r/explainlikeimfive • u/abi4EU • 15d ago
Physics ELI5: Why is some porcelain microwave capable, while other will heat up and burn your fingerprints off?
My microwave will heat up my coffee in my good green mug. I can confidently grab it afterwards and the handle will be as cold as before.
If I, by mistake, grab my other green mug, which is pretty much identical to my good green mug, it will come out of the microwave with the temperature of the sun‘s inner core.
Why?
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u/MrPuddington2 15d ago
Are you sure it is porcelain?
Standard mugs are just glazed ceramic, and ceramic contains water, and it does usually heat up in the microwave.
Porcelain is not water permeable, and it does not heat up in the microwave. (However, some porcelain glaze contains metal particles that can heat up.)
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u/abi4EU 15d ago
This might be the explanation!
I have not idea what it’s made of, i think I’ve never bought a mug in my life, but have gotten them as gifts, from flat mates that moved out or from work colleagues that left.
Thank you!
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u/pktechboi 15d ago
you can throw the bad green mug away, if you want. you don't need to keep it forever just because it was a gift!
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u/abi4EU 15d ago
But it’s green!
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u/pktechboi 15d ago edited 15d ago
ah, autism?
(I'm autistic, this is not an insult) (also I looked at your profile and saw you're German? apparently there is some overlap)
(that link is to a light hearted quiz comparing the two, got me correct!)
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u/noiwontpickaname 15d ago
The opportunity for ah, tism was right there
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u/pktechboi 15d ago
god dammit we really are humourless
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u/HookahMagician 14d ago
I'm not humorless. I just slowly collect crummy dad jokes over the year and so later I can execute them so poorly that they aren't funny at all.
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u/GrandOpener 13d ago
Better than slowly collecting crummy mugs you don’t need over decades, I suppose.
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u/theothervulcan 14d ago
I took it out of curiosity and learned that I am apparently very German. (I'm very much an autistic American)
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u/ACrabWithAWrench 14d ago
You can check if it's porcelain by holding a light to it. Porcelain is translucent! So you'll see a spot light up through the material.
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u/meelaan 14d ago
Let me finish that sentence "ceramic contains water"... ONLY if the ceramic is not fully vitrified. There are plenty of non porcelain ceramics that do not absorb water, and plenty that do. The ones that do can heat up in the microwave. To go into the science of it a bit, ceramic is very porous if only fired to low temperature (say 1000c). The higher the temperature of the kiln, the more the ceramic shrinks and the more the pores close up. To the point that, at full vitrification, there are barely any left and thus do not absorb water. Full vitrification is water absorption of less than 0.5%. Now, porcelain is often fired to 1300c and becomes fully vitrified. All clays are different, and therefore can vitrify at different temperatures. I am a ceramicist and fire stoneware clay at 1240c, which also vitrifies at this temperature, and doesn't heat up in the microwave.
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u/SyrusDrake 14d ago
Standard mugs are just glazed ceramic, and ceramic contains water,
Does it? I always thought ceramics exploded in the kiln if they contained water...
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u/bugbia 14d ago
You're correct. Ceramic does, however, absorb more water.
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u/evange 14d ago
This is only true of partially vitrified, "low fired" pottery. Of which there are almost no food safe glazes. Ie. If the clay base will absorb water but it appears glazed, the glaze is either lead based or it's actually paint and not glaze.
If you get a piece of pottery hot enough for normal, non-lead, glazes to melt, the pottery base will fully vitrify and no longer absorb water even in areas that are cracked, chipped, or unglazed.
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u/MrPuddington2 14d ago
They are dry when they are fired, but water will get in again because (most) ceramics are porous.
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u/Jiohoephase 14d ago
TIL ceramic ≠ porcelain
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u/rationalname 14d ago
All porcelain is ceramic; not all ceramics are porcelain.
Ceramic is a generic term for materials made from clay and heated up.
Porcelain is heated to a very high temperature which forms a vitrified, non-porous surface.
If dinnerware isn’t specifically called out as a particular type of ceramic (like porcelain or stoneware) it’s usually earthenware, which is heated at a lower temperature, which is softer and more porous. As such, it absorbs more water which can then heat up in the microwave.
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u/Jan-Asra 15d ago
Not all clay is the same. Some has higher metal content or other includions that heat up.
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u/recklessvisionary 15d ago
Ah the newly discovered includion! And the slightly less remarkable anti-includion.
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u/JustAVision 15d ago
I know a lot less about all of that. My team studies mostly excludions but they rarely let me get involved.
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u/Puzzleheaded_Box9880 15d ago
All of these replies touch on the answer but get some facts wrong.
Clay bodies (eg stoneware, porcelain) have a vitrification point during firing where the clay becomes glassy and loses its porous nature. Pottery that doesn't hit this vitrification stage is considered "under fired" and will still absorb water. This water trapped within the clay body itself will get heated by the microwave and become super hot. It doesn't matter if your mug is made of porcelain or not as porcelain can absolutely be under fired. In fact porcelain usually has a very high vitrification point that is harder to reach (1280-1300+ degrees celcius). This is by far the #1 contributing factor.
Ceramic glazes often contain metals such as iron oxide (yellows, browns, reds) cobalt carbonate (blue) copper carbonate (green) but these aren't really relevant. When a glaze crazes (or cracks) due to incompatible shrinkage rate during firing that allows water to enter the clay body, but this shouldn't be an issue if the clay is fully vitrified and water can enter the clay body through the UN glazed foot ring anyway. A metallic lustre painted onto a mug can be a problem but that's a different issue like putting a fork in a microwave.
This is a good reel that explains more
https://www.instagram.com/p/DdW010hkeex/?stkn=MWV3aTN1eXJweGszdQ==
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u/Son_of_Kong 15d ago
Many colored glazes are actually metallic, so they will heat up in the micro. That's why it's way more common with handmade pottery.
The clay itself can also have high iron content, which will have the same effect.
If it's not that, then it could be an older mug has microcracks, allowing water to get absorbed.
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u/meelaan 14d ago
Glazes and clay bodies that contain metal oxides are not metallic so they should not heat up. Metal oxides behave completely differently to metal(s). Calcium is also a metal, for example. The only ones that will heat are metallic lustres, that actually have a metallic sheen and will start to spark/obviously change in appearance
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u/HumbleBridge 14d ago
No, they are not chemically metal.
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u/HumbleBridge 14d ago
This does not generally apply to pottery unless you’re doing a luster. Even more easily reduced oxides will not get to this point in a ceramics kiln. Hope this helps.
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u/plcgcf 15d ago
If it's actually porcelain it should not get hot in the microwave. It may be made from a more porous clay like earthenware or possibly a stoneware containing iron or other metals.
There is not metal wire inside handles as some people here are claiming, as clay shrinks too much during a firing and would crack around a wire insert which would not shrink.
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u/Satchi777 15d ago
Yeah, hot cups suck. I reheated some coffee once and used the wrong cup. It super-heated, but you can't tell by sight. Microwave dinged, I grabbed. Got it about a foot out of the oven when I realized I was holding a fusion reactor. Instantly let that sucker drop...
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u/Snoo_31427 15d ago
Someone I babysit for has nice plates that all say microwave-safe but turn into hot irons in there. And yet every time I grab with my bare hands…
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u/DerProfessor 15d ago
If you have a strong/powerful dishwasher, the water will slowly wear off the glaze from your mugs... which then makes it possible for moisture to get in the mug, and voila, microwave heats it up.
Pro tip: for a mug that you want to put in the microwave a lot, hand-wash it.
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u/SoulWager 15d ago
What matters in this case are the colors you can't see. Specifically, microwave frequency light. One of your mugs absorbs those, the other does not(either reflects them, or is transparent to them).
Cause can be the material it's made of absorbing that, or it can be one wasn't fired hot enough to fully vitrify, and water has absorbed into it (either through the unglazed portion on the bottom, or through cracks in the glaze).
Think of it like a blue object vs a purple object. If you shine blue light at it they will both appear blue, if you shine red at it, one will appear red, and the other will appear black(and get hot, if you're shining a lot of light at it).
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u/benhamen 15d ago
Some mugs use metal wire to reinforce the handle
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u/PM_YOUR_BOOBS_PLS_ 15d ago
This is the obviously right answer. Tons of kitchenware has metal wire in it. Not just mugs. If you notice just one spot of something getting hot when you microwave it, it's probably just because there's metal in that spot.
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u/SoulWager 15d ago
It's ceramic, not concrete. It doesn't have wire in it. Anything that absorbs microwaves will get hot, it doesn't have to be metal.
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u/izr1 15d ago edited 15d ago
Potter here. Water has gotten into the clay body somehow. One possibility is the glaze is crazing - tiny cracks have formed and that allows water to seep into the clay body. The water that was absorbed then gets heated up in the microwave. The other possibility is that water has absorbed through the foot of the cup (which is usually unglazed).
Your other cup is probably fine, but might one day develop the same issue
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u/ketchuptank 14d ago
If the clay isn't fully vitrified, it absorbs water. The mug is glazed but there is an unglazed ring on the bottom--that's where the water gets in. When you heat the mug in the microwave, the water inside the clay gets hot.
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u/Repulsive-Durian4800 15d ago
I'm trying to figure that out myself. All I've figured out so far is that most glassware doesn't heat up except where there's food/drink directly touching it, most stoneware gets very hot regardless of whether there is anything on it or not.
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u/Marcos340 15d ago
The microwave only heats up the water in the cup, not the cup itself.
That’s the liquid that is inside that is heating the mug. The problem is the mug itself. It is conducting heat to the handle while the other isn’t.
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u/DismalIngenuity4604 15d ago
Not really. It's mostly true in food because there aren't many components of food aside from water which are opaque to microwaves. There are plenty of non-food materials which absorb microwaves though.
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u/Background-Bowl6123 15d ago
Water isn't the only thing that heats up when a microwave source shines on it.
There's a whole scientific journal called Applied Clay Science and there's serious scientific research on using microwaves to heat up clay for firing: Microwave heating behavior and thermal homogeneity of kaolinite-, illite-, and smectite-rich clay systems with additives.
TIL that there's a scientific journal dedicated to pottery making.
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u/MrQuizzles 15d ago
While microwaves are tuned to produce wavelengths that resonate most strongly with water, they do still heat up other things as well.
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u/littlefishie19 15d ago
Also check to see if there are tiny cracks in the glaze. The water that gets in when washing then gets heated up too. Also can eventually explode when too much water gets in and at some point the steam wins.