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Two Pieces Ruined, Help Needed


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Hi everyone, so Ive just opened the kiln and its not good.

 

I had two large tile pieces which smaller pieces attached  on by scoring , slipping wiggling and pressing down really well. Or so I thought  :( the smaller 'attached' pieces have come away from the surface tile making them only fit for the  ceramic grave yard.

 

Ive made a piece similar before and Is been fine and the thing is I have three more pieces ready be fired. Can anyone shed any light as to why this is happening? Im certain they are dry and  I took extra care in fixing them on, all help much appreciated.

 

Julia

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Since you said the smaller pieces came off the larger piece, I would bet the culprit is uneven drying. The small pieces dried more quickly than the base and contracted at a different rate, causing it to pop off later in firing.

 

Try covering the next one in plastic to even out the moisture distribution as it dries.

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Hi, these are the pieces after I took them from the kiln.

 

Pic 2 seems to have just cracked straight across with the attached pieces coming away from the surface.

 

In the first one the piece has seemingly crumbled in one part as well as the smaller pieces coming away from the surface, very curious as to what has caused the crumbling.

 

Any ideas very welcome.

 

Julia

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Picture 1 damage is from the tile being fired before it was dry. Moisture in the clay blew it up, try candling the tiles overnight, kiln around 185F, before starting to increase temp slowly.

 

Picture 2, cracks could be from cooling too fast. Do you fire these pieces on something like silica sand so they can move in the kiln as they shrink? Few more questions though, what's your firing schedule, how fast does your kiln cool and at what temp do you crack the lid open? Also, the little piece that came away from the surface, I would guess it's from not having it joined really well or is the result of a second cooling dunt.

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Thanks Min,

 

Wow I really thought they were dry, I did the moisture test with my finger and they seemed ok, but I guess not :(

 

I fire to 1140 °over approx. 12hrs about 100°an hour with a half hour soak. I don't open the door until its under 200°and even then just for a peek. I also never used to cover while drying as It takes so long anyway but will start doing this now. I have to confess these two pieces were left out in the rare sunshine for an hour or two just t speed it up so I suppose that didn't help matters.

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So I'm guessing that since you are bisque firing to approx cone 1 or 2 that this is earthenware?

 

Do you have a controller on the kiln? If you do I would slow down parts of the firing and speed up other parts. For thickish pieces I would use something like this:

 

candle overnight at 85C

25C per hour up to 120C

90C per hour up to 540C

then slow down to 40C per hour up to 600C

speed up again to 80C up to around 1000C

then slow it down again to 40C until you hit your final temp of 1140C

 

I wouldn't peek at 200C, leave the plugs in, go have a cup of tea, walk the dog, clean the studio, leave it alone until its about 90C then take the plugs out, but leave it be to cool down the rest of the way. Don't unload until the shelves are cool enough to pick up. (air temp inside kiln is a lot cooler than shelf temp).

 

+1 for what others have said about even drying.

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Hi, thanks for going to all that trouble :rolleyes: 

 

Yes its earthenware and cone 2. It does have a controller so I will attempt to  reprogramme to your recommendations and see if its any better. Its hard not to peek but I'll go walk next doors dog !

 

Thanks again, Julia

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