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.

Chris Hogg wrote:
On Wed, 17 Mar 2021 11:34:12 -0700 (PDT), Murmansk
wrote:

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?

It doesn't have to be moisture making the plates lossy, any sort of
material that makes the pottery conductive will make them get hot.

--
Chris Green
·

On 17/03/2021 18:34, Murmansk wrote:
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.

Some metallic oxides in the glazes can be very lossy and absorb
microwave energy.

I doubt it's down to water as the plate likely crack or possibly shatter.

Murmansk wrote

I have a mixed selection of plates, with some of them
the plate gets really hot when put in the microwave.

None of mine do.

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.

Not moisture, some other stuff absorbs microwaves too.

I have a massive great ceramic thing that is deliberately
designed to heat up in the microwave, used to get nice
crisp pizza bottoms.

There's no metalic element to the glaze as far as I can see.

Metal doesnt heat up in a microwave.

On 17/03/2021 19:41, Chris Hogg wrote:
On Wed, 17 Mar 2021 11:34:12 -0700 (PDT), Murmansk
wrote:

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'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).

Chris Hogg wrote:
On Wed, 17 Mar 2021 20:12:29 +0000, Chris Green wrote:


It doesn't have to be moisture making the plates lossy, any sort of
material that makes the pottery conductive will make them get hot.

Sure. Any suggestions?

Carbon in some shape or form, almost any sort of metallic impurity, etc.

--
Chris Green
·

Chris Hogg expressed precisely :
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.

The rise in temperature from refiring would be slower than from a
microwave, allowing time for the escape of steam perhaps.

Chris Hogg wrote:
On Wed, 17 Mar 2021 21:43:41 +0000, Chris Green wrote:

Chris Hogg wrote:
On Wed, 17 Mar 2021 20:12:29 +0000, Chris Green wrote:


It doesn't have to be moisture making the plates lossy, any sort of
material that makes the pottery conductive will make them get hot.

Sure. Any suggestions?

Carbon in some shape or form, almost any sort of metallic impurity, etc.

But neither of those are found in tableware.

Impurities are found in everything! :-)

Having searched a little there are also some specific possibilities
such as glaze - dark colours are often apparently somewhat conductive.

I also wouldn't be surprised to know that some smeinconductors might
be present in china clay.

--
Chris Green
·

On Thu, 18 Mar 2021 08:26:14 +1100, cantankerous trolling geezer Rodent
Speed, the auto-contradicting senile sociopath, blabbered, again:



None of mine do.

Of course they don't, you clinically insane auto-contradicting senile
cretin!

--
Richard about senile Rodent:
"Rod Speed, a bare faced pig and ignorant ****."
MID:

On 17/03/2021 18:34, Murmansk wrote:
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.

heatimg in microwave is not from water - its dielectric heating

So its all about the ceramic used to make the plate


--
A lie can travel halfway around the world while the truth is putting on
its shoes.

On 17/03/2021 21:54, Chris Hogg wrote:
On Wed, 17 Mar 2021 21:43:41 +0000, Chris Green wrote:

Chris Hogg wrote:
On Wed, 17 Mar 2021 20:12:29 +0000, Chris Green wrote:


It doesn't have to be moisture making the plates lossy, any sort of
material that makes the pottery conductive will make them get hot.

Sure. Any suggestions?

Carbon in some shape or form, almost any sort of metallic impurity, etc.

But neither of those are found in tableware.


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


--
€œA leader is best When people barely know he exists. Of a good leader,
who talks little,When his work is done, his aim fulfilled,They will say,
€œWe did this ourselves.€�

ۥ Lao Tzu, Tao Te Ching

On 17/03/2021 21:51, Chris Hogg wrote:

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'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.

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).

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.



CAN I JUST SAY FINALLY THAT MICROWAVE HEATING IS NOTHING TO DO WITH WATER!!!


--
€œA leader is best When people barely know he exists. Of a good leader,
who talks little,When his work is done, his aim fulfilled,They will say,
€œWe did this ourselves.€�

ۥ Lao Tzu, Tao Te Ching

On 17/03/2021 22:12, Chris Hogg wrote:
I'm not saying it is the moisture in the pores, but I've yet to see a
better explanation.
DIELECTRIC HEATING IS NOTHING TO DO WITH WATER PER SE

you are probably looking at metal ions in the ceramic


--
€œA leader is best When people barely know he exists. Of a good leader,
who talks little,When his work is done, his aim fulfilled,They will say,
€œWe did this ourselves.€�

ۥ Lao Tzu, Tao Te Ching

On 17/03/2021 22:12, Chris Hogg wrote:
On Wed, 17 Mar 2021 21:51:24 GMT, Harry Bloomfield, Esq.
wrote:

Chris Hogg expressed precisely :
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.

The rise in temperature from refiring would be slower than from a
microwave, allowing time for the escape of steam perhaps.

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.

I've not heard of 'breezing' and my Googling has failed me. Can you explain?

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.

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.
https://en.wikipedia.org/wiki/Burn#Pathophysiology

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.

Most explanations concern the glaze. I have said this already.

https://expandusceramics.com/qa/why-...microwave.html

"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."

On Wed, 17 Mar 2021 22:39:49 +0000, The Natural Philosopher wrote:

On 17/03/2021 22:12, Chris Hogg wrote:
I'm not saying it is the moisture in the pores, but I've yet to see a
better explanation.
DIELECTRIC HEATING IS NOTHING TO DO WITH WATER PER SE

you are probably looking at metal ions in the ceramic

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.

Murmansk wrote:

I have a mixed selection of plates, with some of them the plate gets really hot when put in the microwave.

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 ...

On 17/03/2021 18:34, Murmansk wrote:
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.


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.

On 18/03/2021 05:28, jon wrote:
On Wed, 17 Mar 2021 22:39:49 +0000, The Natural Philosopher wrote:

On 17/03/2021 22:12, Chris Hogg wrote:
I'm not saying it is the moisture in the pores, but I've yet to see a
better explanation.
DIELECTRIC HEATING IS NOTHING TO DO WITH WATER PER SE

you are probably looking at metal ions in the ceramic

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.

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


--
"Anyone who believes that the laws of physics are mere social
conventions is invited to try transgressing those conventions from the
windows of my apartment. (I live on the twenty-first floor.) "

Alan Sokal

On 18/03/2021 05:28, jon wrote:
On Wed, 17 Mar 2021 22:39:49 +0000, The Natural Philosopher wrote:

On 17/03/2021 22:12, Chris Hogg wrote:
I'm not saying it is the moisture in the pores, but I've yet to see a
better explanation.
DIELECTRIC HEATING IS NOTHING TO DO WITH WATER PER SE

you are probably looking at metal ions in the ceramic

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.

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:

http://sites.science.oregonstate.edu...diation. html

On 19/03/2021 00:27, The Natural Philosopher wrote:
On 18/03/2021 05:28, jon wrote:
On Wed, 17 Mar 2021 22:39:49 +0000, The Natural Philosopher wrote:

On 17/03/2021 22:12, Chris Hogg wrote:
I'm not saying it is the moisture in the pores, but I've yet to see a
better explanation.
DIELECTRIC HEATING IS NOTHING TO DO WITH WATER PER SE

you are probably looking at metal ions in the ceramic

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.

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

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

"Fredxx" wrote in message
...
On 19/03/2021 00:27, The Natural Philosopher wrote:
On 18/03/2021 05:28, jon wrote:
On Wed, 17 Mar 2021 22:39:49 +0000, The Natural Philosopher wrote:

On 17/03/2021 22:12, Chris Hogg wrote:
I'm not saying it is the moisture in the pores, but I've yet to see a
better explanation.
DIELECTRIC HEATING IS NOTHING TO DO WITH WATER PER SE

you are probably looking at metal ions in the ceramic

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.

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

With respect,

No point in respect with mindless **** like that.

that is their primary purpose.

It is indeed.

It just so happens that organics are made of water molecules.

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

--

This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please
Note this Signature is meaningless.!
"Murmansk" wrote in message
...
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.

Plastic or recyclable paper plates?
Brian

--

This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please
Note this Signature is meaningless.!
"Chris Green" wrote in message
...
Chris Hogg wrote:
On Wed, 17 Mar 2021 20:12:29 +0000, Chris Green wrote:


It doesn't have to be moisture making the plates lossy, any sort of
material that makes the pottery conductive will make them get hot.

Sure. Any suggestions?

Carbon in some shape or form, almost any sort of metallic impurity, etc.

--
Chris Green
·

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

--

This newsgroup posting comes to you directly from...
The Sofa of Brian Gaff...

Blind user, so no pictures please
Note this Signature is meaningless.!
"Chris Hogg" wrote in message
...
On Wed, 17 Mar 2021 21:43:41 +0000, Chris Green wrote:

Chris Hogg wrote:
On Wed, 17 Mar 2021 20:12:29 +0000, Chris Green wrote:


It doesn't have to be moisture making the plates lossy, any sort of
material that makes the pottery conductive will make them get hot.

Sure. Any suggestions?

Carbon in some shape or form, almost any sort of metallic impurity, etc.

But neither of those are found in tableware.

--

Chris

On Fri, 19 Mar 2021 12:35:16 +1100, cantankerous trolling geezer Rodent
Speed, the auto-contradicting senile sociopath, blabbered, again:

FLUSH the trolling senile asshole's latest troll**** unread

--
Marland answering senile Rodent's statement, "I don't leak":
"That¢s because so much **** and ****e emanates from your gob that there is
nothing left to exit normally, your arsehole has clammed shut through disuse
and the end of prick is only clear because you are such a ******."
Message-ID:

On 19/03/2021 07:53, Brian Gaff (Sofa) wrote:
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.

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.

On 18/03/2021 09:14, Chris Hogg wrote:
On Thu, 18 Mar 2021 01:20:22 +0000, Fredxx
wrote:

I've not heard of 'breezing' and my Googling has failed me. Can you explain?

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
https://en.wikipedia.org/wiki/Glaze_defects#Pin-hole.

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. https://tinyurl.com/ykyz5kg4 ). 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.
https://tinyurl.com/yvmbwyuu

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) https://tinyurl.com/43dknh3e . 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.


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.

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.
https://en.wikipedia.org/wiki/Burn#Pathophysiology

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.

Most explanations concern the glaze. I have said this already.

https://expandusceramics.com/qa/why-...microwave.html

"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."

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

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.

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

On Saturday, 20 March 2021 at 13:07:31 UTC, Fredxx wrote:


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.

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.

On 22/03/2021 08:11, polygonum_on_google wrote:
On Saturday, 20 March 2021 at 13:07:31 UTC, Fredxx wrote:


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.

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.

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.



--
The New Left are the people they warned you about.

On 22/03/2021 10:49, The Natural Philosopher wrote:
On 22/03/2021 08:11, polygonum_on_google wrote:
On Saturday, 20 March 2021 at 13:07:31 UTC, Fredxx wrote:


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.

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.

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.

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.