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moh

Troubleshooting thin warping bowls

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Recently I've been getting a lot of warpage on bowls. I'm averaging about 80% loss rate -- very unusual and hoping for some answers :)

1 Pots are thrown on plastic bats and left to dry . I take them off when they come off on their own. Temperature here's about 65-70 so doesn't try super fast by any means (trimmable the day after they're thrown)
2. The walls are very thin -- just thicker than 1/8". 
3. There's no warpage during trimming and don't look warped after or after bisque.
4. ^ 5 glaze, cone  ^05 bisque. Glaze inside only
5. Warps when fired in full kiln, warps when fired in not-to-capacity kiln.

Goodness, what am I doing wrong?

Thank you :)
 

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Edited by moh

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A few thoughts-cut them off the bats with a wire after throwing then let dry.

Glazing the inside only puts stress on the forms and you said they are thin so thicker may cure this.

Remember a pot is stronger by far with both sides glazed.

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Thank you Mark for the reply.
It's unusual specifically because it's something new and I haven't had the issue before. I'm going to try throwing them thicker and trim them thin instead and see if that helps.

 

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I would look at few possibilities, first off the design of the bowls, I would look at how much overhang from the foot you have and the design of the rim. If your aesthetic allows a change in direction with the rim, adding a flared outward small rim flange would help stabilize the rim, if not then beefing up the rim helps. Any small warping that happens during drying the bowls will be magnified during glaze firing so even wall thickness and even drying need to be looked at to prevent warping while drying, as does how you remove the bowls from the batts. Do you put another batt on top of the rim and flip the bowl over while sandwiched between the 2 batts?  Also, when firing is the kiln shelf smooth, no crusty bits of wash for the foot to snag? I would use cones to ensure you are not overfifing which can cause warping especially if you use porcelain or porcelain like clay as it gets soft and prone to slumping. You mentioned that this is a new problem, makes me think something has changed in your process, drying speed, humidity, design, something has changed.

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Also remember if they ever where warping in the making or drying or handling-that clay has memory and will often go back to that warp.Mins suggestion on letting them slide on the bottoms-try a few on a flat  thin waster slab that is made from same clay to see if that fixes the issue.

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I second what Min and Mark are saying about handling the bowl while it’s drying. I learned this a while ago. If I have a bowl with a simple rim that I really want to hold its round/level shape, especially the wider shallower forms, I never pick them up with my hands during the leather hard stage. I only use batts to flip them over. Any flexing that happens to the bowl at that stage, even if the bowl is immediately returned to its original shape, and even if it looks perfect out of the bisque kiln, will come back to life in the glaze kiln. Using batts to flip the leather hard pots made a huge difference for me. 

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You said the problem is something new and you didn't have the issue before...what did you change? Things that were OK before rarely just start to fail on their own. Go back and look at your total process from the clay to the glaze firing and determine what part of the process you changed, no matter how small or insignificant it may seem, and correct that to see if you recover.

JohnnyK

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Where is the foot in all of this?

If not under the abrupt change from side to base, then that would be a point of design I'd look at.

Maybe you trimmed this lot thinner and that side/base area is thinner..break one and find out.

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4 hours ago, Babs said:

Where is the foot in all of this?

If not under the abrupt change from side to base, then that would be a point of design I'd look at.

Maybe you trimmed this lot thinner and that side/base area is thinner..break one and find out.

These forms look like they are not footed.

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