Barney Adams on sun 15 apr 01
Is there also a relationship to the thickness? I've noticed that on some =
of the
large
pieces I've thrown and trimmed the bottom thinner than the sides I get s =
cracks
in the bottom where normally I never get cracks because I cone and compre=
ss
the bottom.
Barney
vince pitelka wrote:
> > Haven't had one since, I really cone the hell out of the clay. Seems=
it
> > kind of wedges it on the wheel head.
>
> Chris -
> In fact I (and many other people) refer to the coning process as "wheel
> wedging." I have been too busy to follow much of this thread, so perha=
ps
> someone has already pointed out why this makes such a difference. Unle=
ss
> you cone wedge in the right direction, and carefully place your clay lu=
mp on
> the wheel so that the wedging spiral conforms with the throwing spiral,
> there are erratic currents of platelets in a wedged lump of clay, and i=
f
> those currents are left undisturbed in the bottom of the pot it creates=
a
> directional grain structure in the bottom. The clay will shrink more a=
cross
> the grain (across the currents) than with the grain. When we wheel wed=
ge up
> and down a few times, the friction of our hands twists the clay mass ri=
ght
> to its core, and that alligns the platelets in spiral currents, creatin=
g a
> spiral grain structure, which causes the shrinkage to occure much more
> evenly throughout the pot.
>
> This is also part of what happens when we compress the bottom of a pot.=
We
> are introducing a level of compression to the bottom which is equal to =
the
> walls, but we are also re-alligning the platelates in a way that is clo=
ser
> to the grain structure in the walls, thus equalizing both the amount as=
well
> as the direction of shrinkage.
>
> Most people do not pay nearly enough attention to grain structure in pl=
astic
> clay. Platelets are little flat disks, and the pressure or wedging and
> forming flattens those disks into a layered, grained structure with ver=
y
> specific shrinkage behavior. The problem is perhaps most prevalent in =
slab
> construction, but it is also extremely imporant in throwing. What we r=
efer
> to as "clay memory" is primarily due to grain structure - the platelets
> allign according to the wedging and forming methods, and if two attache=
d
> pieces have contradicting grain structures, they will likely crack or p=
ull
> appart during shrinkage in drying and/or firing.
> 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/
>
> _______________________________________________________________________=
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vince pitelka on sun 15 apr 01
> Haven't had one since, I really cone the hell out of the clay. Seems it
> kind of wedges it on the wheel head.
Chris -
In fact I (and many other people) refer to the coning process as "wheel
wedging." I have been too busy to follow much of this thread, so perhaps
someone has already pointed out why this makes such a difference. Unless
you cone wedge in the right direction, and carefully place your clay lump on
the wheel so that the wedging spiral conforms with the throwing spiral,
there are erratic currents of platelets in a wedged lump of clay, and if
those currents are left undisturbed in the bottom of the pot it creates a
directional grain structure in the bottom. The clay will shrink more across
the grain (across the currents) than with the grain. When we wheel wedge up
and down a few times, the friction of our hands twists the clay mass right
to its core, and that alligns the platelets in spiral currents, creating a
spiral grain structure, which causes the shrinkage to occure much more
evenly throughout the pot.
This is also part of what happens when we compress the bottom of a pot. We
are introducing a level of compression to the bottom which is equal to the
walls, but we are also re-alligning the platelates in a way that is closer
to the grain structure in the walls, thus equalizing both the amount as well
as the direction of shrinkage.
Most people do not pay nearly enough attention to grain structure in plastic
clay. Platelets are little flat disks, and the pressure or wedging and
forming flattens those disks into a layered, grained structure with very
specific shrinkage behavior. The problem is perhaps most prevalent in slab
construction, but it is also extremely imporant in throwing. What we refer
to as "clay memory" is primarily due to grain structure - the platelets
allign according to the wedging and forming methods, and if two attached
pieces have contradicting grain structures, they will likely crack or pull
appart during shrinkage in drying and/or firing.
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/
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