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DaddyT

Slab cracking

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I am attempting for the third time to make two panels appx 16 x 20 inches. These will have multiple applied pieces and shapes to them.( an oval raised ridge encircling a selection of two fish hanging from a point with ribbons and vines encircling this. the second is similar with a pheasant hanging from the ribbons.) They are a reproduction of something someone has asked me to make. I have made one set with a thickness of 3/8 inch and one of 3/4 inch hand rolled clay. When fired, the 3/8 cracked in multiple places in the bisque. The 3/4 fired fine in the bisque and cracked in every direction on the glaze.

Everything was dried slowly and evenly over several weeks. I am a 40 year experienced potter and never really attempted anything the large in slab and as intricate with details. Someone suggested it might be that the flat plane was pulling against all the different additions in different directions causing the cracking. I don't agree.

I am using a stoneware clay. Firing bisque to 05 and then glaze to 5 in electric kiln.

I would like some suggestions on how to improve my results, since I am currently at 0% satisfaction and completion.

 

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I would be inclined to also believe the cracking is coming from the tension of the slab expanding/contracting on the kiln shelf during firing. When you make the slab, also make either some coils that the slab can lay on that raises it above the kiln shelf, or some flat slats, again to raise the slab above the shelf. Bisque them together; glaze fire them together. The coils/slats will help the slab move more easily during expansion/contraction. You could also lay a level layer of grog on the kiln shelf and put the slab on top.

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I would be inclined to also believe the cracking is coming from the tension of the slab expanding/contracting on the kiln shelf during firing. When you make the slab, also make either some coils that the slab can lay on that raises it above the kiln shelf, or some flat slats, again to raise the slab above the shelf. Bisque them together; glaze fire them together. The coils/slats will help the slab move more easily during expansion/contraction. You could also lay a level layer of grog on the kiln shelf and put the slab on top.

 

 

You could also lay a level layer of grog on the kiln shelf and put the slab on top.-This often works well, but I do something else that others may not have thought of. Old soft firebick can be broken up and powdered fine. This sprinkled on the shelf is courser than grog and gives great movement.

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I would be inclined to also believe the cracking is coming from the tension of the slab expanding/contracting on the kiln shelf during firing. When you make the slab, also make either some coils that the slab can lay on that raises it above the kiln shelf, or some flat slats, again to raise the slab above the shelf. Bisque them together; glaze fire them together. The coils/slats will help the slab move more easily during expansion/contraction. You could also lay a level layer of grog on the kiln shelf and put the slab on top.

 

 

You could also lay a level layer of grog on the kiln shelf and put the slab on top.-This often works well, but I do something else that others may not have thought of. Old soft firebick can be broken up and powdered fine. This sprinkled on the shelf is courser than grog and gives great movement.

 

Fantastic comments thanks I ll post pictures tomorrow..before and after.

 

 

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I have used grog most of my life but in the past 10 years I have tried the coils. Vince Pitelka made this suggestion for a different situation. I adapted it to my needs. I have found that the could really work better for me when I am firing large pieces like tall columns sometimes of paper clay. These are hard to dry out and the coils let the bottoms release moisture better than grog. It is a simple matter of specific needs. Grog can work too. I just like the coils better for me.

Marcia

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I like the idea of getting the large flat piece up off the shelf in the firing too. It gives the heat a better access to both sides of the slab at once. Might try circular kiln shelf supports on their sides IF you already have some. This would get the piece even further off the shelf. If you realy want to take this idea to its extreme you could try firing them standing on edge with a wall of shelf supports or soft bricks blocking the direct heat from the elements from the edges of the piece closest to the elements. This would allow for more even heating of the piece (see below).

 

Question: How large is your kiln? In an electric kiln heat radiates form the elements. The available radiated heat close to the elements is going to be incredibly high compared to that at the middle of the slab. This can lead to an huge difference in temperature within the piece from edge to center. This could lead to cracking. To minimize this you need to make sure any ware that gets close to the elements is not large enough to have its other side vary far from the elements. ie: a coffee mug can comfortably be fired within a couple of inches of the elements but a large slip cast pot that only clears the elements by the same amount may not survive the firing. Had this happen to a friend. He made some wonderful, large slip cast pieces. Made one that had a 30 piece mold. He designed the thing to just clear the elements when green and never could get one to survive the firings. After some research he tried firing one in a much larger gas kiln and it survived just fine. They just couldn't survive the stress created by the temperature differential from edge to center.

 

You may also want to try a different method for making the Slab. I'm afraid that hand rolling may introduce stresses into the slab that other methods may not.

Who, on this forum, makes large slabs and how do you do it?

If it were me, I'd follow the Harry Davis method that he used to make kiln shelves as described in his book "A Potters Alternative". Do you have access to a copy?

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I like the idea of getting the large flat piece up off the shelf in the firing too. It gives the heat a better access to both sides of the slab at once. Might try circular kiln shelf supports on their sides IF you already have some. This would get the piece even further off the shelf. If you realy want to take this idea to its extreme you could try firing them standing on edge with a wall of shelf supports or soft bricks blocking the direct heat from the elements from the edges of the piece closest to the elements. This would allow for more even heating of the piece (see below).

 

Question: How large is your kiln? In an electric kiln heat radiates form the elements. The available radiated heat close to the elements is going to be incredibly high compared to that at the middle of the slab. This can lead to an huge difference in temperature within the piece from edge to center. This could lead to cracking. To minimize this you need to make sure any ware that gets close to the elements is not large enough to have its other side vary far from the elements. ie: a coffee mug can comfortably be fired within a couple of inches of the elements but a large slip cast pot that only clears the elements by the same amount may not survive the firing. Had this happen to a friend. He made some wonderful, large slip cast pieces. Made one that had a 30 piece mold. He designed the thing to just clear the elements when green and never could get one to survive the firings. After some research he tried firing one in a much larger gas kiln and it survived just fine. They just couldn't survive the stress created by the temperature differential from edge to center.

 

You may also want to try a different method for making the Slab. I'm afraid that hand rolling may introduce stresses into the slab that other methods may not.

Who, on this forum, makes large slabs and how do you do it?

If it were me, I'd follow the Harry Davis method that he used to make kiln shelves as described in his book "A Potters Alternative". Do you have access to a copy?

 

 

no i do not have access to this book. your comments are helpful though

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Check your local library and see if they can get it through interlibrary loan (if you have time).

 

The basic method is to make a frame that is the wet thickness and outer dimensions of the slab you want.

Use sloppy,soft clay. Work on top of a flat board covered in newsprint. Lay down your frame and fill it with clay. Throw it down hard then paddle it down until it stands above the edge of the frame. Use a large framed cutoff wire across the top of the frame and remove the excess clay. Cover with news print. When it begins to stiffen, put a board on top and flip it. Remove the frame asap and stand it on edge asap to stiffen and dry.

 

He was using a mix that was about 50% fireclay grog / 50% fireclay to make kiln shelves.

 

With your clay YMMV.

 

Ben

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Hi there, could also be a clay problem. What are you using? I have a very old potters book and describes a great way to make large slabs. I will bring it back from the studio when I go tomorrow and try to scan in the section. T

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Hi there, could also be a clay problem. What are you using? I have a very old potters book and describes a great way to make large slabs. I will bring it back from the studio when I go tomorrow and try to scan in the section. T

 

 

I can't believe the responses I have had in less than 24 hours to this. I shall have to find some more problems to bring to the forum for resolution. I will get to work with these ideas and proceed t to the next attempt toward success. Many thanks to you...

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Hi there, could also be a clay problem. What are you using? I have a very old potters book and describes a great way to make large slabs. I will bring it back from the studio when I go tomorrow and try to scan in the section. T

 

 

I can't believe the responses I have had in less than 24 hours to this. I shall have to find some more problems to bring to the forum for resolution. I will get to work with these ideas and proceed t to the next attempt toward success. Many thanks to you...

 

 

 

This was a pugged clay that I have used before with success. It is a blend of Highwater Moon white and a grogged red clay. Cone 6-10 clays

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You might want to think about firing the slabs on their sides, propped up against the stack of shelves -- providing the sides are not fully glazed. That would greatly minimize the surface area sitting on the shelf. Also, you could put the slabs on a bed of kaowool (of course, use appropriate gloves when handling).

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Thanks for posting the images. Looking at them I can see that you are asking a lot of that clay body ... It's thin and thick, you have pressed down into it, built up on it, impressed on it at all angles ... that's a lot of stress.

I think you need a more forgiving clay body ...

Marcia can help here as she might know what you could add to the actual clay that would help.

My first guess would be grog but Highwater makes some very resilient clays. Call them and ask which clay can take the most abuse. Some clays just can't take all the stuff you are doing and others can ... I think you have to try to fix the clay then use those hints for firing so it moves on the shelf.

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I have a problem with cracking only on my platters. I throw them on the wheel about 11" diam and@ 1" thick bottom and rim with buffalow wallow clay. I have had one platter make it without cracking but in the glaze firing it warped because of the stilts I used. Am I drying the platters to fast, I used to store in pottery barn uncovered for 1 week-cracked down center. Then I put one in uninsulated building 1/2 covered for a week, this one is still drying. Do I need to leave it covered for 2weeks? I don't know?

Also How do I place a fully glazed platter on a kiln shelf without it warping?

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Thanks for posting the images. Looking at them I can see that you are asking a lot of that clay body ... It's thin and thick, you have pressed down into it, built up on it, impressed on it at all angles ... that's a lot of stress.

I think you need a more forgiving clay body ...

Marcia can help here as she might know what you could add to the actual clay that would help.

My first guess would be grog but Highwater makes some very resilient clays. Call them and ask which clay can take the most abuse. Some clays just can't take all the stuff you are doing and others can ... I think you have to try to fix the clay then use those hints for firing so it moves on the shelf.

 

Makes sense. Most of the work is just slip attached though. Some have suggested I make the application and bisque fire it then glaze in place. I don't see this as an answer. Too much detail to manipulate without a fixed surface for one thing and having to make sure everything stays in place in the glaze fire is another. I will check with High water. I also have some grog I can add to a clay that i use regularly Standard 259...Comments from Marica will be appreciated.

 

If you are in Raleigh as noted and can do so, share the word about the Naked/Ferric class with Wally and Sue I mentioned in March with some of your Potters..please.

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Thanks for posting the images. Looking at them I can see that you are asking a lot of that clay body ... It's thin and thick, you have pressed down into it, built up on it, impressed on it at all angles ... that's a lot of stress.

I think you need a more forgiving clay body ...

Marcia can help here as she might know what you could add to the actual clay that would help.

My first guess would be grog but Highwater makes some very resilient clays. Call them and ask which clay can take the most abuse. Some clays just can't take all the stuff you are doing and others can ... I think you have to try to fix the clay then use those hints for firing so it moves on the shelf.

 

Makes sense. Most of the work is just slip attached though. Some have suggested I make the application and bisque fire it then glaze in place. I don't see this as an answer. Too much detail to manipulate without a fixed surface for one thing and having to make sure everything stays in place in the glaze fire is another. I will check with High water. I also have some grog I can add to a clay that i use regularly Standard 259...Comments from Marica will be appreciated.

 

If you are in Raleigh as noted and can do so, share the word about the Naked/Ferric class with Wally and Sue I mentioned in March with some of your Potters..please.

 

Wow after seeing your cracks, I felt really bad for you. It is always such a disapointment when you open the kiln arrrg... Anyway I think you need a groggier clay as well...I live in spain so I can't offer advice about a premix ready clay as I am pretty sure it is different here. So wait for Marica... I didn't make it up to my studio today, but am sure the slab building with a frame book I have will be of some help. Trina

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I agree with Chris from the pictures you are asking a lot of the clay. Are you using a groggy clay body? For something like you are making with all the thin and thick variables, I would reinforce the edge with a thicker band. The edge is thin and the center is thick so the thin edge will dry faster, heat faster and shrink faster than the center. I'd put a similar ribbon from the center design around the edge to beef it up. I still recommend 1/4 inch coils under the piece to allow even heating and movement during shrinking. My coils have broken up and I still spread out 1.5"length coils under pieces. I have had good results doing this.

Hope these suggestions help.

Marcia

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Marcia, I found this thread after two 15" x 15" x 1/2" slabs cracking during bisque firing. I had used ^5 Laguna Bmix with Grog, rolling out the slabs to 1/2" while turning in all different directions. The slabs were completely dry (not cold to the touch) before I did a SLOW (15 hour) bisque firing to ^04 in a very loosely-packed kiln, with the 2 slabs on the cooler bottom portion of of the kiln, resting on full kiln shelves. One of the slabs cracked completely in half and the other cracked 75% across the surface. <sigh>

 

I didn't use coils or kiln posts to elevate the slabs off the shelves because I thought that might cause warping. I considered using silica or sand as "ball bearings" under the slabsm but decided against this after reading a cautionary essay by Vince Pitelka on the practice.

 

Do you still recommend using coils under a large slab? If so, how far apart should they be spaced? Pitelka talks about using short kiln posts in the corners, but all I can imagine is my large slab turning into a bowl as it sinks in the middle during the firing process.

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I've just done some large tiles with sculpted flowers on them, and have done many others up to 15 x 8 . I have never had a problem with cracking. I dry them directly on wire mold shelves so the air circulates on the top and below. I do a slow bisque when they are fully dry.

 

I place tiles on "pins" -- 5" ceramic rods arranged to support the piece every 2 inches or so. I really like the pins because them are the exact same size and provide even support. (buy these from Seattle Pottery Supply)

 

Sounds like you've done everything right except to support the piece. I can't find fault with using the Bmix with grog. Try the wire mold shelf and/or the pins.

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Marcia, I found this thread after two 15" x 15" x 1/2" slabs cracking during bisque firing. I had used ^5 Laguna Bmix with Grog, rolling out the slabs to 1/2" while turning in all different directions. The slabs were completely dry (not cold to the touch) before I did a SLOW (15 hour) bisque firing to ^04 in a very loosely-packed kiln, with the 2 slabs on the cooler bottom portion of of the kiln, resting on full kiln shelves. One of the slabs cracked completely in half and the other cracked 75% across the surface. <sigh>

 

I didn't use coils or kiln posts to elevate the slabs off the shelves because I thought that might cause warping. I considered using silica or sand as "ball bearings" under the slabsm but decided against this after reading a cautionary essay by Vince Pitelka on the practice.

 

Do you still recommend using coils under a large slab? If so, how far apart should they be spaced? Pitelka talks about using short kiln posts in the corners, but all I can imagine is my large slab turning into a bowl as it sinks in the middle during the firing process.

 

I would use the coils. I got the idea from Vince. I applied it to my large architectural pieces of paper clay. I put them about every inch. they break but I still use the bits. I extrude the coils so they are uniform.

Marcia

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