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  #1  
Old 10-18-2012, 12:42 PM
rscott rscott is offline
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Plaques cracking during firing

I am having a problem with my plaques cracking during firing and am hoping someone can give me some suggestions how I can help prevent this. I'm new to this so any help I can get is very much appreciated.

The description of the clay I used for the item pictured is as follows. I chose this clay because there is very little warping during the drying process.
"Dark Brown Clay - Cone 4-6
Gives reduction-like dark brown color in electric kiln. Smooth
and plastic. Good for all techniques.
C/4: Shrinkage 11 .5%, Absorption 2.4%
C/6: Shrinkage 13%, Absorption 1.4%"

The piece itself is about 3/8" even thickness throughout, and about 12" high. The piece was created using a plaster mold which I made. It went into the kiln flat, bisque fire was good, no warping or cracks noted when examined while glazing the bisqueware. It cracked right across the piece in the glaze fire, edges are sharp and splintered (see photo).

My kiln is an L&L ez fire electric kiln and I use the slow bisque setting to cone 5. Both firings were done with the piece flat on the shelf though it is worth noting that this piece is beveled slightly so the middle doesn't touch the shelving. Because of it's size the piece straddles the space between the two 1/2 shelves in the kiln. I leave the vent fan on until firing is complete then turn it off for cooling.



Thank you!
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  #2  
Old 10-18-2012, 01:12 PM
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The Forge The Forge is offline
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Re: Plaques cracking during firing

Since the piece is straddling the space, it will fire and cool unevenly. The shelves will act as a shield and the area of the piece resting on them will be affected by it, while the center part is wide open. You could try putting the piece on stilts so that there is an open space under the piece, that will allow for circulation.
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  #3  
Old 10-19-2012, 08:51 PM
rscott rscott is offline
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Re: Plaques cracking during firing

Thanks for taking the time to reply, I will get the next one up on posts and see what happens.

Do you think it is necessary to do this for the bisque firing as well?

Thanks again
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Old 10-20-2012, 12:40 PM
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The Forge The Forge is offline
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Re: Plaques cracking during firing

If you intend to make more of them, the best would be to get a shelf big enough to support the whole plaque and put in the kiln. Just put stilts under it and the two side shelves for support and to allow the heat to circulate.
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  #5  
Old 11-28-2012, 10:45 AM
rscott rscott is offline
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Re: Plaques cracking during firing

Thanks again for your suggestions, as a follow up I ended up buying a full - hollow core shelf for the larger plaques. That plus stilting the pieces differently has so far been successful.
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