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Plates crack down middle during bisque


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Hello

I have recently acquired a small kiln for use at home (Hobbytech 40). It works and I’m happy with it for the most part, but when I have bisque fired plates, they crack during firing, down the middle to about halfway across the plate.

This must be a fault when I’m making the items. Can anyone tell me what could be causing this so that I don’t waste efforts making anymore to be disappointed when they’re fired? I’ve not had this happen with plates I’ve made in my ceramic lessons.

 

Some are wheel thrown, some are made from slabs. Clay bodies used are lavafleck stoneware, or white special stoneware. Are these plates now only good for the bin?

 

Photos attached.

A1E60E85-3E3A-457F-85D3-0ED2932B506B.jpeg

F535A063-F0B3-4A4B-A379-C826E364C75F.jpeg

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1 hour ago, roobarb said:

Hello

I have recently acquired a small kiln for use at home (Hobbytech 40). It works and I’m happy with it for the most part, but when I have bisque fired plates, they crack during firing, down the middle to about halfway across the plate.

This must be a fault when I’m making the items. Can anyone tell me what could be causing this so that I don’t waste efforts making anymore to be disappointed when they’re fired? I’ve not had this happen with plates I’ve made in my ceramic lessons.

 

Some are wheel thrown, some are made from slabs. Clay bodies used are lavafleck stoneware, or white special stoneware. Are these plates now only good for the bin?

 

Photos attached.

A1E60E85-3E3A-457F-85D3-0ED2932B506B.jpeg

F535A063-F0B3-4A4B-A379-C826E364C75F.jpeg

I should also add that I’m using a plate setter with uprights and pins. Each setter holding about 10 plates.

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Plates can be subjected to a lot of stress during making that doesn’t show up until the firings. Plates that have been made by mostly flattening the clay with an outward motion are most susceptible to cracking, especially if they do not have a foot. There is a tremendous attraction between the clay and the metal wheel head and clay that has been stretched out against the wheel head will be subjected to competing forces. You’re moving the top part of the clay outward while the bottom is doing its best to stick to the wheel. Leaving enough clay to cut a foot on the bottom gets rid of the stressed clay. It also helps to move the hands back inward toward the center at least as much as you move them outward. 

Uneven drying can also cause cracking. Dry slowly, so that the rim does not dry before the center of the plate. A donut of plastic wrap around the rim will help. And make sure the plate is of even thickness. Thick and thin areas will dry unevenly. 

Slabs should be turned and flipped during the making to prevent similar cracking issues. 

 Ruth Ballou

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And even a bad example serves a purpose, or in this case you can use the cracked ware,  or pieces thereof for test work for glaze or decoration,   Not sure you are going to like a color scheme, or how a new glaze will look on a particular body, why waste a good piece when you have test ware you can use.  Think Swatches.  

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  • 3 weeks later...

One thing is for sure-they all need to go into the bin

The stress on a slab plate is huge-It would help if I could see the profile of a broken one in 1/2 to see the thickness.

The sharp upward lip is also an issue but the biggest may be how you are firing them and cooling them.

Just follow all on the input in above posts for better results.

A plate that has a foot and is glazed on two side is less stress than a flat unglazed bottom plate.

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Roobarb-

How large are the platters? Are you placing them into the kiln so they are spanning the crack between 2 shelves? That can be an issue if the shelves have distorted (warped) differently. If you are, I’d recommend putting a “waster” slab under the platters so it can take the abuse and not the platters. Just roll a slab about 1/4-3/8” thick and  is the same size or bigger in diameter as your platters. If the “wasters” crack in the firing, just toss them. Clay is cheap in comparison to the labor involved in finished work. You’ll want to use them in the glaze fire as well, except put a coat of kiln wash on them.

Regards,

Fred

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