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pieces warping during glaze firing

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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.

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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.

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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!

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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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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.

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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.

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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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I am having troubles with platters warping in the glaze fire also. they have a lip, but it is the same thickness as the slab. Bisque to 04 and glaze fire to cone 6. The slabs dry for a good week  to 10 days. I compress slabs on  foam with a board on top, pressing them down.

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18 hours ago, Chukchi said:

I am having troubles with platters warping in the glaze fire also. they have a lip, but it is the same thickness as the slab. Bisque to 04 and glaze fire to cone 6. The slabs dry for a good week  to 10 days. I compress slabs on  foam with a board on top, pressing them down.

Couple thoughts re rims warping when making platters like this. Roll the clay in each direction when making the slab then let the slab firm up a bit before pressing it. The larger the platter the thicker the slab needs to be, it’s fine having thin slabs for small work but for bigger pieces don’t go too thin. As it's already been said, don’t pick up the slab with your hands at any point, sandwich the slab between two boards any time it’s necessary to flip it over or move it. After the slab is rolled out and firmed up a bit put your foam on top of it and holding the board underneath it flip it over onto the foam. If possible leave your pot on the foam and remove the form then don’t touch the pot until it has reached leather hard at which point clean up the edges if necessary. The more the walls are angled outwards and the wider the rim the greater the likelihood of getting some warping. Clay with grog helps too, plus drying the pots evenly. If it’s a large platter try and centre it on the kiln shelves while firing.

Just confirming I've read your question correctly, you are talking about making pots like in this video?

Edited by Min
added a link

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min, video did not work.

i think he is making platters the way i make small things, pressing the clay down into soft foam so the edges come up.    the shape of the item doing the pressing is the center of the piece.  boards work very well this way for smaller things, a large platter works more easily by letting a slab slump into an opening cut into a thick framework.

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I had lots of cone 6 issues with warping (both thrown and slab pieces) a few years ago.  Increasing the thickness helped a little and so did flipping the slab more while rolling out.  But what made the biggest difference was adding about 5-10 % fireclay and fine white sand.  I add a couple pounds of each to a ~35# pug mill load.  Without a pug mill it could be wedged in but might not be worth the trouble.

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Others have hit most of the common issues for warping. One thing that hasnt been mentioned which is part of what causes work to warp is that as clay reaches its maturation point (temp during firing), it begins to vitrify. This stage is where crystal forms of silica are formed and the clay body becomes more glass like; it is at this stage where a lot of warping occurs as the body is in motion. The furniture you fire you work on has a lot to do with how it maintains its original/desired shape. If your shelves are warped, likely your work will be too. Any plates or platters over 4-6" in diameter when fired on my old cordierite shelves would warp no matter what I did. Finally got wonderful advancer shelves and now my work is flat as humanly possible.

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Had the same problem with drooping at ^6:

 

large.5a7d7051192fd_FlattendFish1.JPG.33

 

large.5a7d704ea1b95_FlattendFish.JPG.599

Next one I made sure it was born with a curved tail, not a rolled slab bent upwards.  Which reminds me, I've got another one in WIP that needs to be cut apart and re-joined to avoid the warpage.

 

Edited by Chilly
Picture didn't show, added words.

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About yarn bowls... I don't think a hole in the side is useful to a knitter. I had the same idea to just cut a hole for the yarn to go through because I don't like a big swirly cut out. However, if you just have a hole the bowl always has to stay with whatever they are knitting, and a lot of knitters like to take their projects with them. They usually don't want to take the bowl as well.

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I'm having problems with plates drooping flat.  They are bought bisqueware and glazed. I'm firing to 05.  Am I holding too long? I have a very slow schedule programmed and the plates are coming out flat.  Except for the ones that are now sticking to my kiln shelf, and then breaking.   Any help appreciated. 

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is C 05 the recommended cone for this claybody.

There is a claybody. Vitreous china which fires really low and can melt to a puddle a bit higher up.

There is a thread here on yarn bowls but a lot of the hints to remedy are referring to the making process of which you have no control.

a firing schedule would help with the advice given.

get info from maker for recommended temp for g.firing

 

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