Slab cracking

[kevin]

cracking could be because of low compression of the clay. if you compress your clay very well i'm stumped. also the glaze fire cracking could be because of thermal shock but i don't know the temp. you took it out at.

[Karen B]

Hi Daddy, I also posted about this problem.

Marcia is right. It was most definitely the uneven heat circulation. Apparently the shelf doesn't heat or cool at the same rate as the clay.

As per suggestions, I put a thick layer (3/8th inch) of grog underneath my large slab and it worked.

I also did a test of just a thin layer of grog in case is was just the friction of shrinkage, which yielded in cracks.

The trouble with the grog was that it is impossible to keep from falling on other things, so I plan on creating the coils like Marcia did.

[Karen B]

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

 

 

Hi Marcia,

If possible, could you please post a picture of how you lay the coils on your shelf. I am having trouble picturing it.

Thanks for all your good info.

Karen

[laurelneth]

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?

 

[laurelneth]

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?

 

 

[perkolator]

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

 

 

our students make large slabs like the ones described by the OP all the time with success. we use a relatively heavily grogged stoneware clay body. compact the heck outta the slab during construction and wrap the edges to prevent drying cracks and make sure to flip it when compacting and drying to whatever consistency needed to attach objects. usually slab itself is around 1-2" thick. for firing, we'll make lots of balls of kiln putty (equal parts silica, kaolin, and 20m grog) about 1-2" diameter to "stilt/prop" up the slab and support it evenly. due to size, gas kiln (Bailey downdraft) is usually where they get fired in the mix of all our regular life-size sculpture and the firing is very slow (maybe 48hrs pre-heat, then the climb to temp takes around 20hrs or so) and depending on thickness of slabs may need to be down-fired to prevent cooling cracks. usually we fire our work to earthenware temps, but also do slabs like this at midrange.

 

i've sometimes loaded slabs vertically like described, but usually that's only when i didn't have much room and just stick it in with that orientation. it DOES work, but I much prefer to fire flat and avoid possible problems like warping or falling over, etc.