Joyce Lee on tue 9 may 00
Eureka!!! I think I've got it!.....or at least some part of it. Now,
tell me in VERY SIMPLE TERMS if I'm right or not. Education is necessary
and important, but again it pales when compared with experience. How
many, many times have I read about "heat work" as
measured by cones?? FINALLY, I think I MAY have a tenuous hold on this
concept. So simple. So elusive. As happens too often, once again I fired
to at least ^12 or ^14 when aiming for ^10 ... ^11 was flat, very flat,
and beginning to crumple. Now, drat, how did that happen? I used an
electronic device that measures the temperature of the environment,
right? AND it was on target with the ^10 cone at (for me) 9 o'clock as
ascertained visually. So visually AND electronically I was at ^10. I
decided to soak for 30 minutes, which to me meant that I held the
temperature within five degrees of ^10. Well, yes, that did happen......
but the heat work continued to escalate.. the heat work INSIDE the pots
and thus affecting the cones, right? I've just reread Hamers' section
and it seems somewhat close to a correct judgment.... right? Is this
something like running your finger back and forth in front of the
peephole .... temp doesn't really change ... but with each movement, the
finger gets hotter? Or is that way off target? I think my grasp of "heat
work" is slipping again. Help. Wish I'd paid more attention in Physics
instead of just memorizing as opposed to learning.
Joyce
In the Mojave so glad the wind has lessened to about zip ... blew the
keys right out of my hand when I was opening the car trunk ... not to
mention carrying off all the little plastic peanuts nestled around a box
of pots... lovely whirly design.... supermarket people were thrilled....
smiling while gritting one's teeth signifies "thrilled," right? It's
good that there are laws in the desert against hitting old ladies ......
Cindy Strnad on wed 10 may 00
Hi, Joyce.
If I understand it right, heat work is like this. Say you're cooking a
turkey. Without running to check my cookbooks, I believe the recommended
temperature is 375 F. You want the inside of the bird at 375, not just the
atmosphere of the oven, but that's just part of the picture.
You can cause the desired alteration of protein structure at a lower
temperature--say 300 F. It just takes longer (more heat work). And if you
hold it at 375 F for too long, your bird will dry out and eventually scorch,
even though it never will get above the recommended temperature. Now you
can't carry this to extremes. If you set the oven for 150 F, all you'll do
is make turkey jerky. No matter how long you leave that turkey in there, it
won't actually alter the protein the way cooking it at a higher temperature
will do.
Translated, experiment with holding your kiln at ^9 instead of at ^10. You
should still get the interaction of glazes that soaking is famous for, and
by the end of your hold time, I'll bet you'll be at ^10 or above.
Cindy Strnad
earthenv@gwtc.net
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
Dave Finkelnburg on wed 10 may 00
Oh Joyce, I just love your sense of humor! :-) Really big smile on my
face right now reading your description of what the wind does!
Heatwork:
in scientific terms it's proportional to heat multiplied by time. As
you found, at a high temperature, in a short amount of time you get a lot of
heat work.
If you hold at cone 10, in about 40 minutes or so cone 11 should go
down.
Next time, when you get cone 9 down, cut the burners back so the
temperature is steady, neither rising nor falling, and do your 30 minute
soak. Cone 10 should go down about the end of the soak. You can watch it.
it should start bending during the soak, and be sinking slooowly during the
soak. If it doesn't, just up the gas a little at the end of the soak.
This way you can get in your soak without overfiring.
Sounds like you're having fun! Good potting!
Dave Finkelnburg
-----Original Message-----
From: Joyce Lee
To: CLAYART@LSV.UKY.EDU
Date: Tuesday, May 09, 2000 12:26 PM
Subject: Heat Work
>----------------------------Original message----------------------------
>Eureka!!! I think I've got it!.....or at least some part of it. Now,
>tell me in VERY SIMPLE TERMS if I'm right or not. Education is necessary
>and important, but again it pales when compared with experience. How
>many, many times have I read about "heat work" as
>measured by cones?? FINALLY, I think I MAY have a tenuous hold on this
>concept. So simple. So elusive. As happens too often, once again I fired
>to at least ^12 or ^14 when aiming for ^10 ... ^11 was flat, very flat,
>and beginning to crumple. Now, drat, how did that happen? I used an
>electronic device that measures the temperature of the environment,
>right? AND it was on target with the ^10 cone at (for me) 9 o'clock as
>ascertained visually. So visually AND electronically I was at ^10. I
>decided to soak for 30 minutes, which to me meant that I held the
>temperature within five degrees of ^10. Well, yes, that did happen......
>but the heat work continued to escalate.. the heat work INSIDE the pots
>and thus affecting the cones, right? I've just reread Hamers' section
>and it seems somewhat close to a correct judgment.... right? Is this
>something like running your finger back and forth in front of the
>peephole .... temp doesn't really change ... but with each movement, the
>finger gets hotter? Or is that way off target? I think my grasp of "heat
>work" is slipping again. Help. Wish I'd paid more attention in Physics
>instead of just memorizing as opposed to learning.
>
>Joyce
>In the Mojave so glad the wind has lessened to about zip ... blew the
>keys right out of my hand when I was opening the car trunk ... not to
>mention carrying off all the little plastic peanuts nestled around a box
>of pots... lovely whirly design.... supermarket people were thrilled....
>smiling while gritting one's teeth signifies "thrilled," right? It's
>good that there are laws in the desert against hitting old ladies ......
>
Ivor and Olive Lewis on thu 30 sep 04
As I keep saying "Heat Work" is a tautology, a statement which repeats =
the basic information. In scientific communities Heat is measured on the =
MKS and expressed as Joules. So is Work. So, is "Heat Work" measured in =
Joules Squared
There is an opinion that suggests this parameter, "Heat Work", has the =
value of the integration of the temperature time line and is represented =
by the area beneath the line on the graph. Careful analysis of this idea =
may reveal it to be erroneous thinking. A modern approach is to consider =
that the heat applied to a kiln serves only to bring the ingredients of =
clay and glaze above their reaction activation temperatures and that the =
chemical reactions that happen as our materials move towards maturity =
are Exothermic and once initiated continue unassisted. That is, the =
ingredients themselves begin to release and radiate energy.
An interesting side light to this argument is that the active elements =
inside a kiln, the Ware being fired and the Cones, should, as the cones =
begin to bend, have a greater black body radiation value (Temperature !) =
than their surroundings. This means they should glow more brightly than =
the walls or the furniture in the kiln. Proving this might be difficult =
because we really do not know anything about the chemical reactions in =
such complex systems.
But you may need to adjust your goggles or squint a little tighter just =
in case ! ! ):-^
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
=20
Url Krueger on thu 30 sep 04
On Thu, 2004-09-30 at 00:06, Ivor and Olive Lewis wrote:
> the chemical reactions that happen as our materials
> move towards maturity are Exothermic
>
Ivor,
Was it Boris Karloff who used to say "Eentra-vesting???".
This is contrary to my expectations. Can you provide
some examples of reactions that would be exothermic at
kiln temperatures?
Maybe Ron Roy could develop glazes based on these
reactions so we could have self-heating pots. ;-)
--
Earl K...
Bothell WA, USA
"You may be disappointed if you fail,
but you are doomed if you don't try."
Beverly Sills (1929 - )
Jim Murphy on fri 1 oct 04
Hi Ivor,
I'll share something very important [with respect to "Heat Work"] to think
about.
Prepare yourself for my next statement ... [are you ready ???] ... with the
right tools, "measuring 'Heat Work' is as EASY as measuring the mathematical
expression of 'time'". [You can quote me on that one.]
One needs only to understand what the fundamental mathematical expression of
time measurement really "is" and apply a similar mathematical concept to
measure - or "calculate" - Heat Work.
With "time" measurement [or "mathematical expression" thereof] for example,
there's SIMPLY a "reference standard" [atomic clock] and a means of
"indexing" other time measurements [from clocks, timers, etc.] to that
reference standard.
In ceramics, alot of the "dirty work" has already been done. "Reference
standards" already exist. For example, there's a "Cone 10 [atomic clock]",
"Cone 9 [atomic clock]", "Cone 8 [atomic clock]", etc., etc., etc.
[Notice how these are just "numbers" - but they mean alot in terms of heat &
time.]
So, to mathematically express "Heat Work", all we really have left to do is
a little mathematical wizardry and the "indexing" part.
How ? ... sample [collect] Temp/Time data points during firing, generate a
"curve" from these data points, calculate a "Heat Work Index" [HWI] number
[NO units, no "Joules", etc.] from this mathematically-expressed "curve" and
"index" - or associate - the HWI "number" to the appropriate reference
standard. [I'll keep my HWI mathematical formulas secret - for now.]
In time, one could discover how changes to a firing schedule [heatrise ramp,
hold-time, cooldown ramp, etc.] affect the generated "curve", which impacts
the calculated HWI number, which is indexed to a reference standard.
Ivor, you wrote, "A modern approach is to consider that the heat applied to
a kiln serves only to bring the ingredients of clay and glaze above their
reaction activation temperatures and that the chemical reactions that happen
as our materials move towards maturity are Exothermic and once initiated
continue unassisted. That is, the ingredients themselves begin to release
and radiate energy."
In my own personal research into measuring "Heat Work", I've learned to
separate the actual "physics" from the calculation. As with measuring
"time", I believe there is an atomic clock - somewhere - but I don't need to
concern myself with that clock, or the actual Cesium atoms [in that clock]
to be able to express "time". [Calculating Heat Work is calculating "a
number".]
And to think ... thousands of years ago some thought measuring "time" was
impossible or impracticable. "What a waste of time ?", they must have
declared.
Best wishes,
Jim Murphy
mailtoandrew@FSMAIL.NET on fri 1 oct 04
Hello Ivor,
The term heat work can be criticised as being tautologous, however could
not the use of =91heat=92 just be seen as qualifying =91work=92? Whilst caut=
ion
should be exercised with the term providing it is understood, or perhaps
more so its failings, I think it has some validity.
Certainly the properties of fired ceramic materials are dependent on the
combined effect of both time and temperature and this can be described, if
not defined, as heat work. Is it not therefore convenient to use it to
qualify whilst realising it can not quantify? However, as you state, it
would be erroneous to ascribe too great a significance to the area
underneath a simple time:temperature plot.
Do the reactions that occur during firing simply occur because a certain
activation energy has been reached, these then being exothermic? This
seems to suggest that the reactions will proceed to completion, or at
least equilibrium, irrespective of the subsequent kiln schedule? I would
have thought that the physical and chemical changes that occur across the
firing range are highly endothermic.
Your thoughts?
Regards,
Andrew
Ivor and Olive Lewis on sat 2 oct 04
Dear Andrew,
Thanks for your notes.
I like the idea of "working Heat" as a colloquial phrase. When we are
ramping up the kiln we are working with heat.
Relating to the whole of a firing event. As you know form your own
studies, there are many factors to consider in accounting for the
distribution and utilisation of heat when we process ceramic
articles.
I also believe there is a general agreement that in most instances
concerning the processing of Traditional Ceramics that processing is
curtailed before equilibrium is attained.
It seems clear to me that we explain phenomenon or changes that take
place when we fire ceramic raw materials with Folk Lore, in the
absence of experimental evidence. As I have said on many occasions, I
think the descriptions given by Kingery et al (Ch 10) are worth
digesting.
I believe the terms Endothermic and Exothermic relate only to Chemical
reactions (including changes to lattice structures). When physical
changes take place, that is phase changes, from solid to liquid and
liquid to gas or in the reverse direction that we use the term Latent
Heat to describe the energy which is being transferred.
Best regards,
Ivor
Ivor and Olive Lewis on sat 2 oct 04
Dear Earl ,
When someone sorts out the chemistry so that we know what the
reactions are I might attempt, providing the necessary thermodynamic
information is available, to distinguish between the endothermic and
exothermic reactions.
What you might consider doing is an analysis of Heat utilisation in a
typical glost firing.
Best regards,
Ivor
Ron Roy on sat 2 oct 04
I have listed the different events during firing and cooling in my workshop
book - Nils used them in his book.
There is an endothermic peak somewhere between 450 and 600C as the combined
water leaves clays.
There are exothermic peaks at 980C (during conversion from metakaolin to
spinel) and again at 1150C as mullite is formed.
RR
>This is contrary to my expectations. Can you provide
>some examples of reactions that would be exothermic at
>kiln temperatures?
>
>Maybe Ron Roy could develop glazes based on these
>reactions so we could have self-heating pots. ;-)
>
>--
>Earl K...
>Bothell WA, USA
>"You may be disappointed if you fail,
>but you are doomed if you don't try."
> Beverly Sills (1929 - )
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Ivor and Olive Lewis on sun 3 oct 04
Hello again Jim,
An interesting analogy. Am I right in saying that all we are dealing
with is the interval between events which gives two identifiers, the
'Marks" and the "Periods". Does Heat Energy have two identifiers?
Material things appear to collect or dissipate energy which manifests
itself changes in degrees of intensity. What are those Identifiers.
Remember, our recording instrument has no scale other than arbitrary
placement in a line of similar objects. It can respond only to an
increase in energy. It will not respond to dissipation of energy
You propose << So, to mathematically express "Heat Work", all we
really have left to do is a little mathematical wizardry and the
"indexing" part. How ? ... sample [collect] Temp/Time data points
during firing, generate a "curve" from these data points, calculate a
"Heat Work Index" [HWI] number>> Which puts you in the position of
defining Time and Temperature.
But you seem to be telling us that you have no need of a standard by
which marks, intervals or intensities are scored for the events you
are recording. Will your "Index of Heat Work" be represented by scalar
quantities? It would appear that Energy (either "Heat" or "Work") may
be a Vector, having both magnitude and direction .
Is it worth wasting the Energy ? Could bring on a bout of Topotaxis !
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Ivor and Olive Lewis on sun 3 oct 04
Dear Ron Roy,
Thanks for that reminder.
I do have Nils' book on the shelves and he does give that information,
pp 45 in The Art of Firing.
Any one wishing to research this topic should be looking for the term
"Topotaxis"
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Jim Murphy on sun 3 oct 04
Hi Ivor,
<events which gives two identifiers, the 'Marks" and the "Periods". Does Heat
Energy have two identifiers?>>
Many "events" occur during a firing cycle, however, we do not need to
concern ourselves - mathematically - with all of them. Also, IMHO "Heat
Work" and "Heat Energy" are different entities.
<itself changes in degrees of intensity. What are those Identifiers.
Remember, our recording instrument has no scale other than arbitrary
placement in a line of similar objects. It can respond only to an
increase in energy. It will not respond to dissipation of energy>>
With "Heat Work" measurement, we've really got to separate the "physics"
["dissipation of energy"] from the calculation [mathematics]. Separate the
glaze "result" you're looking at from what it took ["Heat Work" curve] to
get there [create that result]. "Heat Work" IS performed in some sections of
some cooldown ramps and understanding this is key to developing algorithms
and/or equations to mathematically express "Heat Work".
[Sidebar #892 - Inside a mechanical "watch", we "understand" there's kinetic
& potential energy changes happening, BUT, we separate out those "physics".
The work of internal gears & springs controls the movement of a needle
around a scalar dial. We see where the needle tip "is" and "calculate" [in
our mind] what that needle position really "means" with respect to "time".
We do something very similar when "reading" a Cone position.]
<really have left to do is a little mathematical wizardry and the
"indexing" part. How ? ... sample [collect] Temp/Time data points
during firing, generate a "curve" from these data points, calculate a
"Heat Work Index" [HWI] number>> Which puts you in the position of
defining Time and Temperature.>>
Scalar Temp/Time data points, being sampled, are produced in "real-time" -
inside the kiln - and are used to generate a temp/time "Curve". These
Temp/Time data points "shape" the curve which is used to calculate a
mathematical expression for a HWI number. [It's somewhat analogous to
gears/springs shaping the "cumulative sweep" of the watch needle allowing
reading of "time" passage].
Incidently, I do not mean to imply that the HWI number is any more valuable
than the generated curve itself. The generated curve provides a greater
range [time-wise] of temp/time info than a pyrometric cone [~2 hrs] and may
assist to more quickly adjust firing schedules and yet maintain the same HWI
number ["indexed" to a certain PCE].
<which marks, intervals or intensities are scored for the events you
are recording.>>
Apparently things aren't as they "seem".
Numbers are "tools" and we do need to be aware of which Temp & Time "units"
we're using in equations as well as be aware of sampling-rate, etc. Also,
"indexing" to a reference standard [PCE] is very important as I indicated.
<quantities? It would appear that Energy (either "Heat" or "Work") may
be a Vector, having both magnitude and direction.>>
Heat Work Index (HWI) number is definitely scalar Ivor.
For "Heat Work" measurement, here's "the meat" of our quandary. I ask
rhetorically: (a) When a Cone 10 firing is completed, and the pyrometric
Cone 10 [center of the cone pack] tip is level with the Cone's base, what
does the tip's position "mean" ?; and (b) With regard to PCE [pyrometric
cone equivalent] could "cone 10 tip level with base position" be represented
by a scalar quantity [like "10" or some other HWI-like number] ??
Somethin' to think about.
Best wishes,
Jim Murphy
Louis Katz on sun 3 oct 04
I am hoping that Ivor will explain equilibrium from his point of view.
Its an important concept but not a particularly difficult one. I will
explain it from mine.
Equilibirum is an important concept to relate to our lives, our working
rhythm, perhaps mental health, and fiscal state. It is different than
balance in ways that seem trivial but at times are not. I think it is a
concept that should be covered in Junior High Science class or earlier,
a very basic concept with broad application.
If you hold a glaze at a certain temperature it will continue to
dissolve the clay body it sits on until it cannot dissolve any more.
This may take quite some time. After a length of time even the coarse
silica in the glaze may all dissolve. After a time all of the silica in
the clay that is going to change to cristobalite changes. The glaze may
become homogenous. When equilibrium has been reached all of these
changes will be finished and the glaze and clay will stop changing in
any perceptable way. In the unsealed kilns we fire this may never take
place. Some "fluxes" will continue to evaporate albeit at slower and
slower rates as they dissappear.
Because we do not fire until we reach equilibrium phase diagrams that
record the state of a melt at equilibrium are of limited use in
prediciting the state of our glazes. Although they do provide some
information about possibilities and trends.
Louis
On Oct 3, 2004, at 5:57 AM, David Hewitt wrote:
>>
> I have been following this thread with interest and would appreciate
> knowing what you mean by 'equilibrium' in this context.
David Hewitt on sun 3 oct 04
In message , Ivor and Olive Lewis writes
>I also believe there is a general agreement that in most instances
>concerning the processing of Traditional Ceramics that processing is
>curtailed before equilibrium is attained.
>Ivor
I have been following this thread with interest and would appreciate
knowing what you mean by 'equilibrium' in this context. Do you mean when
all oxides have gone into the melt or is it something else, please?
David
--
David Hewitt
Web:- http://www.dhpot.demon.co.uk
mailtoandrew@FSMAIL.NET on mon 4 oct 04
Hello Jim,
Whilst the use of an Index side steps the problem of units that Ivor
initially noted Im still puzzled to the benefit of such a measurement.
Anyway does not HWI already exist? Pyrometric devices, be they cones, bars
or rings, give an indication of heat work and their values are an
arbitrary number: ergo a Heat Work Index?
As useful as these devices are they are crude measures of probably
activity within the ware. The same Pyrometric device value can be obtained
by firing to a low temperature for a long time as results from a high
temperature for a short time. Whilst for the same raw material composition
these two schedules may result in one characteristic, say degree of
vitrification, being the same but many others will not. Even relatively
small changes to the soak or maximum temperature will result in
differences to the microstructure of ceramic for the same Pyrometric
device value.
The area under a time:temperature plot is likely to be even less reliable;
Pyrometric devices are at least composed of ceramic materials and will
therefore share some behaviour with the ware.
The precise measurement of time and temperature may be possible but
applying these nice, simple numbers to the extremely complex systems of
whiteware reactions is, to understate greatly, a little difficult.
Regards,
Andrew
mailtoandrew@FSMAIL.NET on mon 4 oct 04
Hello Ivor,
Yes firing processes are curtailed before completion. To quote your fellow
antipodean J. Mellor, one of the greatest researchers in ceramics, who
wrote nearly 100 years ago:
...The reaction between the different constituents of the body, in
firing, is arrested before the system is in a state of equilibrium. The
chemistry of pottery is therefore largely a chemistry of incomplete
reactions, and many erroneous inferences have been made from the failure
to appreciate this fact ...
Your post picked up my comment about physical and chemical reactions being
exothermic by ... I believe the terms Endothermic and Exothermic relate
only to Chemical reactions (including changes to lattice structures)...
Now not wishing to start an argument but I would have thought the
instanaeous inversion of the various silica phases was a lattice change, a
physical reaction and exothermic.
Regards,
Andrew
Jim Murphy on mon 4 oct 04
Hi there Andrew,
<or rings, give an indication of heat work and their values are an arbitrary
number: ergo a Heat Work Index?>>
An HWI number AND generated "curve" are intended to be used together to
"extend" our understanding [via mathematical expression] of contributions
made by firing schedule heatrise, hold, and cooldown ramp segments to a
final pyrometric cone tip-bent-to-base position. Perhaps then, number
"values" become LESS arbitrary.
<temperature for a long time as results from a high temperature for a short
time. Whilst for the same raw material composition these two schedules may
result in one characteristic, say degree of vitrification, being the same
but many others will not.>>
I absolutely [110%] agree! The very same dichotomy, however, provides the
driving force to try a more mathematical approach at "modifying" firing
schedules to achieve desired results.
If one MUST fire their ware to Cone 10 for example, and the 1st trial fired-
results indicate a viscosity problem ONLY, then "mathematics" may help. The
"benefit" may be to use mathematically generated "curves" [modified firing
schedules] - of certain calculated HWI number - to limit the number of
additional fired test trials required [potential for huge $$$ savings in
labor, material, energy, etc.].
<reliable>>
You may have noticed I didn't go out of my way to disagree with Ivor's
assertion, "erroneous thinking" with regard to area. nudge>
<applying these nice, simple numbers to the extremely complex systems of
whiteware reactions is, to understate greatly, a little difficult.>>
Though the 4 strings of a violin vary in string diameter, the 4 strings are
each "normalized" [by design] to a same fixed length, allowing for beautiful
"controlled" music to be played. [Fibonacii may be smiling upon us.]
Best wishes,
Jim Murphy
mailtoandrew@FSMAIL.NET on tue 5 oct 04
Hello again Jim,
Im still puzzled ...
How does your approach, to quote ... An HWI AND generated curve ... ,
differ from the established use of Pyrometric device value and kiln
schedule (ramp, max. temp., soak.), the latter which is sometimes plotted?
Regards,
Andrew
Ivor and Olive Lewis on tue 5 oct 04
Dear Andrew,
Thanks for your note.
Yes, I did pick up on the idea that Phase Changes which involve
spatial redistribution of atoms incur an energy adjustment, though I
am not sure which direction of change will cause warming or cooling. I
do believe there are technical terms for the evolution or adsorption
of heat, something like Recalescence and Decalescence.
Phase Changes, Solid<>Liquid and Liquid<> Gas, as I understand things,
involve Latent Heat. The evolution or adsorption of heat is not
usually described by the terms of Exothermic or Endothermic.
The other term we might need to familiarise ourselves with in order to
understand how and why a kiln heats when we pump energy into it is
Specific Heat Capacity
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
----- Original Message -----
From:
To:
Sent: Monday, 4 October 2004 8:56
Subject: Re: Heat Work
> Hello Ivor,
>
> Yes firing processes are curtailed before completion. To quote your
fellow
> antipodean J. Mellor, one of the greatest researchers in ceramics,
who
> wrote nearly 100 years ago:
>
> ...The reaction between the different constituents of the body, in
> firing, is arrested before the system is in a state of equilibrium.
The
> chemistry of pottery is therefore largely a chemistry of incomplete
> reactions, and many erroneous inferences have been made from the
failure
> to appreciate this fact ...
>
>
> Your post picked up my comment about physical and chemical reactions
being
> exothermic by ... I believe the terms Endothermic and Exothermic
relate
> only to Chemical reactions (including changes to lattice
structures)...
>
> Now not wishing to start an argument but I would have thought the
> instanaeous inversion of the various silica phases was a lattice
change, a
> physical reaction and exothermic.
>
>
>
> Regards,
>
>
> Andrew
>
>
______________________________________________________________________
________
> Send postings to clayart@lsv.ceramics.org
>
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>
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melpots@pclink.com.
Ivor and Olive Lewis on tue 5 oct 04
Dear Louis Katz,
I hope you found my suggestions about "Equilibrium" helpful
In my terms, for the ceramic examples you give, those activities cease
when there is equilibrium. Conversely, if there is activity
equilibrium has not been achieved.
Phase equilibrium Diagrams show us what will happen when we do not
wish to achieve a glaze. They tell us what can crystallise from the
fully molten mixture should we choose to allow that to happen, Given
that anyone can draw up a phase diagram for half a dozen or more
oxides ! !
We should not forget that in any firing of pottery our intention is
that the clay fabric should always remain solid but the glaze fabric
should always, always become a molten solution. When we start to use
Porcelain, the line separating these contrasting states of matter
becomes blurred.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
----- Original Message -----
From: "Louis Katz"
To:
Sent: Monday, 4 October 2004 2:42
Subject: Re: Heat Work
> I am hoping that Ivor will explain equilibrium from his point of
view.
> Its an important concept but not a particularly difficult one. I
will
> explain it from mine.
> Equilibirum is an important concept to relate to our lives, our
working
> rhythm, perhaps mental health, and fiscal state. It is different
than
> balance in ways that seem trivial but at times are not. I think it
is a
> concept that should be covered in Junior High Science class or
earlier,
> a very basic concept with broad application.
>
> If you hold a glaze at a certain temperature it will continue to
> dissolve the clay body it sits on until it cannot dissolve any more.
> This may take quite some time. After a length of time even the
coarse
> silica in the glaze may all dissolve. After a time all of the silica
in
> the clay that is going to change to cristobalite changes. The glaze
may
> become homogenous. When equilibrium has been reached all of these
> changes will be finished and the glaze and clay will stop changing
in
> any perceptable way. In the unsealed kilns we fire this may never
take
> place. Some "fluxes" will continue to evaporate albeit at slower
and
> slower rates as they dissappear.
>
> Because we do not fire until we reach equilibrium phase diagrams
that
> record the state of a melt at equilibrium are of limited use in
> prediciting the state of our glazes. Although they do provide some
> information about possibilities and trends.
>
> Louis
>
> On Oct 3, 2004, at 5:57 AM, David Hewitt wrote:
>
> >>
> > I have been following this thread with interest and would
appreciate
> > knowing what you mean by 'equilibrium' in this context.
>
>
______________________________________________________________________
________
> 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.
Ivor and Olive Lewis on tue 5 oct 04
Dear Jim,
I am not prepared to spend time sorting through your melange of ideas
to arrive at some form of understanding. Either write a decent well
constructed essay to present your case or don't waste your energies.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Jim Murphy on tue 5 oct 04
Andrew wrote:
<differ from the established use of Pyrometric device value and kiln
schedule (ramp, max. temp., soak.), the latter which is sometimes plotted?>>
Hi Andrew,
1. A "real" "HWI number" [from actual firing] is calculated from "curve"
[generated from actual firing data]. "Indexing" ideally is made to
pyrometric device(s) used [in actual firing].
2. When changes to firing schedule are desired (e.g., heatrise, hold,
cooldown ramp changes), "virtual" [computer] modified curves [with same HWI
number] can quickly be generated and a "best" modified curve selected [to
achieve desired fired results].
3. Mathematical "normalization" is used [for ALL curves - "real" &
"virtual"] to properly index "Heat Work".
Like that guy once stated, "If you can't drag an atomic clock with you
everywhere, just create an index to it."
Best wishes,
Jim Murphy
Louis Katz on wed 6 oct 04
You can come up with any formula you want but it will not accurately
model all work done in the kiln. Glazes from different temperature
ranges and different compositions react differently. Even cones are
not particularly accurate. Glazes fired quickly to cone 04 and slowly
look different.
This does not mean that some formula for simulating cones, or some
other "standard" measure would not be useful, just limited.
Cones and pyrometers small mass also create measurement problems. They
heat up much more quickly than dense brick like pieces.
On Oct 5, 2004, at 12:12 PM, Jim Murphy wrote:
> Andrew wrote:
>
> <> ,
> differ from the established use of Pyrometric device value and kiln
> schedule (ramp, max. temp., soak.), the latter which is sometimes
> plotted?>>
>
> Hi Andrew,
>
> 1. A "real" "HWI number" [from actual firing] is calculated from
> "curve"
> [generated from actual firing data]. "Indexing" ideally is made to
> pyrometric device(s) used [in actual firing].
>
> 2. When changes to firing schedule are desired (e.g., heatrise, hold,
> cooldown ramp changes), "virtual" [computer] modified curves [with
> same HWI
> number] can quickly be generated and a "best" modified curve selected
> [to
> achieve desired fired results].
>
> 3. Mathematical "normalization" is used [for ALL curves - "real" &
> "virtual"] to properly index "Heat Work".
>
> Like that guy once stated, "If you can't drag an atomic clock with you
> everywhere, just create an index to it."
>
> Best wishes,
>
> Jim Murphy
>
> _______________________________________________________________________
> _______
> 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.
>
mailtoandrew@FSMAIL.NET on wed 6 oct 04
Hello Jim,
Thanks for the further details. However Im still at a loss as to what
youre trying to achieve; maybe its the limitations of the format of
ClayArt, maybe its your explanation or perhaps Im just being stupid.
Probably the latter!
Anyway best of luck with it, and Id be interested in the results if youre
successful.
Regards,
Andrew
Jim Murphy on wed 6 oct 04
Ohhhh-kay. Apparently, some of my previous reply text was clipped-out due to
formatting error. Are we all using Plain Text ?
Bottom line ... Heat Work is a relative entity - like time - which is
exactly why it may be measured ... relative to something else.
Best wishes,
Jim Murphy
Michael Wendt on sat 2 oct 10
Eleanor,
The concept of heat work is based on the fact that
crystalline materials
require a certain number of BTUs/ lb (or calories per gram)
to go from the solid to liquid state at the same
temperature. Ice to water is the classic example since it
requires 80 calories/gram to melt ice at 0 degrees Celsius
with no change in temperature.
Knowing that, you can fire your kiln to a slightly lower end
point if you increase the end hold time and if melting of
some of the materials has started, the longer hold will
supply enough calories to melt the glaze just as well as if
it had been fired to a higher temperature for a much shorter
time.
Incidentally, I have found the quality of finish to be much
better now that I have slowed the firings by two hours at
the end.
It's worth a try.
Regards,
Michael Wendt
Eleanor wrote:
Please, dear gurus, discourse on Heat Work. Not so much on
what it is
but more on how to apply it.
Specifically, when I suspected my 13 year-old kiln was
overfiring, I
contacted the the techs at Skutt, who asked a lot of
questions before
they were sure I was correct in my suspicions.
I fire ^6 using the ramp/hold suggested in M^6G with
modifications to
suit my kiln, which worked well until recently. I'm a
dabbler,
hobbyist, and only fire maybe an average of 2-3 times/month,
more
often recently now that I'm selling.
One tech told me to test the elements; I did and they
passed. The
other tech referred to heat work and the fact that my 13
year old kiln
is showing signs of old age---slowing down (!). He advised
me to lower
the top temperature and reduce hold times.
I understood his directions for changing the firing schedule
but when
he went into an explanation of Heat Work, he lost me.
I couldn't find more than a passing reference to Heat Work
in my home
library of clay books.
So I, and maybe others, would appreciate an explanation in
words of
one syllable (or fewer) on the subject of Heat Work and
especially on
how to apply its principles to my elderly kiln and to kilns
in general.
Eleanor Kohler
Centerport, NY
William & Susan Schran User on sat 2 oct 10
On 10/2/10 2:01 PM, "Eleanor" wrote:
> Please, dear gurus, discourse on Heat Work. Not so much on what it is
> but more on how to apply it.
> I fire ^6 using the ramp/hold suggested in M^6G with modifications to
> suit my kiln, which worked well until recently. I'm a dabbler,
> hobbyist, and only fire maybe an average of 2-3 times/month, more
> often recently now that I'm selling.
> One tech told me to test the elements; I did and they passed. The
> other tech referred to heat work and the fact that my 13 year old kiln
> is showing signs of old age---slowing down (!). He advised me to lower
> the top temperature and reduce hold times.
> I understood his directions for changing the firing schedule but when
> he went into an explanation of Heat Work, he lost me.
> I couldn't find more than a passing reference to Heat Work in my home
> library of clay books.
> So I, and maybe others, would appreciate an explanation in words of
> one syllable (or fewer) on the subject of Heat Work and especially on
> how to apply its principles to my elderly kiln and to kilns in general.
Put simply, heatwork is heat applied over time.
The longer you expose your pots to a heating ramp (especially in the final
90 minutes or couple hundred degrees) the lower the top temperature has to
be to get a full melt of the glaze and vitrification of the clay. The faste=
r
you fire, the higher the final temperature has to be to achieve the same
melting of the glaze.
For example, for cone 6 melted at the proper angle, firing at a rate of 27F
per hour in the last 90 minutes the temperature would be 2165F, at 108F per
hour it would be 2232F, at 270 per hour it would be 2269F.
So if your kiln is slowing down, especially in the later stages of the
firing, you don't have to fire as high to get the same heatwork as the kiln
firing faster and requiring a higher temperature to reach the firing cone.
And Eleanor, you ARE using witness cones aren't you?
Even if you don't look at them, witness cones measure heatwork and will be
the best way to see if you have achieved the necessary heatwork to properly
melt the glazes.
Bill
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
Eleanor on sat 2 oct 10
Please, dear gurus, discourse on Heat Work. Not so much on what it is
but more on how to apply it.
Specifically, when I suspected my 13 year-old kiln was overfiring, I
contacted the the techs at Skutt, who asked a lot of questions before
they were sure I was correct in my suspicions.
I fire ^6 using the ramp/hold suggested in M^6G with modifications to
suit my kiln, which worked well until recently. I'm a dabbler,
hobbyist, and only fire maybe an average of 2-3 times/month, more
often recently now that I'm selling.
One tech told me to test the elements; I did and they passed. The
other tech referred to heat work and the fact that my 13 year old kiln
is showing signs of old age---slowing down (!). He advised me to lower
the top temperature and reduce hold times.
I understood his directions for changing the firing schedule but when
he went into an explanation of Heat Work, he lost me.
I couldn't find more than a passing reference to Heat Work in my home
library of clay books.
So I, and maybe others, would appreciate an explanation in words of
one syllable (or fewer) on the subject of Heat Work and especially on
how to apply its principles to my elderly kiln and to kilns in general.
Eleanor Kohler
Centerport, NY
Talk about elderly---I'm approaching 80---now that's elderly!
".... philosophers lay down many precepts fair in argument but not
applicable in use"
--Sir Francis Bacon
Snail Scott on sun 3 oct 10
On Oct 2, 2010, at 1:01 PM, Eleanor wrote:
> Please...discourse on Heat Work...
If you are cooking a casserole, you may intend to
cook it to 350 F, but you know you can't keep it in
the oven just until the oven thermostat tells you it's
hit 350, because you know that more time is needed
for the flavors of the ingredients to blend and to be
sure that the heat has penetrated throughout.
Heat _acting over time_ gives us heat-work. which
may be defined not as temperature, but as the
EFFECTS of temperature over time.
Imagine that you have a fast-heating oven, and it
can reach 350F in just ten minutes. Is your casserole
cooked? Of course not! There has not been time for
the effects of heat to do their work. (Heat-work!)
But, what if you have a slow oven, and it takes an
hour and a half to creep up to 350F? By the time the
thermostat tells you it's at 350F. the whole casserole
might be adequately cooked, because even though
it only just hit 350F, it took its sweet time getting
through 275, 300, 325, etc, and although these aren't
technically the temperature you were shooting for,
they did most of the necessary heat-work because
they had long time to do it.
If you have an oven that runs hot, and you are actually
cooking that casserole at 375, you may get it cooked
in less time than you thought, because enough heat-
work has been done sooner. If your oven is decrepit
and will only reach 325, you can still cook your casserole
but it will take longer.
Can you cook your casserole at 500F and get it done in
half the time? Of course not. Can yo cook it for five hours
at 250F? No, not that either. Time can be substituted for
temperature, but only within a fairly narrow range. Also,
the time spent at lower temperatures is largely irrelevant,
It's how long it spends at temperatures hot enough to
melt your cheese (clay/glaze) that really matter: the last
few cones.
You can run quickly through a burning building, or dip you
hand for an instant in boiling water, and emerge unscathed:
not enough time for serious heat-work on you! Conversely,
people can die at 100F temperatures if trapped in their
apartment for days during a heat wave. Too much heat-
work, even though the temperature was much lower. (This
isn't really a proper use of the term heat-work, but you get
the idea, right?)
This is why every textbook on ceramics has a chart in
back listing temperature equivalents for cones, but there
is always a specification of _rate of heat increase_.
A kiln which is heating at 50F per hour in its upper
range will reach the necessary heat-work at a lower
temperature than one which rockets up at 150F per
hour. That one will have to go hotter to gain the same
level of heat-work for the clay inside.
This is why cones are our gold standard to tell what's
really and truly happening in the kiln. They are utterly
low-tech, but because they are made of actual ceramic
materials, they respond to heat just as our real work does,
and tell us how much heat-work has occurred, regardless
of the temperature.
An electronic kiln takes the temperature reading sent by
the thermocouple and combines it with the readings from
its internal clock, and uses a mathematical algorithm to
convert this into an approximation of heat-work. Usually
this works pretty well, but if the thermocouple is off (bad
data) or the software isn't using a good algorithm (bad
thinking), it may not tell you true. Cones always will, so
it's just cheap insurance to always include a witness cone
in each firing, especially if you suspect 'drift'.
Your kiln, as its elements have aged, may well be taking
longer to get up to that final temperature, and I would not
be surprised at all if more heat-work is occurring by the
time it gets there, resulting in slightly overfired work
With an electronic kiln, however, it's easy to fix. Just lie to
it! Figure out, based on your witness cones, how far off
the calculated heat-work actually is, and program in an
offset to compensate. Also note that more solidly packed
loads will tend to have less heat-work occur at the same
setting because even though the thermocouple is giving
the truth as it sees it near the kiln wall, there's a whole lot
of clay soaking up heat further in, and it needs more heat
than the computer realizes. Lightly loaded kilns are more
likely to correspond accurately to the electronic readout.
I compensate by adding a longer soak to a heavy load.
Manual kilns can also have their kiln-sitters fooled this
way by heavy loads, so a cone pack that's tucked back
into the kiln a bit further is a good check on it. Even if you
can't see it during firing, check afterward and use that
information to offset your next firing with a higher sitter
cone or a manual shutdown. At least a sitter cone, unlike
a thermocouple, really does read heat-work.
-Snail
(Postscript disclaimer: Snail can't cook worth beans, so
do not base any actual cooking on this discussion! All
cooking analogies are to be used for ceramic purposes
only. In other word: this post is NOT FOODSAFE! ;) )
-S.
steve graber on sun 3 oct 10
32F is=3DA0freezing but not necessarily ice.=3DA0 leave it in the fridge lo=
nger=3D
to work =3D0Aitself into ice.=3DA0 freezing "fast" or "slow" changes the s=
tyle=3D
of ice you get.=3DA0 =3D0A=3D0A=3D0Athe cone 6 side of the temp-work issue=
is simi=3D
lar.=3DA0 =3D0A=3DA0Steve Graber, Graber's Pottery, Inc=3D0AClaremont, Cali=
fornia U=3D
SA=3D0AThe Steve Tool - for awesome texture on pots! =3D0Awww.graberspotter=
y.co=3D
m steve@graberspottery.com =3D0A=3D0A=3D0AOn Laguna Clay's website=3D0Ahttp=
://www.l=3D
agunaclay.com/blogs/ =3D0A=3D0A=3D0A=3D0A=3D0A_____________________________=
___=3D0AFrom=3D
: William & Susan Schran User =3D0ATo: Clayart@LSV.CERAMIC=
S.=3D
ORG=3D0ASent: Sat, October 2, 2010 7:24:50 PM=3D0ASubject: Re: Heat Work=3D=
0A=3D0AO=3D
n 10/2/10 2:01 PM, "Eleanor" wrote:=3D0A=3D0A> =
Plea=3D
se, dear gurus, discourse on Heat Work. Not so much on what it is=3D0A> but=
m=3D
ore on how to apply it.=3D0A> I fire ^6 using the ramp/hold suggested in M^=
6G=3D
with modifications to=3D0A> suit my kiln, which worked well until recently=
. =3D
I'm a dabbler,=3D0A> hobbyist, and only fire maybe an average of 2-3 times/=
mo=3D
nth, more=3D0A> often recently now that I'm selling.=3D0A> One tech told me=
to =3D
test the elements; I did and they passed. The=3D0A> other tech referred to =
he=3D
at work and the fact that my 13 year old kiln=3D0A> is showing signs of old=
a=3D
ge---slowing down (!). He advised me to lower=3D0A> the top temperature and=
r=3D
educe hold times.=3D0A> I understood his directions for changing the firing=
s=3D
chedule but when=3D0A> he went into an explanation of Heat Work, he lost me=
.=3D
=3D0A> I couldn't find more than a passing reference to Heat Work in my hom=
e=3D
=3D0A> library of clay books.=3D0A> So I, and maybe others, would appreciat=
e an=3D
explanation in words of=3D0A> one syllable (or fewer) on the subject of He=
at=3D
Work and especially on=3D0A> how to apply its principles to my elderly kil=
n =3D
and to kilns in general.=3D0A=3D0A=3D0A
Luke Nealey on mon 4 oct 10
Eleanor:
>
Two Things (maybe three):
Some definitions: Energy is the capacity to do work. The base unit for Work=
=3D
,
in SI system, is the Newton meter, the measure of a force applied over a
distance. Although when we heat things up we are applying forces on the
molecular level it makes more sense for us to talk about heat energy and th=
=3D
e
unit for this is Joules, one Joule is one Nm. All sorts of reactions and
phase changes that are happening in our pots take energy (work) to happen. =
=3D
As
Michael mentioned, the melting of ice into liquid water requires about 6
Kilojoules per mole. So if you had a mole of ice in a pot at zero Celsius
and started heating it, it would go from being ice at zero to being water a=
=3D
t
zero, the temperature would not increase until all the ice had melted, all
of the heat energy(work) goes into breaking the bonds that hold the water
molecules together in the solid. This is what Michael was referring to whe=
=3D
n
he talked about melting the glazes (or vitrifying clay). This is why cones
are so great, they, like our pots, have to absorbed (have work done on them=
=3D
)
enough energy to have melted. The higher the temperature around our pot
the quicker this may happen, but in the end it just has to have enough
energy applied to it. Of course, there are minimum temperatures for some o=
=3D
f
these things to occur.
Snail=3D92s cooking analogies are the easiest way for me to explain some of
these processes. When our turkey is done the internal temperature is 170 F
or so, even though we cook it in a 325 oven or a 250 degree smoker or a pot
of boiling water (212 F). All these heat deliver systems deliver this heat
work/energy at different rates so the turkey takes different times to
doneness. But we measure the internal temp; that is what tells us that we
have denatured the proteins, boiled the water out etc that leaves us with
the mouth feel we desire (and killed microbes, etc.). The outside of the
turkey looks different because it was exposed to different temperatures
where other stuff happened ( think steak grilled on hot grill, charred
outside, raw inside). These examples bring in another phenomena: heat
transfer, which I will let the engineers on list tackle. But, since it
takes such a long time to fire a kiln, heat transfer is not as big of issue
since our pots are relatively thin especially compared with the mass of the
kilns.
Regards,
Luke Nealey, Rankin Co. MS
ivor and olive lewis on mon 4 oct 10
For raw materials; clay, silica, felspars, carbonates and other sundry
compounds to change into ceramic structures of body and glaze Heat has to
flow into them for a period of time. Heat Work is a colloquial term used by
potters to describe this function. Heat can only flow form a location of
High temperature to a location where the temperature is low.
A brief examination of scientific definitions of "Heat" and "Work" sugges=
t
"Heat Work" is a tautology since both are measured in Joules. Trust me on
this one...the definition of "Joule" is difficult to comprehend !
Perhaps those who teach Kiln Management might like to consider adopting,
with reservations, the term "Specific Latent Heat of Fusion" which relates
the quantity of Heat required to change a unit mass of substance from Solid
to Liquid at its melting point.
Why reservations? The range of melting points within the ingredients in a
recipe can vary from around seven hundred degrees for some frits or even
lower for some borates to over seventeen hundred degrees Celsius for Silica
and almost 2600 deg C for Calcium oxide.
Easier to recognise that Cones bending give the optimal conditions we choos=
e
to select for the product we choose to make.
Regards,
Ivor Lewis,
REDHILL,
South Australia
ivor and olive lewis on tue 5 oct 10
Dear Luke Nealey,
Would you confirm that your statement .....
.... "So if you had a mole of ice in a pot at zero Celsius and started
heating it, it would go from being ice at zero to being water at zero, the
temperature would not increase until all the ice had melted "...
....is true if and only if six joules of energy are applied to the ice. An=
y
less and there will be unmelted Ice in the system. Any more and the
temperature of the water will rise.
Thanks,
Ivor Lewis,
REDHILL,
South Australia
Luke Nealey on tue 5 oct 10
On Tue, Oct 5, 2010 at 1:33 AM, ivor and olive lewis
wrote:
> Dear Luke Nealey,
> Would you confirm that your statement .....
> .... "So if you had a mole of ice in a pot at zero Celsius and started
>
> heating it, it would go from being ice at zero to being water at zero, th=
e
> temperature would not increase until all the ice had melted "...
>
> ....is true if and only if six joules of energy are applied to the ice.
> Any
> less and there will be unmelted Ice in the system. Any more and the
> temperature of the water will rise.
>
>
>
> Thanks,
>
>
> Ivor Lewis,
> REDHILL,
> South Australia
>
Ivor,
I know it's not as consise as it could be. I was trying to explain
that melting takes energy that gets used to break the attraction between
the molecules in the solid, and so, even if heat is being applied the
temperature may not increase. What I really would have liked to have done
was put in a the classic heating curve that appears in textbooks with Temp
on the Y and energy added on the x axis. The temperature of any of the
phases rises linearly (if you know the rate of energy input and the mass of
the sample being heated the slope of the line will give you the specific
heat) until it reaches the tempurature where the phase change occurs, the
line then goes horizontal, until the enthalpy of fusion or evaporation (wha=
t
you termed latent heat) is supplied, the line then starts up again at a new
slope, as the different phases will have different specific heats.
Yes, in my example one would need a pan with a heat capacity of zero that
was perfectly insulated and the pan and water would have instantaneous heat
transfer. But I will tell you that yesterday in class we melted a cube of
ice in tap water in a styrofoam cup and got within 5% of the theoretical
enthalpy of fusion for water.
Best regards,
Luke Nealey, Rankin Co. MS, USA
Robert Harris on tue 5 oct 10
Like Ivor, I've always called it Latent heat energy. In the case of
water the extra energy goes into breaking the hydrogen bonds between
the molecules of water. These are particularly strong bonds when
compared to other bonds (e.g. in non organic compounds) formed when a
liquid becomes solid. One reason why the latent heat capacity for
water is so high.
The requirement for energy to break hydrogen bonds can also been seen
when dissolving certain compounds. For example when urea crystals are
dissolved in water the solution will become very very cold as the
hydrogen bonds are broken, removing energy (as heat) from the water.
Robert
On Tue, Oct 5, 2010 at 2:43 PM, Luke Nealey wrote:
> On Tue, Oct 5, 2010 at 1:33 AM, ivor and olive lewis
> wrote:
>
>> Dear Luke Nealey,
>> Would you confirm that your statement .....
>> .... "So if you had a mole of ice in a pot at zero Celsius and started
>>
>> heating it, it would go from being ice at zero to being water at zero, t=
=3D
he
>> temperature would not increase until all the ice had melted "...
>>
>> ....is true if and only if =3DA0six joules of energy are applied to the =
ic=3D
e.
>> Any
>> less and there will be unmelted Ice in the system. Any more and the
>> temperature of the water will rise.
>>
>>
>>
>> Thanks,
>>
>>
>> Ivor Lewis,
>> REDHILL,
>> South Australia
>>
>
> Ivor,
>
> I know it's not as consise as it could be. I was trying to explain
> that melting takes energy that gets used to break the attraction between
> the molecules in the solid, and so, even if heat is being applied the
> temperature may not increase. =3DA0What I really would have liked to have=
d=3D
one
> was put in a the classic heating curve that appears in textbooks with Tem=
=3D
p
> on the Y and energy added on the x axis. =3DA0The temperature of any of t=
he
> phases rises linearly (if you know the rate of energy input and the mass =
=3D
of
> the sample being heated the slope of the line will give you the specific
> heat) until it reaches the tempurature where the phase change occurs, the
> line then goes horizontal, until the enthalpy of fusion or evaporation (w=
=3D
hat
> you termed latent heat) is supplied, the line then starts up again at a n=
=3D
ew
> slope, as the different phases will have different specific heats.
>
> Yes, in my example one would need a pan with a heat capacity of zero that
> was perfectly insulated and the pan and water would have instantaneous he=
=3D
at
> transfer. =3DA0 But I will tell you that yesterday in class we melted a c=
ub=3D
e of
> ice in tap water in a styrofoam cup and got within 5% of the theoretical
> enthalpy of fusion for water.
>
> Best regards,
> Luke Nealey, Rankin Co. MS, USA
>
--=3D20
----------------------------------------------------------
ivor and olive lewis on fri 8 oct 10
<increase the end hold time and if melting of some of the materials has
started, the longer hold will supply enough calories to melt the glaze just
as well as if it had been fired to a higher temperature for a much shorter
time.>>
Dear Michael Wendt,
This sounds logical, but several things about the concepts cause me concern=
.
First, "End Point" is being used ambiguously if the end point of a firing i=
s
governed by the observed behaviour a cone of specified value. Under those
circumstance "...slightly lower End point..." may mean the next lowest cone
value. Or does it mean at a lower than anticipated temperature ?
The second is an assumption that during an extended dwell period thermal
flux, the flow of calories (or BThU's or Joules if you wish) is continuousl=
y
directed towards the work being fired. This can only happen if the
temperature of the environment is always higher than the temperature of the
product being processed. It will not happen if the temperature rise falters=
.
To achieve what you are suggesting requires that the rate of heat input via
burners or elements is progressively reduced so that the temperature, as
indicated by a pyrometer continues to increase but at a reduced rate. If
temperature is abruptly stabilised heat will be prevented from flowing into
the maturing clay and glaze.
For example, if I achieve a uniform rate of temperature rise by increasing
gas pressure by 2.0 Kpa on the hour over the duration of a firing then, whe=
n
I choose to dwell ( Say, as my first cone sets) I would increase pressure b=
y
only 1.0 KPa and hold for an hour then increase pressure by 0.5 KPa. An hou=
r
later I would increase pressure by 0.25 KPa. With luck within the hour I
would hope the next cone would have almost competed it pyroplastic
movements.
Best regards,
Ivor Lewis,
REDHILL,
South Australia
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