Brant Palley NMCLAY.com on tue 8 apr 08
Is there any record of high iron glazes getting blazing hot in a microwave
oven? We had a bowl here with an iron glaze that seemed to reflect the
microwaves back into the food, but, the bowl got really hot also.
Nobody Special on tue 8 apr 08
On Tue, 8 Apr 2008 11:48:32 -0500, Brant Palley NMCLAY.com
wrote:
>Is there any record of high iron glazes getting blazing hot in a microwave
>oven? We had a bowl here with an iron glaze that seemed to reflect the
>microwaves back into the food, but, the bowl got really hot also.
>
I have several mugs made by a friend, all the same size, shape, and clay
body, but with varying glazes. One of them is glazed with a kaki glaze
called Tom's Red, containing 11% RIO. This mug gets outrageously hot when
microwaved, to the point where it cannot be touched. None of the other mugs
do this.
...James
John Hesselberth on tue 8 apr 08
On Apr 8, 2008, at 12:48 PM, Brant Palley NMCLAY.com wrote:
> Is there any record of high iron glazes getting blazing hot in a
> microwave
> oven? We had a bowl here with an iron glaze that seemed to reflect the
> microwaves back into the food, but, the bowl got really hot also.
>
Hi Brant,
This subject comes up from time to time on Clayart with no clear
resolution. I have personally tested glazes with up to 15% RIO and
cannot get them to heat up in the microwave. I am very careful to use
clays that vitrify (<2% water absorption by standard testing) at my
firing temperature.
On the other hand I have had glazes on mugs made by others with
essentially no RIO that do get extremely hot. It is my opinion that
most of the pots that get burning hot do so because the clay is not
vitrified and have absorbed some water. Microwaves are designed to
heat water, not iron oxide. But I guess we can't rule out the
possibilities that some iron-containing glazes are a problem. I just
have never seen one IF the clay is vitrified--and I have tested a lot
of pots.
In MC6Gs we recommend a test for showing whether or not this is
likely to be a problem with your clay body, glaze, firing
combination. It is probably described in the archives several times
also. It is not hard to do.
Regards,
John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
Earth Nouveau on tue 8 apr 08
I have an uncle who worked on prototype microwaves in the 60's...he told me
back then that the bi polar molecule of H2O oscillates back and forth
generating heat.I often wondered if silica being bi polar could be made to
do the same.
David T.
On Tue, Apr 8, 2008 at 4:46 PM, John Hesselberth
wrote:
> On Apr 8, 2008, at 12:48 PM, Brant Palley NMCLAY.com wrote:
>
> Is there any record of high iron glazes getting blazing hot in a
> > microwave
> > oven? We had a bowl here with an iron glaze that seemed to reflect the
> > microwaves back into the food, but, the bowl got really hot also.
> >
> > Hi Brant,
>
> This subject comes up from time to time on Clayart with no clear
> resolution. I have personally tested glazes with up to 15% RIO and
> cannot get them to heat up in the microwave. I am very careful to use
> clays that vitrify (<2% water absorption by standard testing) at my
> firing temperature.
>
> On the other hand I have had glazes on mugs made by others with
> essentially no RIO that do get extremely hot. It is my opinion that
> most of the pots that get burning hot do so because the clay is not
> vitrified and have absorbed some water. Microwaves are designed to
> heat water, not iron oxide. But I guess we can't rule out the
> possibilities that some iron-containing glazes are a problem. I just
> have never seen one IF the clay is vitrified--and I have tested a lot
> of pots.
>
> In MC6Gs we recommend a test for showing whether or not this is
> likely to be a problem with your clay body, glaze, firing
> combination. It is probably described in the archives several times
> also. It is not hard to do.
>
> Regards,
>
> John
>
>
>
> John Hesselberth
> http://www.frogpondpottery.com
> http://www.masteringglazes.com
>
>
> ______________________________________________________________________________
> Clayart members may send postings to: clayart@lsv.ceramics.org
>
> You may look at the archives for the list, post messages, change your
> subscription settings or unsubscribe/leave the list here:
> http://www.acers.org/cic/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
> melpots2@visi.com
>
Nobody Special on tue 8 apr 08
On Tue, 8 Apr 2008 16:46:44 -0400, John Hesselberth
wrote:
---snip---
>
>This subject comes up from time to time on Clayart with no clear
>resolution. I have personally tested glazes with up to 15% RIO and
>cannot get them to heat up in the microwave. I am very careful to use
>clays that vitrify (<2% water absorption by standard testing) at my
>firing temperature.
>
John...
Here is, I believe, one of your elusive "microwave antenna" glazes:
Glaze name: Adrian's Tom's Red
Cone: 10
Color: Opaque Iron Red
Testing: Tested
Surface: gloss
Firing: oxidation
GlazeType:
Recipe: Percent
Kona F-4 Feldspar 48.71
Bone Ash 9.90
Whiting 7.20
Talc 6.29
EPK 6.29
Silica 21.60
Totals: 100 %
Also add:
RIO 10.79
I have this cone 10 glaze on a mug that gets too hot to touch in a
microwave. I have many other mugs made of the same clay body (Rovin RO-01
"You Betcha Mix") with other glazes, and none get hot at all. My own mugs
are made with this clay body and only fired to cone 6, and they do not get
hot. My mugs are glazed with your black liner glaze from your book
(absolutely wonderful glaze), which contains nearly as much iron as the kaki
above (9%).
Microwaves do heat food by exciting the water molecules. Metal gets hot in
a microwave from, I believe, simple electrical arcing. I microwaved a loaf
of bread once and forgot to remove the baggie tie. The paper on the baggie
tie actually ignited.
I believe, based purely on supposition, that the differing behavior has to
do with the fact that in the kaki glaze at least some of the iron is on or
near the surface thus allowing it to act as an antenna, while in your liner
glaze the iron is tied up somewhere in the matrix. Just a reasoned guess.
BTW, the glaze is a wonderfully bright persimmon color in oxidation but
puppy-poop brown in reduction in case anyone wants to use it.
...James
P.S. Loved your book.
Dave Finkelnburg on wed 9 apr 08
James,
I understand your hypothesis. You have a glaze.
The glaze, once fired, causes the ware to heat in a
microwave. To prove your hypothesis you have simply
to glaze more ware, fire it, and expose the resulting
ware to microwaves. This should be done BEFORE
washing the ware or otherwise permitting it to become
exposed to water which might be absorbed and cause
heating which could obscure the effect of your glaze.
Ideally you would also have other ware glazed
differently but fired the same to be tested in the
same manner. And further, you would have an objective
measure of the heat generated. I know that's hard to
do...most of us don't have an accurate optical
pyrometer just lying about by the microwave...
While I appreciate Bruce Girrell's positive
enthusiasm, I did not reject your hypothesis. I
simply stated the facts which I have observed. ALL
unacceptable microwave heating of pottery which I have
been able to document to date has been due to
absorption of moisture.
Further, while I haven't fired your recipe, I have
made quite a bit of ware glazed with an iron red glaze
using ~10% bone ash and ~10% red iron oxide. Such
recipes are variously called Persimmon, Kaki, and
Tomato Red. I have never observed any unusual heating
of ware I've made with my iron red glaze when the ware
was exposed to microwaves.
Now I am not saying in any way that your
hypothesis is false. I am simply writing what I have
observed.
Regards,
Dave Finkelnburg
Date: Wed, 9 Apr 2008 13:33:36 -0500
From: Nobody Special
I know that as a new guy on the list I should know my
place and just butt
out, but here goes anyway:
Just to cloud things, I attached my VOM
(volt-ohm-milliameter)to my
"antenna" mug with the high iron kaki glaze and to a
mug with Mr. Roy's high
iron black liner glaze. The needle did not move at
all with the black mug,
but did move, minutely but perceptibly, with the kaki
mug. Very high
electrical impedance to be sure, but less than
infinite, and less than the
black glaze.
"Science" is hypothesis + testing + repeatability =
theory. The posters you
cite have stated some scientific principals which no
one has discounted nor
challenged. They, however, are completely discounting
the hypothesis that a
kaki glaze (and possibly other high iron glazes)can be
sufficiently
conducive to cause the observed effects, and with all
due respect, none of
the posters has applied the scientific method to this
hypothesis, and
therefore have no basis for their conclusion other
than their own
conjecture. We use the term "science" much too
loosely.
In my own case, I have several mugs, all in the same
clay body, all fired in
the same kiln to the same temperature. The only mug
that gets hot is the
one with the kaki glaze.
__________________________________________________
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Jeoung-Ah Kim on wed 9 apr 08
Dear James,
Thank you for the interesting hypothesis/response.
How about other glazes contains Mn, Cr, Co, or Cu, etc?
Have you tested any?
Best regards,
Kim
Nobody Special wrote:...Metal gets hot in a microwave from, I believe, simple electrical arcing. ...
I believe, based purely on supposition, that the differing behavior has to
do with the fact that in the kaki glaze at least some of the iron is on or
near the surface thus allowing it to act as an antenna, while in your liner
glaze the iron is tied up somewhere in the matrix. Just a reasoned guess.
...James
---------------------------------
Yahoo! for Good helps you make a difference
John Hesselberth on wed 9 apr 08
Thanks for this information James. Without checking all my notebooks,
I don't believe I have tested glazes that are both high in phosphorus
AND red iron oxide in the microwave. Or, for that matter, ones with
phosphorus, but no or little RIO. I wonder if that could be the
missing link here. Nothing else seems to be unusual about this glaze
vs. ones I have tested. I will add that to my testing program. You
may have given us the info needed to sort this out.
Ah, so many glazes to test, so little time.
John
On Apr 8, 2008, at 10:17 PM, Nobody Special wrote:
> Here is, I believe, one of your elusive "microwave antenna" glazes:
>
> Glaze name: Adrian's Tom's Red
> Cone: 10
> Color: Opaque Iron Red
> Testing: Tested
> Surface: gloss
> Firing: oxidation
> GlazeType:
>
> Recipe: Percent
> Kona F-4 Feldspar 48.71
> Bone Ash 9.90
> Whiting 7.20
> Talc 6.29
> EPK 6.29
> Silica 21.60
> Totals: 100 %
>
> Also add:
> RIO 10.79
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com
John Sankey on wed 9 apr 08
"I have an uncle who worked on prototype microwaves in the 60's...he told me
back then that the bi polar molecule of H2O oscillates back and forth
generating heat.I often wondered if silica being bi polar could be made to
do the same."
Household microwave ovens use a frequency of 2.45 GHz. At this
low a frequency, a molecule has to be unbalanced electrically, a
dipole, to be affected. Water is one of the very few tri-atomic
molecules that is. I'll skip the quantum mechanics, but the H-O-H
of water forms an angle of about 105 degrees. The -ve oxygen
sticks out one side and the +ve hydrogens the other.
Any varying magnetic field produces an emf in a conductor. So, if
you put a piece of aluminum foil in a microwave, it absorbs it so
well the voltage at the ends of the conductor are so high you get
sparks and heat so high it can set nearby things on fire. The
iron molecules in any well-fired glaze or fully-fired body are
isolated from each other so don't form a conductor.
If your pottery heats up in a microwave, it's almost certainly
got water in it.
John Sankey
--------
Include 'Byrd' in the subject line of your reply to get through my spam
filter.
I can only read text mail, no attachments.
Nobody Special on wed 9 apr 08
On Wed, 9 Apr 2008 08:09:21 +0100, Jeoung-Ah Kim wrote:
>Dear James,
>
>
> Thank you for the interesting hypothesis/response.
> How about other glazes contains Mn, Cr, Co, or Cu, etc?
> Have you tested any?
>
>
> Best regards,
> Kim
>
>
>Nobody Special wrote:...Metal gets hot in a
microwave from, I believe, simple electrical arcing. ...
>
>I believe, based purely on supposition, that the differing behavior has to
>do with the fact that in the kaki glaze at least some of the iron is on or
>near the surface thus allowing it to act as an antenna, while in your liner
>glaze the iron is tied up somewhere in the matrix. Just a reasoned guess.
>...James
>
>
Kim...
I have not done any such testing, but it would be an interesting project.
Perhaps replacing the iron in the "Tom's Red" base (to limit the variables)
with the above mentioned metal oxides, and also doing a line blend with
varying amounts of iron in that same base.
Of course, 11% iron is fairly high, and having anywhere near that amount of
any of those other metals would likely render the glaze poisonous. It would
have to be strictly for experimental (or sculptural) purposes.
Be well.
...James
Nobody Special on wed 9 apr 08
On Wed, 9 Apr 2008 07:48:01 -0400, John Hesselberth
wrote:
>Thanks for this information James. Without checking all my notebooks,
>I don't believe I have tested glazes that are both high in phosphorus
>AND red iron oxide in the microwave. Or, for that matter, ones with
>phosphorus, but no or little RIO. I wonder if that could be the
>missing link here. Nothing else seems to be unusual about this glaze
>vs. ones I have tested. I will add that to my testing program. You
>may have given us the info needed to sort this out.
>
>Ah, so many glazes to test, so little time.
>
>John
>
>
John...
In the interest of science, I shall subject my "antenna mug" to an
absorption test, though for reasons stated earlier I believe it is
vitrified. I will try to find time to do this over the next few days, and
will post the result. I feel relatively certain (famous last words) that
precipitation of metallic iron is the key.
All the best.
...James
Bruce Girrell on wed 9 apr 08
Dave Finkelnburg wrote:
I tested a number of problematic mugs ... and I found all the problems had =
to do with porous clay. The clay absorbed water. The water got hot. The =
iron in the glaze just didn't make a noticeable difference.
John Sankey wrote:
...a molecule has to be unbalanced electrically ... to be affected. Water i=
s one of the very few tri-atomic molecules that is.
Any varying magnetic field produces an emf in a conductor. ... The iron mol=
ecules in any well-fired glaze or fully-fired body are isolated from each o=
ther so don't form a conductor.
If your pottery heats up in a microwave, it's almost certainly got water in=
it.
Ah, some real science. Thank you guys.
A very important scientific principle is "Correlation does not imply causat=
ion." In other words, the fact that you observe things together (in this ca=
se, a cup with an iron bearing glaze getting hot in a microwave oven) does =
not mean that one causes the other. It is extremely easy to fall into the c=
orrelation trap. I've done it many times myself and I should know better.
We need a "potters' legends" site (sort of like snopes.com for urban legend=
s) where we can archive posts like Dave and John's to debunk pervasive pott=
ery misunderstandings.
Good work guys!
Bruce Girrell
Steve Slatin on wed 9 apr 08
Kim --
I'm following this as a spectator, but I think the
reason why people want to think that iron is
different is that we saturate our glazes with it --
10 percent, 15 percent, even 20 percent of a
glaze by weight.
At 4 percent, most cobalt glazes will run right
off the piece and onto the kiln shelf. Copper
likewise. As for Chrome and Manganese, I
only use under 1 percent of the former, and
don't use the latter.
As for metal arcing, it's certainly possible,
but I've never seen it with a cup in a microwave.
And while my highest iron glaze is about 12
percent, not any higher, It's never gotten hot
in the microwave.
Best wishes -- Steve Slatin
Jeoung-Ah Kim wrote:
Dear James,
Thank you for the interesting hypothesis/response.
How about other glazes contains Mn, Cr, Co, or Cu, etc?
Have you tested any?
Best regards,
Kim
Nobody Special wrote:...Metal gets hot in a microwave from, I believe, simple electrical arcing. ...
__________________________________________________
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Tired of spam? Yahoo! Mail has the best spam protection around
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Nobody Special on wed 9 apr 08
On Wed, 9 Apr 2008 10:41:34 -0400, Bruce Girrell
wrote:
>
>
>Ah, some real science. Thank you guys.
>
debunk pervasive pottery misunderstandings.
>
>Good work guys!
>
I know that as a new guy on the list I should know my place and just butt
out, but here goes anyway:
Just to cloud things, I attached my VOM (volt-ohm-milliameter)to my
"antenna" mug with the high iron kaki glaze and to a mug with Mr. Roy's high
iron black liner glaze. The needle did not move at all with the black mug,
but did move, minutely but perceptibly, with the kaki mug. Very high
electrical impedance to be sure, but less than infinite, and less than the
black glaze.
"Science" is hypothesis + testing + repeatability = theory. The posters you
cite have stated some scientific principals which no one has discounted nor
challenged. They, however, are completely discounting the hypothesis that a
kaki glaze (and possibly other high iron glazes)can be sufficiently
conducive to cause the observed effects, and with all due respect, none of
the posters has applied the scientific method to this hypothesis, and
therefore have no basis for their conclusion other than their own
conjecture. We use the term "science" much too loosely.
In my own case, I have several mugs, all in the same clay body, all fired in
the same kiln to the same temperature. The only mug that gets hot is the
one with the kaki glaze.
For what it's worth, and respectfully submitted.
...James
Nobody Special on thu 10 apr 08
On Wed, 9 Apr 2008 22:14:26 -0700, Dave Finkelnburg
wrote:
>James,
> I understand your hypothesis. You have a glaze.
>The glaze, once fired, causes the ware to heat in a
>microwave. To prove your hypothesis you have simply
>to glaze more ware, fire it, and expose the resulting
>ware to microwaves.
---snip---
Dave...
I appreciate your response, and am quite familiar with this glaze type, the
scientific method, and principals surrounding the construction of a valid
experiment.
Firstly, I cannot quite figure out how I became the poster child for this
thread. As I do not make much functional ware, and no longer fire to high
temperatures (except when wood-firing), I do not even have a dog in this fight.
Having said that, here is a bit of background, much of which was mentioned
in my previous posts: The kiln in question is at a local college, so has
fired probably 100 different glazes on a dozen different clay bodies, all to
cone 10. While much of the ware is "art", there is a fair proportion of
functional work. None of the functional work with other glazes heats up
regardless of clay body, and the potter who made the mug in question said
that ALL of his work glazed with that particular glaze heated up, and NONE
of his other work did. Something is clearly going on. This gentleman
probably makes close to 1000 pieces during a semester.
My beef isn't with any of the posters' opinions, it is with dogmatically
calling an opinion a fact then using it as a club, and with the invoking of
"science" to frame as theory, or worse yet as fact, what is at best
well-reasoned conjecture . I wonder how many of those who dismiss the
microwave hypothesis out of hand even in the face of slightly more than
anecdotal evidence and limited rudimentary testing readily accept the global
warming hypothesis as a theory or a fact even in the face of no hard
evidence, no testing, no repeatability, and not even a preponderance of the
anecdotal evidence. Consensus isn't science. Belief isn't science. Both
are merely opinion, and are often tyranny. (And no, I am not challenging
anyone's faith in global warming, nor in any other belief system.)
I think it would be perfectly fair to say that, in light of the testing that
has been performed by many of the posters, in the very great majority of
cases, and with a very high degree of certainty, the microwave heating of
wares is due solely to excessive absorption of moisture by the clay body,
but to summarily rule out any other well reasoned possibility is just as
dogmatic as to summarily accept the premise. Perhaps a standard response to
such questions in the future could be "In every case I have ever tested, the
problem turned out to be excessive absorption by the clay body, and had
nothing to do with the glaze. Test your ware following standard procedures,
and on the off chance you do NOT see absorption in excess of X%, please let
us know of your results. If, as is likely, you do find excess absorption,
we will try to help you reformulate your clay body."
I agree with a previous poster who stated that we need a website devoted to
debunking ceramics mythology, but one cannot use "an opinion I agree with"
as a counter to "an opinion I do not share". What we truly KNOW about
ceramics as a science is as a mote in the sea compared to what we merely
believe.
Respectfully,
...James
Nobody Special on thu 10 apr 08
On Wed, 9 Apr 2008 22:14:26 -0700, Dave Finkelnburg
wrote:
---snip---
To prove your hypothesis you have simply
>to glaze more ware, fire it, and expose the resulting
>ware to microwaves. This should be done BEFORE
>washing the ware or otherwise permitting it to become
>exposed to water which might be absorbed and cause
>heating which could obscure the effect of your glaze.
---snip---
Dave...
Experiment completed. I just nuked my "antenna" mug empty. After 5 seconds
it was warm, and after 10 seconds it was too hot to hold, but could still be
briefly touched. This mug has not been used in well over a year, so was
quite dry. I next nuked a control mug made of the same clay body and fired
to the same temperature and in the same kiln, but with a non-iron-bearing
glaze (a calcium matte). Even after 20 seconds the mug was not even
discernibly warm.
I don't know if you are following the other thread on this topic, but Mr.
Sankey has duplicated this effect with his own iron-bearing glaze test
tiles, curiously, one containing phosphorus as mine does (good observation,
John H.). We now have hypothesis, testing, and repeatability. Perhaps one
of the physicists on the list can elevate this to theory.
The microwave effect could still conceivably be attributed to unknown
factors rather than glaze condition, but I do not believe the effect can be
discounted henceforth.
This has been great fun, and an intellectual challenge. Thank you all. Mr.
Sankey, if you do figure out what is going on from a physics standpoint
please let me know, and credit me in your Nobel Prize acceptance speech!
All the best.
...James
Eva Gallagher on thu 10 apr 08
Hi - I've been following this thread with great interest. We happen to have
one of the world's microwave experts living here in our small town - (he
does analyses of materials (like ore samples) using microwaves so I assume
that he may have some insight into this. I will forward him the
correspondence and see if he can help.
Eva Gallagher
Deep River, Ontario
----- Original Message -----
From: "Nobody Special"
To:
Sent: Thursday, April 10, 2008 2:54 PM
Subject: Re: more iron results and MICROWAVES!
> On Wed, 9 Apr 2008 22:14:26 -0700, Dave Finkelnburg
>
> wrote:
>
> ---snip---
> To prove your hypothesis you have simply
>>to glaze more ware, fire it, and expose the resulting
>>ware to microwaves. This should be done BEFORE
>>washing the ware or otherwise permitting it to become
>>exposed to water which might be absorbed and cause
>>heating which could obscure the effect of your glaze.
> ---snip---
>
> Dave...
>
> Experiment completed. I just nuked my "antenna" mug empty. After 5
> seconds
> it was warm, and after 10 seconds it was too hot to hold, but could still
> be
> briefly touched. This mug has not been used in well over a year, so was
> quite dry. I next nuked a control mug made of the same clay body and
> fired
> to the same temperature and in the same kiln, but with a non-iron-bearing
> glaze (a calcium matte). Even after 20 seconds the mug was not even
> discernibly warm.
>
> I don't know if you are following the other thread on this topic, but Mr.
> Sankey has duplicated this effect with his own iron-bearing glaze test
> tiles, curiously, one containing phosphorus as mine does (good
> observation,
> John H.). We now have hypothesis, testing, and repeatability. Perhaps
> one
> of the physicists on the list can elevate this to theory.
>
> The microwave effect could still conceivably be attributed to unknown
> factors rather than glaze condition, but I do not believe the effect can
> be
> discounted henceforth.
>
> This has been great fun, and an intellectual challenge. Thank you all.
> Mr.
> Sankey, if you do figure out what is going on from a physics standpoint
> please let me know, and credit me in your Nobel Prize acceptance speech!
>
> All the best.
>
> ...James
>
> ______________________________________________________________________________
> Clayart members may send postings to: clayart@lsv.ceramics.org
>
> You may look at the archives for the list, post messages, change your
> subscription settings or unsubscribe/leave the list here:
> http://www.acers.org/cic/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
> melpots2@visi.com
>
>
Nobody Special on thu 10 apr 08
On Thu, 10 Apr 2008 18:45:33 -0400, Eva Gallagher wrote:
>Hi - I've been following this thread with great interest. We happen to have
>one of the world's microwave experts living here in our small town - (he
>does analyses of materials (like ore samples) using microwaves so I assume
>that he may have some insight into this. I will forward him the
>correspondence and see if he can help.
>Eva Gallagher
>Deep River, Ontario
>
Eva...
Yes, please do! Also the related thread started by Mel entitled, I believe,
Microwave Idea. Fred Paget's last post in that thread referencing
dissipation factors sounds interesting.
Best regards.
...James
Dave Finkelnburg on fri 11 apr 08
James,
Your enthusiasm is admirable. Your experimental
technique is not quite perfected, though.
Fired, glazed clay has the ability to absorb
moisture from the air or from wash water, etc through
the unglazed spots where the ware sat on the shelf.
It is very difficult to drive that moisture out, once
it has entered the ware. I am afraid I learned this,
like most of my pottery lessons, the hard way. :-(
I once refired a stoneware bowl that had sat inside
another bowl, outdoors beside the kiln, for a week in
rainy weather. The whole load was candled for 12
hours at about 180F, so the refired piece was very
dry. It had absorbed enough water through the foot,
though, that it literally exploded quite dramatically
all around the foot when the kiln got above the
boiling point of water. When I described what had
happened my mentor at the time simply asked, "Ah, you
refired a bowl that had been wet, didn't you?"
The fact that your cup was dry on the outside
doesn't assure you that the clay could not have
absorbed any moisture sometime in the past. And due
to the glaze, the moisture has a hard time getting out
again.
Also, unless test pieces are fired on the same
shelf, with a witness cone (or possibly better yet, an
unglazed test tile which can be tested for
vitrification), just because they are fired in the
same kiln does not guarantee they will be fired to the
same degree of vitrification. Especially in a group
studio it may be possible to have 2 or 3 cones
difference between different spots in the kiln.
So, I still think the best possible test would be
to make fresh pieces with different glazes, fire them
together so you know for certain the glazes are the
only difference, and nuke them BEFORE they get wet.
Good testing. Your information helps us all.
Dave Finkelnburg
Date: Thu, 10 Apr 2008 13:54:23 -0500
From: Nobody Special
Experiment completed. I just nuked my "antenna" mug
empty. After 5 seconds
it was warm, and after 10 seconds it was too hot to
hold, but could still be
briefly touched. This mug has not been used in well
over a year, so was
quite dry. I next nuked a control mug made of the
same clay body and fired
to the same temperature and in the same kiln, but with
a non-iron-bearing
glaze (a calcium matte). Even after 20 seconds the
mug was not even
discernibly warm.
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Jennifer Boyer on tue 15 apr 08
So as I've said before, I have a Kaki glaze that gets hot in the
microwave and with testing I DID rule out moisture as a cause. I
microwaved the tests right out of the glaze firing. I also had Ron
test my clay for absorption.
Here's the recipe
Anderson Ranch Red from John Britt's book
Cone 10
Custer Spar 45
Flint 20
Whiting 7
EPK 8
Talc 8
Bone Ash 12
RIO 13.5
How does this glaze relate to the issue of Phosphorus? The unity
formula in Glazemaster reads P2O3 .104
Jennifer
On Apr 10, 2008, at 2:54 PM, Nobody Special wrote:
> On Wed, 9 Apr 2008 22:14:26 -0700, Dave Finkelnburg
>
> wrote:
>
> ---snip---
> To prove your hypothesis you have simply
>> to glaze more ware, fire it, and expose the resulting
>> ware to microwaves. This should be done BEFORE
>> washing the ware or otherwise permitting it to become
>> exposed to water which might be absorbed and cause
>> heating which could obscure the effect of your glaze.
> ---snip---
>
***************************
Jennifer Boyer
Thistle Hill Pottery
Montpelier, VT
http://thistlehillpottery.com
http://jboyerdesign.com
http://artisanshand.com
***************************
Nobody Special on tue 15 apr 08
On Tue, 15 Apr 2008 16:14:56 -0400, Jennifer Boyer wrote:
>So as I've said before, I have a Kaki glaze that gets hot in the
>microwave and with testing I DID rule out moisture as a cause. I
>microwaved the tests right out of the glaze firing. I also had Ron
>test my clay for absorption.
>
>Here's the recipe
>Anderson Ranch Red from John Britt's book
Jennifer...
Your recipe is almost identical to the "Tom's Red" that I posted earlier
that also heats up, within one or two percent on every ingredient. The only
real difference is your potash spar to my soda spar.
...James
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