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Ruchi

Developing A Hair Line Crack In Third Firing

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Hi,

 

I slip cast my work using low fire clay. I fire three times, Bisque and glaze at cone 05 and decals at cone 010. I am having problem with one particular slip casted platter consistently. In third firing (cone 010) this style develops hairline crack. Not all the pieces develop the crack but lately 3 out of 4 have, which implies I am doing something wrong, but I am not able to figure out what? 

 

I glaze the whole platter ( inside out) balancing it on triangular firing stilt and they are fine after glaze firing. It is the last cone 010 firing for decal that the crack appears. As the crack is not sharp and glaze seems to bend inwards I gather it happens at the early stage of firing. 

 

Please advice.

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It is not consistently developing at one spot. It is at times in the centre, or around the edge. Also it at times develops just at one side of the platter ( not visible the other side).

 

How do I attach image to this post??

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Use the More reply options-but 1st read the FAQ at top of main page-read about poting photos-its easy.

Mark

 

PS my guess is the decal fire is to fast going up or to fast going down.Platters need to go slow-put a shelve over them to trap the heat.

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If you think its from firing and not a cooling crack then slow down the temp rise during the inversion zones. (I would also slow it down when cooling through these temps as platters like to dunt when refired) So slow down the firing starting at about 1000F up to about 1100F and to be safe you could slow it down from 400F to 500F also.

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Thanks for the response.

 

I have 12 key kiln and I use slow glaze auto setting, should I be using slow bisque option??

 

So far I have got 5 platters develop cracks, I studied all of them today and found that crack is at different spots, they did not crack through n through(except one).

 

Would it matter if the platter was not kept in the center of the kiln but on the side??

 

attaching pic to show the shape of the platter n crack.

 

Thanks!!

Ruchi

post-6946-0-97969400-1429148000_thumb.jpg

post-6946-0-05730600-1429148013_thumb.jpg

post-6946-0-62123800-1429148109_thumb.jpg

post-6946-0-97969400-1429148000_thumb.jpg

post-6946-0-05730600-1429148013_thumb.jpg

post-6946-0-62123800-1429148109_thumb.jpg

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Try using your slow bisque firing program for the decal firing.  It may take a bit longer to fire the third time, but hopefully it will be a less stressful firing on the platters.  And, for the third firing, add some clay cookies or slats underneath the platters (unless the bottoms are glazed).  Getting the platters, with large surface areas, above the kiln shelf will allow for more even cooling of the platter.

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