Twirt at Hutchtel on sat 15 oct 05
We're not looking carefully at what's going on here in regard to "s" cracks.
When you apply pressure to clay, you change it from a more or less solid to
a liquid. If you are centering properly, the clay is flowing throughout the
hump.
As you apply pressure at the bottom in coning up, clay actually flows up
through the whole hump. If you're using soft clay, and watch the center of
the top of the clay, you'll see a whorl forming, of the clay coming up
through the hump, and flowing down through the center. Very much like the
motion of the fluid in a blender. As it does this, it forms shear lines
where the clay meets and flows back down into the center
. Several "conings", and/or compressions of the bottom, help get rid of
these shear lines. I've found that no matter what the weight of the clay,
something happens on the third coning to improve the homogeneity of the
clay.
Listen to Ivor...he's done more actual research on the issue of S-crack than
anyone. He had a series of articles in Ceramics: Art and Perception
magazine a couple of years back.
When you "compress" the clay, you are not really compressing it (as has
been discussed here many times...you cant compress a solid and a liquid) you
are realigning the particles, and most importantly getting rid of those
shear lines.
As you cone up clay, start looking at what's happening. If you are doing it
right, the center of the clay comes up as much as the sides...which means
there is clay flowing from the bottom up thru the center. Same happens when
you press the cone down...you press at the top but the bottom
widens...again, if you are doing it right.
The most important principal here is that the clay becomes a fluid when you
apply pressure. If you want to see how well you're doing, cut 1/4" disks
off the top of the clay you're centering and put them in the freezer. It
accelerates the formation of s cracks or any other shear lines.
Tom Wirt
Clay Coyote Pottery
twirt@hutchtel.net
>>if you get down to the detail grains of clay they are connected by their
>>little fish scale like platelets. a good centering doesn't disrupt that.
>>the shear size of the "S" crack means the local clay (center spot) was
>>never fully joined & the fundemental flaw shows thru later as the classic
>>"S" crack. that "S" spot is a fracture (like an earthquake fault line)
>>just waiting to be brought visible from the firing. good centering yields
>>no "S". that's why we seldom see it in small pots. they are easy to
>>actually over center if there was such a condition. larger pots are
>>harder to center & we tend to say "good enough" and get rolling with big
>>pots. ~ yet the heart is not fully centered.
>>
>>you get the "S" shape because of the not-fully-centered clay twisting
>>around like it does thru throwing. the helix that is easy to see when you
>>throw two color claybodies. at the floor it forms an "S". a left handed
>>thrower gets the oposite "S". it looks like an S because that's where the
>>center of the helix starts. center really well & you get a blur at the
>>"S" area rather then a strata of the claybodies.
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