cracks on wide bottom forms

[Mudfish]

  I make a lot of these dog-dish type of forms. Some are as large as 16" diameter. Not all of them crack like this, but it sure is a heartache when it is a large piece that is cracked. Sometimes the cracks develop in the bisque fire also. Some are "s" cracks and some are razor straight. I have tried to elevate firing these forms on wads of clay to distribute the heat from the kiln shelf but they get warpy. I really do not think it is a compression issue - I'm obsessive about that. I am thinking its possibly an issue with either uneven drying in the greenware state, or it is an issue with  a large surface exposured to the kiln  shelf that is so much hotter than the kiln atmosphere during the cooling cycle of the firing.  I know cracks can develop and not be evident until the glaze firing. I have not been doing a slow cool with the bisque fire, but I'm thinking this would help. Thanks 

[oldlady]

cannot see photos.   what cone, what clay, etc.

[Babs]

2 hours ago, Mudfish said:

  I make a lot of these dog-dish type of forms. Some are as large as 16" diameter. Not all of them crack like this, but it sure is a heartache when it is a large piece that is cracked. Sometimes the cracks develop in the bisque fire also. Some are "s" cracks and some are razor straight. I have tried to elevate firing these forms on wads of clay to distribute the heat from the kiln shelf but they get warpy. I really do not think it is a compression issue - I'm obsessive about that. I am thinking its possibly an issue with either uneven drying in the greenware state, or it is an issue with  a large surface exposured to the kiln  shelf that is so much hotter than the kiln atmosphere during the cooling cycle of the firing.  I know cracks can develop and not be evident until the glaze firing. I have not been doing a slow cool with the bisque fire, but I'm thinking this would help. Thanks 

Your up loads did not work.

Could try sitting your forms on fine grog.

Get your pots off the bat and onto rims as soon as poss.

Check bottoms not a lot thinner than walls.

Are they footed?

 

[Bill Kielb]

7 hours ago, Mudfish said:

I am thinking its possibly an issue with either uneven drying in the greenware state, or it is an issue with  a large surface exposured to the kiln  shelf that is so much hotter than the kiln atmosphere during the cooling cycle of the firing

My sense is compression, especially for large items. Anytime something is stretched out compressing downward  often does not put things back from which they came. Often if something is stretched away from itself it needs to be pushed back in upon itself to be compressed again. The physics or thermodynamics of a kiln really make it hard for a shelf to mysteriously overheat something. Energy flows downhill like water until the levels are even so I have never found the shelf to be at issue unless the piece does not slide smoothly along the shelf while changing size. Just an educated guess, and I could be totally wrong in speculating, but how do you compress? 

This may give you a new idea about compression https://youtu.be/jVNJELUpclk

 

[Callie Beller Diesel]

The thing that I’ve found helps the most with a wide, flat bottomed form is to surround the upright parts with assorted small items or even spare kiln posts to create a heat sink. It slows the cooling rate of the walls of the pot enough that you don’t get that level of cracking. 

As others have noted, your images didn’t quite work (dm me if you need some assistance resizing images). If the forms don’t have a foot ring, adding one could be another solution to the problem.