Joseph Herbert on sun 22 apr 01
I have a prejudice that ALL cracking in pottery is caused by differential
expansion/contraction. It can be either thermal changes or changes during
drying that cause the difference. To my mind this is a statement of the
obvious and should not cause debate. (Fat chance) The problem is to
determine what causes the differential expansion or contraction and then
plan effective steps to eliminate it.
In the case of S cracks, I believe that the center of the bottom of the
vessel dries more or faster than the foot ring and this sets up the tension
that results in the cracks. If the center of the bottom is wetter than the
foot rim when the pot is removed from the wheel, that part of the object can
experience more shrinkage than the rest of the object. This could be
remedied by not allowing evaporation for a period of time that permitted
diffusion of the additional water to other areas, making them wetter and the
bottom somewhat drier. I think this falls in the category of forever. The
same thing happens to the bottom if the center is allowed to dry much faster
than the foot ring, the center is smaller resulting in tension and cracks.
Whether or not the mechanical working of the bottom of the vessel changes
the platelet orientation, it certainly does some de-watering. If the
process of applying the rib or sponge or finger makes or keeps the bottom’s
moisture content closer to that of the rest of the object, it is an
effective way of reducing S-cracking.
It would be possible to end the debate over whether there is an effect on
the platelet’s orientation by X-ray crystallography. A wafer of
“compressed” clay is cut from the bottom of one vessel and a similar wafer
of “uncompressed” clay is cut from another. The clay used should be the
same in both wafers and the upper surfaces should be as flat as possible.
They really don’t have to be dry, just handled the same. When your friendly
neighborhood x-ray crystallographer subjects these to powder
diffractometery, the relative heights of the peaks in the two scans should
be different. The peaks caused by the presence of feldspar should be the
same in both samples since those grains are chunky rather than platy. The
x-ray reflection from the main clay platelet crystal direction should be
relatively much higher in a sample that has greater alignment of clay
particles. If there is not much discernable difference in the two charts,
the myth of compression (if it is one) would be demonstrated.
My personal x-ray diffractometer is currently in the shop, so someone else
will have to undertake the task.
This is a time when the art department can interact meaningfully with the
physical chemistry people or geologists or whomever it is that has the
machines at your school. Those physical science folks love to show off
their toys and, since this is actually a question they can answer, they
should be glad to give it a try.
Joseph Herbert
Ruth Ballou on mon 23 apr 01
I think it is important to notice that cracks in the bottoms of pots
preferentially take the shape of an S. If cracks occurred just as a result
of drying differential, then I think we could expect to find cracks of many
different configurations, some straight lines or perhaps something that
looked like a dried up creek bed in the center bottom. Also, it is
interesting to note that s cracks are more of a problem with pots thrown
off the hump or that have a trimmed foot, than with pots thrown against the
wheel head. If it was just uneven drying, then pots thrown against the
wheel head should have just as many problems with s cracks as those thrown
off the hump. Pots thrown right on the wheel head have the advantage of
having their bottoms pressed against the metal.
Once, long ago, when I was troubled by s cracks in a particular clay body,
I had one pot that had a severe s crack that went all the way through and
was so large that I could easily break the pot along the fault line. That
crack revealed an area where the clay was not joined to itself; it swirled
away in opposite directions and formed the basis of the s crack. That lack
of cohesiveness had been there from the wedging and I had failed to correct
the fault in the throwing. Water left in the bottom of the pot may have
seeped in and helped things along, but I use water sparingly. All this has
been said before by others in different ways. After that I took a slice
from a bag of commercial clay and one from a freshly wedged block and flung
them obliquely, several times, against the table. Both opened up along the
fault lines created in either pugging or wedging. Stiffer clays show this
more readily than softer clays, probably due to the inferior blending of
the stiffer clay. I think freezing the slices would show the same thing. Or
freezing a freshly thrown pot. But I haven't tried that. It's just a guess.
Maybe we don't need an x - ray crystallography.
After that s crack, I went back to my college notes and studied the stuff
on s crack prevention. There was no clayart to ask then,and now that I
think about it, there weren't even computers. I had gotten lazy and was not
paying attention to throwing back across the bottom, at the beginning of
each pull, from the base of the interior wall to the center bottom. This
move counteracts the outward force used in opening which stresses the clay
at its weakest point. Think of it as a rebound move, a counterbalance. And
I was neglecting to recenter the foot at the beginning of each pull, which
amounts to squeezing (compressing) the clay under the pot. If you're going
to do this, you have to plan ahead and start with a foot that is slightly
larger than you want to end up with. Finally, I resolved to be even more
vigilant of water in the bottom of the pot. These three things cured the s
crack problem. I dry my pots with no extra-ordinary care. Cracks have to
have a point of weakness from which to start. Uneven drying can take
advantage of that weakness, but if there is no point of weakness, then you
can dry your pots as unevenly as you please and you will not get an s crack.
Ruth
Iveragh Ceramics on mon 23 apr 01
I don't usually contribute to clayart but would like to add my bit here. I
was taught pottery in 1965 and have been more or less throwing ever since, I
went into a pottery partnership with my husband Bob Hollis in 1981. We throw
the same clay but he gets S cracks and I don't and never have. Bob is
convinced that it's all in the centring of the clay, in which I am very
thorough, coning several times and making sure every last bit is centred
before proceeding further.
Emma.
----- Original Message -----
From: "Joseph Herbert"
To:
Sent: Monday, April 23, 2001 5:07 AM
Subject: S cracks and others
> I have a prejudice that ALL cracking in pottery is caused by differential
> expansion/contraction. It can be either thermal changes or changes during
> drying that cause the difference. To my mind this is a statement of the
> obvious and should not cause debate. (Fat chance) The problem is to
> determine what causes the differential expansion or contraction and then
> plan effective steps to eliminate it.
>
> In the case of S cracks, I believe that the center of the bottom of the
> vessel dries more or faster than the foot ring and this sets up the
tension
> that results in the cracks. If the center of the bottom is wetter than
the
> foot rim when the pot is removed from the wheel, that part of the object
can
> experience more shrinkage than the rest of the object. This could be
> remedied by not allowing evaporation for a period of time that permitted
> diffusion of the additional water to other areas, making them wetter and
the
> bottom somewhat drier. I think this falls in the category of forever.
The
> same thing happens to the bottom if the center is allowed to dry much
faster
> than the foot ring, the center is smaller resulting in tension and cracks.
>
> Whether or not the mechanical working of the bottom of the vessel changes
> the platelet orientation, it certainly does some de-watering. If the
> process of applying the rib or sponge or finger makes or keeps the
bottom's
> moisture content closer to that of the rest of the object, it is an
> effective way of reducing S-cracking.
>
> It would be possible to end the debate over whether there is an effect on
> the platelet's orientation by X-ray crystallography. A wafer of
> "compressed" clay is cut from the bottom of one vessel and a similar
wafer
> of "uncompressed" clay is cut from another. The clay used should be the
> same in both wafers and the upper surfaces should be as flat as possible.
> They really don't have to be dry, just handled the same. When your
friendly
> neighborhood x-ray crystallographer subjects these to powder
> diffractometery, the relative heights of the peaks in the two scans should
> be different. The peaks caused by the presence of feldspar should be the
> same in both samples since those grains are chunky rather than platy. The
> x-ray reflection from the main clay platelet crystal direction should be
> relatively much higher in a sample that has greater alignment of clay
> particles. If there is not much discernable difference in the two charts,
> the myth of compression (if it is one) would be demonstrated.
>
> My personal x-ray diffractometer is currently in the shop, so someone else
> will have to undertake the task.
>
> This is a time when the art department can interact meaningfully with the
> physical chemistry people or geologists or whomever it is that has the
> machines at your school. Those physical science folks love to show off
> their toys and, since this is actually a question they can answer, they
> should be glad to give it a try.
>
> Joseph Herbert
>
>
____________________________________________________________________________
__
> 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.
>
iandol on wed 25 apr 01
Dear Ruth Ballou,=3D20
Thank you for your contribution to this discussion. You reinforce many =3D
of the points which lead to good craft skill when throwing clay.
A clay which is prone to cracking will give "Star" cracks if it has only =
=3D
been centred and not thoroughly coned. Frank Hamer remarks that this can =
=3D
occur if the base is excessively thin or excess water is not mopped from =
=3D
the inside of the pots. Unequal drying may cause a straight line crack =3D
in similar circumstances.
But I still believe cracks originate a points of weakness such as gaps =3D
or air pockets in the clay fabric or segregation of coarse particles. =3D
The energy which causes the crack to propagate comes from differential =3D
shrinkage during drying or firing. Solid materials are prone to crack =3D
under stress if they contain defects.
Best regards,
Ivor Lewis. Redhill, South Australia
| |
|