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Cracks In Bowl Base


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do you dry them bottom up?  do you leave them on a bat after throwing for extended period of time? how are you drying pots?  how long does it take to get to leather hard?

 

uneven contraction is probably the answer...... although many will point the finger @ compression while throwing

 

in my opinion (imho) (new)  compression evens the clay density lessening the chance of less dense area drying to fast......

 

i haven't seen swirl cracks in years  .... this may be a clue

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The times, that I have had wares develop cracks like that, were towards the end of throwing.  Fairly certain it was caused by me over-working/ stressing the clay, which created weak spots.

 

Biglou's explanations are just as likely.  Quick/ uneven drying will cause stress cracks, as will lack of compression.  Though, when I don't compress, I usually get the typical "S-crack".  The good news is, at least the cracks formed while drying, instead of showing up after firing.  

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From the sponge marks in the bowl, you might be using too much water when throwing . . . possibly also allowing it to pool in the bottom of the bowl while working the sides.  That leads to uneven drying. 

 

Might be helpful if you could also post a picture of the bottom of the bowl from the outside, to get a better idea of thickness, profile, etc. 

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As others have alluded too, your problem is partly drying, partly throwing as I can see. I would be interesting to see how your foot ring looks as well as the base of the pot. I use a large spoon end to compress and smooth my bowls using very little water. At the same time once set up, they are turned upside down for drying before trimming, and after trimming. Think of water being affected by gravity. the thin areas of the pot dry first and rims as surrounded by air. The base dries last. However, when you dry any pot upside down the water will travel somewhat downward by the drying of the rim area, and so slows the drying of the pot all around letting it dry more evenly. Upside down drying of bowls works especially well as the weight of the bowl on the foot ring may inhibit shrinkage of the ring and the bottom of the pot causing some cracking.

 

 

best,

Pres

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I would also ask when do you flip the bowls upside down? If the rim is fairly dry and bottom still quite damp and if it’s thick it can cause cracks. When the rim has already shrunk from drying and the base hasn’t much it creates stress in the pot, stress equals cracks. 


 


I flip pots over as soon as the rim has set up and i can’t wiggle it. Moisture from the base will slowly work it’s way down to the rim and the clay moisture content will even out, cover the pot with plastic if necessary to help even the moisture out. If the base is really thick then I would flip the pot over onto its rim, wrap the rim in plastic on the outside and leave the base exposed until it's dry enough to trim.


 


"S" cracks can develop when there are not enough clay platelets in the base of the pot aligned parallel with the circular direction of the wheel. The crack starts to form when the base and walls of the pot have different rates of shrinkage in the drying stage. While your cracks are not “S†cracks the same issue of different shrinking and drying rates apply.


 


Having a puddle of water in the base of the pot while throwing or not ribbing/sponging it out will exacerbate the problem.


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