Charles on mon 16 apr 01
I have a theory, perhaps I can get some opinions on this. If a pot is mad=
e
with walls/floor that are a uniform thickness, I have found it is less
likely to crack, those walls can be 1 inch or .25 inch thick. The reason =
I
think this happens is because of the normal expansion and contraction of
materials as they heat and cool. If a pot is being heated from the outsid=
e
in, the walls of the pots which are usually thinner will begin to change,
expand and then shrink faster than the bottom of the pot, which is usuall=
y
somewhat thicker (at least for my beginners). I imagine the heat source
simply takes longer to penetrate the thicker areas causing uneven expansi=
on
and contraction throughout the pot and it tears the clay apart. I did a
series of 550 small bowls all hump thrown for a customer last summer (ove=
r 1
weekend) , by the end of the trimming I was tired and getting a little
sloppier and I found that when I fired them the pots that had S cracks we=
re
the ones that had a difference in wall and floor thickness. I wasn't sure=
I
could rely on this information since it did have the wild variable of bei=
ng
hump thrown, but I have a technique using a metal rib to open and compres=
s
the hump thrown pots, then open air drying them until the rims are stiff,
then flipping them over to dry the feet to leather hard. I also usually t=
rim
carefully to even out the thickness in the walls and compress the foot of
the pot while trimming with a rib to compact the particles as much as
possible. THEN I dry the pots to bone dry upside down. I usually don't ha=
ve
problems.
Later, when my students would bring me pots and ask my why it had
cracked, I checked the difference between wall and floor thickness and in
each of them, there was an obvious difference. So I tell them, make sure =
the
bottoms of the pot have no standing water, be sure you compress the botto=
m
while you throw, and do a good job trimming the outside of the form to ma=
tch
the inside. Make sure the walls are even. It usually solves the problems.
-Charles
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----- Original Message -----
From: "Barney Adams"
To:
Sent: Sunday, April 15, 2001 10:21 PM
Subject: Re: "S" Crack musings - grain structure/clay memory
> Is there also a relationship to the thickness? I've noticed that on som=
e
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
compress
> the bottom.
>
> Barney
>
>
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