Margie Pankhurst on wed 24 may 06
I am a long time reader of the postings and find myself with an inquiry. =
I have recently read that if fired slowly a closed form with out a hole =
will not explode in firing. I have made some of those forms recently and =
need to now bisque them. My question is how slow do I need to fire them? =
What are the key temperatures that I should be cautious when nearing to =
avoid a possible explosion? I have a programable paragon. =20
Thanks for your help.=20
Margie=20
Grapevine Tx
www.pankhurstpottery.com
Susan P on wed 24 may 06
Why do you want to fire them with no hole?
Susan near Seattle
On 5/24/06, Margie Pankhurst wrote:
>
> I am a long time reader of the postings and find myself with an inquiry. =
I
> have recently read that if fired slowly a closed form with out a hole wil=
l
> not explode in firing. I have made some of those forms recently and need =
to
> now bisque them. My question is how slow do I need to fire them? What are
> the key temperatures that I should be cautious when nearing to avoid a
> possible explosion? I have a programable paragon.
> Thanks for your help.
> Margie
> Grapevine Tx
> www.pankhurstpottery.com
>
>
> _________________________________________________________________________=
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> Send postings to clayart@lsv.ceramics.org
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>
Crista Nelson on wed 24 may 06
Yes why do you want to take a chance of them blowing up when all you have to
do is put a small pin hole in them to avoid this???
Crista in Mississippi anticipating hurricane season........
In a message dated 5/24/2006 8:41:05 A.M. Central Daylight Time,
susanlists@GMAIL.COM writes:
Why do you want to fire them with no hole?
Susan near Seattle
On 5/24/06, Margie Pankhurst wrote:
>
> I am a long time reader of the postings and find myself with an inquiry. I
> have recently read that if fired slowly a closed form with out a hole will
> not explode in firing. I have made some of those forms recently and need to
> now bisque them. My question is how slow do I need to fire them? What are
> the key temperatures that I should be cautious when nearing to avoid a
> possible explosion? I have a programable paragon.
> Thanks for your help.
> Margie
> Grapevine Tx
> www.pankhurstpottery.com
Susan P on wed 24 may 06
I was thinking that there might be alternatives - and the knowledgable ones
would know - but would there be less danger if there were a very tiny hole
or two during bisque firing that was closed up before the glaze firing?
Susan near Seattle
On 5/24/06, Crista Nelson wrote:
>
> Yes why do you want to take a chance of them blowing up when all you
> have to
> do is put a small pin hole in them to avoid this???
> Crista in Mississippi anticipating hurricane season........
>
> In a message dated 5/24/2006 8:41:05 A.M. Central Daylight Time,
> susanlists@GMAIL.COM writes:
>
> Why do you want to fire them with no hole?
>
> Susan near Seattle
>
> On 5/24/06, Margie Pankhurst wrote:
> >
> > I am a long time reader of the postings and find myself with an
> inquiry. I
> > have recently read that if fired slowly a closed form with out a hole
> will
> > not explode in firing. I have made some of those forms recently and
> need to
> > now bisque them. My question is how slow do I need to fire them? What
> are
> > the key temperatures that I should be cautious when nearing to avoid a
> > possible explosion? I have a programable paragon.
> > Thanks for your help.
> > Margie
> > Grapevine Tx
> > www.pankhurstpottery.com
>
>
> _________________________________________________________________________=
_____
> 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.
>
Elizabeth Priddy on wed 24 may 06
If the reason you don't want to put a small hole in
them is that they are bone dry and you don't know how
without making a mess:
Use a drill with a small drill bit and a slow speed,
or a drill bit in your hand that will pull the clay
away and toward you cleaning the hole as you go.
Try a three hole triangle, less than a centimeter
across to make it decorative.
It's got to sit on something, so there is somewhere on
it for the holes.
Consider them marks of making (it right) if it makes
you feel better about them.
Next go round, put a bead or three wrapped in paper
inside before you close it down and add an aural
aspect to your work.
But for gods sake put a hole in it somewhere. Steam
expansion experiments are for people with plenty of
pots to kill, or kiln, depending on your accent.
Really, put a hole in it and plan for a hole in the
future work.
E
Elizabeth Priddy
Beaufort, NC - USA
http://www.elizabethpriddy.com
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Snail Scott on wed 24 may 06
At 06:17 AM 5/24/2006 -0500, you wrote:
>...I have recently read that if fired slowly a closed form with out a hole
will not explode in firing....My question is how slow do I need to fire
them? What are the key temperatures that I should be cautious when=
nearing...
How fast will depend on several things:
1. the openness of your clay body
2. the volume of the air pocket
3. the thickness of the clay wall
A very open body with a baseball-sized volume=20
can almost be fired normally. With a very=20
tight (fine-grained) body, I don't think I'd=20
even try it at all.=20
The critical temperature is around 200=BAF,=20
depending on your altitude. Precision is=20
pointless here, as most folks' controllers=20
(or oven thermometers, in my case) are not=20
that accurate. So, I candle at about 180=BAF
for a few hours, then gradually ease it up=20
to above 225. This gets it past the main=20
'hump', the danger point when the water of=20
plasticity turns to steam. All clay has this=20
issue; it'snot special to closed forms.=20
They're just more vulnerable to it, if the=20
inner cavity contains moisture when that=20
critical temperature is reached. (Water=20
of hydration will continue to dissociate for=20
many hundred degrees higher, but this seldom=20
causes catastrophe.)=20
Above this temperature, you just need to keep=20
it slow and steady, so that the expanding warm=20
air can escape through the interstices of the=20
open-textured clay body. Air is small; it will=20
manage if given time. My electric kiln isn't=20
too fast anyway, but if yours is, keep it to=20
maybe 100=BA per hour, or less, until red heat.=20
(I don't use a pyrometer, so that's a seat-
of-the-pants estimate based on observation)=20
Once the clay has sintered, it is stronger, and=20
can better withstand the internal pressure of=20
the air even after the body begins to vitrify=20
and 'tighten up'.=20
Again, though, the factors listed above will=20
change this, and the texture of the claybody is=20
(I think) the most important. A really open body=20
may require little babying, but a tight one=20
will need to go very, very slow.=20
As for size, it's the surface-to-volume ratio=20
that's relevant: bigger forms increase their=20
internal volume more than their surface area=20
(presming an identical shape), so more air has=20
to squeeze out though a relatively smaller 'filter'=20
(the clay). (Obviously, some forms will have more=20
surface, and others more volume, but regardless,=20
more surface per unit volume is better.)=20
Thickness is a matter of extremes: too thick and=20
the air just can't escape very well (too far to=20
travel) but very thin work is fragile and=20
vulnerable. I do believe that (within 'normal'=20
ranges,) thickness is the least critical=20
element, though.=20
Now, what we really need is a pressure cooker=20
that can handle 2400=BAF! (NOT!)
-Snail
Margie Pankhurst on thu 25 may 06
Ok Ok so I will unload my kiln and attempt to put a small hole in each =
form. I didnt want the hole to be too conspicuous. I guess it will cover =
with glaze. Thanks for the smack in the head and the possiblity of =
having to vacuum Out my kiln.=20
Margie=20
www.pankhurstpottery.com
Snail Scott on fri 26 may 06
At 06:46 AM 5/25/2006 -0500, Margie wrote:
>Ok Ok so I will unload my kiln and attempt to put a small hole in each
form. I didnt want the hole to be too conspicuous. I guess it will cover
with glaze. Thanks for the smack in the head and the possiblity of having
to vacuum Out my kiln.
Let's clarify the hazard, here! Blowing
into smithereens is a sign of moisture,
only. When a closed form is unable to
release the increasing pressure of the
warm air inside it, it cracks. That's
all. No flying shards. Just a split at
whatever joint was the weakest. As soon
as it splits somewhere, the pressure is
released and that's the end of the damage
unless the split was in a structurally
critical spot.
Moisture = shards.
Stress, whether due to internal pressure
or unequal drying rate or whatever, causes
cracks. Depending on the form, it might
even crack all the way around and fall in
half and stick to whatever was next to it,
but no flying shards.
By the way, a hole covered in glaze may not
help. Dry glaze is often as tough as clay.
I have seen a narrow-necked (bisqued) bottle
clogged with glaze broken in half during a
fast firing, from (apparently) the internal
pressure. If the clay can't release the
pressure anyway, then a hole plugged with
dry glaze may not save it.
Why not fire one in a saggar, and see? We
can type all day about likelihoods and our
own experiences, but only a test with your
clay, with your forms, under your firing
conditions, will be adequately relevant to
your work.
-Snail
moriartymym on tue 30 may 06
I make little rattles of my teapot lid knobs. These are closed forms
with little balls of clay inside and I do not make a pinhole. My
understanding is that if the walls are thin enough and the clay is dry
enough there will be no explosions. I've never had one blow up yet.
My first teacher told me that air is not the enemy. Moisture is the
enemy. Make sure the clay is completely dry, and preheat and bisque
slowly.
Works for me.
MJ
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