Carol Ross on fri 19 nov 04
I'm having a problem with a hand built figure that's drying. [I searched the archives but couldn't
find anything that applied]. There are four S-shaped cracks, two front and two back - nearly
opposite each other and at the same distance from the bottom. They appear to go all the way
thru.
I've never had this type of crack in my work before and am stumped. I did build this piece a little
differently than I usually do - I used "cheese cutter" slabs off the top of a new bag of clay for the
bottom of the piece, thinking it would give me height, quickly. I connected them by scoring and
slipping thoroughly and coil-built the rest from there... The cracks are probably near the top edge
of those slabs.
So... what I'd like to know is... what do you think caused them? and can i repair them now? I've
tried a thick mixture of vinegar, corn syrup and dry clay body but the cracks just keep
reappearing. The piece is not quite dry, somewhere around leather hard and dry...
Help would be much appreciated!
Carol
Marcia Selsor on sat 20 nov 04
Put some toilet paper in with that mix. Soak the paper overnight, throw
the mix in a blender and patch it.
Repeat if necessary. This works with bisque also. You must refire the
bisque, but it works. If the crack is caused by a wieght strain then it
may not work. Sometimes you need to engineer the structure for carrying
weight. You can add the paper patch mix pretty thick on the inside for
reinforcement.
Marcia Selsor
in Montana
Sam Hoffman on sat 20 nov 04
Hello Carol-
I believe what is happening to you is that the s-cracks are following
the spiral from the pug mill. Before premixed clay is bagged, most
clay manufacturers pug the clay in a single auger pug mill, which gives
the clay a certain "memory." By making your slabs from slices right
out of the bag, the spiral is oriented in the worst possible way. The
s-cracks probably form as the clay shrinks and pulls away from itself
along the spiral lines of the pugging process.
Solution- Wedge the clay first, then pound it into a block from which
you can cut slabs.
I think that wedging is just an important part of the process when
hand building with slabs, coils, or pinching, as it is when throwing.
Moisture is distributed more evenly throughout the clay and, more
importantly, the particles get mixed up. By wedging the clay first,
you will eliminate the spiral from the pug mill. (Yes I know, it would
seem that wedging orients the clay particles in a spiral. This may be
true, but wedging clay right before it is used creates much less of a
memory than pugged clay that has sat in a bag for who knows how
long...) Also, by pounding the clay in a block, it is being compressed
and the slabs will be better. You can create as large or small a block
as you like, as opposed to the predetermined square coming straight
from the bag o' clay.
It seems like after having sat for a while, clay needs to be woken up
again. There is a HUGE difference working with a slug of clay cut
right out of the bag versus one that has been wedged a little bit. I
think its also a good process for "waking up" the potter...
Happy wedging,
Sam
S.L. Hoffman Pottery
Corvallis, Oregon
www.samhoffman.com
On Nov 19, 2004, at 10:53 AM, Carol Ross wrote:
> I'm having a problem with a hand built figure that's drying. [I
> searched the archives but couldn't
> find anything that applied]. There are four S-shaped cracks, two
> front and two back - nearly
> opposite each other and at the same distance from the bottom. They
> appear to go all the way
> thru.
>
> I've never had this type of crack in my work before and am stumped. I
> did build this piece a little
> differently than I usually do - I used "cheese cutter" slabs off the
> top of a new bag of clay for the
> bottom of the piece, thinking it would give me height, quickly. I
> connected them by scoring and
> slipping thoroughly and coil-built the rest from there... The cracks
> are probably near the top edge
> of those slabs.
>
> So... what I'd like to know is... what do you think caused them? and
> can i repair them now? I've
> tried a thick mixture of vinegar, corn syrup and dry clay body but the
> cracks just keep
> reappearing. The piece is not quite dry, somewhere around leather
> hard and dry...
> Help would be much appreciated!
>
> Carol
>
> _______________________________________________________________________
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>
katetiler on sun 21 nov 04
I have to agree - the only time I have experienced S-cracks was when I
was cutting slabs of the top of bags of clay & rolling them to make
tiles. About 1 in 5 developed huge S cracks - either during the
biscuit or even the glaze process - very frustrating after I'd spent a
day carving a design into the surface & filling with slip!
When I make my tiles by pounding the clay into a mould, I can still
use the clay without wedging or doing much to it, but if I'm working
with students, I always get them to throw theclay around, slam it into
the table a few times, handle it well before they roll it out.
Sorry I can't suggest any ideas for fixing the work - other than
trying the paper clay method suggested earlier?
Kate
Sam Hoffman wrote:
> Hello Carol-
>
> I believe what is happening to you is that the s-cracks are
following
> the spiral from the pug mill.
> Solution- Wedge the clay first, then pound it into a block
from which
> you can cut slabs.
>
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