Bruce Girrell on tue 29 sep 98
Background
I have had discussion with Clayarters both on the list and privately
regarding the firing of wet ware. I found some of the proposed explanations
of why a wet piece can withstand a firing difficult to believe from a
physical standpoint and some explanations were contradictory. For example,
one person stressed the importance of lots of air motion, while another
stressed the importance of a quiescent atmosphere.
I am an engineer[1] and an experimenter at heart - I like to know how and
why things work the way they do. In some of the private exchanges I proposed
an experiment that might help determine the mechanism by which a wet piece
might escape damage from internal steam pressure. This past weekend my wife
and I were going to do a bisque firing, so I had an opportunity to try
something.
The experiment
In my view, there had to be some mechanism by which the steam from the
interior of the wet clay could escape to the surface of the piece without
building to a pressure that would exceed the tensile strength (which is not
very high) of clay. I felt that it might be possible that the water in the
clay was holding the clay particles far enough apart to allow steam to pass.
Because the water is evaporating so quickly, there is no time for the clay
particles to rearrange and close the passages. So as the next layer of water
toward the interior of the piece is evaporated, there are still channels
open to the surface to allow the steam to pass safely to the surface of the
piece.
If this were true, it would imply that there would be less shrinkage in a
piece fired wet compared to one dried first because the clay particles of
the wet piece would set up and harden in the position that allowed steam
passage whereas a piece dried normally would have time for the clay
particles to cozy up against their neighbors, attempting to balance
electrical charges and the like as the in situ water disappeared. Of course,
it would be important not to fire the clay to maturity, as the melting would
obscure any differences, so a bisque was perfect.
Method
I rolled a piece of porcelain to a thickness of about 3/8 inch or 10 mm. I
trimmed the width to fit around half the circumference of a 40 watt
fluorescent lamp. This created a U shaped form to allow good air
cirulation/steam expulsion on all sides of the form. The length of the piece
was just over 40 cm and I marked the piece every 10 cm in three different
places. Time was of the essence. The bisque was due to fire up the next
morning, so I used a fan along the axis of the piece to help dry it
overnight. Not an ideal situation, but as close as I could come to a normal
drying within the time frame at hand.
By morning the marks measured just over 38 cm, about 5% shrinkage. The piece
went in on the top shelf (of three) in our simple 21 cu ft updraft kiln[2].
The preheat took four hours to get safely above 100C. From there it was a
pretty smooth ride to ^06, though the top shelf really only hit about ^07.
After cooling the marks on the piece measured 37.8 cm. It had shrunk only 2
mm during the bisque. Total shrinkage 5.5%.
The kiln was still warm from the bisque, so I had no fear of taking it back
up to temperature quickly. I formed another U shaped piece of porcelain the
same size as the first one from the same block of clay, using the same slab
roller setup - nothing else had been rolled in the meantime. I placed the
form on a kiln shelf and made the marks as before. Even though the kiln was
empty, I stacked up three shelves as before so that the piece was in the
same position in the kiln as the first piece had been. Tossed in a cone
placque and bricked it up as quickly as I could. Since this kiln is a simple
updraft, the piece remained visible through the exhaust port.
Kiln log
8:08 Lit kiln. Both burners on at 5 PSI.
8:13 Incresed pressure to 10 PSI[3]. Steam visible at side of roof edge[4].
8:16 Some small steam explosions occur in the piece.
8:17 Explosions increasing.
8:18 710C Popcorn in the kiln[5].
8:19 Piece has now broken in half lengthwise.
8:21 Experiment terminated. No measureable pieces remaining.
Analysis
Not the results I was looking for, but results nonetheless. The hypothesis
is neither proved nor disproved. However, it is clear that certain
conditions must be met for the wet firing technique to work. Perhaps my form
was too thick. Perhaps porcelain was not the best choice for a body to
test[6]. Perhaps it is necessary to have a kiln load of wet ware, not just
one piece.
If any of you have any input on this, I'd appreciate it. As you (hopefully)
can tell, I tried to control my variables. If you see a problem in my
approach or methods, please let me know. I'm not offended by constructive
criticism. I've been through a lot of peer reviews and have lived.
Bruce "Damn the torpedoes, more propane" Girrell
[1] It pays for the clay. My wife, Lynne, is the artistic one. Look at the
pieces we make and you can tell which ones were thrown by an engineer.
[2] Fired by two of Marc's MR100 burners. Getting this puppy up to ^06 in a
time period comparable to the commercial kilns would not be a problem.
[3] God, I love that roar. I've loved fire since I was a kid, much to my
parents' dread.
I've got to attend a wood firing some day.
[4] I don't know where this steam was coming from. The kiln had just been
fired. Any humidity in the bricks should have been long gone. And I would
have thought that steam from the piece would be carried off with the exhaust
gas.
[5] You could see little puffs as the pieces blew off. I don't know whether
the puffs were water vapor (my best guess) or zillions of microscopic pieces
of clay being blown out.
[6] I chose porcelain because of the high maturing temperature.
Grimmer on wed 30 sep 98
Bruce,
Very interesting experiment; I love it when scientists get in on the
act. Question: was the piece not wet enough? Perhaps the clay needs to be
freshly thrown for this to work, not just rolled out. Throwing puts a lot of
HOH into the clay.
steve grimmer
marion illinois.
----------
>From: Bruce Girrell
>To: CLAYART@LSV.UKY.EDU
>Subject: Firing wet clay - an experiment
>Date: Tue, Sep 29, 1998, 7:59 AM
>
>----------------------------Original message----------------------------
>Background
>I have had discussion with Clayarters both on the list and privately
>regarding the firing of wet ware. I found some of the proposed explanations
>of why a wet piece can withstand a firing difficult to believe from a
>physical standpoint and some explanations were contradictory. For example,
>one person stressed the importance of lots of air motion, while another
>stressed the importance of a quiescent atmosphere.
>
>I am an engineer[1] and an experimenter at heart - I like to know how and
>why things work the way they do. In some of the private exchanges I
proposed
>an experiment that might help determine the mechanism by which a wet piece
>might escape damage from internal steam pressure. This past weekend my wife
>and I were going to do a bisque firing, so I had an opportunity to try
>something.
>
>The experiment
>In my view, there had to be some mechanism by which the steam from the
>interior of the wet clay could escape to the surface of the piece without
>building to a pressure that would exceed the tensile strength (which is not
>very high) of clay. I felt that it might be possible that the water in the
>clay was holding the clay particles far enough apart to allow steam to
pass.
>Because the water is evaporating so quickly, there is no time for the clay
>particles to rearrange and close the passages. So as the next layer of
water
>toward the interior of the piece is evaporated, there are still channels
>open to the surface to allow the steam to pass safely to the surface of the
>piece.
>
>If this were true, it would imply that there would be less shrinkage in a
>piece fired wet compared to one dried first because the clay particles of
>the wet piece would set up and harden in the position that allowed steam
>passage whereas a piece dried normally would have time for the clay
>particles to cozy up against their neighbors, attempting to balance
>electrical charges and the like as the in situ water disappeared. Of
course,
>it would be important not to fire the clay to maturity, as the melting
would
>obscure any differences, so a bisque was perfect.
>
>Method
>I rolled a piece of porcelain to a thickness of about 3/8 inch or 10 mm. I
>trimmed the width to fit around half the circumference of a 40 watt
>fluorescent lamp. This created a U shaped form to allow good air
>cirulation/steam expulsion on all sides of the form. The length of the
piece
>was just over 40 cm and I marked the piece every 10 cm in three different
>places. Time was of the essence. The bisque was due to fire up the next
>morning, so I used a fan along the axis of the piece to help dry it
>overnight. Not an ideal situation, but as close as I could come to a normal
>drying within the time frame at hand.
>
>By morning the marks measured just over 38 cm, about 5% shrinkage. The
piece
>went in on the top shelf (of three) in our simple 21 cu ft updraft kiln[2].
>The preheat took four hours to get safely above 100C. From there it was a
>pretty smooth ride to ^06, though the top shelf really only hit about ^07.
>After cooling the marks on the piece measured 37.8 cm. It had shrunk only 2
>mm during the bisque. Total shrinkage 5.5%.
>
>The kiln was still warm from the bisque, so I had no fear of taking it back
>up to temperature quickly. I formed another U shaped piece of porcelain the
>same size as the first one from the same block of clay, using the same slab
>roller setup - nothing else had been rolled in the meantime. I placed the
>form on a kiln shelf and made the marks as before. Even though the kiln was
>empty, I stacked up three shelves as before so that the piece was in the
>same position in the kiln as the first piece had been. Tossed in a cone
>placque and bricked it up as quickly as I could. Since this kiln is a
simple
>updraft, the piece remained visible through the exhaust port.
>
>Kiln log
>8:08 Lit kiln. Both burners on at 5 PSI.
>8:13 Incresed pressure to 10 PSI[3]. Steam visible at side of roof edge[4].
>8:16 Some small steam explosions occur in the piece.
>8:17 Explosions increasing.
>8:18 710C Popcorn in the kiln[5].
>8:19 Piece has now broken in half lengthwise.
>8:21 Experiment terminated. No measureable pieces remaining.
>
>Analysis
>Not the results I was looking for, but results nonetheless. The hypothesis
>is neither proved nor disproved. However, it is clear that certain
>conditions must be met for the wet firing technique to work. Perhaps my
form
>was too thick. Perhaps porcelain was not the best choice for a body to
>test[6]. Perhaps it is necessary to have a kiln load of wet ware, not just
>one piece.
>
>If any of you have any input on this, I'd appreciate it. As you (hopefully)
>can tell, I tried to control my variables. If you see a problem in my
>approach or methods, please let me know. I'm not offended by constructive
>criticism. I've been through a lot of peer reviews and have lived.
>
>Bruce "Damn the torpedoes, more propane" Girrell
>
>[1] It pays for the clay. My wife, Lynne, is the artistic one. Look at the
>pieces we make and you can tell which ones were thrown by an engineer.
>
>[2] Fired by two of Marc's MR100 burners. Getting this puppy up to ^06 in a
>time period comparable to the commercial kilns would not be a problem.
>
>[3] God, I love that roar. I've loved fire since I was a kid, much to my
>parents' dread.
>I've got to attend a wood firing some day.
>
>[4] I don't know where this steam was coming from. The kiln had just been
>fired. Any humidity in the bricks should have been long gone. And I would
>have thought that steam from the piece would be carried off with the
exhaust
>gas.
>
>[5] You could see little puffs as the pieces blew off. I don't know whether
>the puffs were water vapor (my best guess) or zillions of microscopic
pieces
>of clay being blown out.
>
>[6] I chose porcelain because of the high maturing temperature.
Don Prey on wed 30 sep 98
In a message dated 09/29/98 6:03:28 AM, you wrote:
<can tell, I tried to control my variables. If you see a problem in my
approach or methods, please let me know. I'm not offended by constructive
criticism. I've been through a lot of peer reviews and have lived.
>>
Bruce,
You might consider taking the experiment back to a simpler level as the next
step. Here is what I'm thinking. Forget the shrinkage for now and test clay
thickness and body type to see what will survive, then go the next step and do
some shrinkage tests. For porcelain, 3/8 ths may be too thick. I would be
tempted to try porcelain and a moderately open stoneware body at 1, 2 and 3
eights. Have fun. (maybe I'll join you on my next bisque).
Don Prey in Oregon
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