. They'll see some fundamental truth that takes a sideways perspective to see, and be told they're wrong, hacking off an interesting way of perceiving the world. It's unfortunate, but it's also something most adults are bad at, including myself. And, I very often microwave my spoons, forks, even metal bowls, without them heating up at all. But I also have a basic understanding of resonance, absorption, insulation, apertures, and the wavelengths involved. I don't do it at work, because it's not worth the effort responding to "you're going to break the microwave", even if you show them that the food is hot and the fork is cool. That said, I wouldn't melt metal in an expensive microwave. The metal has poor absorption relative to the magnetron antenna itself, so both will heat. I wouldn't be surprised if the antenna is also glowing a bit (at least early on)!

Dylan16807 2y ago
> That said, I wouldn't melt metal in an expensive microwave. The metal has poor absorption relative to the magnetron antenna itself, so both will heat. I wouldn't be surprised if the antenna is also glowing a bit (at least early on)! You don't make the metal absorb the microwaves. Either your crucible or the inner lining of your insulator will be designed to do the absorbing.
- krisoft 2y ago
  > the inner lining of your insulator will be designed to do the absorbing. That is exactly the case with our setup.
- nomel 2y ago
  Ah, well I'm guilty! The videos I saw, in the past, were just a ball of fairly fine wire on a plate. This was probably 20 years ago, when I was looking into melting black beach sand in the microwave. For a rant about that, it turned out pretty neat. I put the beach sand in a thin layer on a dinner plate that was a half wavelength above the bottom of the microwave, turntable removed. The beach sand would start to melt where the standing waves were, but then quickly grow into a wire structure, with the growth of the tips driven by where absorption would be maximized, allowing the thing to heat more, and grow more. I tried to model the resulting "antenna", but none of the open source tools at the time could handle the complexity.

fwip 2y ago

Exactly. All models are wrong, some are useful. And a simpler model is easier to teach and remember.

LoganDark 2y ago

I suppose what you're doing is insulating the metal and also obscuring the reflective surface, right? Or does it not reflect microwaves? Curious.

krisoft 2y ago
The way I understand it. (and here I'm just telling you what I have heard from other people) is that the brick we use has two layers. So going from the outside to the inside the structure is: layer1, layer2, graphite crucible, metal to be melted. Layer1 is thermally insulating. Keeps the heat in the brick. It is still 300ish celsius at the end of the melt, and it increases further as the whole structure "cools". Layer2 "captures" the microwave radiation and is the thing the microwave oven heats. All that energy is captured inside the brick and layer1 doesn't let it out so it heats up very quickly. And then layer2 heats up the crucible via mainly heat conduction (so the interface has to be neat between them) and only to a lesser extent via heat radiation. The main dangers of the operation is the same as with any other process which results in tens of grams of molten metal. Those would be not new to anyone who casts metal. (But entirely outside of the experience of someone who never worked with molten metal before.) Everyone with normal household experience would know that molten metal is hot, what might surprise someone without specific training or very good intuitive grasp of physics is that if it contacts water the water immediately turns into steam and that will make the molten metal fly everywhere. [1] So that would be the first unexpected turn of events. Hopefully you have a bucket of sand ready to put out the resulting fire, because water is not your friend in this situation. It is also important as you pack the crucible to not form a "bridge" between the walls with the metal. Because before the metal melts it expands. And if you pack the to-be-melted metal too tight it will crack the graphite crucible. Which would ruin the brick. Then the other surprise is that the layer1 keeps heating up even after you have removed the brick from the microwave. (because layer2 is very hot, and that heat is slowly dissipating through layer1) And if you use the brick too much without letting it cool down between melts it might crack, or it might crack the glass plate of the microwave. 1: this is how that looks on a much bigger scale https://youtu.be/oWSHEC1N770?si=lc1ZKXxQ9kOsuhz0 https://youtu.be/oWSHEC1N770?si=lc1ZKXxQ9kOsuhz0

foobarian 2y ago

This whole thread is the most significant thing I've learned in the last 5 years. Thank you.

dekhn 2y ago

There'a really funny chapter in "Good Eats" by Alton Brown where he has you make your own microwave popcorn (basically put unpopped kernels in a paper bag). It contains a long section on whether it's safe to use a staple to keep the popcorn bag shut in the microwave; he structures it as a dialog with his father, who is a physicist. He concludes that this specific instance is not unsafe (if you have a turntable, the mass of the staple is low, etc). But then, I've put grapes in a microwave and produced plasma.

Dylan16807 2y ago

"Don't put aluminum foil in the microwave" works pretty well as a safety rule, and it's plenty simple. It's not exhaustive but neither is a rule against metal.

hoosieree 2y ago

"No metal in the microwave" is good general-purpose advice but a reasonable person might notice that the microwave interior is basically all metal so there must be more to that advice than meets the eye.[0] [0] Microwaves also contain transformers.

leobg 2y ago

Just had this exact discussion with the kids at dinner yesterday. In our case, we talked about the simplified traffic light rule “go on green, stop on red“. This to come up is wrong, of course. Because if the light turns red while you are crossing, stopping would be the worst thing you could do.

vel0city 2y ago
Also, just because the light is green does not mean it is safe to go. You should still look and make sure the other traffic is actually coming to a stop.

goodrubyist 2y ago

If you're using a heuristic, it can be fine, but only if you also know that it's only a heuristic, instead of thinking that it's truth and lambasting anyone who suggests otherwise.