Debbie Penley on fri 9 apr 10
I just recently started using a stoneware clay after using earthenware fo=
=3D
r a=3D20
couple of years. I LOVE the way the clay feels when working it, very stur=
=3D
dy=3D20
and durable.
However, I've noticed that in very simple forms I'll see cracks appear du=
=3D
ring=3D20
drying that don't seem related to any obvious stress on the clay (not in =
=3D
joins or=3D20
places where the clay was bent, etc.)
Then when I bisque fire, more cracks appear, and when glazing (to cone 6)=
=3D
,=3D20
even more cracks appear or get larger.
I'm wondering about two things. Could this clay have more "memory" than m=
=3D
y=3D20
earthenware? Similar to a porcelain? How do I know how far I can "push" t=
=3D
he=3D20
clay when it's in a chees-hard state without getting cracks later on?
When I fired my kiln the last two times, the firing was super-fast, I thi=
=3D
nk due to=3D20
a kiln shelf being removed from near the bottom of the kiln thus making i=
=3D
t more=3D20
efficient as it's emptier. Anyway, I'm not sure if that might have also=3D2=
0=3D
contributed to my recent problems or not.
Any ideas would be helpful.
Debbie Penley
Debbie Penley on sat 10 apr 10
"You'll need to be a bit more specific where the cracks are occurring on =
=3D
the
pot to allow us to answer your question.
How are the pots formed? Wheel thrown? Hand built?"
It's Standard Clay 240=3D20
(http://www.standardceramic.com/PorcelainsWhiteStoneware.html)
On small pieces, I sculpt the piece, then carve it out, and the cracks ap=
=3D
pear=3D20
inside the piece while drying or after firing. Small cracks, but ones tha=
=3D
t weren't at=3D20
all there when the clay was leatherhard.
On the larger pieces, also sculpted and then carved out, the cracks were =
=3D
mostly=3D20
on the bottoms of the pieces, where the bottoms tried to push up into the=
=3D
=3D20
pieces, but some were on the sides of the pieces just randomly, little cr=
=3D
acks (not=3D20
where the joins were from cutting the piece in half, though.
Debbie
paul gerhold on sat 10 apr 10
Debbie,
What is the clay? Are you working with slabs or throwing and then
modifying? What do you mean bu the term "pushing the clay when it is
cheese-hard" and is this where the cracks appear. If you push on the clay
from one side do you compress back on the other side?
Paul
On Fri, Apr 9, 2010 at 6:57 PM, Debbie Penley wrote:
> I just recently started using a stoneware clay after using earthenware fo=
r
> a
> couple of years. I LOVE the way the clay feels when working it, very stur=
dy
> and durable.
>
> However, I've noticed that in very simple forms I'll see cracks appear
> during
> drying that don't seem related to any obvious stress on the clay (not in
> joins or
> places where the clay was bent, etc.)
>
> Then when I bisque fire, more cracks appear, and when glazing (to cone 6)=
,
> even more cracks appear or get larger.
>
> I'm wondering about two things. Could this clay have more "memory" than m=
y
> earthenware? Similar to a porcelain? How do I know how far I can "push" t=
he
> clay when it's in a chees-hard state without getting cracks later on?
>
> When I fired my kiln the last two times, the firing was super-fast, I thi=
nk
> due to
> a kiln shelf being removed from near the bottom of the kiln thus making i=
t
> more
> efficient as it's emptier. Anyway, I'm not sure if that might have also
> contributed to my recent problems or not.
>
> Any ideas would be helpful.
>
> Debbie Penley
>
William & Susan Schran User on sat 10 apr 10
On 4/9/10 6:57 PM, "Debbie Penley" wrote:
> However, I've noticed that in very simple forms I'll see cracks appear du=
ring
> drying that don't seem related to any obvious stress on the clay (not in =
joins
> or
> places where the clay was bent, etc.)
You'll need to be a bit more specific where the cracks are occurring on the
pot to allow us to answer your question.
How are the pots formed? Wheel thrown? Hand built?
Stoneware usually is a bit denser than earthenware, so it is a good idea to
dry slower to prevent cracks.
Post back with more specifics.
Bill
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
William & Susan Schran User on sat 10 apr 10
On 4/10/10 9:11 AM, "Debbie Penley" wrote:
> It's Standard Clay 240
> (http://www.standardceramic.com/PorcelainsWhiteStoneware.html)
>
> On small pieces, I sculpt the piece, then carve it out, and the cracks ap=
pear
> inside the piece while drying or after firing. Small cracks, but ones tha=
t
> weren't at
> all there when the clay was leatherhard.
>
> On the larger pieces, also sculpted and then carved out, the cracks were
> mostly
> on the bottoms of the pieces, where the bottoms tried to push up into the
> pieces, but some were on the sides of the pieces just randomly, little cr=
acks
> (not where the joins were from cutting the piece in half, though.
Cracks are areas of stress are relieved, as the clay dries and shrinks.
Some clays, like porcelains, that are composed of small particles, will hav=
e
more of a tendency to have this happen.
Earthenware and stoneware clays containing grog and multiple sized clay
particles will have less of a tendency to crack.
Two things that you must do.
#1 - have as equal a wall thickness as possible. I would bet if you checked
carefully, all of your cracking is occurring in areas where the clay is
thinner than other adjacent areas.
#2 - dry slowly. You must try to have the entire piece dry at all the same
rate. Covering with plastic, not exposing to air currents, perhaps even
spraying plastic with water will help prevent cracking, but only id #1 is
adhered to.
You might want to consider changing to a sculpture clay, having higher
amount of grog, that will be less prone to cracking. If you want a nice
smooth surface, the sculpture clay can be coated with slip.
Bill
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
David Finkelnburg on sat 10 apr 10
Debbie,
Isn't it frustrating to have cracks develop in your work? They must be
one of the most frustrating things about making ceramic art.
Every clay body is different in terms of how it can best be worked. Yo=
u
are learning about this new clay. :-(
Firing speed can be an issue, but it really depends on the shape of you=
r
work and the thickness of your work. If some work is surviving and some is
not, then you can judge from what is different about the work that makes
it through OK. In general, though, 150 to 300 degrees F/hour is not an
unreasonable heating rate for work that is 1/2-inch or less thick.
If you are working the clay when it is cheese hard, you may want to try
doing more when the clay is even softer, if possible.
As you seem to recognize, how you work the clay matters. A good way to
try to understand the cracking problem is to remember that ALL cracks (and
warps too) are due to parts of a piece shrinking at different rates or by
different percentages, or both. Clay is very weak in tension and is easily
pulled apart so we see cracks as a result. Clay that has been compressed b=
y
working it will shrink less than clay which has not been. Thus the clay tha=
t
has been worked the least becomes most likely to crack. Clay which dries
fastest, such a rims or handles or other attachments, will tend to throw
moister areas of a piece into tension so that the last part to dry tends to
be the first part to crack.
Have you tried looking at any of your greenware with a magnifying
glass? A simple, cheap 10X hand lens is excellent for this. Examine the
areas where, based on experience, you expect cracks to form. Often the
cracks in greenware are very small and hard to see with the naked eye but
are there. They just become more evident after the first firing, which
isn't really at fault, but just reveals the cracks that formed in the
drying. Unlike with porcelain, stoneware cracks get worse in a second
firing. Again, examine the bisque closely.
Hope this helps. If you keep having these problems try taking some
photos so others can see what has happened.
Good potting!
Dave Finkelnburg
http://www.mattanddavesclays.com
-----------------------------
Date: Fri, 9 Apr 2010 18:57:51 -0400
From: Debbie Penley
Subject: Large cracks appear during drying/firing with handbuilt forms
I just recently started using a stoneware clay....I've noticed that in very
simple forms I'll see cracks appear during drying that don't seem related t=
o
any obvious stress on the clay (not in joins or places where the clay was
bent, etc.)
Then when I bisque fire, more cracks appear, and when glazing (to cone 6),
even more cracks appear or get larger....
paul gerhold on sun 11 apr 10
Dear Debbie,
Stoneware is going to be more difficult to work with than earthenware
because you are firing higher and probably going to be more vitreous. That
said the clay you are using is very plastic so probably has a high shrinkag=
e
which is asking for trouble. Strongly suggest standard 295 or equivalent o=
r
other good sculpture clay. You want clay with as low a shrinkage as
possible which usually means lots of grog which also promotes more even
drying.
Work the clay as wet as possible. Push against outside of piece when
carving out inside to reduce stress. Foam rubber can be useful here. Do
nor rewet the piece if at all possible and dry very slowly. Burnishing
won't hurt and may help unless you are doing it too dry and having to use a
lot of force.
You can also fire lower which can help reduce shrinkage. You don't say if
you are doing pieces that go outside which may cause different concerns tha=
n
work that doesn't need to be weatherproof.
Paul
On Sat, Apr 10, 2010 at 9:11 AM, Debbie Penley wrote=
:
> "You'll need to be a bit more specific where the cracks are occurring on
> the
> pot to allow us to answer your question.
> How are the pots formed? Wheel thrown? Hand built?"
>
> It's Standard Clay 240
> (http://www.standardceramic.com/PorcelainsWhiteStoneware.html)
>
> On small pieces, I sculpt the piece, then carve it out, and the cracks
> appear
> inside the piece while drying or after firing. Small cracks, but ones tha=
t
> weren't at
> all there when the clay was leatherhard.
>
> On the larger pieces, also sculpted and then carved out, the cracks were
> mostly
> on the bottoms of the pieces, where the bottoms tried to push up into the
> pieces, but some were on the sides of the pieces just randomly, little
> cracks (not
> where the joins were from cutting the piece in half, though.
>
> Debbie
>
Snail Scott on sun 11 apr 10
On Apr 9, 2010, at 5:57 PM, Debbie Penley wrote:
> I just recently started using a stoneware clay after using
> earthenware for a
> couple of years. I LOVE the way the clay feels when working it, very
> sturdy
> and durable.
> However, I've noticed that in very simple forms I'll see cracks
> appear during
> drying that don't seem related to any obvious stress on the clay...
Personally, I more often have trouble with
earthenware. Every clay body is different,
though, and 'stoneware' describes a lot of
territory. I'd try samples of other stonewares,
and see if your problem is diminished.
You say you are 'sculpting, then hollowing'.
'Sculpting' is an awfully general term, but I
assume you mean working solid and then
scooping out the thick middle and reassembling
the form afterward. This method has its pros
and cons, but it's important to watch out for
uneven thickness. This may or may not be
relevant to your cracking issue, but uneven
drying (often due to thickness discrepancies)
and its resulting stresses are probably the
biggest contributors to cracking clay at all
phases, from greenware to vitrification.
-Snail
Michael Wendt on sun 11 apr 10
Debbie,
I recommend wire wedging in a side by side trial with clay
right out of the bag.
Why?
There is a common misconception that pugged clay is ready to
use right out of the bag. Generally, this could be true for
freshly pugged clay. I use it without wedging to throw
because I mix it myself and pug it again just before use.
Stored clay starts to develop moisture differences due to
condensation on the inside of the bag.
Many manufacturers add bentonite in the dry form to their
clay bodies so after a while, the bentonite takes on the
water around it and if it is the high swelling variety,
increases shrinkage in localized areas leading to odd and
intermittent cracking issues.
Manufacturers could reduce this effect by blunging the
bentonite with the mixing water first and then blending that
water into the rest of the dry mix. Since I went to blunging
the V-Gum T I add to my clay, cracking has been virtually
nonexistent.
I think even 10 doublings for sculpture would make a huge
difference in the cracking rate.
Please try it
Regards,
Michael Wendt
The question:
How are the pots formed? Wheel thrown? Hand built?"
It's Standard Clay 240
(http://www.standardceramic.com/PorcelainsWhiteStoneware.html)
On small pieces, I sculpt the piece, then carve it out, and
the cracks appear
inside the piece while drying or after firing. Small cracks,
but ones that weren't at
all there when the clay was leatherhard.
On the larger pieces, also sculpted and then carved out, the
cracks were mostly
on the bottoms of the pieces, where the bottoms tried to
push up into the
pieces, but some were on the sides of the pieces just
randomly, little cracks (not
where the joins were from cutting the piece in half, though.
Debbie
Snail Scott on mon 12 apr 10
> On Apr 9, 2010, at 5:57 PM, Debbie Penley wrote:
>
>> I just recently started using a stoneware clay after using
>> earthenware for a
>> couple of years. I LOVE the way the clay feels when working it, very
>> sturdy
>> and durable.
>> However, I've noticed that in very simple forms I'll see cracks
>> appear during
>> drying that don't seem related to any obvious stress on the clay...
Another thought on this issue:
You say you love the feel of the clay; different
from your earthenware. So, it presumably has
different handling characteristics, i.e. different
plasticity, etc.
A common problem when people build solid
and then hollow the forms out is that the clay
wall can get bent, stretched, altered, etc from
handling at this phase, when the clay may be too
stiff to accommodate bending without cracking.
These cracks will tend to be spidery cracks in
apparently random areas, especially on broad
areas with minimal structure to reinforce them.
They may not be evident or obvious at the time
if the piece gets patted back into shape during
reassembly, but the stresses are there, like a
cracked brick wall under wallpaper, waiting to
manifest later on during drying and firing.
If this is the source of your trouble, you might
try other clay bodies, or wait longer before
hollowing out, or be very careful not to flex
the clay walls. Solid-built clay seems to have
more issues this way than hollow-formed
shapes, perhaps because when the clay is
formed hollow to begin with, the particles get
aligned more thoroughly and deeply into the
clay wall by joining coils or rolling slabs, etc.
Solid-formed clay seems to retain a more
random particle structure and is thus a bit
weaker at the same thickness than the same
shape when hollow-formed. (No data to
support this, just personal observation.)
I'd try letting the clay get stiffer, first, then try
some other options.
-Snail
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