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Ginny C

How To Dry A Slab Built Dish Evenly

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I'm having trouble with cracks forming at the edge of big (14 inches in diameter, that's big for me!) hand-built dishes while they are drying. Even thickness, about 1//4 to 1/3 inch, with an added foot ring slightly thicker.  Formed from a slab, with a pattern pressed in from a basketball net, draped over a bowl covered in T-shirt material or over a pillow made from a stretchy material with polystyrene beads inside. I gently ruffle the edges, i.e. 6 ups and 6 downs on the perimeter.  Clear?

 

I drape a lightweight cloth over it and then drape plastic over that and leave it for a couple of days.  Once it's dry enough to turn over, I do that, and continue to keep it all covered.  Still, the edges get completely dry while the center stays damp, and then a crack forms.  Of course!  In both cases, the crack came where the lines had been incised, so now I'm trying with no lines near the edge.  

 

If this one cracks, I'm in trouble!  I've promised it for a charity auction, to go with a smaller piece. made the same way, but deeper.  That one didn't crack!

 

I can't figure out how to get the center to dry more quickly, to keep up with the edges.  Should I use a heat gun on the foot ring and center?  Or should I spritz the edges frequently until the center catches up??

 

Ideas, potter friends & helpers? I'll try to post 2 photos. One of the last one with a clothes pin next to one of the cracks (not bisqued) and the other of the current one, made yesterday, draped over the pillow thing. 

 

Ginny 

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Options might include adding a reinforcing coil rim at the edge as it seems the cracking is coming from the impression thickness being so much less than the rest of the slab. You could add a coil or slab to underside and blend it into the form. Better yet might be to roll the edge of the form under and join or thicken the edge by paddling or sponging like one would do with a chamois on a thrown piece.

 

As to drying, I used to use a trick with some of the student pots where I would coat them with a thin coat of shellac around the edges. I have also seen someone use baby oil painted on the edges to retard drying.

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Good ol' wax resist on the edges can help slow down the drying there, too.

 

It's hard to tell from the photos, but is there any chance the mold/form underneath is restricting the shrinkage of the pot as it dries? Those types of cracks are common in that situation. For that reason it is typically safer to build on the inside of a form rather than the outside.

 

What kind of clay are you using? Smooth white stoneware clays are much picker about even drying that something with fireclay or grog in it, sometimes even worse than porcelain.

 

Rather than drying it really slowly, try drying one quickly and see what happens. Sometimes when they dry quickly they actually dry more evenly. Wax the edges and just leave it open to dry naturally. Get it off the form as soon as possible, even if it's a little wet. I actually like to put leather hard porcelain mugs with fresh handles into the kiln to dry. The fast drying makes them dry more evenly and the handles don't pull away at all.

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I came here to say "put wax on the edges"... But was beat to the punch!

 

Seriously can make a huge difference.

 

Another thing I do on my platters (usually thrown ones) is to just wrap plastic wrap around the edges and leave the interior uncovered. It usually evens out the drying enough so that both sections "get there" at the same time.

 

Good luck!

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lightly plastic bag it with those worthless (to most) plastic bags from the market. cut a few small holes close to the center and let it sit. This has eliminated warping an splitting for pieces being dried in a hot, dry, closed room ( that's all we have) that usually drier out the edges and cause  BIG problems.

 

Probably worth trying all the ideas posted here at the same time and see what works for you

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Two things come to mind one Neil said does the form crack while inverted on the pillow as this is when the clay shrinks the most. A form that shrinks onto vs away will cause issues -another idea is your clay body. This form needs a durable body that can take all this tweeking-you may have the wrong body. You gave no info on your body or what temp its fired to-all helpful for more ideas on the fix.

Mark

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Ooohh nuts!  I just wrote an update on the dish, but before posting it I tried to look at emptynester's photo, and when I came back my message was gone.   Well, in brief, no crack yet. Have waxed and partially covered it, now taken it off the pillow. Fine so far. I'll post a photo of the finished piece, cracked or not!  I hope not!!!

Thanks

ginny

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