Mugs and microwaves.

VicS

VicS

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Not a comment about Forumites and radar /forums/images/graemlins/grin.gif but a question about drinking vessels and kitchen appliances.

Like, I guess, many people we have an assortment of mugs in the kitchen cupboard. Some we've bought, some were gifts, some were free giveaways etc. We often stick them in the microwave when making hot drinks but we have found that the handles of some of them get hot, very hot.
How can this be, any ideas?
I'm talking pottery mugs not un-microwaveable things like melamine or tin mugs.
 
That doesn't really answer the question tho... i can't think of a good explanation - by my understanding microwaves work by causing water molicules to resonate. Ceramic shouldn't have any water in it so something odd is happening. Maybe some other substance also 'resonates' such as the glaze.
 
It does answer the question. Some mugs are made of porous material, and will have liquid inside the ceramic, causing the material of the mug to heat up at the same time as the contents.

There could be metallic glaze on some mugs which would also heat up
 
I doesn't explain why the "problem" affects some mugs , the minority , but not others. Loads of stupid comments on that page as well! Clearly though something that other people have noticed.

The instruction book for the microwave says that glass, porcelain, ceramic, stoneware or temperature proof plastic are suitable materials to use. It warns about metallic decoration but thats not our problem.


Sorry crossed with your reply. I was wondering about water trapped in the material but I seemed unlikely. It would not have survived the original firing so could only have got in later. We'll have to stick one of the offending mugs in the ordinary oven one day when its on for a long spell (With the turkey next Xmas perhaps.) and see if it will dry out.
 
It's a popular view that water is the only material to absorb microwaves. The reality is that everything absorbs microwaves to some extent. Water is especially good at it. So it's my guess that different ceramics will have differing compositions and consequently different microwave absorbing capacity.
I don't know enough about pottery to be more specific, sorry.

Derek
 
FWIW, I'm inclined to agree. We DON'T put stoneware type mugs in the microwave, well not after spilling the reheated tea all over the worksurface, the floor, my trousers...... /forums/images/graemlins/shocked.gif
 
I have always thought it was water in the cement between the handle and the mug that got hot and convection took over down the handle. Cheaper mugs have handles stuck on with resin, the water can't get into the resin and they don't get hot.
 
[ QUOTE ]
what type of mugs are we talking about here

[/ QUOTE ] I dont know but I dont think any of the decent china ones are affected but equally it does not affect all the cheap giveaways. Those given away with Nescafé some years ago are OK except that the dishwasher is removing the glaze.
 
I've always understood it to be something to do with the type of clay the thing is made from. We've got a bowl that heats up in a microwave - it says 'ironstone' on the bottom.
I've also read or been told that microwaves are most effective on sugar and fat, with water coming in third place. Ice is not particularly affected by microwave energy which is why it takes so long to defrost frozen foods with a high water content, and why mince pies will take the roof of your mouth off after a mere 20 second blast.
 
Heating the liquid with micro-waves leads to th eliquid heating the container by convection. On some mugs, the point where the handle is joined to the body is thinner than the rest of the body (there can be a noticeable dent visible from inside).

Perhaps some clays are better convectors than others.
 
People report mugs where if the water if only heated for a short while, and is merely warm, not hot, the handle is already very hot. So not convection in these cases.
 
It's because some of the mugs absorb microwaves, and some don't!

This can be because of water absorbed back into the ceramic after firing (Due to faults in the glaze, etc.);

Because metal ions, etc. in the ceramic make it conductive, to a greater or lesser degree - any vitreous phases will act as electrolytes;

Or because the composition of the ceramic has hydroxyl groups as part of its make-up which absorb at microwave frequencies. Commonly described as the 'water content' of glasses / ceramics. Even window glass, fired at > 1400 degC contains a few hundred ppm 'water'. Interestingly (if you're a nerd, like me), this small 'water' content has a major effect on whether a molten mixture will form a glassy or crystaline solid. The presence of water promotes crystallisation. Naturally occurring glasses (like obsidian) are relatively rare on Earth, but are much more common on the moon, and have an extremely low 'water' content.

The glass dishes in microwaves are made of a special, low conductivity composition to avoid them heating up.

/rambling

Andy
 
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