Roger Graham on wed 18 jun 03
I've been following the posts about whether or not to provide a pinhole
air-escape when firing a totally enclosed pot. For years I've taken the
cautious approach, and made a tiny vent hole in any hollow form. But... in a
bisque firing last year, I had half a dozen terracotta garden cats, big fat
ones with a hollow thrown body & head, and attachments of ears and tail and
feet etc. Total enclosed volume maybe 2 litres. Made a pinhole in every
one, or so I thought. But missed one.... woops!
You've seen those awful bits of television footage, where a suicide bomber
explodes something in a crowded bus? That's what I got in the big kiln. Arms
and legs and heads and ears from other cats strewn about. Kitset cats.
Shrapnel embedded deep in the fibre of walls and door and roof.
The kiln log shows I was firing slowly, with due caution. But a bisque
firing involves not only rising pressure as the entrapped air gets hotter,
but also production of steam as the chemically combined water is driven out
of the clay. In a sealed pot, that's a big ask.
You can calculate the likely pressure rise due to air trying to expand as it
gets hot. It's not moon-rocket science. The physics is, that if you heat a
gas in a sealed container the pressure will be proportional to the Absolute
temperature. That is, proportional to the temperature on the Absolute or
"Kelvin" scale, which has its zero at minus 273 degrees C.
Let's start with an enclosed pot at 27 degrees C, which is (27 + 273) = 300
degrees on the Kelvin scale. They write this now as 300K. Now let's heat the
enclosed pot to 1227 degrees C, about cone 9. That's (1227 + 273) = 1400K
What's happened to the pressure? It started at a value of one atmosphere,
about 14.7 pounds per square inch. But by the time it reaches 1400K, the
pressure will be bigger by a factor of (1400/300) which is about 4.7 times
bigger. So the new pressure in the hot pot will be (14.7 x 4.7) = 69 pounds
per square inch. Say 70 psi, round figure.
Now here's the rub. A sturdily made pot of modest size will probably
withstand an internal pressure of 70 psi, no problem. But a terracotta
garden cat? Hmm.
Sure, the calculation above is a bit approximate. The clay is probably a bit
porous, and some gas probably escapes, pinhole or not. And the pot itself
expands a bit, but only by a trivial amount. But if the pot is truly
airtight, there's no escaping the fact that the pressure inside must rise to
a fairly high value when the kiln is hot. Not making a pinhole seems to me
like a ceramic version of Russian roulette.
Roger Graham, near Gerringong, Australia
http://members.optusnet.com.au/~rogergraham
Rod Wuetherick on wed 18 jun 03
Roger,
Be that as it may and I agree your logic is flawless... yet... I fire
hundreds of hollow
handles, feet a year. I rarely have ever had a problem. If I'm playing
Russian roulette
I have uncanny good luck..
I think this is all directly proportional to how fast one fires.
Cheers,
Rod
> -----Original Message-----
> From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG]On Behalf Of Roger Graham
> Sent: Wednesday, June 18, 2003 2:33 AM
> To: CLAYART@LSV.CERAMICS.ORG
> Subject: Firing a totally enclosed pot.... will it explode?
>
>
> I've been following the posts about whether or not to provide a pinhole
> air-escape when firing a totally enclosed pot. For years I've taken the
> cautious approach, and made a tiny vent hole in any hollow form.
> But... in a
> bisque firing last year, I had half a dozen terracotta garden
> cats, big fat
> ones with a hollow thrown body & head, and attachments of ears
> and tail and
> feet etc. Total enclosed volume maybe 2 litres. Made a pinhole in every
> one, or so I thought. But missed one.... woops!
>
> You've seen those awful bits of television footage, where a suicide bomber
> explodes something in a crowded bus? That's what I got in the big
> kiln. Arms
> and legs and heads and ears from other cats strewn about. Kitset cats.
> Shrapnel embedded deep in the fibre of walls and door and roof.
>
> The kiln log shows I was firing slowly, with due caution. But a bisque
> firing involves not only rising pressure as the entrapped air gets hotter,
> but also production of steam as the chemically combined water is
> driven out
> of the clay. In a sealed pot, that's a big ask.
>
> You can calculate the likely pressure rise due to air trying to
> expand as it
> gets hot. It's not moon-rocket science. The physics is, that if you heat a
> gas in a sealed container the pressure will be proportional to
> the Absolute
> temperature. That is, proportional to the temperature on the Absolute or
> "Kelvin" scale, which has its zero at minus 273 degrees C.
>
> Let's start with an enclosed pot at 27 degrees C, which is (27 +
> 273) = 300
> degrees on the Kelvin scale. They write this now as 300K. Now
> let's heat the
> enclosed pot to 1227 degrees C, about cone 9. That's (1227 + 273) = 1400K
>
> What's happened to the pressure? It started at a value of one atmosphere,
> about 14.7 pounds per square inch. But by the time it reaches 1400K, the
> pressure will be bigger by a factor of (1400/300) which is about
> 4.7 times
> bigger. So the new pressure in the hot pot will be (14.7 x 4.7) =
> 69 pounds
> per square inch. Say 70 psi, round figure.
>
> Now here's the rub. A sturdily made pot of modest size will probably
> withstand an internal pressure of 70 psi, no problem. But a terracotta
> garden cat? Hmm.
>
> Sure, the calculation above is a bit approximate. The clay is
> probably a bit
> porous, and some gas probably escapes, pinhole or not. And the pot itself
> expands a bit, but only by a trivial amount. But if the pot is truly
> airtight, there's no escaping the fact that the pressure inside
> must rise to
> a fairly high value when the kiln is hot. Not making a pinhole seems to me
> like a ceramic version of Russian roulette.
>
> Roger Graham, near Gerringong, Australia
>
> http://members.optusnet.com.au/~rogergraham
>
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Gail Dapogny on wed 18 jun 03
Roger,
I don't know the answer to this, but I've made lots of closed pots. I
have, in almost all cases* (*missed one or two), left a tiny needle-hole
for the bisque firing, but in recent years have not bothered to make sure
that it remains unclogged during the cone 10 glaze firing. This was
because someone I know and respect (with an MFA) told me that it wasn't
necessary. In fact, she said it would not be a problem past the
greenware-to-bone drying stage.
No incidents yet, but I mostly have the pinhole during the bisque..
I'll be interested to know what knowledgeable responders have to say.
---Gail Dapogny in Ann Arbor
>I've been following the posts about whether or not to provide a pinhole
>air-escape when firing a totally enclosed pot. For years I've taken the
>cautious approach, and made a tiny vent hole in any hollow form. But... in a
>bisque firing last year, I had half a dozen terracotta garden cats, big fat
>ones with a hollow thrown body & head, and attachments of ears and tail and
>feet etc. Total enclosed volume maybe 2 litres. Made a pinhole in every
>one, or so I thought. But missed one.... woops!
>
>You've seen those awful bits of television footage, where a suicide bomber
>explodes something in a crowded bus? That's what I got in the big kiln. Arms
>and legs and heads and ears from other cats strewn about. Kitset cats.
>Shrapnel embedded deep in the fibre of walls and door and roof.
>
>The kiln log shows I was firing slowly, with due caution. But a bisque
>firing involves not only rising pressure as the entrapped air gets hotter,
>but also production of steam as the chemically combined water is driven out
>of the clay. In a sealed pot, that's a big ask.
>
>You can calculate the likely pressure rise due to air trying to expand as it
>gets hot. It's not moon-rocket science. The physics is, that if you heat a
>gas in a sealed container the pressure will be proportional to the Absolute
>temperature. That is, proportional to the temperature on the Absolute or
>"Kelvin" scale, which has its zero at minus 273 degrees C.
>
>Let's start with an enclosed pot at 27 degrees C, which is (27 + 273) = 300
>degrees on the Kelvin scale. They write this now as 300K. Now let's heat the
>enclosed pot to 1227 degrees C, about cone 9. That's (1227 + 273) = 1400K
>
>What's happened to the pressure? It started at a value of one atmosphere,
>about 14.7 pounds per square inch. But by the time it reaches 1400K, the
>pressure will be bigger by a factor of (1400/300) which is about 4.7 times
>bigger. So the new pressure in the hot pot will be (14.7 x 4.7) = 69 pounds
>per square inch. Say 70 psi, round figure.
>
>Now here's the rub. A sturdily made pot of modest size will probably
>withstand an internal pressure of 70 psi, no problem. But a terracotta
>garden cat? Hmm.
>
>Sure, the calculation above is a bit approximate. The clay is probably a bit
>porous, and some gas probably escapes, pinhole or not. And the pot itself
>expands a bit, but only by a trivial amount. But if the pot is truly
>airtight, there's no escaping the fact that the pressure inside must rise to
>a fairly high value when the kiln is hot. Not making a pinhole seems to me
>like a ceramic version of Russian roulette.
>
>Roger Graham, near Gerringong, Australia
PurpleLama@AOL.COM on thu 19 jun 03
I make lots of enclosed forms. I "always" leave a pinhole in the form. This
semester I took a class at the local community college and fired some there
using cone 10 stoneware. As I was about to glaze one a few weeks ago, I noticed
that there was no pinhole. Since several pots (none of mine) had exploded
during a previous firing, I asked my teacher (who has been teaching there since
the mid-seventies) whether to fire it. He told me that as long as it had
survived the bisque firing without a pinhole, there would be no problem with the cone
10 glaze firing.
Shula
in night time Redondo Beach, CA
I survived two graduation ceremonies today - my grandson graduated from
elementary school - he received a certificate of release - and my granddaughter
graduated from middle school - they called it a promotion ceremony. The
promotion ceremony was very nice. They kept the speeches short and interesting. Then
they gave out certificates to 300 children in 15 minutes. Each name was called
and each child had a moment in the lights. It was amazing. My hat is off to
those teachers.
> I don't know the answer to this, but I've made lots of closed pots. I
> have, in almost all cases* (*missed one or two), left a tiny needle-hole
> for the bisque firing, but in recent years have not bothered to make sure
> that it remains unclogged during the cone 10 glaze firing. This was
> because someone I know and respect (with an MFA) told me that it wasn't
> necessary. In fact, she said it would not be a problem past the
> greenware-to-bone drying stage.
> No incidents yet, but I mostly have the pinhole during the bisque..
> I'll be interested to know what knowledgeable responders have to say.
>
DEBBYGrant@AOL.COM on thu 19 jun 03
This subject comes up from time to time. I can assure you that it is
possible to
fire enclosed forms and I do it all the time. Trapped air does not cause a
pot to
explode. THIS IS A MYTH! What causes a pot to explode is moisture. If a
pot
is not completely dry and the firing is too fast steam forms and the pot
explodes.
The problem with completely enclosed forms is that they do not always dry
completely and, believe it or not, putting in a pin hole helps the drying.
Debby Grant in NH
John Hesselberth on thu 19 jun 03
>
The vast majority--and probably all--of the 'explosions' of pots that
occur during firing occur at or about 100-120 deg C because the pot is
not dry. Lots of people fire closed forms without incident IF they
carefully dry their pots first. The pressure calculations presented a
day or two ago would be the absolute worst case and impossible to
achieve in the real world. At the temperature where combined water is
released the clay is still very porous. Think about it. Does bisqued
pottery leak water? Yes. Then you know for certain it will also leak
gases. And it is significantly more porous at 450-700 deg C or so where
combined water is released. There is simply no way to build up much
pressure in a porous closed form with the small amount of combined
water that is given off gradually over a broad temperature range as you
heat the kiln through that temperature range over a period of a couple
hours or more.
Now it is true that closed forms are more difficult to dry than open
forms, although a pinhole will have a negligible effect on the rate of
drying. Most, if not all, of the bad press closed forms get for
'exploding' is simply due to the fact that people fire them before they
are dry. And wet pots fired too quickly will 'explode' whether or not
they are a closed form--and, more than likely, whether or not that
closed form has a pinhole in it.
Regards,
John
http://www.frogpondpottery.com
http://www.masteringglazes.com
Vince Pitelka on sat 21 jun 03
> 14.7) or about 55 psi. Like pumping up your car tyre to a gauge pressure
of
> 55 psi. Less than 70, but still a lot of pressure.
> Is a big enclosed pot more likely to go pop? Yes indeed.
Roger -
I appreciate your thorough explanation, but I do not think we can give much
credence to the likelihood of explosion due to expanding air in a normal
firing. Steam becomes gas rather abruptly during the heating cycle, and
thus steam explosions are common even when the air pockets are very small.
But air expansion occurs slowly over the entire heating ramp, and it is far
more likely that the pressure could diffuse gradually over that period of
time.
It is true that porous bisque-fired clay would allow air pressure to escape
from a closed space, but it seems to me that this is irrelevant, because the
obvious question is "How do you get it bisque-fired in the first place?" In
my experience, bone-dry clay is often almost completely air-tight, with
little capacity for relieving trapped pressure. If the firing is slow
enough, and if the trapped space is small, then I know it can work, but it
does involve special accommodation.
I appreciate your comment about the size of the space in proportion to the
surface area - the larger the space, the more pressure must escape from
proportionally less wall surface, and thus the greater the chances of
failure. It seems to me that the notion of firing forms with completely
closed spaces is practical only with smaller closed spaces.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Craft
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Office - wpitelka@tntech.edu
615/597-6801 x111, FAX 615/597-6803
http://iweb.tntech.edu/wpitelka/
Roger Graham on sat 21 jun 03
I'm still following the anecdotes about totally sealed pots which have or
have not exploded when firing. Can't argue with the genuine experience of
potters who have been doing this for years without disaster. But...
Several posts have used dismissive words like "MYTH". There seem to be two
extremes of belief here. One says "If you leave a little bubble in your
clay the pot will explode!". The other extreme says "Not true. It's all a
myth. Hot air doesn't make pots explode. It's steam pressure". I think the
truth is a bit more complex. Even if there's no steam, the size of the pot
comes into it, and so does the shape
Some facts about steam first. When one gram of water in your damp pot is
driven off as steam, the volume it produces at 100 degrees C is about 1550
millilitres... about a litre and a half. So if you fast-fire a damp enclosed
pot, expect a huge increase in inner pressure due to steam. Nobody seems to
disagree about that. If the pot goes pop, it serves you right. But I don't
think the facts about air pressure are as well understood.
Let's assume for now that steam doesn't come into it. The enclosed pot
contains just air. Also let's pretend the pot is totally airtight, which it
probably isn't really. A bisqued pot is porous enough to soak up water, so
it surely must be porous enough to let air diffuse through, but slowly. But
assume it really doesn't leak. Will the air pressure build up enough to
burst the pot when it's hot?
I've been expecting somebody to point out an omission in an earlier post
about this, where I calculated that the air pressure in such a pot would
rise from 14.7 psi at room temperature, to about 70 psi at Cone 9. Expected
to be told (correctly) that the figure of 70 psi is just the calculated
pressure inside, but of course there's still 14.7 psi of atmospheric
pressure outside, so the difference between the two would be only (70 minus
14.7) or about 55 psi. Like pumping up your car tyre to a gauge pressure of
55 psi. Less than 70, but still a lot of pressure.
Is a big enclosed pot more likely to go pop? Yes indeed. Let's make two
enclosed ball-shaped pots, sized like a golf ball and a soccer ball. To keep
the maths simple, let's pretend their diameters are 4 cm and 20 cm. They
both have the same wall thickness, equally strong. Only one is five times
bigger than the other.
Now pump them both up to 55 psi. Imagine a line drawn around the equator of
each pot, and think about the area of the circle enclosed by this equator
line, like an imaginary horizontal slice through the pot. If we call this
area in the golf-ball pot "One unit", then the area in the soccer-ball pot
will be 25 units. It varies with the SQUARE of the diameter. If you've
followed all this, it's plain that the upward force trying to pop the top
off the soccer ball is 25 times as big as the force trying to pop the golf
ball.
Sure, the soccer ball pot is bigger. But the length around its equator isn't
25 times as big as the golf ball. It's only 5 times further around the
equator. So there's 25 times the force being applied to clay with only 5
times the strength. In engineer-speak, the "stress" is five times as big.
Which seems to say, that the stress on the wall of the pot increases in
proportion to the size of the pot. Big pot, bigger stress. And bigger pop...
ask me how I know.
All of which says to me, that continued success in firing relatively small
enclosed forms indicates chiefly that the pots were competently made and
sensibly fired, with sufficient strength to withstand the bursting forces
involved. But it doesn't follow at all that you can extrapolate this
experience to pots of bigger size, as a kind of sweeping generalization
"It's all a myth".
If leakage of air by diffusion through the wall is involved, the story is
much the same. The area available for air to diffuse away increases with the
SQUARE of the diameter, But the volume of trapped air in the pot increases
with the CUBE of the diameter. So, pressure relief by diffusion will be
more effective in a small pot. But that's another story.
One more thought, about some shapes being stronger than others. You've
noticed how LP gas bottles and compressed-air tanks are made with
spherically curved ends to withstand the pressure. No flat surfaces. Same
with pots, I'd expect. Rounded spherical shapes are strong. Pussy-cat shapes
with an egg-shaped body, narrow neck, big round head and a pointy face, all
in the same pot... well, they go pop. Again, ask me how I know.
Unless there's a good reason not to, pinholes for me. Every time.
Roger Graham, near Gerringong, Australia
http://members.optusnet.com.au/~rogergraham
Snail Scott on sun 22 jun 03
At 05:23 PM 6/21/03 -0500, you wrote:
>I appreciate your comment about the size of the space in proportion to the
>surface area - the larger the space, the more pressure must escape from
>proportionally less wall surface, and thus the greater the chances of
>failure. It seems to me that the notion of firing forms with completely
>closed spaces is practical only with smaller closed spaces.
I concur with this conclusion. Smaller pieces CAN be fired
without pressure-relieveing holes, but I suspect that
the surface-to-volume-ratio problem would make this less
successful as the size increases. But that's not a problem,
in my mind. Small pieces that can be picked up and turned
about in the hand might be esthetically compromised by a
single pinhole, and it might thus be desirable to take the
small risk of omitting the vent. With larger work, though,
the point at which it becomes hazardous to omit the vent is
also the point at which it becomes inconvenient to pick up
the piece and scrutinize all sides. So, why not put a pin
hole (or several) on the underside or in a crevice of the
texture? A pinhole that seems huge on a hand-sized piece
will be proportionally much smaller on a barrel-sized piece.
On larger work, I often don't even have a complete bottom,
to reduce the stresses of uneven drying. Who's gonna see
it? If there were any reason to have the piece fully sealed,
I'd use a few pinholes for firing, then seal them afterward.
With sufficiently slow firing, it may be possible to fire
even huge pieces without venting, but why bother? There
is little to be gained. The stuff that really benefits
from being entirely closed is the small stuff, which
(coincidentally) is the very stuff that can be reliably
fired that way.
-Snail
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