Dennis, Rhonda, Rachel Oldland on sat 27 may 00
I responded to an email regarding s-cracks. I had the problem years ago
(1988) and found by turning my pots upside down after the upper part had
dried seemed to cure my problem, but while skipping through the potter's
primer my eyes went directly to this peice of information that might be the
source of the problem or not."Tension THE BASE" Press the base down firmly
and move your thumb or forefingers from the center out to the sides two or
three times, taking care not to thin the base too much.This presses the
clay particles in the same way that the clay walls will be pressed
,aligning them so they can dry more evenly , and preventing S-shaped cracks
from forming in the base during drying.
Maybe I began doing that the same time I began to flip my pots !!!! either
way I haven't had the problem since and maybe this will help...best regards
Rhonda
iandol on thu 1 jun 00
Subject: s-shaped cracks update
I suppose every one who throws clay for a prolonged period will at some =
time or other run into the problem of S-cracks. Perhaps Rhonda Oldland =
would give more information about the clay primer from which she quotes =
to describe a way of tensioning the base of a pot. The term Tension is a =
change from Compress. I know about the technique she describes but have =
always found that when I apply the sort of pressure which I discern is =
needed I finish up with my finger or thumb going through the clay to the =
wheelhead if I move from the centre to the inner base of the wall.=20
There is another thing which continues to concern me. It is descriptions =
of the way clay particles align themselves as parallel sheets in =
response to pressure. Although there are idealised diagrams in several =
books which show this, not one author has come forward with =
photomicrographs of unfired pots which clearly illustrate that this =
happens as a fact. Therefore I tend to consider their information to be =
assumption, conjecture or imaginative.
Like Rhonda, I have an open mind on the matter, until better evidence is =
presented. But having said that, I find it difficult to accept that =
platelets of clay in any clay body (which may have more than forty =
percent non plastics whose particle size is two or three orders of =
magnitude greater, that is a hundred to one thousand times the size of =
clay particles) are going to respond to pressure from a thumb which has =
an area a million times larger that that of the majority of clay =
crystals.
Should anyone wish to take this discussion further I would be pleased =
hear what they have to say.
Ivor Lewis. Often doing things in a contrary way to gain understanding.
Sheron Roberts on thu 1 jun 00
The issue of S-shaped cracks
was addressed in one of the
many lectures/discussions
held in the advanced=20
throwing class I was=20
taking at college. My=20
instructor demonstrated
pretty much the same
technique Rhonda mentions.
The only difference was,
he moved his thumb or
finger back and forth
from inside to edge and
back again. He talked
of clay particles, alignment
and compressing the clay
particles. He never=20
mentioned where or from
whom he learned this, just
that it seemed to work.
If the pot was large enough
a rib tool worked just as well,
much the same as when
throwing a plate.
He also instructed us to
make sure we didn't leave
water standing in the bottom
of the pot, not even a little,
as it could cause uneven
drying and result in s cracks.
And one more thing, if the
walls of the pot were thicker
than the floor uneven drying
would occur and possibly
cause cracks in the bottom,
not necessarily S-cracks
though. So the bottom line
was: throw even walled pots
with not too thin floors, clean
the water out of the bottoms
and compress the bottoms,
dry in a damp cabinet until
you could turn the pots over
and trim the bottoms, after
trimming, dry upside down on
a kiln cart. All this worked for
us, the folks taking the class,
and still works for me today.
Sheron
PS Ivor, I will ask my former=20
teacher if he has ever seen
any printed information on=20
platelets and their alignment.
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