Dave Finkelnburg on sat 19 aug 06
Brad,
I do not know the recipe for B-mix, at cone 5 or
10, but no, I don't believe it can be fired that high
and still be a high talc body.
No, porcelains are not high talc. Most have little
to no talc.
More important, S-cracks are caused by uneven
drying. ANY clay body will S-crack if the rim dries
too much before the base.
The potters' admonition, the last place to dry is
the first to crack, is definitely true with S-cracks.
The problem is, the rim dries and shrinks, and the
foot hasn't, so the uneven shrinkage puts the foot in
tension. Brittle materials always fail in tension.
The foot can take a little uneven drying, but not a
lot.
The cures for S-cracks are, in no particular order,
even drying (wrap the rim and expose the foot, if
necessary) (flip the pot upside down as soon as
possible to allow the foot to dry while slowing drying
of the rim), trimming the foot to a similar thickness
to the rest of the pot (helps the foot dry faster, by
the way), and burnishing the foot to fill in
micro-tears in the clay caused in opening.
I am not saying one should struggle with a bad clay
body. By all means change to a better clay when you
can. BUT, any clay, badly handled, can be made to
S-crack.
Good potting,
Dave Finkelnburg
--- Brad Carter wrote:
I
> understand B-mix is a porcelainious clay. But how
> similar is it to porcelain? Is
> B-Mix a hgigh-talc clay? Mel recently wrote that
> high-talc clays have a strong
> tendency to S-crack. Is porcelain a high-talc
> clay? If so, does it also
> have the tendency to S-crack?
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Gene Somdahl on mon 21 aug 06
Besides the dry rim, the foot ring drying out stresses the bottom of the
pot. When the foot ring is dry, the shrinking clay in the bottom is
pulled toward the ring causing it to split in the middle. After trimming
put the pot back into damp storage for a day or two. Also, wrapping the
pot on cloth or paper towels allows moisture to be wicked from moist to
dry areas promoting even drying.
Dave Finkelnburg wrote:
> More important, S-cracks are caused by uneven
>drying. ANY clay body will S-crack if the rim dries
>too much before the base.
> The potters' admonition, the last place to dry is
>the first to crack, is definitely true with S-cracks.
>The problem is, the rim dries and shrinks, and the
>foot hasn't, so the uneven shrinkage puts the foot in
>tension. Brittle materials always fail in tension.
>The foot can take a little uneven drying, but not a
>lot.
> The cures for S-cracks are, in no particular order,
>even drying (wrap the rim and expose the foot, if
>necessary) (flip the pot upside down as soon as
>possible to allow the foot to dry while slowing drying
>of the rim), trimming the foot to a similar thickness
>to the rest of the pot (helps the foot dry faster, by
>the way), and burnishing the foot to fill in
>micro-tears in the clay caused in opening.
>
>
Ivor and Olive Lewis on tue 22 aug 06
Dear Gene Somdahl ,
You say, <bottom of the pot. When the foot ring is dry, the shrinking clay in the =
bottom is pulled toward the ring causing it to split in the middle. >>
An interesting proposition. Have you cemented strain gauges to samples =
and measured the forces in the pots to verify this proposition. From =
your description I would suggest an annular fracture separating the base =
form the footring is equally plausible.
In my imagination I can see that, as the footring dries, it will reduce =
in diameter putting the clay of the interior into compression. To =
accommodate the stress, strain will be normal to the force and the =
vertical thickness will increase. Assuming that the interior clay =
remains plastic, shrinkage will draw on this material and the thickening =
will subside. When this drying is completed and the interior clay starts =
to dry the question to be answered is "Which of these clay fabrics has =
the greater and which has the lesser modulus of rupture. Or should this =
value be constant in any sample ? ?"
I would suggest that if the fractures have an "S" conformation there is =
some sort of segregation or fundamental flaw in the structure of the =
clay that responds to shrinkage stress. And I still ponder on the role =
of Carbon Dioxide in this scenario.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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