I have a mixed selection of plates, with some of them the plate gets really hot when put in the microwave.
For instance I am in the habit of defrosting a couple of slices of bread for 40 seconds and with the plates in question the bread remains largely still frozen but the plate is almost too hot to hold!
I reckon it must have something to do with the amount of moisture that's somehow inside the plate but how that can be so I don't know given that it'll have been fired in a kiln.
There's no metalic element to the glaze as far as I can see.
You are quite probably right in your thinking that there may be moisture trapped within some plates. Certainly, when fired at typically around 1000°C or higher, any moisture in the plate will have been driven off at much lower temperatures. But, and this depends on the type of pottery or porcelain (earthenware pottery, porcelain, bone china, etc), over the years, moisture will be re-absorbed into pores in the body of the plate through the unglazed foot or through the pin-marks that have been ground off, or through microcracks in the glaze. Older tableware often exhibits a network of fine cracks in the gaze which have developed over years (crazing), and these will very slowly let in moisture into the pores within the plate, for example every time it's washed. Plates that are not porous, such as porcelain or bone china, seldom show the effect.
But whether this absorbed moisture is causing the plates to get hot, I'll pass. Are those plates crazed? Are they old plates that have been in use for a long time? Are the plates that don't get hot made of porcelain or bone china, or perhaps just bought more recently?
I'm not denying it, but as I said to CG - suggestions?
That would depend on whether the steam pressure generated in the pores exceeds the strength of the pottery. Refiring an old plate to the upper hundreds centigrade will cause a fault (I think it's known as 'breezing'), where the glaze is blasted off by the steam pressure, leaving tiny pits and pinholes in the glaze, but the plate doesn't necessarily break, and the temperatures involved are a lot higher that you'd get by heating in a domestic microwave.
I'd have thought that this would dry out with regular microwaving (although it does depend a bit on the porosity of the plate and the amount of damaged glaze).
I've always attributed my plates and mugs that get "warm" to a slightly conductive glaze (on the basis that the colouring is often given by metal oxides some of which are slightly conducting).
I'm nor sure that it does. The pores are small, in the order of a micron or less, the vapour pressure of water in fine pores is significantly reduced, and diffusion paths to the outside are narrow and tortuous.
They may be conducting when in solid crystalline form, but many colouring oxides dissolve in the glaze and are present as isolated ions. Although having said that, some pigments that are used these days are insoluble, but whether they're conductive, even weakly so, I don't know.
If that were the case, 'breezing' wouldn't happen. But it's a recognised potential problem in the pottery industry when firing plates etc that have stood around in the factory for any length of time before being decorated. Not often seen these days because it is understood and avoided.
Murmansk said his plates get almost too hot to hold in about 40 seconds, which is probably about 70°C, maybe bit more. The steam pressure generated at those temperatures (not even boiling) aren't going to break anything.
I'm not saying it is the moisture in the pores, but I've yet to see a better explanation. If Murmansk comes back with more detail - types of plate (pottery or porcelain etc); old or fairly new; highly coloured glaze or white etc. then some explanations can be eliminated.
Not so sure, and certainly glazes can have metals in them earthenware and stoneware clays are heavily adulterated with plenty of metals like iron and manganese oxides
Pure kaolin clay may be less prone to heating I often heat my stonewatre plates in the microwave
I've not heard of 'breezing' and my Googling has failed me. Can you explain?
Pain threshold is ~45C Anything more than a few seconds and it's not going to be much hotter than this. Skin damage starts to occur at ~44C.
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Most explanations concern the glaze. I have said this already.
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"The most likely reason your ceramic dishes are getting hot has to do with the glaze. Improperly formulated glazes often contain heavy metals that are microwave absorptive."
Microwave ovens are specifically designed to heat water molecules by excitation of the dipolar moments of the H2o ion and to heat them very rapidly. This can end up heating the water faster than it can turn into vapour, causing it to become superheated. Bubbles of water vapour can then quickly form when the water is jostled as the cup is taken out of the microwave.
OK; not come across that before. Sounds much more likely than water in the pores, which I wasn't convinced by anyway, just trying to explain that there will be water in the pores where they exist.
So Murmansk should be able to relate which plates get hot and which don't, to the type of plate. Porcelain and bone china use much purer raw materials than stoneware, for example, although bone china contains a large amount of calcium phosphate, so may behave differently to other types of ceramic.
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