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circular cracks in greenware

updated sun 23 jan 05

 

Tomodachi Tomodachi on tue 18 jan 05


Hi!

I keep getting odd cracks in my porcelain greenware bowls after trimming. The cracks are horizontal along the outer wall of the pot, closer to (but not at) the footring than the rim, and they generally "girdle" the pot, nearly defining the entire circumference. Sometimes there is more than one, but they are on virtually the same plane and nearly meet.

I dry my bowls on their rims, very slowly under plastic. I then trim them at leather hard, or sometimes a bit beyond (as with the doctor, total honesty helps with diagnosis...), and occasionally must moisten them a bit with a wet sponge as I trim. I then continue to let them dry on their rims. This is when the cracks appear.

I have actually had the entire bottom of the pot lift off in my hand, separating in a nearly perfect circle. Again, it is not at the footring, but generally about an inch or two above it, spanning the circumference.

I've checked Hamer & Hamer and the Clayart archives, but so far I haven't found a mention of anything similar in greenware.

My guess is that I'm drying it too quickly after trimming, although I haven't had much luck varying this. Or, because I'm trimming away more material near the bowl's footring, there is a difference in wall compression and perhaps the cracks are occuring at the point where the difference is greatest. Someone also suggested it may just be "short" clay.

Has anyone else heard of this/had this happen? If so, do you know the causes?

Thanks so much for your help. Have a great week!

Linda
In our nation's capital, where "inauguration" translates to "road closures."

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Israel Shmueli on thu 20 jan 05


On Tue, 18 Jan 2005 22:49:56 -0500, Tomodachi Tomodachi
wrote:

>Hi!
>
>I keep getting odd cracks in my porcelain greenware bowls after trimming.
The cracks are horizontal along the outer wall of the pot, closer to (but
not at) the footring than the rim, and they generally "girdle" the pot,
nearly defining the entire circumference. Sometimes there is more than one,
but they are on virtually the same plane and nearly meet.
>
>I dry my bowls on their rims, very slowly under plastic. I then trim them
at leather hard, or sometimes a bit beyond (as with the doctor, total
honesty helps with diagnosis...), and occasionally must moisten them a bit
with a wet sponge as I trim. I then continue to let them dry on their rims.
This is when the cracks appear.
>
>I have actually had the entire bottom of the pot lift off in my hand,
separating in a nearly perfect circle. Again, it is not at the footring,
but generally about an inch or two above it, spanning the circumference.
>
>I've checked Hamer & Hamer and the Clayart archives, but so far I haven't
found a mention of anything similar in greenware.
>
>My guess is that I'm drying it too quickly after trimming, although I
haven't had much luck varying this. Or, because I'm trimming away more
material near the bowl's footring, there is a difference in wall
compression and perhaps the cracks are occuring at the point where the
difference is greatest. Someone also suggested it may just be "short" clay.
>
>Has anyone else heard of this/had this happen? If so, do you know the
causes?
>
>Thanks so much for your help. Have a great week!
>
>Linda
>In our nation's capital, where "inauguration" translates to "road
closures."
>
>_______________________________________________
>Join Excite! - http://www.excite.com
>The most personalized portal on the Web!
>
>___________________________________________________________________________
___
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.

I would suspect the wetting procedure while trimming.
To my expirience (not particulary with porclain but with some clays), if
you wet them at trimming stage when they too dry you get cracks only at the
outer surface.

Sometimes they apear only after buisqe.

maybe it is because wet surface expands while absorbing water and contract
back while drying without having enough flexibility throuout the entire
wall thickness.

matbe it is some thing to do with the open nature of dry or almost dry
porclain body

I suggest trimming sooner or trim dry (dangerous,toxic dust),like some
traditional porclain makers do at china
http://www.ceramicstoday.com/articles/classical_porcelain.htm

Israel Shmueli
who
Cann't write to clayart after so long time without remembering Ababi and
miss him so much.

Petach Tikva
Israel
www.botzpottery.co.il

Carol Gardner on sat 22 jan 05


Hi! I once had the same problem, especially with large bowls. On =
advice from a friend I tried using sharper trimming tools and it solved =
the problem. I think my tools were dull enough so that the pressure I =
was applying in the trimming was weakening the place near the foot ring =
and, in drying, it was cracking. This may not be your problem, but just =
a suggestion. (this could be a plug for Bison Tools-as it was a good =
addition to the trimming tool collection-along with a better sharpener.)

Sincerely, Carol Gardner
On a lovely sunny Denver day


>
>_______________________________________________
>Join Excite! - http://www.excite.com
>The most personalized portal on the Web!
>
=
>________________________________________________________________________=
___
___
>Send postings to =
clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from =
http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.

I would suspect the wetting procedure while trimming.
To my expirience (not particulary with porclain but with some clays), =
if
you wet them at trimming stage when they too dry you get cracks only =
at the
outer surface.

Sometimes they apear only after buisqe.

maybe it is because wet surface expands while absorbing water and =
contract
back while drying without having enough flexibility throuout the =
entire
wall thickness.

matbe it is some thing to do with the open nature of dry or almost dry
porclain body

I suggest trimming sooner or trim dry (dangerous,toxic dust),like some
traditional porclain makers do at china
=
http://www.ceramicstoday.com/articles/classical_porcelain.htmceramicstoday.com/articles/classical_porcelain.htm>

Israel Shmueli
who
Cann't write to clayart after so long time without remembering Ababi =
and
miss him so much.

Petach Tikva
Israel
www.botzpottery.co.il

=
_________________________________________________________________________=
_____
Send postings to =
clayart@lsv.ceramics.org

You may look at the archives for the list or change your subscription
settings from =
http://www.ceramics.org/clayart/

Moderator of the list is Mel Jacobson who may be reached at =
melpots@pclink.com.