william schran on sun 27 mar 05
David wrote:>I have had "circumferential cracking" problems which
sound similar to those Liz described. I make soup bowls about 6
inches in diameter and lose about 1/3 of them to cracks roughly
horizontal about an inch outside the foot ring.<
David - Write a bit more how you throw/trim your bowls. If you are
losing 1/3 of them and it happens with different clay bodies, then it
has to do with the way you're throwing and/or trimming, or even
drying the bowls. Bowls & plates thrown with too much water, made
with excess bottom thickness, trimmed unevenly (thick & thin areas
abutting) or dried too fast can cause cracking even at a later time.
I can never remember having this type of cracking occurring with any
bowl I've thrown, no matter the size.
When I throw bowls, I try to get the centering and throwing of the
basic form done as quickly as possible so I'm not adding too much
water to the clay. The more water, the greater the shrinkage and more
chance of cracking.
After completing the basic form (widened cylinder, curved interior
bottom) I trim away excess clay thickness near exterior bottom,
sponge away all water and wipe my hands dry.
I compress the exterior with a flexible rib, fingers on the interior,
without altering the shape.
I form the bowl by stretching the clay out from the interior with a
flexible rib (I use a stainless steel curved rib), from the top lip
down to the bottom, using NO water. This is a gradual stretching
process that may involve several passes to get the form I want. After
each stretch, I compress the rim with a damp sponge making certain no
water drips into the bowl. By forming the bowl using this dry
stretching process, I can pick up bowls off the wheel head without
them warping. I can also demonstrate to my students the entire
throwing/trimming process, by taking the just thrown bowl, turning it
upside down and immediately trimming the foot rim!
I allow bowls to dry slowly, turning them upside down when the rims
are stiff enough.
After trimming, I dry the bowls upside down on a sheet of newspaper,
slowly under plastic.
Hope this helps you solve/discover a solution to your cracking issue.
Bill, in Fredericksburg, Va, where I think I've finally recovered
from the Baltimore flu.
David Cowdrill on thu 31 mar 05
Thanks for the advice; that is also my hunch that the source of my problem is throwing/trimming
technique. Referring to your step-by-step description, I follow the exact set of steps (the way you
taught me) with one exception: I have not been compressing the exterior with a flexible rib; that
lack of compression could well be the problem. I will do that with the next batch. Thanks again!
david
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