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the x factor in firing -long and painful

Fred Parker on wed 30 jan 08

I have never fired a fuel kiln. Understanably, I suspect many will read
no further; however, I will write this anyway because I consider it to be
very important in firing whether the kiln is heated with fuel or electric
resistance. It might also be helpful to potters whose backgrounds avoided
the physical sciences.

The branch of science that deals with heat and heat transfer
is =93thermodynamics.=94 Most sane human beings never sit in a thermodynami=
cs
classroom. Ever since I did I have believed it (or a layman=92s version of
it) should be required even in high school. It explains much about
everyday life from hot coffee to the space shuttle to the arguments men
have with wives over thermostat settings. I found it especially useful in
explaining welding to metal sculpture students. Maybe it can help potters.

Thermodynamics proves that =93heat=94 is a form of energy that has specific
measurable characteristics. As such, heat can and does =93flow=94 through a=

body =96 ALWAYS moving from the warmer to the cooler areas. =93Cold=94 neve=
r
flows anywhere. =93Cold=94 is not energy. "Cold" is simply the absence (or=

lessening)of energy. When you place your hand on a cold window pane and
feel =93cold=94 moving into your hand that is not what actually happens. Wh=
at
really happens is =93heat=94 from your hand flows INTO the window pane (warm=
er
to cooler), and that specific quantity of heat your hand lost diminishes
the total amount of =93heat=94 in your hand by the same amount, resulting in=
a
cooling sensation.

It takes a very specific amount of =93heat=94 to raise the temperature of a
single gram of a material (clay, for example) one degree. It takes ten
times that amount to raise it ten degrees and so on. In a kiln the
burners or elements provide heat AT A SPECIFIC RATE depending on type of
fuel, burner size, setting, element resistance/voltage etc.

It is theoretically possible to apply engineering calculations to a
kilnload of pottery and determine precisely how long it would have to be
fired to reach a specified temperature or cone. To do this would also
require more information =96 for example, how much of the heat supplied by
the burners or elements is lost by various escape routes such as radiation
through walls, cracks around lids/doors, the exact mass of
clay/shelves/posts in the kiln and how much =93internal insulation=94 (my
term) is realized by having closed or almost closed forms such as bottles
versus open forms like bowls and mugs. As Mel pointed out EVERYTHING in
the kiln must be heated to TO THE SAME temperature -- pottery, shelves,
posts, the hampster carcass you want to cremate.

Not many of us would go to the trouble.

Think of it as filling a big, thousand gallon leaky water tank with a
pump. The pump puts water in at a specific rate -- say a gallon a
minute. Should take a thousand minutes -- right? But the tank has leaks
which slow the progress so it actually takes longer than a thousand
minutes because the pump has to make up the water that's lost through
leaks. A bigger pump fills it faster. A smaller pump takes more time.
Both still have to compensate for leaks.

Kilns are like tanks, except instead of water you're filling it with units
of heat. It=92s all about how many one-degree temperature steps you have to=

accomplish to raise the temperature of the total kiln interior mass from a
starting temperature to whatever you want it to be -- and remember, it
takes a given amount of heat to raise the temperature of something one
degree. If the kiln starts out 20 below, it takes longer than if it
starts at 75 degrees because it has more of those little one-degree steps
to take. If it=92s windy more heat gets =93blown away=94 (more leaks in the=

tank) so it will take longer than if it is calm.

There is even more to it =96 for instance, it takes a certain amount of heat=

to evaporate a quantity of water. Pottery that has been sitting in high
humidity in South Florida prior to firing will not fire the same as
pottery sitting at the exact same temperature in the Sonoran Desert. It
will take an additional amount of heat to drive out the moisture in the
more humid zone. Even the hampster carcass with have an effect because it
probably contains more water than a dried clay pot does.

I believe this is essentially what Mel refers to as the =93X Factor.=94
Thinking of heat in terms of something provided in specific, measurable
quantities =96 like gallons of water =96 helps explain some of the strange
results a firing can produce, and it also helps devise corrections.

I hope this benefits anyone not so fortunate to have been cruelly tortured
by Dr. Mengele=92s distant relative, the thermodynamics professor I had at
Auburn=85

Fred Parker

On Tue, 29 Jan 2008 08:54:56 -0600, mel jacobson wrote:

>there is always a BUT, IN firing.
>
>an X factor.
>
>no one way, no perfect answer.
>
>here are a couple of scenarios:
>
>i have a big plate order...sets of dishes.
>want them all to match...so i have to stack
>that kiln full...yes...every kiln shelf, every post
>i own. they all get crammed into the kiln.
>26 kiln shelves...all in one firing.
>
>how long will it take to fire?
>
>who knows?
>you have to heat everything in the kiln to cone 11.
>that is a great deal of mass. a great deal of weight.
>nils points this out very well in his book.
>
>weight, mass, heat, time.
>
SNIP

Bruce Girrell on wed 30 jan 08

Well done Fred.

Bruce Girrell

Fred Parker wrote:

The branch of science that deals with heat and heat transfer
is "thermodynamics." Most sane human beings never sit in a thermodynamics
classroom. Ever since I did I have believed it (or a layman's version of
it) should be required even in high school. It explains much about
everyday life from hot coffee to the space shuttle to the arguments men
have with wives over thermostat settings. I found it especially useful in
explaining welding to metal sculpture students. Maybe it can help potters.

Thermodynamics proves that "heat" is a form of energy that has specific
measurable characteristics. As such, heat can and does "flow" through a
body - ALWAYS moving from the warmer to the cooler areas. "Cold" never
flows anywhere. "Cold" is not energy. "Cold" is simply the absence (or
lessening)of energy. When you place your hand on a cold window pane and
feel "cold" moving into your hand that is not what actually happens. What
really happens is "heat" from your hand flows INTO the window pane (warmer
to cooler), and that specific quantity of heat your hand lost diminishes
the total amount of "heat" in your hand by the same amount, resulting in a
cooling sensation.

It takes a very specific amount of "heat" to raise the temperature of a
single gram of a material (clay, for example) one degree. It takes ten
times that amount to raise it ten degrees and so on. In a kiln the
burners or elements provide heat AT A SPECIFIC RATE depending on type of
fuel, burner size, setting, element resistance/voltage etc.

It is theoretically possible to apply engineering calculations to a
kilnload of pottery and determine precisely how long it would have to be
fired to reach a specified temperature or cone. To do this would also
require more information - for example, how much of the heat supplied by
the burners or elements is lost by various escape routes such as radiation
through walls, cracks around lids/doors, the exact mass of
clay/shelves/posts in the kiln and how much "internal insulation" (my
term) is realized by having closed or almost closed forms such as bottles
versus open forms like bowls and mugs. As Mel pointed out EVERYTHING in
the kiln must be heated to TO THE SAME temperature -- pottery, shelves,
posts, the hampster carcass you want to cremate.

Not many of us would go to the trouble.

Think of it as filling a big, thousand gallon leaky water tank with a
pump. The pump puts water in at a specific rate -- say a gallon a
minute. Should take a thousand minutes -- right? But the tank has leaks
which slow the progress so it actually takes longer than a thousand
minutes because the pump has to make up the water that's lost through
leaks. A bigger pump fills it faster. A smaller pump takes more time.
Both still have to compensate for leaks.

Kilns are like tanks, except instead of water you're filling it with units
of heat. It's all about how many one-degree temperature steps you have to
accomplish to raise the temperature of the total kiln interior mass from a
starting temperature to whatever you want it to be -- and remember, it
takes a given amount of heat to raise the temperature of something one
degree. If the kiln starts out 20 below, it takes longer than if it
starts at 75 degrees because it has more of those little one-degree steps
to take. If it's windy more heat gets "blown away" (more leaks in the
tank) so it will take longer than if it is calm.

There is even more to it - for instance, it takes a certain amount of heat
to evaporate a quantity of water. Pottery that has been sitting in high
humidity in South Florida prior to firing will not fire the same as
pottery sitting at the exact same temperature in the Sonoran Desert. It
will take an additional amount of heat to drive out the moisture in the
more humid zone. Even the hampster carcass with have an effect because it
probably contains more water than a dried clay pot does.

I believe this is essentially what Mel refers to as the "X Factor."
Thinking of heat in terms of something provided in specific, measurable
quantities - like gallons of water - helps explain some of the strange
results a firing can produce, and it also helps devise corrections.

miriam on fri 1 feb 08

I hope this benefits anyone not so fortunate to have been cruelly tortured
by Dr. Mengele's distant relative, the thermodynamics professor I had at
Auburn.

Fred Parker

Hmm... The guy who woks my poor old body out at the gym is a close
relative of your thermodynamics prof, I am sure.

I hope it is ok if I send your email to some of my students who, like me,
have never had a theromodynamics class. You make it so clear.

miriam

Fred Parker on mon 4 feb 08

Hello, Miriam:

Please feel free to use it however it might be helpful. Are you sure
these aren't "problem" students you want to torment?...

Fred

On Fri, 1 Feb 2008 15:51:25 -0500, miriam
wrote:

>I hope this benefits anyone not so fortunate to have been cruelly tortured
>by Dr. Mengele's distant relative, the thermodynamics professor I had at
>Auburn.
>
>Fred Parker
>
>
>Hmm... The guy who woks my poor old body out at the gym is a close
>relative of your thermodynamics prof, I am sure.
>
>I hope it is ok if I send your email to some of my students who, like me,
>have never had a theromodynamics class. You make it so clear.
>
>miriam
>
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