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pieces warping during glaze firing


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#1 wildfire

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Posted 30 March 2013 - 07:43 PM

Some of my pieces are warping during the glaze firing. I've made some yarn bowls that were pretty round before glaze firing, but afterward, the rim is a bit off. Also, my platters are rocking from side to side, rather than sitting nice and flat on the table. The platters were made from a slab that was about 1/4 inch thick, give or take a little. What's happening? I bisque to Cone 04 and the glaze firing is to Cone 6.

#2 bciskepottery

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Posted 30 March 2013 - 09:47 PM

Does your yarn bowl design include a cut in the rim to allow threading the yarn? If so, that might be contributing to the rim distortion . . . as the clay expands and shrinks during glaze firing, the cut in the rim may be allowing it to distort. Because your bisque temperature is lower, it does not occur at that firing, only when you hit the higher temperatures of a glaze fire. You might think about altering your design, perhaps foregoing the cut in the rim for just a hole in the side that allows the yarn to come through. Or, you could switch to glazing at low fire temperatures.

As for the platters, in general, make sure you are compressing the slabs, rolling them in all directions . . . especially if you are using a slab roller to make them. Also, after rolling, take the ware board with slab on it and drop it flat against the floor from about mid-waist high -- that will compress the slab. Handle slabs minimally to prevent warping. If you could tell how you make your slabs, it might be easier to diagnose what is going wrong.

#3 wildfire

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Posted 31 March 2013 - 06:17 PM

Thanks so much for your reply!Yes, the yarn bowls do have a cut in the rim. Just cutting a hole for the yarn would certainly simplify the process!

The platters -- I roll out my slab with a slab roller. I reposition the slab every couple of rolls, turning it at right angles and sometimes flipping it over. I sometimes drop the ware board from about waist to chest height, sometimes just drop it onto a table a few times until it has "settled in". Should the slab be thicker than 1/4 inch? I guess dropping it once from waist high is preferable to 4 drops on a table...

If I'm glazing at Cone 6, could I bisque hotter than 04?

Lots of learning to do!

#4 Mark C.

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Posted 31 March 2013 - 07:05 PM

A few thoughts -just remember clay has memory-as you are dropping the slabs this should cancel out that.
The other obvious things are to thin of slabs and or non flat warped shelves??
Maybe the table is not flat as well?Many wood tables are not flat- all these things can be checked out.
What size are these platters? are they round or square and do they have a lip?

Mark
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#5 TJR

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Posted 31 March 2013 - 09:03 PM

Wildfire;
As Mark has said, your platters, and all clay shrinks as it dries. I dry all of my pots upside down so the rims dry slower. My platters, which are thrown, I dry on flat particle board batts. I also cover my leather hard pieces under plastic. You might be drying your pieces too quickly. The bigger the piece, the more drying issues.If your pieces are hanging over a kiln shelf, they will also warp.
TJR.

#6 wildfire

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Posted 01 April 2013 - 06:18 AM

Thank you all so much for your advice. Hopefully the next group of pots will be better!

#7 AtomicAxe

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Posted 02 April 2013 - 10:13 AM

as others have said, it's the memory of the clay, with the bowls it's going to be harder since just holding them in a leather hard stage can give the clay enough motivation to move into an ovalish shape let alone cutting them lets them move freely without the support of the rim trying to hold it in shape.

As a general rule, I always follow the mantra 'the flatter it is, the slower it dries' also, if you have to invert, or do something to assist with drying (waxing handles and lugs, waxing rims, inverting and drying on foam, etc) ... do it.

#8 neilestrick

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Posted 02 April 2013 - 05:17 PM

Unevenness will also contribute to warping. Thin lip and thick bottom will result in a warped bowl.
Neil Estrick
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#9 nairda

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Posted 04 April 2013 - 06:46 PM

With yarn bowls it helps to rest the bowl on its rim when cutting the opening for the yarn. Drill a hole and then start the channel from there. Cut through the rim section last and dry the bowl upside down and lightly covered to let it dry slowly. Also helps to use a good quality knife (Dolan for example) and lightly oil it so it slips easily through the clay body.




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