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They can happen at any point, really.  The clay is shrinking from the moment it begins to dry to the moment it reaches maturity in the kiln even during cooling.  Those stresses can express themselves as cracks anywhere along that timeline.

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I agree, drying, bisque or final firing tho the chances are less during the final than during drying or bisque. I always say never count your pots til they are glazed fired.

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I agree, drying, bisque or final firing tho the chances are less during the final than during drying or bisque. I always say never count your pots til they are glazed fired.

 

And even past that point.  I had a vase crack a couple months ago (glaze issue) and I took it out of the kiln, showed some students later on in the day, then looked a couple hours later and it had cracked.  Aint ceramics grand?

 

I've  never worked with porcelain, but with the clay bodies I have worked with, the s-crack presented itself in the late leatherhard stage.  I haven't had an issue with s-cracks in some time.  I started using a wood rib to compress the bottom, as opposed to just using my fingertips in conjunction with a sponge.  Since then, I haven't had a s-crack.  It's even a rare occurrence that my students have a s-crack, as I teach them to use the same method.  

 

Coning also helps prevent the cracks, though my students, starting off, aren't always good about coning, but still don't have any cracks, as long as they compress with the rib.

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Mastering/coning was one of the first things I required students to do when learning to throw. When they had "mastered" this I let them go on to centering. I found that in the end they learned to center quicker.

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I compress with a rubber rib. I find students tend to get s cracks when they compress but then decide to modify the piece, accidentally getting water inside the vessel and forgetting to dry and compress again.

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I wish I had the time, to teach students to do many things.....  With wheel throwing being one part of a class, I really can't.  I tried to create a specific "Pottery" class, but it's just not in the cards.  I barely have time in the schedule, to fit the courses I have now.

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