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more iron tests, including microwave - byrd

updated sat 19 apr 08

 

Neon-Cat on sun 13 apr 08


John wrote: "What we need is the conductivity of the compounds in our
glazes: oxides or more complex molecules in almost all cases. A search is in
progress - any help is appreciated."

John and all, to make this simple and comprehensible, let's take a different
approach to why some mugs with their decorative glazes feel hotter than
others after microwaving.

First, who cares how the microwaves heat? Let's just delight ourselves with
the knowledge that they do heat materials and substances.

Second, let's work with the Specific Heat Capacities (Cp or cp - in constant
pressure values) of our substances, a very normal and valid scientific
approach to this problem. Recall that the specific heat capacity of a
substance is the amount of heat needed to raise the temperature of one gram
of a substance by 1 degree C (1 gram of water requires 4.18 joules of heat
to raise its temperature by 1 degree C). Calorimeters are used to calculate
Cp for substances and tables and individual values are obtainable from
various reference sources. Below are some Specific Heat Capacity (Cp) values
for common ceramic materials in J/(gK). Here is a handy calculator that will
show values for microwave heating, etc. (select "heat change"). All we
really need to do is simply compare specific heat capacity values for the
elements and compounds in our various glazes to see why we are getting
various results out of the microwaves. The web site is somewhat useful if
you want to vary the microwave kilowatt input to arrive at different
possible heat changes.
See: http://www.allmeasures.com/Formulae/

My point is that it will simply require more energy (heat from whatever
source) to increase the outside surface temperature of a microwaved mug if
the glaze is one containing substances with high specific heat capacities.
For example, it will take 8 times the heat energy to get a medallion of
magnesium hotter than a same-sized medallion of lead. You don't need to know
how the microwaves are pinging around the glaze materials or whether or not
the materials are electronegative, conductive, or whatever. You just need
the value for the specific heat capacity of your glaze ingredient and the
amount (weight) in play. Glazes composed of a number of materials having low
specific heat capacities will heat up sooner than glazes containing
materials with high specific heat capacities. Improperly sealed mugs that
have picked up water will take longer to become hot to the touch than
properly sealed mugs.

The formula for heat change:
Q = mc?T
where Q is the heat in joules or calories
m is the mass in grams
c is the specific heat capacity
?T is the change in temperature

Specific Heat Capacities (in J/gC) for selected materials
(compiled from various sources)

Aluminum 0.897
Apatite (as Calcium Phosphate) 0.84
Barium 0.204
Barium oxide 1.05
Bone 0.44
Boron 1.3
Brick (common) 0.84
Calcium 0.647
Calcium carbide 1.00
Clay 0.92
Cobalt 0.421
Concrete 0.88
Copper (solid) 0.385
Glass (silica) 0.84
Glass (flint) 0.503
Glass (Pyrex) 0.753
Iron 0.450
Lead 0.127
Lithium 3.58
Magnesium 1.02
Magnesium carbonate 0.837
Magnesium hydroxide 1.31
Manganese 0.480
Nickel 0.433
Paper 1.4
Phosphorus 0.769
Potassium 0.757
Sand 0.835
Selenium 0.320
Silica (fused) 0.703
Silicone carbide 0.686
Silicon dioxide 0.749
Sodium carbonate 1.14
Strontium 0.300
Tin 0.228
Water (liquid @ 25 degrees C) 4.18
Zirconium 0.278
Zinc 0.387

See if Specific Heat Capacity is the ticket for you. I've got to get back to
a clay cactus...

Marian
neoncat@flash.net
neon-cat.com

jonathan byler on mon 14 apr 08


Wouldn't that imply that it was probably more a claybody issue than a
glaze issue? If it were merely specific heat capacity, then the
glaze itself would probably have a lot less to do with it than the
clay body, given the thickness of glaze application (on average) and
the thickness of the clay portion of the mug's wall. A more relevant
question is if the mugs are about the same thickness, and fired about
the same, and have absorbed the same amount of water. the common
glaze materials that have specific heat capacities at the lower range
of the spectrum are in such relatively small quantities in the glazes
as to not make much difference, percentage wise. most of the glaze
is made up of materials similar in specific heat capacity to the
claybody itself.

And then you can say, that the mug itself has a specific heat
capacity of about .75-.9 (according to the numbers provided). Why
then does the cup not usually heat up, while the water which has a
much higher specific heat capacity (4.18) does? I would argue because
the claybody and the glaze, normally are not affected by the ability
of the microwaves to excite the molecules that make up the clay and
glaze, whereas they are readily able to excite the molecules of
water, heating it up in the process.

it's late, maybe someone else can make this point a bit more clearly
in the morning?

-jon


jon byler
3-D Building Coordinator
Art Department
Auburn University, AL 36849

On Apr 13, 2008, at 4:01 PM, Neon-Cat wrote:

> John wrote: "What we need is the conductivity of the compounds in our
> glazes: oxides or more complex molecules in almost all cases. A
> search is in
> progress - any help is appreciated."
>
> John and all, to make this simple and comprehensible, let's take a
> different
> approach to why some mugs with their decorative glazes feel hotter
> than
> others after microwaving.
>
> First, who cares how the microwaves heat? Let's just delight
> ourselves with
> the knowledge that they do heat materials and substances.
>
> Second, let's work with the Specific Heat Capacities (Cp or cp - in
> constant
> pressure values) of our substances, a very normal and valid scientific
> approach to this problem. Recall that the specific heat capacity of a
> substance is the amount of heat needed to raise the temperature of
> one gram
> of a substance by 1 degree C (1 gram of water requires 4.18 joules
> of heat
> to raise its temperature by 1 degree C). Calorimeters are used to
> calculate
> Cp for substances and tables and individual values are obtainable from
> various reference sources. Below are some Specific Heat Capacity
> (Cp) values
> for common ceramic materials in J/(gK). Here is a handy calculator
> that will
> show values for microwave heating, etc. (select "heat change"). All we
> really need to do is simply compare specific heat capacity values
> for the
> elements and compounds in our various glazes to see why we are getting
> various results out of the microwaves. The web site is somewhat
> useful if
> you want to vary the microwave kilowatt input to arrive at different
> possible heat changes.
> See: http://www.allmeasures.com/Formulae/
>
> My point is that it will simply require more energy (heat from
> whatever
> source) to increase the outside surface temperature of a microwaved
> mug if
> the glaze is one containing substances with high specific heat
> capacities.
> For example, it will take 8 times the heat energy to get a
> medallion of
> magnesium hotter than a same-sized medallion of lead. You don't
> need to know
> how the microwaves are pinging around the glaze materials or
> whether or not
> the materials are electronegative, conductive, or whatever. You
> just need
> the value for the specific heat capacity of your glaze ingredient
> and the
> amount (weight) in play. Glazes composed of a number of materials
> having low
> specific heat capacities will heat up sooner than glazes containing
> materials with high specific heat capacities. Improperly sealed
> mugs that
> have picked up water will take longer to become hot to the touch than
> properly sealed mugs.
>
> The formula for heat change:
> Q = mc?T
> where Q is the heat in joules or calories
> m is the mass in grams
> c is the specific heat capacity
> ?T is the change in temperature
>
> Specific Heat Capacities (in J/gC) for selected materials
> (compiled from various sources)
>
> Aluminum 0.897
> Apatite (as Calcium Phosphate) 0.84
> Barium 0.204
> Barium oxide 1.05
> Bone 0.44
> Boron 1.3
> Brick (common) 0.84
> Calcium 0.647
> Calcium carbide 1.00
> Clay 0.92
> Cobalt 0.421
> Concrete 0.88
> Copper (solid) 0.385
> Glass (silica) 0.84
> Glass (flint) 0.503
> Glass (Pyrex) 0.753
> Iron 0.450
> Lead 0.127
> Lithium 3.58
> Magnesium 1.02
> Magnesium carbonate 0.837
> Magnesium hydroxide 1.31
> Manganese 0.480
> Nickel 0.433
> Paper 1.4
> Phosphorus 0.769
> Potassium 0.757
> Sand 0.835
> Selenium 0.320
> Silica (fused) 0.703
> Silicone carbide 0.686
> Silicon dioxide 0.749
> Sodium carbonate 1.14
> Strontium 0.300
> Tin 0.228
> Water (liquid @ 25 degrees C) 4.18
> Zirconium 0.278
> Zinc 0.387
>
> See if Specific Heat Capacity is the ticket for you. I've got to
> get back to
> a clay cactus...
>
> Marian
> neoncat@flash.net
> neon-cat.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 mon 14 apr 08


On Mon, 14 Apr 2008 03:17:43 -0500, jonathan byler wrote:
---snip---
>
>And then you can say, that the mug itself has a specific heat
>capacity of about .75-.9 (according to the numbers provided). Why
>then does the cup not usually heat up, while the water which has a
>much higher specific heat capacity (4.18) does? I would argue because
>the claybody and the glaze, normally are not affected by the ability
>of the microwaves to excite the molecules that make up the clay and
>glaze, whereas they are readily able to excite the molecules of
>water, heating it up in the process.
>
>it's late, maybe someone else can make this point a bit more clearly
>in the morning?
---snip---
>
Jon...

Your reasoning is essentially correct: The minor variations amongst the
glazes are relatively insignificant in terms of the total mass, or volume
of "stuff", of the target mug, so the specific heat idea could not explain
the radical variation in results. Your observation that the water gets MUCH
hotter than the mug further demonstrates that this ideas does not explain
the observations.

If you reread John Sankey's posts and my own, you will see that the clay
body per se has to be ruled out as the culprit. In both of our tests, all
test pieces were composed of the same clay body and subjected to the same
firing conditions. Some pieces got hot while others didn't, with the only
difference being glaze composition. Certain glazes are producing a metallic
state while others do not. The problem now is to determine how and why.

Be well.

...James

Nobody Special on mon 14 apr 08


On Sun, 13 Apr 2008 16:01:52 -0500, Neon-Cat wrote:

---snip---
>John and all, to make this simple and comprehensible, let's take a different
>approach to why some mugs with their decorative glazes feel hotter than
>others after microwaving.
>
>First, who cares how the microwaves heat?
---snip---

Marian...

Man, I swore I was going to stay out of this and let the scientists on the
list take over. With all due respect, and again acknowledging that I could
very well be wrong, I believe your approach to the problem is incorrect.

I believe you are looking at transfer of heat energy from a source to a
target. Your approach would be correct for, say, heating in a conventional
oven or a kiln, where the heating elements in the oven get hot, and this
heat is then conducted or convected to the target. Microwave ovens do not
transfer heat energy. They transmit very low frequency electromagnetic
radiation, far, far, far lower than mid-infrared radiation(heat), THis
radiation stimulates molecules in the target in one of two ways which John
Sankey has outlined, and the heat is actually produced by and within the
target itself. With dipole "heating", the microwaves cause the molecules to
vibrate rapidly, and this vibration produces the heat. In effect, the food
heats itself. With resistive "heating", the microwaves energize a metallic
target, causing the metal to actually become a heating element in the
conventional sense. This is why metal becomes so much hotter than food when
subjected to microwaves.

Under certain conditions, a metallic state is being produced in certain
iron-bearing glazes, causing resistive heating. The key seems to be to
figure out what conditions cause this metallic condition so that they may be
avoided in functional ware.

All the best.

...James

Nobody Special on tue 15 apr 08


On Tue, 15 Apr 2008 16:38:33 +0930, Ivor and Olive Lewis
wrote:

>Dear James,
>
><<...Certain glazes are producing a metallic state while others do not. The
problem now is to determine how and why...>>
>
>
>I suggest you have a look at the way Iron oxides work when reduced in the
region of 700 to 1000 deg C.
>

Ivor...

My mugs were fired in a gas kiln, in a (likely)neutral to lightly reducing
atmosphere (no oxy probe). I do not know how John Sankey or Jennifer
Boyer's tiles were fired.

I appreciate your insight, but it has been many moons since I studied
chemistry, so at this point the nuts and bolts research is quite beyond me.
I'm afraid I shall have to rely on you, John S, and the other scientists on
the list. I believe I know what is happening, but lack the ability to
determine the how or why.

Excelsior.

...James

Ivor and Olive Lewis on tue 15 apr 08


Dear James,

<<...Certain glazes are producing a metallic state while others do not. =
The problem now is to determine how and why...>>


I suggest you have a look at the way Iron oxides work when reduced in =
the region of 700 to 1000 deg C.

So far I do not recall any mention of the way the mugs were processed. =
If they are fired in reduction I might anticipate there could be =
elemental Iron distributed through the glaze.

Best regards,

Ivor Lewis.
Redhill,
South Australia.

Jennifer Boyer on wed 16 apr 08


My kiln fires to 2300 in a neutral atmosphere more or less. I try not
to go over .12 on the Oxyprobe.
Jennifer
On Apr 15, 2008, at 9:27 PM, Nobody Special wrote:
> On Tue, 15 Apr 2008 16:38:33 +0930, Ivor and Olive Lewis
> wrote:
>
>> Dear James,
>>
>> <<...Certain glazes are producing a metallic state while others do
>> not. The
> problem now is to determine how and why...>>
>>
>>
>> I suggest you have a look at the way Iron oxides work when reduced
>> in the
> region of 700 to 1000 deg C.
>>
>
> Ivor...
>
> My mugs were fired in a gas kiln, in a (likely)neutral to lightly
> reducing
> atmosphere (no oxy probe). I do not know how John Sankey or Jennifer
> Boyer's tiles were fired.
>
> I appreciate your insight, but it has been many moons since I studied
> chemistry, so at this point the nuts and bolts research is quite
> beyond me.
> I'm afraid I shall have to rely on you, John S, and the other
> scientists on
> the list. I believe I know what is happening, but lack the ability to
> determine the how or why.
>
> Excelsior.
>
> ...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

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

Ivor and Olive Lewis on thu 17 apr 08


Dear James,

Some of the things we observe are often difficult to explain. In some =
instances it is also difficult to describe what we are experiencing. If =
I can bring reasonable knowledge to the table and It helps others I am =
satisfied. Your candour is appreciated.

Best regards,

Ivor