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tomhumf

Stop clay 'unwinding' during firing

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I've been trying to make some yarn bowls. Each time the clay kind of unwinds during firing - it moves a bit during drying too.

I'm thinking I may push it the other way while raw next time... Just wondered if there are any secrets to help avoid this problem?

IMG_20190404_184843.jpg

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Keep a small part of the lip joined together when you cut the slot. After the piece has dried completely, carefully remove that last bit. Another option is to cut the entire slot, then fold a piece of clay across the cut to hold it in place during drying. If it continues to move during firing, consider making the pot thicker, or experiment with making the cut in different shapes/directions.

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When I read your title, I pictured the clay cracking in a spiral, as this happened to me way back with several large vase/jar pots. Changes in bisque solved my problem.  I think Neil's solution would work, but you could also play with bisque schedules.

 

best,

Pres

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@JohnnyK, just cutting a hole solves the warping issue but all my customers want the slot so they can take one colour of yarn out of the bowl and replace it with another colour without having to cut the yarn.

I make the cuts all the way through the bowl then take part of what I've cut out and wedge it up, I also press in the upper left side of the cut, looks like yours is warping the other way round. Firing a little cooler than the clay maturity temperature helps also. Keeping the cut out channel part as short as possible also helps, less sag from the severed rim.

717210023_Untitled2.png.d006fddae08526d1bce39db103ed0059.png

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6 hours ago, Min said:

@JohnnyK, just cutting a hole solves the warping issue but all my customers want the slot so they can take one colour of yarn out of the bowl and replace it with another colour without having to cut the yarn.

I make the cuts all the way through the bowl then take part of what I've cut out and wedge it up, I also press in the upper left side of the cut, looks like yours is warping the other way round. Firing a little cooler than the clay maturity temperature helps also. Keeping the cut out channel part as short as possible also helps, less sag from the severed rim.

717210023_Untitled2.png.d006fddae08526d1bce39db103ed0059.png

 

Thanks, I'll try this, and probably Neils method too. Out of interest which way does your wheel spin? Maybe that's why they move different ways...

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3 hours ago, tomhumf said:

Out of interest which way does your wheel spin?

Counter clockwise. If mine are going to warp it's always the part on the left above the cutout that warps outwards. I think it's just because of the weight of the clay that is being suspended, if yours kick out on the right side of the cut then try press that inwards just a tiny bit after doing the cutout. 

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I think there are a combination of things going on, there is the sagging issue plus the warping. How the bowl is thrown and ribbed likely evens out the compression from the spiral throwing pressures within the clay. Without going on too much of a tangent and getting into teapot spouts here, with spouts there is an issue of spiral shrinkage from the throwing process which doesn't happen to a noticeable degree with larger forms or cast or slab built spouts. If you think about it when larger pots (like bowls) are decorated with vertical stripes on the outside the bowl doesn't come out of the glaze fire with the stripes on angles. If there is a bending, twisting, warping force the cut out slot in yarn bowls will relieve most of those pressures and permit you to counter the inherent "spring" in the form by pressing the area inwards while soft leatherhard.

Edited by Min
clarity

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I don't think it's a teapot spout thing. That really only happens with narrow tubes like spouts. And they don't unwind, they continue to wind. There are definitely stresses at work here, though, but it's more of a warping thing than a twisting thing.

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The best yarns take place on the deck of a windjammer on a trip around the horn. My pappys pappy use to talk about rounding up them whales in the middle latitudes as well as discovering Hawaii back in the day. Nothin' better than a Nantucket sleigh ride. Them were the days.

 

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5 hours ago, neilestrick said:

I don't think it's a teapot spout thing. That really only happens with narrow tubes like spouts. And they don't unwind, they continue to wind. There are definitely stresses at work here, though, but it's more of a warping thing than a twisting thing.

 I did not know that the spouts continue to wind, rather than unwind, just knew how to offset for "twist" without thinking which direction it was winding. Thanks! Never too old to learn ;)

 

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