Kate Johnson on mon 23 aug 04
Thanks, Snail--
> At 09:48 AM 8/20/2004 -0500, you wrote:
> >I would be scared to DEATH of air pockets, however...
>
>
> Air pockets don't blow up pots. It's moisture
> (in the form of steam) that blows up pots.
> Air pockets can create weak spots that the
> expanding steam will exploit, but they are
> not the cause of the explosion by themselves.
I'm afraid I was guilty of speaking shorthand! What I meant was moisture
trapped in air pockets and expanding when heated (though I still don't know
why the legs blew off those pipkins, they were BONE dry...)
>
> Fairly large air pockets, such as might be
> created by an overlying attached slab, can
> cause expanding air to be trapped. This air
> can expand enough that the attached clay may
> be popped off by the pressure inside the
> pocket. This is NOT an explosion, though.
Is there a phenomena that could be described as an explosion, and if so,
what causes that?
>
> How a big a trapped pocket must be to cause
> trouble depends on several factors. One is
> the speed of firing. With a sufficiently
> slow firing, the expanding air will find its
> way out through the porosity of most clays
> without causing harm. The porosity of the
> clay matters, too, since air can escape from
> a coarse clay far more readily than from a
> 'tight', fine-grained body.
I'm mostly using a grogged terra cotta, at the moment...
So,
> to fire work which is likely to have air
> bubbles, dryness and slow firing (through
> the lower temperature ranges) are key. If
> it's dry enough, and slow enough, you can
> fire damn near anything successfully.
Oh God. Once again the Universe reminds me I am sadly lacking in
PATIENCE. Thank you...I'll have to check with Keith and see how long
he warms the kiln. Unfortunately I'm sort of dependent on what he's able to
do and the time available...anyway, thank you!
So, yet another related question--because of my paranoia about moisture
bearing bubbles, I've been very reluctant to use my own wedged clay. I
thought the purpose of wedging was largely to rid the clay of Those Demon
Bubbles...and truthfully, due to that patience thing, I've always been
afraid I didn't do it LONG enough. (I generally use it straight from the
box and then use up the scraps doing small things or making slip.)
So...from what I'm reading here, it sounds as though I can use my own wedged
clay, even if it has bubbles left, and just dry the heck out of it, yes?
Regards,
Kate Johnson
Graphics/Fine Arts
http://www.cathyjohnson.info/
http://www.epsi.net/graphic/
Snail Scott on mon 23 aug 04
At 07:57 AM 8/23/2004 -0500, you wrote:
...I still don't know
>why the legs blew off those pipkins, they were BONE dry...
Well, nothing is ever truly dry. For one thing,=20
it will never be drier than the humidity level=20
that it's stored at. For another, the clay molecule=20
itself has H2O bonded to it chemically, which must=20
also be driven off, but won't dissociate from the=20
molecule until higher-than-boiling temperatures.=20
>Is there a phenomena that could be described as an explosion, and if so,
>what causes that?
I suppose that when I use the term, I mean a=20
result which happens suddenly and with force,=20
resulting in many splintered shards. This is=20
typical with moisture-related damage.
Joined-on sections can also be damaged by=20
steam, but since they usually just split off=20
at their joint, in one piece, I wouldn't call=20
that an explosion.
Pockets that are filled with trapped air may=20
explode if there is moisture involved, but if=20
the air is dry, and the heat expansion merely =20
exceeds what the porosity of the clay can=20
release, you tend to get cracks and splits,=20
but not multiple shards.
Also, damp clay often tends to blow apart in=20
scale-like layers, roughly parallel to the=20
surface of the work. Air-pressure problems=20
(or shrinkage cracks, or dunting) tend to create
cracks that run perpendicular to the surface.
>I'm mostly using a grogged terra cotta, at the moment...
That sounds nice and coarse - and forgiving!
>...i'll...see how long
>he warms the kiln. Unfortunately I'm sort of dependent on what he's able=
to
>do and the time available...
If in doubt, can you put the work in a home=20
oven just before loading? Overnight at 200=BAF
will do nicely, then ramp it up a bit, and=20
deliver it to the kiln wrapped in a cotton=20
towel to keep it warm. Most explosions happen=20
in the very low temperature range, below=20
400=BAF, so even a modest pre-heat can help.
>...I've been very reluctant to use my own wedged clay. I
>thought the purpose of wedging was largely to rid the clay of Those Demon
>Bubbles...and truthfully, due to that patience thing, I've always been
>afraid I didn't do it LONG enough. (I generally use it straight from the
>box and then use up the scraps doing small things or making slip.)
That is one reason to wedge; the other is=20
to align the particles. This is useful for=20
wheel-throwing, and can make a discernable=20
difference in the throwing properties of a=20
clay. For handbuilding, however, this alignment=20
is much less critical.=20
Most commercially-processed clay bodies are=20
pugged and de-aired before sale, so you are=20
generally fine in using such clays directly=20
from the bag. In fact, unskilled wedging can=20
actually introduce air bubbles into previously=20
airless clay (though it may still align the=20
particles).=20
Personally, I seldom wedge any clay intended for=20
handbuilding, especially coarse bodies. Clay=20
that has been frozen can benefit from some=20
wedging, as can unpugged clay, but otherwise, I=20
skip it - it offers no benefit to my work. I=20
haven't wedged in a decade, except to stay in=20
practice for teaching, or for (very occasional)=20
throwing.
A lot of teachers talk about wedging as though=20
it were mandatory. It's probably a good idea=20
to start that way - otherwise half the students=20
would skip learning it - but though it is quite=20
helpful for throwing, its benefit to handbuilding=20
is negligible in most cases.
>
>So...from what I'm reading here, it sounds as though I can use my own=
wedged
>clay, even if it has bubbles left, and just dry the heck out of it, yes?
Dry is good. Slow firing is even better. Coarse=20
clay helps.
-Snail Scott
Kate Johnson on mon 23 aug 04
Well, nothing is ever truly dry. For one thing,
it will never be drier than the humidity level
that it's stored at.
Ah, forgot about that, and Missouri in the summer is HUMID.
For another, the clay molecule
itself has H2O bonded to it chemically, which must
also be driven off, but won't dissociate from the
molecule until higher-than-boiling temperatures.
Erf...
>Is there a phenomena that could be described as an explosion, and if so,
>what causes that?
I suppose that when I use the term, I mean a
result which happens suddenly and with force,
resulting in many splintered shards. This is
typical with moisture-related damage.
We see that at school occasionally, so I usually beg him to put my stuff on
a shelf by itself.
>I'm mostly using a grogged terra cotta, at the moment...
That sounds nice and coarse - and forgiving!
It also does better when using the pots over embers, to cook in them...
>...i'll...see how long
>he warms the kiln. Unfortunately I'm sort of dependent on what he's able
to
>do and the time available...
> If in doubt, can you put the work in a home
oven just before loading? Overnight at 200ºF
will do nicely, then ramp it up a bit, and
deliver it to the kiln wrapped in a cotton
towel to keep it warm. Most explosions happen
in the very low temperature range, below
400ºF, so even a modest pre-heat can help.
Excellent, thank you, yes I can! That's where I usually store my greenware
anyway, to keep it away from the monster kitties.
>...I've been very reluctant to use my own wedged clay. I
>thought the purpose of wedging was largely to rid the clay of Those Demon
>Bubbles...and truthfully, due to that patience thing, I've always been
>afraid I didn't do it LONG enough. (I generally use it straight from the
>box and then use up the scraps doing small things or making slip.)
> That is one reason to wedge; the other is
to align the particles. This is useful for
wheel-throwing, and can make a discernable
difference in the throwing properties of a
clay. For handbuilding, however, this alignment
is much less critical.
Good! I've certainly not had any problems so far, anyway...
> Personally, I seldom wedge any clay intended for
handbuilding, especially coarse bodies. Clay
that has been frozen can benefit from some
wedging, as can unpugged clay, but otherwise, I
skip it - it offers no benefit to my work. I
haven't wedged in a decade, except to stay in
practice for teaching, or for (very occasional)
throwing.
The only stuff I wedge is left over from what I'm building--trimmings and so
forth, so I can reuse them, or I make very small stuff with it.
>A lot of teachers talk about wedging as though
it were mandatory. It's probably a good idea
to start that way - otherwise half the students
would skip learning it - but though it is quite
helpful for throwing, its benefit to handbuilding
is negligible in most cases.
And so I did, at age 7, and still thought it was mandatory unless you bought
boxed clay ready to go!
>
>>So...from what I'm reading here, it sounds as though I can use my own
wedged
>clay, even if it has bubbles left, and just dry the heck out of it, yes?
> Dry is good. Slow firing is even better. Coarse
clay helps.
Great! I haven't had any problems for a couple of years, but am having
rather a lot of leftover stuff right now and I hate to waste it...
Thanks again!
Best--
Kate
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