search  current discussion  categories  techniques - misc 

high-iron glazes in microwave

updated mon 20 feb 06

 

Bob Masta on sun 5 feb 06


The "Answers" column in the Feb 2006 Ceramics Monthly
contains a statement that high amounts of iron oxide in
glazes should be avoided, because "The metal will be an
antenna, focusing the microwaves, resulting in the
buildup of heat and electricity in the ware, which can
result in sparking on edges."

I'm skeptical of this. Sparking in a microwave requires
a conductor, but the iron oxide is bound into a glassy
matrix and thus *should* be an insulator. (Think about all
those classic brown high voltage tower insulators!)

Normal microwave heating acts on water molecules, which
have a charge separation that allows them to be dragged
back and forth by the waves, imparting energy. So even if
the iron oxide had a charge separation (anyone know?),
the fact that it is bound into a glassy matrix should
prevent any heating action at non-molten temperatures.

This topic has appeared on Clayart before, and as I recall
the general consensus has always been that ceramic wares
only got hot when they had absorbed water. That fits
with my understanding of microwaves. But reality is
sometimes stubbornly resistant to democracy, so what
we really need here is some hard data. Does anyone
have anything that is conclusive, one way or the other?

It would seem that the actual testing in a microwave oven
should be simple enough: Otherwise-identical test tiles
prepared with high-iron and no-iron glazes, starting at
room temperature but having been first baked in a
conventional oven (or fresh from the kiln) to insure
there is no absorbed moisture. A separate cup of water
would also be placed in the microwave, since some ovens
are reputed to be damaged if run without a
microwave-absorbent load. After a minute, check the
temperature of the tiles... I'm betting that they can be
held in the hand and will both feel the same.

The problem is that there are a *lot* of possible glazes,
bodies, and firing temperatures. If there is a negative outcome,
as I suspect, then a critic could always say "Yeah, but
you used high-boron cone 6 glazes on stoneware. I'm sure
you would have seen a big difference with cone 10 glazes
on porcelain."

So, can we reach a consensus about what would be accepted
as a valid test? If there *was* going to be any differential
heating, what would be the best situation to show it?
Can anyone think of any conceivable reason why the base
glaze or body would make a difference? How much iron
oxide would be enough satisfy the sharpest critic?

Inquiring minds want to know!




Bob Masta

potsATdaqartaDOTcom

John Hesselberth on mon 6 feb 06


On Feb 6, 2006, at 6:12 PM, Jennifer Boyer wrote:

> My question is do I just avoid pot types that could be microwaved
> when using this glaze? I could just do lamps, vases, bigger platters
> etc.

Hi Jennifer,

What is the absorption of your fired clay body? Not the one the
manufacturer claims--the one you have carefully measured?

Regards,

John

John Britt on tue 7 feb 06


Jennifer,

Always feel free to post any recipes I have in the book, or that I have
ever given to anyone!

President of the Glaze Free Trade Society

John Britt
www.johnbrittpottery.com

Ivor and Olive Lewis on tue 7 feb 06


Dear Bob Masta=20

It will be a week or two before I get my copy of CM but I would like to =
know if the person who has answered the original question is a scientist =
or an artist.

In the statement <<"The metal will be an antenna, focusing the =
microwaves, resulting in the
build up of heat and electricity in the ware, which can result in =
sparking on edges.">> the issue is clouded by confusing a free metal =
with the compound of a free metal. In a glaze that contains Iron oxide =
of one species or another there is no free metallic iron. I would like =
to know what a Physicist has to say about this.

My experience is that pots coated with Tenmoku glazes do get hotter than =
those with clear glazes, given the same volume of fluid in each. But in =
twenty years of heating milk for coffee in a microwave oven I have never =
encountered any sparking.

You will need to check my understanding on this. All Atoms have =
electrons. At each quantum shell there can be one or two electrons but =
no more. Iron is an element whose atoms have four quantum shells =
containing only one electron. When there are two electrons of opposite =
spin acting in an electromotive field (Microwave Radiation) their =
effects cancel each other. But Iron has Four Unpaired Electrons. These =
act in consort in an electromotive field. The result is the generation =
of heat energy. But do please check my science because I may be =
misunderstanding what I have read.=20

Yes, testing is a good idea.

Best regards,

Ivor Lewis.
Redhill,
South Australia.

Steve Slatin on tue 7 feb 06


Jennifer -- There's lots and lots of metal used inside all of the
microwave ovens I've seen. The reason why they tell you not
to use metal is because SOME metal can be dangerous --
thin foils can get very hot and even in some cases ignite, and
sharply tipped metal pieces can spark.

Lots of people put thick rounded pieces of metal (spoons) into
microwave ovens without ill effect. In our litigious society,
though, and bearing in mind the obvious fact that half of all
people are below average, manufacturers just sell the idea "never
put metal objects in a microwave" and never mind the consequences.

(It's easier to sell a simple process than a hard one. Think of
bananas -- "Never store bananas in your refrigerator!" Why? Because
in the fridge, the 'nana skin turns black. Also, ripening of the fruit
slows or stops. The skin, usually an excellent indicator of ripeness,
becomes unreliable. Of course if it's Friday and you're at work (and
not a potter, so you won't be there Saturday) you can take a perfectly
ripe banana you don't then want and put it into the refrigerator ... you
will have a perfectly ripe banana ready for you on Monday when you
get back to work, though the peel will be ... unappealing. So why
didn't Chiquita advertise "Remember, if storing a banana in the
refrigerator, the time the fruit is refrigerated must be subtracted
from the total time the fruit ages, or you may inadvertently eat an
unripe astringent banana although the peel will be black and gnarly?"
30 words vs. 6! When aiming for the
American consumer, it's no contest. Likewise with metal in
microwaves.)

I tested the highest-iron-content glaze I use in the traditional fashion
(empty test mug in microwave along with known good mug, good mug
with water and microwave until the water's boiling) and at 14% RIO had no
noticable warming. The glaze itself may have some iron-bearing
ingredients, so even though your RIO content was similar, your
overall iron content could have been higher. And I believe John Britt is
the founder of the 'free glaze exchange' and a major promoter of the
idea that glaze recipes are to be shared. If you're uncomfortable
with providing the recipe, though, you can always provide the reference
and let us find it at the source.

Best wishes and thanks for raising this issue -- Steve Slatin


Jennifer Boyer wrote:
Well, it's one from John Britt's book, so I'm not sure I should list
it here. But I think its heat in the microwave fits with my
understanding of microwave problems potters can have:
Lowfire pots can get hot when porous earthenware has moisture in it
and the water in the clay heats up. But I'm using cone 10 clay and
glazes

Any glaze with a high metal content can heat up, since metals aren't
supposed to be used in a microwave and must hold heat. Since my glaze
is somewhere around 14% iron, it makes sense to me that the iron is a
problem, causing my pots to heat up. I used the same claybody
(whiteware cone 10) that I use with my 3 glazes that don't produce
any heat when microwaved.

Steve Slatin --

In watermelon sugar the deeds were done and done again
as my life is done in watermelon sugar.

---------------------------------
Relax. Yahoo! Mail virus scanning helps detect nasty viruses!

Bob Masta on tue 7 feb 06


Jennifer, could you pass along the recipe for that glaze,
along with claybody and firing particulars? I'd like to
understand what's going on, since this seems to defy
conventional models of ceramics and microwaves.

Thanks!

Bob Masta
-----------------------------------

On 7 Feb 2006 at 0:00, Jennifer Boyer wrote:

> One thing about high iron glazes I have been testing in the
> microwave: I've used several from John Britt's book on
> stoneware glazes. Most have 12-15 % iron and the one I've
> put on mugs gets HOT. My other glazes don't change temp, but
> these iron red mugs are hard to touch. No sparks though! My
> question is do I just avoid pot types that could be
> microwaved when using this glaze? I could just do lamps,
> vases, bigger platters etc. But I've already gotten an order
> for dinnerware. I explained the limitations to the couple
> and they were fine with it.
>
------------------------------------------------

Jennifer Boyer on tue 7 feb 06


Well, it's one from John Britt's book, so I'm not sure I should list
it here. But I think its heat in the microwave fits with my
understanding of microwave problems potters can have:
Lowfire pots can get hot when porous earthenware has moisture in it
and the water in the clay heats up. But I'm using cone 10 clay and
glazes

Any glaze with a high metal content can heat up, since metals aren't
supposed to be used in a microwave and must hold heat. Since my glaze
is somewhere around 14% iron, it makes sense to me that the iron is a
problem, causing my pots to heat up. I used the same claybody
(whiteware cone 10) that I use with my 3 glazes that don't produce
any heat when microwaved.

What about all this doesn't make sense to you?
Just curious,
Jennifer
On Feb 7, 2006, at 8:59 AM, Bob Masta wrote:

> Jennifer, could you pass along the recipe for that glaze,
> along with claybody and firing particulars? I'd like to
> understand what's going on, since this seems to defy
> conventional models of ceramics and microwaves.
>
> Thanks!
>
> Bob Masta
> -----------------------------------
>
> On 7 Feb 2006 at 0:00, Jennifer Boyer wrote:
>
>> One thing about high iron glazes I have been testing in the
>> microwave: I've used several from John Britt's book on
>> stoneware glazes. Most have 12-15 % iron and the one I've
>> put on mugs gets HOT. My other glazes don't change temp, but
>> these iron red mugs are hard to touch. No sparks though! My
>> question is do I just avoid pot types that could be
>> microwaved when using this glaze? I could just do lamps,
>> vases, bigger platters etc. But I've already gotten an order
>> for dinnerware. I explained the limitations to the couple
>> and they were fine with it.
>>
> ------------------------------------------------
>
> ______________________________________________________________________
> ________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
> melpots@pclink.com.

*****************************
Jennifer Boyer
Thistle Hill Pottery
Montpelier, VT
http://thistlehillpottery.com
*****************************

Ron Roy on tue 7 feb 06


I have made a lot of tenmoku dinner ware - the glaze has 7.5% red iron in
it - I use those pots all the time - and have tested them - and have never
seen the problem.

I'm also sure you can get enough iron into a clay or a glaze to get the
effect of heating up but - It is a simple test and someone should do the
work to see which glazes will be most likely to heat up.

It would make some sense to me that - if you could get enough iron to
condense out on the surface - at some point you would get some sparking -
but I would not expect that in any kind of glaze or clay that I have seen.

RR

>The "Answers" column in the Feb 2006 Ceramics Monthly
>contains a statement that high amounts of iron oxide in
>glazes should be avoided, because "The metal will be an
>antenna, focusing the microwaves, resulting in the
>buildup of heat and electricity in the ware, which can
>result in sparking on edges."

Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513

Jennifer Boyer on tue 7 feb 06


okey dokey
This glaze is Anderson Ranch Red
Custer Feldspar 45
Silica 20
Whiting 7
Kaolin 8
Talc 8
Bone Ash 12
Red Iron Oxide 13.5

I fire my propane kiln without reduction: under .3 on the oxyprobe
Most of the other kaki's in John's book didn't go as red for me as
this one.
Bailey's went red but it also crazed and Ron's suggested adjustment
that changed the color.

Any ideas why this glaze gets hot? There's definitely no sparking.
Jennifer

On Feb 7, 2006, at 8:11 PM, John Britt wrote:

> Jennifer,
>
> Always feel free to post any recipes I have in the book, or that I
> have
> ever given to anyone!
>
> President of the Glaze Free Trade Society
>
> John Britt
> www.johnbrittpottery.com

*****************************
Jennifer Boyer
Thistle Hill Pottery
Montpelier, VT
http://thistlehillpottery.com
*****************************

Bob Masta on wed 8 feb 06


Jennifer (and Ivor):

First of all, thanks for passing along the glaze recipe.

Regarding what doesn't make sense to me about this:
There are two different issues about microwave heating.
As I noted in my original post, normal microwave heating
acts on water or other polar molecules (like oils). This
has nothing to do with conductivity (oils are insulators),
only with the fact that the molecules have some charge
separation: One side or end of the molecule is more
positive than the other, so the microwaves can get hold
of them and drag them back and forth, which causes
friction-type heating.

A conductor in a microwave gets hot for a completely different
reason: There is induction of current into the metal, and little
short-circuited loops of current heat the metal by resistive
heating, just like current through an electric heating element.
This shouldn't be applicable to glazes, since they are not
conductive. True, there may be metals in them, but they are
oxides, not raw metals, and more importantly they are bound
in a glassy matrix. Again, you KNOW they aren't conductive
if you've ever seen those brown (iron) glazed high-voltage
insulators. They use those on towers at many 1000s of volts.

So, what can be happening in a high-iron glaze? OK, maybe
there is some charge separation in the molecules, but since
they are supposedly immobile, how can they be dragged around?
They *might* have some strange crystalline formation like
piezolectric crystals, where a trapped ion can "pop" between
two sides of the crystal like a kid's tin cricket. I have never
heard anything like this about iron, however.

Or maybe these high-iron glazes are actually conductive. Again,
seems unlikely, but needs further testing. The best I can come
up with would be that large conductive crystals might be forming,
crystals large enough to hold a small circulating current but still
isolated from each other so there is no bulk conductivity. But
that is wild speculation at this point.

Now, please don't take this the wrong way, but it would help
if we could put your observations on a rigorous scientific
footing, and remove as many confounding variables as
possible. I can't replicate your conditions exactly at the
moment, so....

Would you be willing to do a side-by-side controlled test, with
otherwise-identical test bars using a base glaze and your
high-iron glaze? You would need to pre-dry them in a
conventional oven (or take them fresh from the kiln) to insure
that trapped moisture is not causing any heating. (They'd need
to be cooled in dry surroundings to reduce airborne water
being absorbed.) Then put the bars in the microwave along with
a cup of plain water, heat on high for 1 minute (or whatever time
would insure the phenomenon, based on your experience)
and then touch both bars (carefully). If there is a clear and
obvious temperature difference, then that should begin to answer
most skeptics (me included).

In the meantime, I will see what I can discover on my end,
with my clay at cone 6. I will try similar amounts of RIO
in a couple of different bases, and report back.

If anyone else wants to join in the testing, your efforts
will be most welcome. This may take a *lot* of tests
to pin down, if it is a phenomenon of crystal formation
with fussy glaze formulation and cooling regimes.

Best regards,

Bob Masta

potsATdaqartaDOTcom

Jennifer Boyer on wed 8 feb 06


Hi Steve,
Here's a pic of the red and a similar tan mug. It's the top picture.
I did measure how hot the water gets in each and the tan one's water
gets hotter. The red glaze must be stealing heating energy from it's
water...
The liner glaze is the same as the tan mug's glaze. 1 or 2% iron. The
clay is white stoneware. The recipe for the red is on the web page.

http://www.thistlehillpottery.com/brittglazetests.html

I don't really understand why one 14% iron glaze would heat up and
another wouldn't. Any ideas? I posted the recipe to Clayart last night.


I'm testing the glaze on a mug with a thicker handle. My theory is
that maybe more clay mass would allow the handle to stay cooler.
Jennifer, firing today. Started the day at 5AM in jammies fixing a
propane leak...brrrrrr


> I tested the highest-iron-content glaze I use in the traditional
> fashion
> (empty test mug in microwave along with known good mug, good mug
> with water and microwave until the water's boiling) and at 14%
> RIO had no
> noticable warming. The glaze itself may have some iron-bearing
> ingredients, so even though your RIO content was similar, your
> overall iron content could have been higher. And I believe John
> Britt is
> the founder of the 'free glaze exchange' and a major promoter of the
> idea that glaze recipes are to be shared. If you're uncomfortable
> with providing the recipe, though, you can always provide the
> reference
> and let us find it at the source.
>
> Best wishes and thanks for raising this issue -- Steve Slatin
>

*****************************
Jennifer Boyer
Thistle Hill Pottery
Montpelier, VT
http://thistlehillpottery.com
*****************************

Jennifer Boyer on wed 8 feb 06


Hi Bob,
Thanks for the explanation of all this!
I have some tests in this firing that's happening today. I did a line
blend so I have a test tile of the base glaze without iron. I also
have a tile with 80 % iron and 20% base. If it's red,
I'll try your test. Only problem is the tiles aren't the same size.
If I get no info from this test, I'll put some similar tiles in the
next kiln with iron glaze and base.

Also I have a another mug in the kiln with iron glaze. Would it help
if I did another test using this mug and another mug using my
standard Temple White? or are you mainly interested in comparing the
iron red to it's base minus iron? I'll probably heat water in the
iron red mug and the Temple mug straight out of the warm kiln anyway.

I'll let you know what I find..
Jennifer

>
> Or maybe these high-iron glazes are actually conductive. Again,
> seems unlikely, but needs further testing. The best I can come
> up with would be that large conductive crystals might be forming,
> crystals large enough to hold a small circulating current but still
> isolated from each other so there is no bulk conductivity. But
> that is wild speculation at this point.
>
> Now, please don't take this the wrong way, but it would help
> if we could put your observations on a rigorous scientific
> footing, and remove as many confounding variables as
> possible. I can't replicate your conditions exactly at the
> moment, so....
>
> Would you be willing to do a side-by-side controlled test, with
> otherwise-identical test bars using a base glaze and your
> high-iron glaze? You would need to pre-dry them in a
> conventional oven (or take them fresh from the kiln) to insure
> that trapped moisture is not causing any heating. (They'd need
> to be cooled in dry surroundings to reduce airborne water
> being absorbed.) Then put the bars in the microwave along with
> a cup of plain water, heat on high for 1 minute (or whatever time
> would insure the phenomenon, based on your experience)
> and then touch both bars (carefully). If there is a clear and
> obvious temperature difference, then that should begin to answer
> most skeptics (me included).
>
> In the meantime, I will see what I can discover on my end,
> with my clay at cone 6. I will try similar amounts of RIO
> in a couple of different bases, and report back.
>
> If anyone else wants to join in the testing, your efforts
> will be most welcome. This may take a *lot* of tests
> to pin down, if it is a phenomenon of crystal formation
> with fussy glaze formulation and cooling regimes.
>
> Best regards,
>
> Bob Masta
>
> potsATdaqartaDOTcom
>
> ______________________________________________________________________
> ________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
> melpots@pclink.com.

*****************************
Jennifer Boyer
Thistle Hill Pottery
Montpelier, VT
http://thistlehillpottery.com
*****************************

Steve Slatin on wed 8 feb 06


Jennifer --

It appears that the wall thickness on your mugs are similar. Given
that the mug on the right doesn't heat up (you said it was from the
same clay, right? Presumably fired similarly or identically?) I would,
like you, suspect the glaze. (BTW, excellent test idea -- the
very simple experiments are often the best.)

The additional information that you gave us is that the water gets
hotter in the tan mug. That means it's absorbing more energy, and
the only apparent reason for that would be that more energy is
crossing the mug with the lower-iron content glaze. Until a better
idea comes along, I'm going to have to go with your conclusion
and say that higher iron-content glazes may indeed have a serious
problem with microwave usage.


(I'll also get some of my older, high-iron glaze stuff from the
garage and run side-by-side tests for heat transmission and
see what I get. I've got some old 5-6% iron glazes somewhere;
with a little luck I can get results that may provide some useful
information. Now I have to find a good thermometer,
too. Hmmm. It may be that my high-iron stuff is heating,
just not sufficiently to be noticed. Or, it may be that the metal
is reflecting the microwaves somewhat, or attenuating their
strength. The question of why one would heat up and another
wouldn't, though, remains a good one.

(My glazes are Redwine's Black with extra iron (mid-iron)
and a very slight variation on Lili K's reformulation of Tony
Yeh's brown speckled with orange.)

We should pursue this -- Steve Slatin




Jennifer Boyer wrote:
Hi Steve,
Here's a pic of the red and a similar tan mug. It's the top picture.
I did measure how hot the water gets in each and the tan one's water
gets hotter. The red glaze must be stealing heating energy from it's
water...
The liner glaze is the same as the tan mug's glaze. 1 or 2% iron. The
clay is white stoneware. The recipe for the red is on the web page.

http://www.thistlehillpottery.com/brittglazetests.html

I don't really understand why one 14% iron glaze would heat up and
another wouldn't. Any ideas? I posted the recipe to Clayart last night.


I'm testing the glaze on a mug with a thicker handle. My theory is
that maybe more clay mass would allow the handle to stay cooler.
Jennifer, firing today. Started the day at 5AM in jammies fixing a
propane leak...brrrrrr



Steve Slatin --

In watermelon sugar the deeds were done and done again
as my life is done in watermelon sugar.

---------------------------------
Relax. Yahoo! Mail virus scanning helps detect nasty viruses!

Joan Klotz on fri 10 feb 06


Since I have a small bowl I glazed with Bailey's red I ran into the
kitchen as soon as I read your message and microwaved some water in
it. The water was to hot to touch but the rim of the bowl was
cool. Just a little more data.

Joan Klotz,
Venice, CA.

At 05:22 PM 2/10/2006, you wrote:
>So here's the scoop on my hot-in-the- microwave pots:
>I thought it was the Anderson Ranch that was getting hot but I
>realized it's the Bailey's Red. I have a mug on which the Bailey's
>has a sort of scum on the surface that looks muddy brown(the mug is
>mainly a nice Kaki red) and the scummy areas are drier than the rest
>of the pot, which is quite glossy.

Jennifer Boyer on fri 10 feb 06


So here's the scoop on my hot-in-the- microwave pots:
I thought it was the Anderson Ranch that was getting hot but I
realized it's the Bailey's Red. I have a mug on which the Bailey's
has a sort of scum on the surface that looks muddy brown(the mug is
mainly a nice Kaki red) and the scummy areas are drier than the rest
of the pot, which is quite glossy. There's quite a lot of scum on the
handle, which gets hotter than the rest of the mug. So, thinking
about Bob's discussion of how microwaves work(below) I'm wondering if
the scum has a high concentration of iron crystals that have
migrated to the surface of the mug and that the iron concentration is
what's getting hot rather than the glaze itself....just a theory.

When I fired some more Anderson Ranch mugs they didn't get hot, so
only the Bailey's mug did this. Maybe it was a fluke! Interesting
that there is a higher % of iron in the Anderson Ranch Red which
didn't get hot.

I may do some tests on tiles and microwave them next to a mug of
water. I let you know if anything interesting shows up....
Jennifer

Bailey's Red:
Custer Feldspar 47
Flint 13
EPK 13
Talc 10.5
Bone Ash 14
Lithium Carbonate 2.5
RIO 8

Anderson Ranch Red
Custer Feldspar 45
Silica 20
Whiting 7
EPK 8
Talc 8
Bone Ash 12
Red Iron Oxide 13.5


On Feb 8, 2006, at 10:01 AM, Bob Masta wrote:

> So, what can be happening in a high-iron glaze? OK, maybe
> there is some charge separation in the molecules, but since
> they are supposedly immobile, how can they be dragged around?
> They *might* have some strange crystalline formation like
> piezolectric crystals, where a trapped ion can "pop" between
> two sides of the crystal like a kid's tin cricket. I have never
> heard anything like this about iron, however.
>
> Or maybe these high-iron glazes are actually conductive. Again,
> seems unlikely, but needs further testing. The best I can come
> up with would be that large conductive crystals might be forming,
> crystals large enough to hold a small circulating current but still
> isolated from each other so there is no bulk conductivity. But
> that is wild speculation at this point.

*****************************
Jennifer Boyer
Thistle Hill Pottery
Montpelier, VT
http://thistlehillpottery.com
*****************************

Bob Masta on sat 11 feb 06


Jennifer:

Hmm, I am intrigued by the "scum" aspect. I am wondering
if this could somehow be glomming on to water vapor
floating around in the microwave oven, assuming that you
do indeed have another mug of water in there for protection
of the oven. If the water adsorbed onto/into the scummy
areas, it might heat up in the usual way, unrelated to iron
in the glaze. On the other hand, I haven't heard any other
claims that matte surfaces heat up more.

In trying to think of how to test this, I am having trouble
coming up with ideas that are both safe and that will
resolve the issue of possible surface water. One thought
is that if the scummy parts heat up well before the
container of liquid water gets hot enough to raise the
humidity too much, that would argue against this hypothesis.
Another, possibly more dangerous, idea would be to put in a cup
of cooking oil instead of water. But we are always warned
to be careful of oils in the microwave because they can
apparently heat up very hot, very fast. So if you do try
this, try with caution!

Thanks for the recipes. I will be making test bars today,
but it might be a couple of weeks before they are bisqued, glazed,
fired, and tested in the microwave. And of course, they
won't be your exact glazes anyway, since I will modify
them for cone 6.

Ain't science fun!!!?

--------------------------
Jennifer Boyer wrote:
>
> So here's the scoop on my hot-in-the- microwave pots:
> I thought it was the Anderson Ranch that was getting hot but
> I realized it's the Bailey's Red. I have a mug on which the
> Bailey's has a sort of scum on the surface that looks muddy
> brown(the mug is mainly a nice Kaki red) and the scummy
> areas are drier than the rest of the pot, which is quite
> glossy. There's quite a lot of scum on the handle, which
> gets hotter than the rest of the mug. So, thinking about
> Bob's discussion of how microwaves work(below) I'm wondering
> if the scum has a high concentration of iron crystals that
> have migrated to the surface of the mug and that the iron
> concentration is what's getting hot rather than the glaze
> itself....just a theory.
>
> When I fired some more Anderson Ranch mugs they didn't get
> hot, so only the Bailey's mug did this. Maybe it was a
> fluke! Interesting that there is a higher % of iron in the
> Anderson Ranch Red which didn't get hot.
>
> I may do some tests on tiles and microwave them next to a
> mug of water. I let you know if anything interesting shows
> up.... Jennifer
>
> Bailey's Red:
> Custer Feldspar 47
> Flint 13
> EPK 13
> Talc 10.5
> Bone Ash 14
> Lithium Carbonate 2.5
> RIO 8
>
> Anderson Ranch Red
> Custer Feldspar 45
> Silica 20
> Whiting 7
> EPK 8
> Talc 8
> Bone Ash 12
> Red Iron Oxide 13.5
>
-----------------------------


Bob Masta

potsATdaqartaDOTcom

Steve Slatin on tue 14 feb 06


Jennifer --

I also pursued the iron-heating question with a slightly
different test. I took 4 mugs, a pitcher of water fresh
from the tap, and a microwave oven and did the following --

Ran a glass of water up to boiling, to make sure the
oven itself was heated up (this took 1 minute, 55
seconds), and then took the three mugs,
put a measured cup of water in each, and heated in
the microwave (on the same setting) until the water
audibly was boiling.

Results were as follows --

Mug 1 -- made from Seattle Pottery Supply 'Goldstone'
clay (high in iron, with flecks, presumably manganese)
glazed with a nephelene syenite glaze with 1%Fe2O3
per GlazeMaster. (Also about 10x as much tin oxide
by weight.) Result -- 2 min 26 sec to boil, handle slightly
warm, body above the water line warm, below water line
very hot.

Mug 2 -- made from Tacoma Clay Art Center 'Bennett'
clay (moderate to low iron content; no flecks) glazed with
essentially Lili K's variation on Tony Yeh's brown with
orange fleck glaze -- per Glaze master, 12.3% Fe2O3.
Result -- 2 minutes 12 seconds to boil, handle slightly
warm, body above water line maybe warmer than with
Mug 1, body below water line very hot.

Mug 3 -- 'Goldstone' clay, the same nephelene syenite
glaze as Mug 1 but without the iron oxide and with
a small amount of copper -- 1 minute 51 seconds to
boil. Handle somewhat warm, body slightly warm
above water line, very hot below.

Mug 4 -- Seattle Pottery Supply 'Sea Mix 5' clay
(very low iron) with an Alberta-slip based floating
blue glaze, less than 1/2% FeO3. Result -- 1 minute
57 seconds to boil. Handle slightly warm when I
picked it up, body slightly warm above water line,
very hot below, but I didn't dump this one right
away and when I went to dump it a few minutes
later the handle was rather hot.

Observation -- There is more of a gap between results
than one might expect if the body and glaze made
no difference (that is, if the microwaves acted only
on the water in the vessel and not on the vessel).
On the other hand, the anticipated relationship between
iron oxide in glazes and microwave issues did not
appear.

I'm going to again change my mind, Jennifer, and
say that maybe iron in a glaze isn't too significant,
at least up into the low teens in concentration. I'll
withhold an opinion on the influence of clay bodies,
though, and if I remain interested will test more,
later.

Best wishes -- Steve Slatin


Jennifer Boyer wrote:
Hi Steve,
Here's a pic of the red and a similar tan mug. It's the top picture

Steve Slatin --

In watermelon sugar the deeds were done and done again
as my life is done in watermelon sugar.

---------------------------------
Yahoo! Mail
Use Photomail to share photos without annoying attachments.

Bob Masta on wed 15 feb 06


Steve:

Thanks for reporting on your tests. One possible
confounding factor is that you put water in the
vessels you were testing. The problem is that
the different bodies may have different thermal
conductivities. So, even if they are totally
unaffected by microwaves themselves, they
will conduct heat away from the water in them,
and at differing rates. Then the temperature of
the body when the water finally reaches boiling
will depend upon how much heat it has absorbed,
versus how much it has lost by radiation, conduction,
or convection to the rest of the oven. Too many
variables!

On the other hand, if the vessel is empty (and
another vessel holds the water) then any heating
in the test vessel indicates some interaction directly
with the microwaves.

Best regards,


Bob Masta

potsATdaqartaDOTcom

Jennifer Boyer on wed 15 feb 06


So I did a test of mugs using your method below, with a mug of water,
but no water in the mugs that I wanted to test.
I actually put 4 mugs in the microwave with one water filled mug. The
Bailey's Red mug still got hotter than the others.
Go figure....I'm glad the Anderson Ranch doesn't have the same problem.
Jennifer
On Feb 15, 2006, at 9:11 AM, Bob Masta wrote:

> Steve:
>
> Thanks for reporting on your tests. One possible
> confounding factor is that you put water in the
> vessels you were testing. The problem is that
> the different bodies may have different thermal
> conductivities. So, even if they are totally
> unaffected by microwaves themselves, they
> will conduct heat away from the water in them,
> and at differing rates. Then the temperature of
> the body when the water finally reaches boiling
> will depend upon how much heat it has absorbed,
> versus how much it has lost by radiation, conduction,
> or convection to the rest of the oven. Too many
> variables!
>
> On the other hand, if the vessel is empty (and
> another vessel holds the water) then any heating
> in the test vessel indicates some interaction directly
> with the microwaves.
>
> Best regards,
>
>
> Bob Masta
>
> cobson who may be reached at melpots@pclink.com.

*****************************
Jennifer Boyer
Thistle Hill Pottery
Montpelier, VT
http://thistlehillpottery.com
*****************************

Steve Slatin on wed 15 feb 06


Bob --

You're quite right that it's a confounding factor.
I had already tested each of these mugs in
the traditional fashion -- empty, in a microwave
with a bowl of water, heating the water and then
checking to make sure that the mug was still
cool. (This is just a test for interaction between
the empty vessel and the microwave emissions.)
All passed.

What I had never done before, even in such a
casual way, was to check to see if the mugs
were interfering in some way with the microwaves.
(I had always presumed good microwave transmission.)
As we've discussed here before, smooth, thick
pieces of metal reflect microwaves, and if you
put something in a cup of smooth, thick metal
and cover it with a smooth, thick metal lid you
won't have any substantial heating at all of
liquid within (and if you do this with nothing
inside the microwave oven to absorb the
energy, you'll possibly damage the tube).

This was really just a test to see what happens
with the thought that if a high-iron glaze is a
problem it should be immediately evident, and
probably a low-iron glaze should also differ a
bit. The lack of correlation points to (but does
not completely confirm) a finding that an iron-rich
glaze is fairly innocuous.

What you're really absolutely spot on with here
is that there are too many variables. Different
clays, different glazes, differently sized/shaped
vessels, different wall thickness, etc. etc.

This was just a first step, using what was
available in my kitchen while my bisque firing
was cooling down in the studio -- a quick and
dirty to start off with. If materials are at hand
and I have some time free, I'll try to do a better
controlled test.

Best wishes -- Steve Slatin



Bob Masta wrote:
Steve:

Thanks for reporting on your tests. One possible
confounding factor is that you put water in the
vessels you were testing. The problem is that
the different bodies may have different thermal
conductivities. So, even if they are totally
unaffected by microwaves themselves, they
will conduct heat away from the water in them,
and at differing rates. Then the temperature of
the body when the water finally reaches boiling
will depend upon how much heat it has absorbed,
versus how much it has lost by radiation, conduction,
or convection to the rest of the oven. Too many
variables!

On the other hand, if the vessel is empty (and
another vessel holds the water) then any heating
in the test vessel indicates some interaction directly
with the microwaves.

Best regards,


Bob Masta

Steve Slatin --

In watermelon sugar the deeds were done and done again
as my life is done in watermelon sugar.

---------------------------------
Brings words and photos together (easily) with
PhotoMail - it's free and works with Yahoo! Mail.

Steve Slatin on wed 15 feb 06


Jennifer --

A different test, but a good one. As the mugs rotate they are exposed to
slightly differing intensities of microwaves, so the overall average if they
are in a circle around the water filled mug should be more or less equal.
(This kind of testing can't be done too well in a microwave without a
carousel, as different areas within the over will be 'hotter' than others.)

Clearly there's an issue with that one Bailey's Red mug. Whether it
relates to glaze or clay body or how well vitrified the clay is or whatever
I can't possibly say, but I'm approaching a moderately high degree of
confidence that it's not specifically iron in the glaze.

Like Bob says, too many variables ....

Best wishes -- Steve Slatin


Jennifer Boyer wrote:
So I did a test of mugs using your method below, with a mug of water,
but no water in the mugs that I wanted to test.
I actually put 4 mugs in the microwave with one water filled mug. The
Bailey's Red mug still got hotter than the others.
Go figure....I'm glad the Anderson Ranch doesn't have the same problem.
Jennifer

Steve Slatin --

In watermelon sugar the deeds were done and done again
as my life is done in watermelon sugar.

---------------------------------
Yahoo! Autos. Looking for a sweet ride? Get pricing, reviews, & more on new and used cars.

Bob Masta on thu 16 feb 06


Jennifer Boyer wrote:
> So I did a test of mugs using your method below, with a mug
> of water, but no water in the mugs that I wanted to test. I
> actually put 4 mugs in the microwave with one water filled
> mug. The Bailey's Red mug still got hotter than the others.
> Go figure....I'm glad the Anderson Ranch doesn't have the
> same problem.

Jennifer, can you give a rough idea of just how much hotter
you are talking here, like "Slightly warm" versus "YOW!"?
Also, when you say that the Bailey's Red was hotter than
Anderson Ranch, how hot was Anderson Ranch? In this
test, assuming dry mugs, the "null hypothesis" would
be no heating at all. If the Anderson had even moderate
heating then it would still indicate something is going
on in that glaze that I'd like to understand. Or, perhaps
the claybody itself could be getting warm, the Anderson
has no effect on its own, and the Bailey's adds to the
problem. Gee, I love a mystery! Where's my Deerstalker
and Meerschaum? -;)

Best regards,





Bob Masta

potsATdaqartaDOTcom

Jon Pacini on thu 16 feb 06


Greetings All ---Hi Steve---

I wondered when this thread would come around to how the clays react in the
microwave. A few years ago when Ron and I were sparing about earthenwares in
the mirco wave, I tested a whole bunch of various bodies in the mirco wave
oven here at work. what I found was that the more dense bodies got hotter
than the more porous ones. these were unglazed test bars that we do our QA
tests with.

Somewhere in the archives is the post with the results of my tests-- I did
about 20 clays of various composition red, white and buff at 06 5 and 10. I
wish I had the results handy but I can't seem to find it this morning.

If anybody feels ambitious --- try searching the archive for the post--
Best regards,
Jon Pacini
Clay Manager
Laguna Clay Co

Taylor, in Rockport TX on thu 16 feb 06


Hey Jon:

Not too difficult to find. Here is the link (might break):

A2=ind0402D&L=CLAYART&P=R33833&D=0&O=D>

Taylor, in Rockport TX


On Thu, 16 Feb 2006 10:08:18 -0800, Jon Pacini
wrote:

>Greetings All ---Hi Steve---
>
>I wondered when this thread would come around to how the clays react in the
>microwave. A few years ago when Ron and I were sparing about earthenwares
in
>the mirco wave, I tested a whole bunch of various bodies in the mirco wave
>oven here at work. what I found was that the more dense bodies got hotter
>than the more porous ones. these were unglazed test bars that we do our QA
>tests with.
>
>Somewhere in the archives is the post with the results of my tests-- I did
>about 20 clays of various composition red, white and buff at 06 5 and 10.

Carole Fox on fri 17 feb 06


On Thu, 16 Feb 2006 18:14:20 -0500, Taylor, in Rockport TX
wrote:

>Hey Jon:
>
>Not too difficult to find. Here is the link (might break):
>
>>A2=ind0402D&L=CLAYART&P=R33833&D=0&O=D>

I couldn't get Taylor's link to work - it seems to have some unneeded
characters at the end - but this one worked for me:

http://lsv.ceramics.org/scripts/wa.exe?A2=ind0402D&L=CLAYART&P=R33833

Jon, I am just wondering, since you did these tests, why doesn't Laguna
include data on suitability for microwave in the usage PDF file that one
can download from their web site? Seems like it would be helpful.

Thanks,
Carole Fox
Dayton, OH

Jennifer Boyer on fri 17 feb 06


It's, how should I put it....? YOWEEEEE. You know, potters who have
been at it as long as I have always laugh when the waiter at a
restaurant says "this plate is hot", right?

Well I just microwaved a Bailey's vase from the studio that had never
been near water(eliminating a lack of virtification/water absorption
theory). I put it in there with a mug of water and microwaved it for
quite a bit. I, "potter of no nerves left in fingers" had to remove
it with an oven mit...

And this vase had fired hotter than the infamous mug I've been
testing. That mug has some mat areas on it and I wondered if
something was going on with iron concentrations on the surface mat
areas. Maybe the mat areas were absorbing water in the microwave?
But this vase is all gloss, no mat at all. Just HOT.

The Anderson Ranch pot gets SLIGHTLY warmer than some of my other
misc mugs with lower % iron with several different glazes and saw no
real difference , but it's nothing compared to the Bailey's. I
compared a batch of my friends' mugs to a batch of my own made from
the same clay I've been talking about- no difference in heating of
the low iron ones. I feel confidant considering the Anderson Ranch
glaze microwave safe.

Yeah, this IS a mystery, no?

Jennifer
On Feb 16, 2006, at 11:24 AM, Bob Masta wrote:

> Jennifer Boyer wrote:
>> So I did a test of mugs using your method below, with a mug
>> of water, but no water in the mugs that I wanted to test. I
>> actually put 4 mugs in the microwave with one water filled
>> mug. The Bailey's Red mug still got hotter than the others.
>> Go figure....I'm glad the Anderson Ranch doesn't have the
>> same problem.
>
> Jennifer, can you give a rough idea of just how much hotter
> you are talking here, like "Slightly warm" versus "YOW!"?
> Also, when you say that the Bailey's Red was hotter than
> Anderson Ranch, how hot was Anderson Ranch? In this
> test, assuming dry mugs, the "null hypothesis" would
> be no heating at all. If the Anderson had even moderate
> heating then it would still indicate something is going
> on in that glaze that I'd like to understand. Or, perhaps
> the claybody itself could be getting warm, the Anderson
> has no effect on its own, and the Bailey's adds to the
> problem. Gee, I love a mystery! Where's my Deerstalker
> and Meerschaum? -;)
>
> Best regards,
>
>
>
>
>
> Bob Masta
>
> potsATdaqartaDOTcom
>
> ______________________________________________________________________
> ________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
> melpots@pclink.com.

*****************************
Jennifer Boyer
Thistle Hill Pottery
Montpelier, VT
http://thistlehillpottery.com
*****************************

Bob Masta on fri 17 feb 06


Jon, thank you for those test results!
As the character on Laugh-In used to say:
"Verrrrry Interrrrethting!". About the only
thing that makes sense to me is the
raku with reduced copper. The metallic
copper would be a (poor) conductor, so would
get hot.

Incidentally, I have been reading up on
microwave browning technology since
Janet's informative post a couple days ago.
(Thanks, Janet!) The reason the browning
dishes use a coating of tin oxide is that it
is a poor conductor. In the trade, these
things are called "susceptors" and include
silicon carbide and other semiconductors.

Yes, these are "semiconductors", even
though they are not useful for electronics
(so far), because they are somewhere
between an insulator and a conductor...
basically, a poor conductor, one with
fairly high resistance. It turns out that
this it just what you need for microwave
heating. The microwaves try to cause
a current to circulate in a conductor.

A perfect conductor would have a high
current induced, but no heating because
it has no resistance. (Heating power
is resistance times the square of current).

A perfect insulator with infinite resistance
would have no current at all, so no heating there.

But somehere in the baby-bear intermediate
range the resistance is low enough to
get a healthy current, but high enough to
give a lot of heat.

But it's still not clear what is going on in the
bodies. All I have been able to find so far
is that "some materials absorb microwaves"
without any real physics-type explanation.

It sure seems unlikely that there is any global
circulating current since they seem like insulators.
But maybe local grains have tiny circulating
currents, maybe in iron inclusions or something.
Or maybe there are polar molecules that have
enough mobility to vibrate in their matrix, giving
the same effect as normal water does (as opposed
to semicondutor heating).

Anyway, maybe the focus of the mystery should move from
glazes to bodies!

Best regards,

Bob Masta

potsATdaqartaDOTcom

Steve Slatin on fri 17 feb 06


Bob --

2 things --

1 -- I'm with you on the glaze-to-bodies issue.
I was cleaning up in the studio, at the 'where's
my hammer'* level, and I studied the relative
thickness of glaze and bodies on several different
samples of body and glaze. After firing, my
glaze thickness was rarely more than 1 mm and
often only a half mm or less. The body thickness,
though, was often around 6 mm. With many times
more volume of material there's a good chance
that the body is more active than the glaze.

2 -- With regard to microwaves, (a) in addition
to insulating and conducting, there's also
reflecting, and (b) When energy is absorbed
prior to either insulating or conducting, it
still goes somewhere. Insulation heats up
or radiates on a different frequency or
whatever, and so on. When I read more on
physics I could comment more sensibly, but
my weak recollection now is just that absorbtion
of RF current like microwaves usually leads to
an atom being kicked up to a higher energy
level -- and when it goes down it usually gives
off heat.

The 'trick' to microwaves is, of course, that they're
'tuned' to radiate on the native frequency of the
OH bond in water, so if it hits water it will turn
directly to molcular (heat) energy. Jennifer's
found something weird abut a single glaze, and
while an iron-link doesn't seem to be controlling,
there's something about that glaze that is.


Best perplexed wishes -- Steve S






*Which reminds me of a joke the punchline to
which is "Oh ... I thought you said 'thumbtacks' "
but it's too vulgar for this venue, so I won't repeat
it.

Bob Masta wrote:
Jon, thank you for those test results!
As the character on Laugh-In used to say:
"Verrrrry Interrrrethting!". About the only
thing that makes sense to me is the
raku with reduced copper. The metallic
copper would be a (poor) conductor, so would
get hot.

Incidentally, I have been reading up on
microwave browning technology since
Janet's informative post a couple days ago.
(Thanks, Janet!) The reason the browning
dishes use a coating of tin oxide is that it
is a poor conductor. In the trade, these
things are called "susceptors" and include
silicon carbide and other semiconductors.

Yes, these are "semiconductors", even
though they are not useful for electronics
(so far), because they are somewhere
between an insulator and a conductor...
basically, a poor conductor, one with
fairly high resistance. It turns out that
this it just what you need for microwave
heating. The microwaves try to cause
a current to circulate in a conductor.

A perfect conductor would have a high
current induced, but no heating because
it has no resistance. (Heating power
is resistance times the square of current).

A perfect insulator with infinite resistance
would have no current at all, so no heating there.

But somehere in the baby-bear intermediate
range the resistance is low enough to
get a healthy current, but high enough to
give a lot of heat.

But it's still not clear what is going on in the
bodies. All I have been able to find so far
is that "some materials absorb microwaves"
without any real physics-type explanation.

It sure seems unlikely that there is any global
circulating current since they seem like insulators.
But maybe local grains have tiny circulating
currents, maybe in iron inclusions or something.
Or maybe there are polar molecules that have
enough mobility to vibrate in their matrix, giving
the same effect as normal water does (as opposed
to semicondutor heating).

Anyway, maybe the focus of the mystery should move from
glazes to bodies!

Steve Slatin --

In watermelon sugar the deeds were done and done again
as my life is done in watermelon sugar.

---------------------------------

What are the most popular cars? Find out at Yahoo! Autos

Ron Roy on sat 18 feb 06


Hi Jennifer,

The vase does not have to have been near water - if the clay is not
vitrified enough it will absorb water from the air - enough over time to be
as saturated as the air.

I'm suggesting that the clay may not be vitrified enough and that may be
the cause of the heating in the microwave.

RR


>Well I just microwaved a Bailey's vase from the studio that had never
>been near water(eliminating a lack of virtification/water absorption
>theory). I put it in there with a mug of water and microwaved it for
>quite a bit. I, "potter of no nerves left in fingers" had to remove
>it with an oven mit...
>
>And this vase had fired hotter than the infamous mug I've been
>testing. That mug has some mat areas on it and I wondered if
>something was going on with iron concentrations on the surface mat
>areas. Maybe the mat areas were absorbing water in the microwave?
>But this vase is all gloss, no mat at all. Just HOT.
>
>The Anderson Ranch pot gets SLIGHTLY warmer than some of my other
>misc mugs with lower % iron with several different glazes and saw no
>real difference , but it's nothing compared to the Bailey's. I
>compared a batch of my friends' mugs to a batch of my own made from
>the same clay I've been talking about- no difference in heating of
>the low iron ones. I feel confidant considering the Anderson Ranch
>glaze microwave safe.
>
>Yeah, this IS a mystery, no?
>
>Jennifer
>On Feb 16, 2006, at 11:24 AM, Bob Masta wrote:
>
>> Jennifer Boyer wrote:
>>> So I did a test of mugs using your method below, with a mug
>>> of water, but no water in the mugs that I wanted to test. I
>>> actually put 4 mugs in the microwave with one water filled
>>> mug. The Bailey's Red mug still got hotter than the others.
>>> Go figure....I'm glad the Anderson Ranch doesn't have the
>>> same problem.
>>
>> Jennifer, can you give a rough idea of just how much hotter
>> you are talking here, like "Slightly warm" versus "YOW!"?
>> Also, when you say that the Bailey's Red was hotter than
>> Anderson Ranch, how hot was Anderson Ranch? In this
>> test, assuming dry mugs, the "null hypothesis" would
>> be no heating at all. If the Anderson had even moderate
>> heating then it would still indicate something is going
>> on in that glaze that I'd like to understand. Or, perhaps
>> the claybody itself could be getting warm, the Anderson
>> has no effect on its own, and the Bailey's adds to the
>> problem. Gee, I love a mystery! Where's my Deerstalker
>> and Meerschaum? -;)
>>
>> Best regards,
>>
>>
>>
>>
>>
>> Bob Masta
>>
>> potsATdaqartaDOTcom
>>
>> ______________________________________________________________________
>> ________
>> Send postings to clayart@lsv.ceramics.org
>>
>> You may look at the archives for the list or change your subscription
>> settings from http://www.ceramics.org/clayart/
>>
>> Moderator of the list is Mel Jacobson who may be reached at
>> melpots@pclink.com.
>
>*****************************
>Jennifer Boyer
>Thistle Hill Pottery
>Montpelier, VT
>http://thistlehillpottery.com
>*****************************
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.

Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513

Jennifer Boyer on sat 18 feb 06


But why would I never have seen this heating in any of the pots I've
used that are made out of this clay? I reheat food regularly in the
microwave using my seconds bowls. I've used the clay for at least 3
years and have just microwaved 6 of the pots I have in the house with
no unusual heating. So I'm saying that 9 pots were microwaved,
including various no-iron glazes, and the Bailey's and Anderson
Ranch Reds. Only the Bailey's got hot and it happened with 2 Bailey's
pots. And one of the Bailey's pots was overfired compared to the
other, with the glaze very glossy, so I can be sure it was not
underfired and undervitrified since the hot part of my kiln goes to
cone 11.

Believe me I'll have some tests in the next firing including the
weighing/boiling/weighing test for vitrification on this clay.
Jennifer
On Feb 18, 2006, at 12:37 AM, Ron Roy wrote:

> Hi Jennifer,
>
> The vase does not have to have been near water - if the clay is not
> vitrified enough it will absorb water from the air - enough over
> time to be
> as saturated as the air.
>
> I'm suggesting that the clay may not be vitrified enough and that
> may be
> the cause of the heating in the microwave.
>
> RR
>
>
>> Well I just microwaved a Bailey's vase from the studio that had never
>> been near water(eliminating a lack of virtification/water absorption
>> theory). I put it in there with a mug of water and microwaved it for
>> quite a bit. I, "potter of no nerves left in fingers" had to remove
>> it with an oven mit...
>>
> Jacobson who may be reached at melpots@pclink.com.

*****************************
Jennifer Boyer
Thistle Hill Pottery
Montpelier, VT
http://thistlehillpottery.com
*****************************

Daniel Semler on sat 18 feb 06


Hi Jennifer, All,

I've been wondering about this absorption versus glaze composition thing as
these messages have been going past. You could test the pieces for absorption.
While this does not conclusively prove of course that absorption is the
culprit
it would provide a indication. If I was doing this (and I've been
thinking about
it) I'd weigh each of the mugs/vases as they are, dry them in the oven
carefully
(don't go nuts with the temp, and start 'em cold), weigh them again,
and then do
a "standard" boil and weigh absorption test.

That would at least demonstrate if there is, in this case, a correlation
between heating observed and the water content.

One other comment I'd like to make. Jon Pacini's tests were very interetsting
in respect of the more porous bodies. I found the result interesting at the
time as it seemed to be that a porous body was acting like an IFB - this would
make sense, in respect of the current proposition, if the body was dry. Thus a
test suggests itself. It would be interesting to take body samples of
differing
absorptions and get them wet (as part of the absorption test for example) and
measure the degree of heating in an effort to quantify the impact of
absorption.

Just thoughts

Thanx
D

Ron Roy on sat 18 feb 06


Hi Jennifer,

I was just saying - clay does not have to be near water to get some - thats all.

Some clay makers don't continually test their clay bodies and raw materials
do drift - so it can happen that a clay can vary between batches. Some
companies don't continually adjust their clay bodies to make sure they are
not under or over vitrified.

I'm not saying this has happened - I'm saying we have to keep in mind all
the variations if we are to solve the problems.

I think you are doing something important here - trying to figure this out
on clayart - and we are all learning something important - I do hope you
get to the bottom of it.

I will help if I can - RR

>But why would I never have seen this heating in any of the pots I've
>used that are made out of this clay? I reheat food regularly in the
>microwave using my seconds bowls. I've used the clay for at least 3
>years and have just microwaved 6 of the pots I have in the house with
>no unusual heating. So I'm saying that 9 pots were microwaved,
>including various no-iron glazes, and the Bailey's and Anderson
>Ranch Reds. Only the Bailey's got hot and it happened with 2 Bailey's
>pots. And one of the Bailey's pots was overfired compared to the
>other, with the glaze very glossy, so I can be sure it was not
>underfired and undervitrified since the hot part of my kiln goes to
>cone 11.
>
>Believe me I'll have some tests in the next firing including the
>weighing/boiling/weighing test for vitrification on this clay.
>Jennifer
>On Feb 18, 2006, at 12:37 AM, Ron Roy wrote:
>
>> Hi Jennifer,
>>
>> The vase does not have to have been near water - if the clay is not
>> vitrified enough it will absorb water from the air - enough over
>> time to be
>> as saturated as the air.
>>
>> I'm suggesting that the clay may not be vitrified enough and that
>> may be
>> the cause of the heating in the microwave.
>>
>> RR
>>
>>
>>> Well I just microwaved a Bailey's vase from the studio that had never
>>> been near water(eliminating a lack of virtification/water absorption
>>> theory). I put it in there with a mug of water and microwaved it for
>>> quite a bit. I, "potter of no nerves left in fingers" had to remove
>>> it with an oven mit...
>>>
>> Jacobson who may be reached at melpots@pclink.com.
>
>*****************************
>Jennifer Boyer
>Thistle Hill Pottery
>Montpelier, VT
>http://thistlehillpottery.com
>*****************************
>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.

Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513

The Chapel of Art on sun 19 feb 06


Hi Bob! You are welcome... Just glad someone
actually reads my mail these days. Feeling so
isolated now that I am totally devoid of clay and
am even deprived of the odd second-hand clay fix
when talking to potters delivering work! Alisa in
Denmark was the last potter I actually talked to
face-to-face and that was way back last August!

But seriously... Your addition on
"semiconductors" etc. has answered a couple of
the questions which popped into my head as I was
reading about microwaves... I must admit that
microwave ovens have been around so long that I
have forgotten all the basics, even if I ever
took the finer points on board in the first
place!! I am no techie and it is really similar
to the "doings" in cars... However interesting
they may be, you do not actually need to know how
the they work to be able to drive! Same goes for
a lot of new-fangled technology, including that
used to communicate here in Clay Town and as you
have so kindly helped me sort out in the past.
LOL!

I sincerely wish we had some engineers attached
to a research department in either a physics lab
or a military research unit actively partaking in
Clay Art!! They would be able to answer so many
questions that we can only give educated guesses
on. Like microwave behaviour and the actual
physical as well as mechanical effects they have
on other materials. Until there are some
definitive answers to some of our questions,
"mere potters" can really only ever guess what is
happening, but never really know for sure. Don't
you agree?

Cheers

Janet

*** IN REPLY TO THE FOLLOWING MAIL:
>Incidentally, I have been reading up on
>microwave browning technology since
>Janet's informative post a couple days ago.
>(Thanks, Janet!) The reason the browning
>dishes use a coating of tin oxide is that it
>is a poor conductor. In the trade, these
>things are called "susceptors" and include
>silicon carbide and other semiconductors.
>
>Yes, these are "semiconductors", even
>though they are not useful for electronics
>(so far), because they are somewhere
>between an insulator and a conductor...
>basically, a poor conductor, one with
>fairly high resistance. It turns out that
>this it just what you need for microwave
>heating. The microwaves try to cause
>a current to circulate in a conductor.
>
>A perfect conductor would have a high
>current induced, but no heating because
>it has no resistance. (Heating power
>is resistance times the square of current).
>
>A perfect insulator with infinite resistance
>would have no current at all, so no heating
there.
>
>But somehere in the baby-bear intermediate
>range the resistance is low enough to
>get a healthy current, but high enough to
>give a lot of heat.
>
>But it's still not clear what is going on in the
>bodies. All I have been able to find so far
>is that "some materials absorb microwaves"
>without any real physics-type explanation.

*** PREVIOUS MAIL ENDS HERE ***
THE CHAPEL OF ART - or - CAPEL CELFYDDYD
8 Marine Crescent : Criccieth : GB-Wales LL52 0EA

Plan visiting The International Potters Path?
Contact: Janet Kaiser
Tel: ++44 (01766) 523122
http://www.the-coa.org.uk



--
No virus found in this outgoing message.
Checked by AVG Free Edition.
Version: 7.1.375 / Virus Database: 267.15.11/264 - Release Date: 17/02/2006