Warping

[meisie]

I have been making yarn bowls in clay for about a year and am now learning to fire on my own. I recently had a yarn bowl warp during the glaze firing and I'm not sure what the cause would be. Can any one give me a hand? This bowl is not yet bisqued but is an example of what the other one looked like. The cut in the bowl is for the yarn. The cut in my other bowl came out fine in the bisque. It was then glazed and fired and the left hand side of the pot warped down and curled in just a bit. What might cause this? Some of the glaze did melt into the space between the sides of the bowl and blocked the opening. Could that be enough to cause it to warp? If so then that's my mistake in putting the glaze on incorrectly, If not then something else caused it to warp. Thanks for the help

Renee

[Seasoned Warrior]

I'm really not the best person to be replying to you and hopefully Marcia Selsor or John Baymore or one of the other experts will chime in here. The bowl looks thrown, is it? I rarely throw and that is the reason why I don't think I am a good source but you seem not to have gotten any other responses. It is my understanding that sometimes the way that you throw and the amount of torque you induce into the walls of the bowl will cause residual stresses to remain in the clay so that when you approach the vitrification temperature the stresses are released and it causes warpage. I know that this can be the case with with slabs and I undertand that it is also appears as "S" cracking in the base of thrown items (I don't know if that has shown up in your work) but that is a thought. I suspect expecially since you make a cut in the wall that is not linear that some of the built up stresses cause the warpage. Another consideration might be the way you place your appliques on the wall, perhaps you are causing uneven stresses as a reslt of the appliques.

 

Best regards,

Charles

[Deb Evans]

Hi Meisie,

need a bit of info - what cone? what clay?

You never had worping or cracking before?

 

Since you don't have to worry about food safety go / ^ 04, more forgiving, less warping.

 

you probably just undercut it a bit more than usual. It's tough because clay likes to be supported and once there's more clay being suspended than clay supporting the piece = crack and or warp.

Hope this helps, Deb

 

 

 

 

[meisie]

I'm really not the best person to be replying to you and hopefully Marcia Selsor or John Baymore or one of the other experts will chime in here. The bowl looks thrown, is it? I rarely throw and that is the reason why I don't think I am a good source but you seem not to have gotten any other responses. It is my understanding that sometimes the way that you throw and the amount of torque you induce into the walls of the bowl will cause residual stresses to remain in the clay so that when you approach the vitrification temperature the stresses are released and it causes warpage. I know that this can be the case with with slabs and I undertand that it is also appears as "S" cracking in the base of thrown items (I don't know if that has shown up in your work) but that is a thought. I suspect expecially since you make a cut in the wall that is not linear that some of the built up stresses cause the warpage. Another consideration might be the way you place your appliques on the wall, perhaps you are causing uneven stresses as a reslt of the appliques.

 

Best regards,

Charles

 

 

Thanks

The bowl is thrown and this is the first time it's happened although I did have an s crack in a piece once before in the base. Since it happened while I was running the kiln for my first time I wondered if it had something to do that. But what you say is food for thought thank you

Renee

[meisie]

Hi Meisie,

need a bit of info - what cone? what clay?

You never had worping or cracking before?

 

Since you don't have to worry about food safety go / ^ 04, more forgiving, less warping.

 

you probably just undercut it a bit more than usual. It's tough because clay likes to be supported and once there's more clay being suspended than clay supporting the piece = crack and or warp.

Hope this helps, Deb

 

 

 

 

 

 

Thank you for your reply and no it's never happened before and I've made several. The glaze fire was at cone 6 . I consider myself a beginner and am just getting into using a kiln on my own. I had one piece with a crack in the base but for the most part everything comes out fine. The clay is Laguna 90. That's what they gave me in the class I took. Thank you It may be that the bowl that warped was a bit too thin and unsupported You may be right.

Renee

[bridgespottery]

Hi - I am a potter on Long Island and I make yarn bowls too. I sometimes get a little warping - doesn't really bother me too much but I think that any warping that appears in the glaze run is from the piece being unsupported and just wilts a bit. Doesn't always happen and I agree with firing at a lower cone for less warp. Happy knitting. Patricia @BridgesPottery.com

[meisie]

Hi - I am a potter on Long Island and I make yarn bowls too. I sometimes get a little warping - doesn't really bother me too much but I think that any warping that appears in the glaze run is from the piece being unsupported and just wilts a bit. Doesn't always happen and I agree with firing at a lower cone for less warp. Happy knitting. Patricia @BridgesPottery.com

 

 

Thanks for the reply. I'm just a beginner and your pots are lovely. I came across the yarn bowl idea purely by accident and mostly because I'm a knitter and have been for many years. I had never even seen one. I explained the idea to my instructor (he hadn't seen one either. ) and we came up with the design that was in my photo. Lo and behold after making one I found tons online. But they are fun to make and I have several surrounding my chair for all my yarn balls and projects.

Thanks again

Renee

[Username]

I am not a knitter, so excuse what is probably an ignorant question.

Why do you need the slot on the side of the bowl? Couldn't you just have a smooth hole for the yarn to go through? I guess the slot makes it easier to put the yarn in.

[Ben]

Did you use a witness cone in that firing or just a cone in the kiln sitter?

[Guest JBaymore]

Hi.

 

Looks like a case of a combination iof variables might be at work here. That is the thing with ceramics.... there are a lot of variables to deal with...............many we are aware of... and some that we might not be. Learning about and controlling these variables is a lot of the process of learning that comes with experience and training.

 

Here's an example that might be one thing that could have contributed to the warping: If this piece was located on the very outer edge of the kiln shelf, and the unsupported section of the cutout to the left of the slot was on the side of the piece that was facing directly toward the kiln's elements, it is going to get slightly more heat energy applied to it than the part that is "away" from the elelment (radiant heat transfer and the Inverse Square Law on energy transmission). The very slight highler level of vitirfication of the cone six clay body, which makes it a little softer at the peak of the firing (pyroplasticity). Gravity then takes over and does what it does, and the unsupported sectuion tries to move downward. But part of it is connected....... so the upper edge moves inward relative to the original position.

 

Since this is your first firing, it is also possible that you fired just a tad hotter / longer (clay vitrifies not according to temperature, but to the amount of heat work) than your teacher / class does in general. So even if the piece was not near the outer edge of the shelf... it still was slightly more vitreous overall. So the same structure and wall geometry might not support itself as well.

 

If you add in something like the fact that the wall section was thrown/trimmed just a tad thinner than you usually make it, particularly in the belly of that curve....... that will also add to the possibility. As will the fact that the position of the leaf could affect the leverage on the top edge. Also the thickness of the leaf addition will affect the weight of it on the wall (mass). The higher the mass and the higher the position pf that mass relative to the bottom of the cut-out, the more it will tend to warp the edge inward for a given amount of "softness" (pyroplasticity) in the lower part of the cut wall.

 

Now we get to the glaze......... and that can potentially add to the issue too.

 

If the glaze application on the inside was thicker than it was on the sucessful pieces, that could be contributing to the issue also. The mass of the glaze itself adds a little weight to the inner face of the cutout. If the insides are glazed and the outsides not... this makes the relationship even more likely to possibly cause some issues.

 

Although unlikely, you also have the "fit" between the freezing glaze (when liquid glaze sets up it is said to "freeze") and the clay body. If the glaze is tending to shrink more than the clay underneath it (has a higher Reversible Coeffieicnt of Thermal Expansion -C.O.E.-) then the glaze is sort of "pulling" on the unsupported clay wall as it contracts as the kiln cools. This likely is not happening enough when the clay is still pyroplastic here in this case... but it is possible that is contributing something to the cause. If the glaze on the inside has a high C.O.E. and the glaze on the outside has a low C.O.E., this makes the forces on the underlying clay greater.

 

Even the EXACT geoometry of the location and size of the cutout could cause issues if the clay, fired to exactly the correct and proper cone, is pyroplastic. That gets into basic physics of the support of the free-hanging section. So it is possible the warping is solely caused by the variation in the cutout on this particular piece. If you are "living on the edge" of warping with all of these designs of yarn holders...... then this will happen occasionally when your cutout is making the support fall outside the necessary level.

 

It could even be that the ball of clay that you used for this one was unevenly mixed material from the supplier........ and contained a bit more fluxing (melting) material in it than the usual clay. SO it was more pyroplastic. VERY unlikely that this would be the isolated case on just one piece.... but still possible. If it was reclaimed clay,... that makes it a bit more possible.

 

And so on......

 

Lots of possibilities........ you'll now have to think and test to see which one(s) is/are the culprit(s) and then know that you need to control those variables. I hope one thought in here pops out as a potential cause/solution for you.

 

best,

 

................john

[Riorose]

I have been making yarn bowls in clay for about a year and am now learning to fire on my own. I recently had a yarn bowl warp during the glaze firing and I'm not sure what the cause would be. Can any one give me a hand? This bowl is not yet bisqued but is an example of what the other one looked like. The cut in the bowl is for the yarn. The cut in my other bowl came out fine in the bisque. It was then glazed and fired and the left hand side of the pot warped down and curled in just a bit. What might cause this? Some of the glaze did melt into the space between the sides of the bowl and blocked the opening. Could that be enough to cause it to warp? If so then that's my mistake in putting the glaze on incorrectly, If not then something else caused it to warp. Thanks for the help

Renee

 

[meisie]

OMG! Kidding :-) Lots of info and what makes me really happy is that I actually understood most of what you wrote. Thank you very much I am going to pay attention to several of those variables. It makes a lot of sense. I certainly appreciate the time as well. I'm a late starter to clay and firing, kind of learning as I go. I'm an art teacher by trade and have been exposed to it although it was many, many years ago. All of the schools I have taught in did not encourage clay at the elementary level. Had a kiln in my room but they removed it because the teacher prior to me decided it was not appropriate for the age level. I wish I had spoken up more about that. Again thanks so much.

 

 

Hi.

 

Looks like a case of a combination iof variables might be at work here. That is the thing with ceramics.... there are a lot of variables to deal with...............many we are aware of... and some that we might not be. Learning about and controlling these variables is a lot of the process of learning that comes with experience and training.

 

Here's an example that might be one thing that could have contributed to the warping: If this piece was located on the very outer edge of the kiln shelf, and the unsupported section of the cutout to the left of the slot was on the side of the piece that was facing directly toward the kiln's elements, it is going to get slightly more heat energy applied to it than the part that is "away" from the elelment (radiant heat transfer and the Inverse Square Law on energy transmission). The very slight highler level of vitirfication of the cone six clay body, which makes it a little softer at the peak of the firing (pyroplasticity). Gravity then takes over and does what it does, and the unsupported sectuion tries to move downward. But part of it is connected....... so the upper edge moves inward relative to the original position.

 

Since this is your first firing, it is also possible that you fired just a tad hotter / longer (clay vitrifies not according to temperature, but to the amount of heat work) than your teacher / class does in general. So even if the piece was not near the outer edge of the shelf... it still was slightly more vitreous overall. So the same structure and wall geometry might not support itself as well.

 

If you add in something like the fact that the wall section was thrown/trimmed just a tad thinner than you usually make it, particularly in the belly of that curve....... that will also add to the possibility. As will the fact that the position of the leaf could affect the leverage on the top edge. Also the thickness of the leaf addition will affect the weight of it on the wall (mass). The higher the mass and the higher the position pf that mass relative to the bottom of the cut-out, the more it will tend to warp the edge inward for a given amount of "softness" (pyroplasticity) in the lower part of the cut wall.

 

Now we get to the glaze......... and that can potentially add to the issue too.

 

If the glaze application on the inside was thicker than it was on the sucessful pieces, that could be contributing to the issue also. The mass of the glaze itself adds a little weight to the inner face of the cutout. If the insides are glazed and the outsides not... this makes the relationship even more likely to possibly cause some issues.

 

Although unlikely, you also have the "fit" between the freezing glaze (when liquid glaze sets up it is said to "freeze") and the clay body. If the glaze is tending to shrink more than the clay underneath it (has a higher Reversible Coeffieicnt of Thermal Expansion -C.O.E.-) then the glaze is sort of "pulling" on the unsupported clay wall as it contracts as the kiln cools. This likely is not happening enough when the clay is still pyroplastic here in this case... but it is possible that is contributing something to the cause. If the glaze on the inside has a high C.O.E. and the glaze on the outside has a low C.O.E., this makes the forces on the underlying clay greater.

 

Even the EXACT geoometry of the location and size of the cutout could cause issues if the clay, fired to exactly the correct and proper cone, is pyroplastic. That gets into basic physics of the support of the free-hanging section. So it is possible the warping is solely caused by the variation in the cutout on this particular piece. If you are "living on the edge" of warping with all of these designs of yarn holders...... then this will happen occasionally when your cutout is making the support fall outside the necessary level.

 

It could even be that the ball of clay that you used for this one was unevenly mixed material from the supplier........ and contained a bit more fluxing (melting) material in it than the usual clay. SO it was more pyroplastic. VERY unlikely that this would be the isolated case on just one piece.... but still possible. If it was reclaimed clay,... that makes it a bit more possible.

 

And so on......

 

Lots of possibilities........ you'll now have to think and test to see which one(s) is/are the culprit(s) and then know that you need to control those variables. I hope one thought in here pops out as a potential cause/solution for you.

 

best,

 

................john

 

[Riorose]

yes, i guess there are many variables to consider, weights, stresses and tweaking the materials during forming or construction but here is an idea even with all the potential variables. Use a glaze formulated for a slightly lower temperature then use orton cones to measure heat work on the piece. You can fire a 1250 degree centigrade clay at 1220 and hold it for an hour or more and the heat work will reach the 1250 centigrade mark. Clays stay more stable this way but remember to adjust the glaze because a glaze that needs 1250 won't properly mature at 1220.

[meisie]

To be honest you don't really need the slot. In fact you can just have a bowl on the ground and put the ball of yarn in it and it rolls around in the bowl and not on the floor gathering dust. You could even use a cereal bowl from your kitchen. Many of the yarn bowls I've seen have the slot and it directs the yarn just a little better. If you have just a hole and no slot once the yarn is in the bowl and then through the hole you can't take your knitting anywhere with you unless you cut the yarn or put the bowl in your bag as well. No good knitter is going to want to cut the yarn that's a big no no. Lots of us travel with knitting sort of like taking a good book with you. I don't take my bowl but have my yarn then unwind from a carry bag.

Knitting trivia :-)

 

I am not a knitter, so excuse what is probably an ignorant question.

Why do you need the slot on the side of the bowl? Couldn't you just have a smooth hole for the yarn to go through? I guess the slot makes it easier to put the yarn in.

 

[meisie]

Just the kiln sitter.

 

 

Did you use a witness cone in that firing or just a cone in the kiln sitter?