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Seedy Potter

Large Flat Pieces Cracking During Glaze Firing

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I am having trouble with successfully firing large flat pieces in a glaze kiln.  When I bisque fire them, I put a little sand under them to reduce the friction on the shelf as they shrink and move during firing.  This has been very successful.  However, I am hesitant to put sand in a glaze kiln especially with a vent master drawing air downward.  I am afraid that the sand will be drawn into molten glaze.    My question is:  is that a reasonable concern?  If so, what would any of you suggest to use to keep large pieces from breaking apart at ^6.  Might the problem be solved if I brought the kiln temp up much more slowly? What temp range is the most critical? 

 

Is it possible that all the moving sand will have "found a home" by the time the glaze is molten?  This still feels if-y to me... I could use some sage advice! 

Chantay likes this

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You should be okay using sand during the glaze firing, too. Just make sure you leave some space between the sand and where your glaze starts. The vent should not be moving the sand around during the firing. Options to using sand include clay cookies allow the platter to sit above the kiln shelf, clay slats, or clay coils. But the key is to get the platter/flat item above the shelf so it cools evenly. The kiln shelf retains heat and cools more slowly as it is thicker than your pottery (except for sculpture). Allowing the item to cool above the shelf minimizes the chance for uneven cooling and cracking.

Seedy Potter likes this

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Thanks, bciskepottery.  I appreciate your quick reply.  I'll give it a try and hope for the best.  This piece is a first of a kind for me and after a lot of experimentation, I really want this to go well.  

 

Grype: My pieces this time are about 20' round.  I have never had a problem with plates that are under 12".  There's always a first time, tho. I use medium fire stoneware.  

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How close together/dense should the coils be so that the bottom is sufficiently supported and does not distort on the glaze firing (for a mid fire stoneware clay)?  Should the coils be kiln-washed to prevent sticking?

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Thanks, these are great suggestions. A couple of questions: I am concerned with the lage flat piece warping. It seems like it would take a lot of coil to make sure it doesn't. Do you create the coil support for every piece you do? It seems logical that using the same clay body would insure continuity of movement, but that's a lot of material to waste. Am I seeing this correctly? Sounds like the cookies solution is a reuseable solution, for a 20" disc that's still a lot of cookies.

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we sprinkle grog on the glaze kiln shelf for large flat pieces and it seems to work.  Once I made a larger flat piece and was advised to make a "waste slab" to lie underneath the piece during both bisque and glazing. the waste slab cracked but the main piece didn't.  i also cut some grooves in the back of large flat pieces so a little air can move. You might also use a "groggier" clay. rakuku

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The piece I fired, with the coils, we spaced them 1-2" apart.  So max, we only had twenty coils, but it was probably closer to a dozen.  Keep in mind, only  a couple of them are really as long, as the work.  Then they get progressively shorter, as you go towards the outside.  So it's not wasting that much clay.  

 

If you are concerned about wasted clay, just reuse them.  I didn't reuse those I made, because I generally don't have students make a slab relief sculpture, the size of a full kiln shelf...

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Re: Warping (or slumping). What type of claybody are you using? Stoneware (smooth or groggy) or Porcelain? How thick is the piece? And, is your kiln shelf flat? If you took care in keeping the piece flat while forming, rolling, making and drying, you have minimized the likelihood of warping. Your big challenge is uneven heating/cooling. My guess is you are firing in an electric kiln with a 22 or 23 inch diameter -- the piece will take up a full shelf. Electric kilns heat from the perimeter (where the elements are) to the center. So, in a piece as big as yours, the piece will start hot from the edges and proceed to the middle. That is a fair amount of stress.

 

You can create coils/cookies each time, or you can save and re-use. Using the same clay body does insure compatibility, but I've re-used cookies from other clay bodies without any issues (but my items are much smaller). If you save and re-use, you are not wasting a lot of material. Besides, they coils/cookies are made from clay and that is the cheapest part of the equation. The cost of the clay is miniscule compared to the amount of time you spent making the item, decorating/glazing it, and firing it.

 

The cookies/coils can be spaced and do not have to touch each other. You just want to be sure they are not spaced so far apart that the form ends up slumping. If you make cookies, use a variety of sizes. A variety of sizes also work well when you need a glaze drip catcher for your future firings.

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I have stacks of bisqued disks in varying sizes that I place under my plates and platters. I reuse them again and again and in different combinations depending on what is needed to give support. I have not had an issue with stuff cracking so I guess they are working. Oh and some of them I kiln washed but most of them not. I use them on just about everything because I like to glaze allllll the way down and placing a slightly smaller disk than the base of a piece under it lets me do this with no risk of drips on the shelf.

 

T

LeeU likes this

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Clay lover, I sometimes uses coils to explain to students, that while fragile, ceramics are quite strong. I take a bone dry piece and break it, it with no effort at all. Then I try a bisqued piece. With a quarter inch coil, I can't safely break it.

 

As long as they don't get dropped from more than several inches high, they should stay in good shape, while stored.

clay lover likes this

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I am working on larger ( greater than 8 inches) bonsai pots.  What I have learned to do it make extra feet to support the bottom of the pot.  The bonsai pots have feet, but they are heavy and have the same issues with cracking as a flat plate.  The feet are not attached and travel with the pot from making through drying, bisque, and glaze firing.  They are then thrown away as each pot is unique.  I have also done this with a large 24 inch platter that I made with feet.  I place the extra feet about every four inches.

Roberta12 likes this

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I am a newbie to pottery about 6 months into this exciting media. My knowledge is limited so am still learning...... I am using stoneware and mid fire porcelain clays firing to cone 6.  My Big problem is i have many pieces crack after the glaze firing.  I make lots of flat heart shaped wall hangings and so many of these are cracking in the same place every time like in the  dip of the heart or like the centre of a 'm'.  I am very careful with my slabs and slow drying etc, to keep everything flat.  they come out of the bisque fire ok, i then underglaze and clear glaze before the final firing.  I can have three or four hearts on the same shelf and two will crack and two won't !!!!!  i do not put anything under the hearts is this my problem?  I have stilted porcelain before and had some sagging.  so have not used anything since.  I have tried them lower in the kiln, in the middle or up the top and it doesn't seem to matter where in the kiln they are  i can still get cracking on all levels.  I have a electric kiln with a electronic controller. Some crack some don't,  my hearts are about 1 1/2cm thick.  Can anyone help me here or have any suggestions as to how to stop this happening?  I can't seem to upload a pic of one sorry.

 

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jojess, it sounds like they are cracking at points of stress, in this case sharp angles/ points.  You could try rounding those points a bit, so stress won't build up there as much.

 

The crack could be present during drying and bisque, but just not visible, until the higher heat of the glaze firing.

 

Another possibility is from the glaze and clay body expanding/ contracting at different rates, leading to the cracks.

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Is the crack line sharp with no glaze rounding down into the cracks or does it look like the glaze went into the crack?

Can you post your glaze recipe?

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Everyone will be happy (I'm sure :-) ) to hear that the two large pieces I fired came out with no cracks. Yea! Used sand under each. None traveled to molten glaze. One piece was a 20" disc and the other was a platter -oval shaped- 24" x 20". After rolling each base on a flat ware board, I did not pick up either until they were bone dry. I did all my base clean up after bisque. Thanks for all your reassurances!

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Everyone will be happy (I'm sure :-) ) to hear that the two large pieces I fired came out with no cracks. Yea! Used sand under each. None traveled to molten glaze. One piece was a 20" disc and the other was a platter -oval shaped- 24" x 20". After rolling each base on a flat ware board, I did not pick up either until they were bone dry. I did all my base clean up after bisque. Thanks for all your reassurances!

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