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Cracks in large, thick stoneware tiles


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#1 bethkam

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Posted 21 January 2013 - 04:35 PM

I recently bisqued 4 large, thick (about 1 1/4" in some places) tiles in a propane updraft kiln. Bisqued to my normal 08 in about 8 hours. Two of the tiles cracked to tiny pieces ( I am thinking they must have had some moisture in them, but they had changed color and dried for a month before firing). The other two tiles have cracks in them that go from the outside edge inward. Not sure what is going on. I am using a clay that I make myself and have used for dozens of years. Have even made other large tiles with that clay. Kiln is different from the one long ago. Kiln is up-draft, propane. I had the kiln on pilots for about 12 hours, top was hot when I removed spy hole.

Not sure how to limit the variables. Fire on grog? Take longer for bisque firing? Put up a baffle wall? I can't afford to loose anymore tiles. Have any suggestions?

#2 bciskepottery

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Posted 21 January 2013 - 04:46 PM

Did you fire the tiles flat or on edge? My guess is flat . . . and the tile may have started to cool while the kiln shelf was still hot. Firing on grog may help; also roll out some coils or slats and place them under the tiles so the tiles sit above the kiln shelf; that will help compensate for the temperature difference and allow the tiles to shrink and expand during firing. Consider firing on edge; it reduces the amount of contact space between the tile (just an edge rather than full surface) and kiln shelf.

#3 Chris Campbell

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Posted 21 January 2013 - 05:31 PM

Can you post images?
When a tle just crumbles I think materials ... When it explodes I think water.
Would love to see the cracks from the edges in the others ... I assume they were uneven since you said they were a certain height in places.

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#4 Marcia Selsor

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Posted 21 January 2013 - 09:22 PM

Recommend drying pieces on sticks and firing them on 1/4" clay coils to help with heat circulation.
How large are they?

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#5 bethkam

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Posted 21 January 2013 - 09:39 PM

Recommend drying pieces on sticks and firing them on 1/4" clay coils to help with heat circulation.
How large are they?

Marcia

They dried on drying racks made of slats. I will try to fire on bed of grog possibly in an electric kiln??? I once made some super large slabs, about 40" x 24" they cracked similarly and I fired them on beds of fiberfax.

#6 bethkam

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Posted 21 January 2013 - 09:43 PM

Can you post images?
When a tle just crumbles I think materials ... When it explodes I think water.
Would love to see the cracks from the edges in the others ... I assume they were uneven since you said they were a certain height in places.


Two images, one from back (on edge) the other flat looking down. Both were bisqued. I will fire next time on a bed of grog. Did that for the glaze fire and noticed the cracks after that fire. I am thinking the cracks came with the bisque.

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#7 Chris Campbell

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Posted 21 January 2013 - 11:13 PM

Does that crack go all the way through? Can't tell from the images.

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#8 bethkam

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Posted 22 January 2013 - 07:37 AM

Does that crack go all the way through? Can't tell from the images.



#9 bethkam

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Posted 22 January 2013 - 07:40 AM

You can't see the crack from the top, only from the bottom side. However, the first time I fired sample tiles (and did not notice the cracks on the bottom until after the glaze fire) the cracks did go through only not as obviously. Interesting thought on your part. What's on your mind?.....

#10 neilestrick

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Posted 22 January 2013 - 10:01 AM

Even dry pieces can blow up in a bisque if they are very thick. 1 1/4" is very thick. They'll need a nice long preheat to dry out the last bit of remaining water.

Most clay bodies won't handle being that thick unless they have a lot of grog in them. At the very least you'll need to prop them up off the shelf to they eat more evenly. You'll also want to even out the thickness by doing some carving from the backside.
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#11 Chris Campbell

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Posted 22 January 2013 - 11:13 AM

I have made similar tiles and I agree with Neil that you need a very forgiving clay body ... there is just so much stress in all parts of the making and firing that arise from the varied thicknesses. Places I would look to for problems during process would be ...
forming the tile ... slab roller? Hand rolled? Thrown?
Carving the tile ... Being more aware of how I am carving pressure wise and thin/thick ratios
Drying without any movement until leather hard
Firing on its side or on clay rollers or some support to keep air moving across the bottom

Also, have you considered adding some paper pulp to your clay body?

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#12 trina

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Posted 22 January 2013 - 11:50 AM

Hi there,
I have also found that if possible when making large tiles it helps to use a tool and thinly cross hatch a grid across the back. I have found that some times you might get a small hairliner crack on the edge but the grid prevents it from traveling through the whole tile. I also agree with firing on grog or coils and not directly on the kiln shelves. T

#13 bethkam

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Posted 22 January 2013 - 02:16 PM

Hi there,
I have also found that if possible when making large tiles it helps to use a tool and thinly cross hatch a grid across the back. I have found that some times you might get a small hairliner crack on the edge but the grid prevents it from traveling through the whole tile. I also agree with firing on grog or coils and not directly on the kiln shelves. T



#14 bethkam

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Posted 22 January 2013 - 02:35 PM

Thank you all for your thoughtful replies. The are appreciated.

I have already completed making the tiles. They have been drying for the past month, there's no way I can change anything. I have used this clay body to make other large tiles many years ago, so there's a certain amount of faith I have about the clay. Having said that, everything and anything seems to be a variable right now. I will fire the next couple of sample tiles in an electric kiln with grog under each. See what happens there. Then proceed. I am going with the idea of direct interaction with the kiln shelves and possibly to quick a bisque. Will see. Again thank you for your thoughts.

#15 neilestrick

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Posted 22 January 2013 - 02:55 PM

Rather than firing on grog, try silica sand. It has smoother, rounder particles that will act like little ball bearings and allow the tile to expand and contract. Grog has sharp jaggedy edges. Fiber will not work at all. In fact it will probably insulate the bottom of the tile and exacerbate the problem. Take your grog and wedge it into the clay to open up the body more.
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#16 bethkam

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Posted 22 January 2013 - 03:17 PM

Sounds good. Silica sand. Have to deal with the body the way it is....all the tiles are made. I used to feel confident about the body because I have a long history using it.
Thanks again.

#17 Ben

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Posted 22 January 2013 - 03:47 PM

Try a presoak below boiling. Say 180 f for several hours. This will bake out any left over moisture that sitting on the shelf cannot. Then bisque fire.
Hope they work out.
Ben




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