OT. Plates that get hot in the microwave

[Chris Hogg]

Are you sitting comfortably? Then I'll begin :-)

I've not been involved with the ceramic industry for some twenty years plus, and it was a pretty rare occurrence even then, having been sorted out decades before that. I may not even have the correct term, as I can't find anything about it either. 'Spit out' may be another term for it, although that may also apply to other glaze faults such as simple pin-holing

But my understanding is as follows: Traditionally, tableware went through three firings (probably only one these days to reduce the energy used and hence the cost). They were a biscuit fire, where the plate or whatever is fired from 'green' to 'biscuit' to sinter the various components (clays, quartz and feldspathic flux). They would then be glazed; in the past this would be a lead-based glaze, but for H&S reasons this is now borosilicate based. 'Earthenware' ('pottery' in the retail trade) would not be fired to as high a temperature as porcelain or bone china (lumped together as 'porcelain' by the retail trade). After the glaze (glost) firing, the tableware would go through a third firing, the decorating fire, during which high-temperature enamel decorations applied either by hand or via transfers, would become permanently fused to the underlying glaze. Each firing would be one or two hundred degrees lower than the previous. So: biscuit, say

1100°C, glost, say 1000°C and decoration, say 800°C, all ball-park figures.

Tableware, and in particular, earthenware, contains free quartz. Also, earthenware is not fired to as high a temperature as porcelain or bone china. Consequently, earthenware is slightly porous, and for the most part relies on the glaze to keep it clean and hygienic. But there are usually small areas that are either deliberately not glazed, such as the raised foot-ring (if they stood on a kiln shelf with the glaze on the foot, they would fuse to the shelf), or supported on three points (stilts) in a setter (e.g.

). But the point contacts leave sharp blemishes where they contact the glaze which are subsequently ground off, exposing small patches of unglazed pottery where moisture can gain access. See e.g. The third, decoration fire, fuses the enamels to the glaze, and is usually done at a lower temperature, high enough to melt the enamel but not the glaze.

Also:-

On firing and again on cooling, quartz goes through at least two phase changes. In particular, quartz experiences a significant expansion on heating and contraction on cooling at around 600°C (the alpha - beta quartz phase change)

. The contraction on cooling caused microcracks around the quartz particles, and where those particles sit on the surface of the fired clay (but still covered by the glaze) they provide a pathway for steam to escape. Normally of course, those pathways are closed off by the glaze. But if a plate is re-fired after it has absorbed moisture, then the steam generated within the item will blow a little crater in the glaze immediately above the quartz particle. Taking the temperature higher allows the glaze to re-melt and heal the hole, but this can't be done in the decorating firing as it would destroy the decoration.

It was a big problem in the industry many decades ago until they worked out what was causing it. Tableware that had been through the glaze fire and was waiting to be decorated would 'breeze' if it was allowed to stand too long in the factory before being decorated, simply by absorbing moisture from the air. A few weeks was all it took, but then it had to go through the glaze fire again to drive off the moisture and settle the glaze before being decorated.

I remember many years ago that a lady had bought a white undecorated earthenware plate, and she had painted it with enamel colours and was going to present it to a friend in commemoration of some event (birthday, long service, whatever, I don't remember). She asked if we could fire it in our laboratory kiln to fuse the colours. She'd put a lot of work into it and it was lovely. Except that we knew that after all this time between manufacture and the enamel fire it would have absorbed moisture and it would be ruined. This was explained to the lady, who said to go ahead anyway, which we did, and sure enough, the plate was covered in tiny craters. Such a pity.

Most commercially available glazes are not 'improperly formulated', although it may happen with glazes made by amateur or studio potters who mix their own.

[Andy Burns]

Quick, sell the idea; microwave plate warmer sounds more impressive than a hostess trolley, even if it's less efficient, marketing can soon gloss over that ...

[mm0fmf]

There is some material in the plate, probably the glaze, acting as a susceptor and absorbing the microwave energy and releasing it as heat.

They add susceptors to some microwave food packaging where you are meant to cook the food in the packaging and the purpose of the susceptor is to allow browning or crisping.

You can have ceramic susceptors, you don't have to use metals.

[The Natural Philosopher]

Try reading up on how microwaves heat. They are not 'specifically designed to heat water'

[Fredxx]

Water is a polar molecule and has broadband adsorption in the microwave region. The heating is the dipoles rotating imparting energy to the lattice vibration we call heat. It's usually called dielectric absorption.

If your interested in absorption characteristics of water this is an article that might assist:

[Fredxx]

With respect, that is their primary purpose. It just so happens that organics are made of water molecules.

[Brian Gaff (Sofa]

Well there still may be metallic substances in the mix of the material making it. Some plates of course are designed to get warm as hot food in a cold plate tends to get cold very fast. I tend to use nothing for the bread, or in an open plastic bag, depending on how clean the microwave is for defrosting bread.

Obviously, the point about microwave power is that defrost is still full power, its just that it cycles this on and off over a period to allow the heat to dissipate through the food, so you need to allow longer if you are using a plate that heats up, I'm also assuming no metallic decoration on the plate or pigment for that matter. I cracked a casserole because of the latter. Brian

[Brian Gaff (Sofa]

Plastic or recyclable paper plates? Brian

[Brian Gaff (Sofa]

Well my casserole mentioned earlier was grey all the way through. I had originally thought it was in the glaze. Obviously the edge of the broken bits showed this to a sighted person. Imagine how annoying it is for me, when you cant even guess if its going to be good or bad, the only way is a bit of trial and error, sadly, then careful stacking of good and bad plates and other receptacles. I had a cup once where the handle only got very hot in the microwave and eventually fell off. Brian

[Steve Walker]

Many manufacturers mark items as being microwave safe. Okay you can't see them, but maybe you could get someone to go through yours and see what is and isn't marked. If not many are unmarked, you could then replace those few and know that everything you have is safe.

Just a thought, but is it worth contacting your MP and seeing if you can push for manufacturers to have tactile markers in the base? You often find a manufacturer's mark or country of origin in raised or indented formats? That could cover microwave safe and dishwasher safe.

[Fredxx]

I'm sorry for not replying earlier and thanking you for your excellent description. Breezing seems to be be a term that has gone out of fashion, hence no hits.

I do some bronze work, and so aware of gentle heating of ceramic mould through the 100C mark, to drive out moisture and minimise cracking. I wasn't aware that pottery, once fired, absorbs water though common sense suggests it would. I accept that glazes can enclose the ceramic, but in the lady's example, could some patches of unglazed ceramic be left to allow the escape of water vapour?

Of course Ceramic moulds can be intentionally made with increased porosity to assist the pour.

I hear what you say about 'improperly formulated' and I read this as glazes that would absorb microwaves, rather than making a choice where absorption is minimal.

[Tricky Dicky]

We had some Thomas China crockery just plain white porcelain but all the plates had a thin silver band on the edge. If you put them in the microwave it was like a firework sparkler along the edges, you only did it once!

Richard

[Chris Hogg]

Glad you found it helpful.

I suspect not. As I said, most earthenware pottery has small unglazed areas on it, either the foot-ring or where stilt marks have been ground off, and that didn't seem to prevent the problem within the ceramic industry. The then BCRA in Stoke (British Ceramic Research Association, later called Ceram Research, now Lucideon) did work on it and IIRC produced a report for their members, possibly in the 1950's, which gives you an idea as to how long ago it was a problem. IIRC the little craters occurred immediately above quartz particles, demonstrating the connection.

No doubt they can be so formulated if required, although a glaze that gets hot faster than the underlying ceramic substrate will expand differentially, and that expansion between the two will put the glaze into severe compression and may cause it to flake off, a problem known as 'peeling' in the industry. It's usually a problem of differential contraction on cooling rather than expansion on heating, but the end result is the same.

[polygonum_on_google]

Some pottery is fully vitrified, hence unable to absorb water - such as stoneware and porcelain.

Years ago, a place of work had a hotch-potch mixture of crockery in their kitchen. One particular range, a very dark brownish red, was positively dangerous. They would get hotter than their contents.

[The Natural Philosopher]

yes. I have some stoneware that verges on that

I think it all seems to boil down to what impurities are in the clay. red clay is presumably full of iron oxide...yep that seems to be what te experts say. also clay can contain aluminium oxide (alumina).

I suspect these are what causes the heating.

[Fredxx]

Iron oxide is an absorbing oxide so could account for the heating, but really Alumina? Alumina is a very low loss dielectric, and used as a substrate in microwave circuits.

[Chris Hogg]

It is the convention in all mineralogical and ceramic chemical analyses to express the components as oxides, but it doesn't mean they are actually present as free oxides.

Clays used for making stoneware are usually very impure, but contain no free aluminium oxide i.e. no alumina. In fired stoneware it will all be combined as an aluminosilicate such as mullite, or included in the glassy phase. Other components will be alkalis soda and potash, and alkaline earths lime and magnesia (Na2O, K2O, CaO and MgO respectively), also all combined in the glassy phase, i.e. not as free oxides. In stoneware, there will also be small amounts of iron and titanium oxides. In the raw clay these may actually be free oxides, or present as ilmenite, FeTiO3 (or FeO.TiO2, ferrous titanate, if you prefer it written that way).

Depending on the firing conditions (low or high temperature, in either an oxidising or a reducing atmosphere), the iron may be in the ferric or ferrous state, or a mixture of both. Ferrous iron is a strong divalent flux and contributes to the glassy phase along with the lime and magnesia.

The titania and iron together are responsible for the grey colouration of most stoneware clays when fired, due to the absorption of light across the visible spectrum via charge-transfer or electron hopping interactions, something along these lines: Fe2+ - O - Ti4+ <-> Fe3+ - O - Ti3+ but I'm not very sure of the details and may not have got that quite right.

I leave it to others to decide which of that lot are responsible for the microwave heating.