Bruce Girrell on mon 25 jan 99
I would like to add thisto the wedging discussion:
Most texts that I have read mention wedging as something that you need
to do.
A few attempt to describe the process, but devote little more that a few
sentences to it. Maybe I missed a tape here or there, but I haven't seen
the wedging process in any of RobinHopper's tapes and Stephen Jepson
covers
it for about fifteen seconds in his - no close-ups or slo-mo.
Such a fundamental operation, and it seems as though we're simply
supposed
to know how it works.
I attempted to follow the directions in the books and I watched the
Jepson
segment time after time. No matter what I did, I seemed to do something
in
the wedging process that would cause a flap of clay to fold over the
main
lump that I was working. To me, that means that I just trapped some more
air
into the lump.
I have developed a rhythm of my own now that seems to work as evidenced
by
encountering fewer bubbles during throwing, but I would ask anyone who
is
considering doing a book or video to please spend some time on this
subject.
Many of us, particularly the beginners, do not process sufficient volume
or
have sufficient money to warrant the purchase of a pug mill.
Bruce "my rant for the day" Girrell
Jim and Marge Wade on thu 29 mar 01
One more question, and you can be sure I will NOT mention the word
"sanding". Do air bubbles in pots cause them to explode/crack during a
bisque firing? I've heard different answers from different potters.
Thanks.
Marge
CINDI ANDERSON on thu 29 mar 01
Oh goody. The experts are at NCECA and there is a question I know the answer to!
No. That is a myth. The only thing that causes explosions / cracks is moisture.
Cindi
Jim and Marge Wade wrote:
> One more question, and you can be sure I will NOT mention the word
> "sanding". Do air bubbles in pots cause them to explode/crack during a
> bisque firing? I've heard different answers from different potters.
>
> Thanks.
>
> Marge
>
vince pitelka on fri 30 mar 01
> One more question, and you can be sure I will NOT mention the word
> "sanding". Do air bubbles in pots cause them to explode/crack during a
> bisque firing? I've heard different answers from different potters.
Marge -
The air bubbles do not cause any problem at all. However, if there are
air-pockets large enough to allow steam pressure to gather, then you run
into problems. The worst situation is when there is a network of air
pockets where a piece has been joined together with insufficient pressure
and/or slurry, and thus the gathering steam pressure in air pockets along a
common plane will cause the whole addition to pop off in the firing.
The only time you get anything resembling "explosion" is when you heat a
very thick piece up too quickly, and that has only to do with steam
pressure, or when you heat a piece which has significant-sized closed-air
spaces with no vent holes at all. In the latter case the piece can explode
with sufficient power to destroy other pieces in the kiln.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
Bruce Girrell on fri 30 mar 01
> Do air bubbles in pots cause them to explode/crack during a
> bisque firing?
Provided that you fire sufficiently slowly, air bubbles in the clay will
have no effect.
The expansion of air as a function of temperature is very small compared to
the increase in volume as water turns into steam. If your firing schedule
allows steam to escape from the interior (middle of the clay wall) of your
pot at an appropriate rate, then air can certainly escape, too.
Having said that, I would also have to agree that a pot wall with an air
bubble in it will be a weaker wall than one without the bubble.
First, the bubble represents absence of clay material that otherwise would
help hold the piece together. A cross-section of the pot wall through the
air bubble would show that the wall has become two thinner walls. This
certainly must be weaker than the full wall thickness.
Secondly, the bubble serves as a place where steam can accumulate and can
serve as a stress concentrator that help promote failure. On the other hand,
since the distance from the air pocket to the outer surface of the wall
(where the steam can escape) is necessarily less than the distance from the
center of the wall to the outside, then the steam has an easier time
escaping.
So the final answer is that air bubbles can weaken a pot wall, but the
expansion of air trapped in the bubble is not the cause of breakage and that
if you fire your pots at a rate that allows steam to escape, the air bubble
should have little or no effect.
BTW steam is produced in large quantities at _two_ points in the firing. The
obvious one is at 100 oC, where the water of plasticity is driven off. The
second point is near 600 oC, where the hydroxyl radicals are driven from the
clay and the clay becomes pot. Your firing schedule should include a
slowdown starting about 500 oC and ending about 650 oC. This not only allows
the steam to escape during the dehydroxylization, but also decreases thermal
gradients that could cause cracking due to alpha/beta quartz inversion. Most
cracking due to free silica inversion happens during cooling, though.
Bruce "confusing enough?" Girrell
Jay Jensen on fri 30 mar 01
I have believe they can cause blow outs. But I also
belive if something is bisqued slowly air bubbles
shouldn't be a problem. And yes I dont want to talk
about sanding anymore
Jay
www.mrpots.net
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David Eickholt on fri 30 mar 01
Hi Marge, I high fire and seems that the air bubbles go though a slow bisque
just fine, [if the piece is bone dry] then after the glaze they appear as
bloats and ruin the piece. Have had pieces explode when rushing work a
several years back, no longer rush work and am more careful to eliminate any
air bubbles when throwing also have had toswitched to deaired plugged clay in
the box carpel tunnel trouble Dave
Burns Christina on fri 30 mar 01
As I understand it IF the air bubble has moisture in it, it can cause the
pot to explode in the kiln. The problem is, moisture can and sometimes does
stay in an air bubble and you have no way of knowing it is there. If you
think your pieces have air bubbles that may contain moisture - candle them
in the kiln in hopes that the heat will evaporate the water.
-----Original Message-----
From: Jim and Marge Wade [mailto:wadeosd@EARTHLINK.NET]
Sent: Thursday, March 29, 2001 10:55 PM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Air bubbles
One more question, and you can be sure I will NOT mention the word
"sanding". Do air bubbles in pots cause them to explode/crack during a
bisque firing? I've heard different answers from different potters.
Thanks.
Marge
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Charles on fri 30 mar 01
Absolutely NOT! Air bubbles are often ugly, make a piece more likely to be
chipped because of the thin walls around an air bubble, and they act as
little speedbumps when you throw. The only thing that causes pots to explode
in the kiln is water vapor. If a pot explodes in the kiln, it is because the
operator of the kiln has misjudged the time required to drive off moisture.
During our summer camps, I often load still flexible pieces into the kiln
and fire them. The only pieces that have ever blown up in my kilns are my
own pieces, because I am often willing to take risks with my own work that I
wouldn't with someone elses.
BURST the air bubble myth!
Best Wishes,
Charles Hughes
Check out our AMAZING workshop series
Pottery in Central PA will never be the same!
Next is Ian Stainton, a potter with technical skills surpassing ANYONE'S I
have ever seen
This is the man who can shore up ANY throwing problems you may have.
Check him out!
http://www.thecreativeoasis.com/SP2001Workshops/workshop1.html
Visit my webpage...
http://www.thecreativeoasis.com/Hughes/hughes.html
>
>
Snail Scott on fri 30 mar 01
At 10:54 PM 3/29/01 -0600, you wrote:
>One more question, and you can be sure I will NOT mention the word
>"sanding". Do air bubbles in pots cause them to explode/crack during a
>bisque firing? I've heard different answers from different potters.
>
>Thanks.
>
>Marge
>
It's not the air bubbles that do it, it's the steam that
accumulates in those bubbles as the moisture is heated.
The steam pressure is the force that blows things up.
Thorough drying and slow candling past the boiling point
of water will reduce explosions significantly. 'Open'
clay bodies, with coarse texture, will allow more of the
steam to escape through the porosity ofthe clay than a
'tight' or smooth clay will, also.
Air bubbles alone aren't the culprit; they are a facil-
itator, though. No air bubbles = no place for large
volumes of steam to gather in one spot under pressure.
I often work in multi-part forms, which are joined after
firing. To 'true up' the connection, I often use a big
angle-grinder. I have often found air pockets half an inch
across after grinding down the surface, because my working
technique is loose and rather casual. I've never had an
explosion because of them, though. This is because my clay
is very groggy, and the pressure simply escapes through
the clay itself.
Some people (like Mary Barringer) even make closed forms
without vent holes, enclosing volumes as large as a quart
or so of air. They rely on the open texture of their clay
body to allow the pressure to escape. Every clay has a
different tolerance for this, but thorough drying and
slow candling will improve the chances of any clay to
withstand a few air pockets.
-Snail
william schran on fri 30 mar 01
Air bubbles do not cause the kind of explosion that destroys the pot
and others around it. That's from water still in the clay. Air
pockets can cause small pops, splits, cracks.
Bill
kruzewski on sat 31 mar 01
Yes they do if there is no way the air can get out as it expands when it is
heated.A tiny pin hole is enough to allow trapped air to escape. I used to work in
a commercial pottery that also ran classes. Occasionally there were explosions that
ruined a lot of other work too - you could often see from the remains of the
exploded pot where the air bubble had been.
Jacqui Kruzewski
Wales, UK
Jim and Marge Wade wrote:
> One more question, and you can be sure I will NOT mention the word
> "sanding". Do air bubbles in pots cause them to explode/crack during a
> bisque firing? I've heard different answers from different potters.
>
> Thanks.
>
> Marge
>
> ______________________________________________________________________________
> 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.
kruzewski on sat 31 mar 01
Before I get jumped on for my earlier comment about air bubbles - I should have
read everyone elses's comments before putting my two-penny worth in - I'm new to
Clayart, forgive me.
I'd always accepted that as well as firing damp work too quickly, air pockets could
cause explosions - not just in class work but also in some of the joined and
sprigged pieces. They always seemed to be absolutely dry - some of the class stuff
hung around for months before it was fired - but, as someone said, you never know
completely.
It's only happened to me once - a sprig blew off and I could see the air pocket
that was responsible. My clay is fairly fine.
I stand corrected and educated.
Jacqui Kruzewski
Wales, UK
iandol on sun 2 dec 01
Dear Lajos Kamocsay,
Perhaps Steve Branfmann is using a fairly broad brush to paint his =
picture here but I think he is justified.
It is basically true that free water present in clay will cause minor to =
major explosions which destroy pots even when the rate of heating is =
slow.
The science behind this is interesting. Eighteen grams of water(about =
two thirds of an ounce) can change instantaneously into about twenty =
eight litres (almost six gallons)of steam at 100 deg Celsius. So, =
roughly, a tenth of an ounce of water can give about a gallon of steam, =
which is a lot of space to find in the base of a five pound pot. I =
understand the noise is a muffled KKRRUUUMMMMPPPP!!! and I know from =
experience that the result is a lot of ceramic shrapnel, showing that =
steam seeks out any and every weakness is the fabric of clay
An air bubble expands by about 1/273 rd of its volume for every degree =
(C) rise in temperature. Since this is such a slow process in most =
firings the air has time to diffuse from the clay which is porous. But =
the cavity remains and might cause pain if it fills with water drawn =
from a glaze. If this is not allowed to dry fast firing of a raku might =
lead to a disaster.
I also feel that the water which is released during the decomposition of =
clays and other materials which contain Hydroxyl (OH) groups can cause =
pots to suffer from fractures if time is not allowed for the water =
vapour to diffuse from the clay. In a similar fashion, we assume that =
oxygen from the air in a kiln will diffuse into the clay and assist in =
burning organic residues.
Most of us take adequate precautions, allow time for drying and fire =
slowly. As you are finding out, such disasters are fairly rare events.
Best regards and enjoy your claywork,
Ivor Lewis. Redhill, South Australia.
Mud Hen on tue 4 dec 01
I was wondering if this air bubble/water problem works the same with
sculpted pieces. I share our high school art room with another teacher who
has her grade 12 class create clay heads. There is often a problem where
the students have built up areas like the chin or cheeks. The air pocket
can usually be seen after the breakage in the kiln.
I usually test pieces for dryness by temperature of the touch, but the
pieces my students create are alot smaller than the grade 12's work. How
does one tell with bigger pieces when they are dry?
thanks for all of the insight and knowledge.
mudhen
Cindy Strnad on wed 5 dec 01
Dear Mudhen,
First, the air pockets: Yes, they can cause
breakage, particularly if firing is not slow
enough. They don't cause explosions, as water
vapor can, but they can rupture a piece. Best, in
my opinion, to place an unobtrusive pinhole into
closed air spaces to prevent this, even though it
is possible to fire closed forms without this
precaution.
Second, how to tell if a large piece is dry: it
just takes a lot of time. In addition, the larger
pieces have thicker walls which hold more
atmospheric moisture even after dry. The only way
to be safe is to soak these pieces at below
boiling for sufficient time. For some very large,
thick pieces, this may be several days or longer.
Best wishes,
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
cindy@earthen-vessels-pottery.com
http://www.earthen-vessels-pottery.com
Snail Scott on wed 5 dec 01
At 10:06 PM 12/4/01 -0500, you wrote:
>I was wondering if this air bubble/water problem works the same with
>sculpted pieces...
It's usually less severe, mainly because sculpture
clays tend to be more open and coarse than throwing
clays. Also, (my personal, unfounded theory) throwing
seems to somewhat compact the clay particles, aligning
and 'nesting' them together, so even if the clay body
is the same, steam has a harder time escaping safely
from thrown clay. On the other hand, sculpture is often
made LOTS thicker than thrown pottery, so the steam has
a lot further to travel, and the parts that blow off
tend to be bigger and more damaging to the other work
in the kiln.
>How does one tell with bigger pieces when they are dry?
Clay has the wonderful property of distributing its
moisture fairly evenly as it dries, (more so than most
materials,) if it dries slowly enough. So, the 'touch
test' is still valid. Beware of clay that's been fast-
dried, though, where the moisture on the inside hasn't
had time to migrate out, but the outside surface is
deceptively dry. When in doubt, candle low and slow!
Note: I've noticed that when those over-thick, built-up
student sculptures explode, the split usually occurs
at the site of a joint - where one layer was applied
over another, and the outer layer blew off. Sometimes
these joints show a lot of score marks. Many students
treat the 'score and slip' procedure as a sort of
ritual/voodoo process, and don't understand that if the
slip doesn't fully fill the score marks, they're just
making lots of air pockets.
Deep scoring does NOT create a stronger joint, and
scores made with a needle tool have narrow pointy
bottoms that never fully fill with slip. Let them
score only with blunt tools! (Forks, wood tools, or
serrated ribs, etc!)
More personal unsubstantiated theory: I think that
the lack of a really good connection between the outer
and inner layer of clay is substantially to blame for
such explosions. The moisture from the inner parts of
the sculpture is trying to migrate out and evaporate,
but is trapped in the inner layer by the lack of a
continuous clay structure. (Sort of a geological
discontinuity.) So, since the moisture can't flow freely
out, some of it gets trapped behind the joint, and all
those little unfilled score marks and gaps between the
layers give the steam a place to build up pressure.
Any comments on this idea, pro or con?
I try to get students to remove the section and rebuild
rather than applying extra thickness, but they don't
always know they're going to get that thick when they
start! If the inner surface of the form is accessible,
I try to get them to carve it back from the inside.
When that's not possible, this is what I do:
Pierce the thick part all over, about every half-inch
or so, all the way through to the inner cavity of the
piece. Then just smooth over all those holes on the
outside. This way the steam can vent to the inside of
the piece, and the inside itself can be adequately
vented with just a couple of small holes in unobtrusive
places. If the standard needle tool is too short to
pierce all the way into the inner cavity, a sharpened
bit of thin welding rod does very well instead.
It's VERY important that the vents go all the way in!
(If they don't, they just make another air pocket to
trap steam in.) For venting the inner cavity to the
outside, a single needle-tool hole is usually
sufficient even for medium-sized (1-2 cu.ft. interior)
sculpture, though I generally use two or three, for
insurance.
-Snail
Andi Fasimpaur on wed 5 dec 01
At 10:06 PM 12/4/01 -0500, you wrote:
>I was wondering if this air bubble/water problem works the same with
>sculpted pieces. I share our high school art room with another teacher who
>has her grade 12 class create clay heads. There is often a problem where
>the students have built up areas like the chin or cheeks. The air pocket
>can usually be seen after the breakage in the kiln.
Talk to the grade 12 sculpture teacher about possibly switching her students
over to paper clay... there is less concern with blowups from trapped
steam (which in my experience is the real culprit with the air pockets) since
the paper burns out and leaves a very open claybody...
Another essential of firing heavier sculpture work is slow firing, I always
candled my sculpture firings overnight, and then did the slowest possible
bisque firing, all the spys open etc... the slow firing schedule really seemed
to minimize the loss with any student works... (an important lesson when
firing the work of very young children who haven't yet learned that pots break.
and that they shouldn't be attached to anything that isn't completely finished,
and sometimes not even then...)
Best wishes,
Andi
who went to a book signing today for a young adult book written by a friend
of my husband (Beatnik Rutabagas From Beyond the Stars, by Quentin Dodd)
and who then came home and couldn't even produce a decent pinch pot in
2 hours in the studio... I've given up... I'm going to catch up on clayart,
and
enjoy quality time with Alpheus, my 19 week-old kitten, currently asleep in
a bag slung over my sholders... his favorite place to be, and good practice
for me I'm sure... I think we're going to go take a nap...
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