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Large Flat Pieces Cracking During Glaze Firing

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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?

Hi Min   the crack has no glaze going down into it  it just looks like a clean break.  I use commercial underglazes and commercial clear glaze over the top   am too new to start mixing my own yet!!!

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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.

Hi Benzine,  wow there are helpful people on this site , this is the first time that i have asked a question and am so thankful that people are happy to offer advise.  I did change the shape of the heart so there is a softer angle at the top and not a sharp 'V' but some of them still crack.  I am handcutting them so is this the problem??  should i lift them off the shelf with stilts, i am just worried about sagging tho.

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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?

hi Min  i replied further down,  not sure how to use this forum yet!!!   :)

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i typed a nice long answer to you and it became lost when my cat came up and told me to play with her.  yes, cut your hearts without a sharp inward cut at the top.  you can also compress the sides of a flat slab by smacking it with a stick.  the easiest way is to cut the slab, put it on a flat surface like a table and slide a yardstick (that is at least as thick as the slab) smartly against it all the way around the slab.  you will notice a distinct difference in the size of the finished heart because you are compressing it so much.  the dip at the top of the heart should be rounded and compressed with a thumb in all directions.  oops, the cat is back.  she knows it is hug your cat day.

 

glad you know about sand under the pots.  do not use stilts, use sand.  if you think there are cracks in bisque, a quick dip in water will show the crack as it dries.  look at stress points.

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i typed a nice long answer to you and it became lost when my cat came up and told me to play with her.  yes, cut your hearts without a sharp inward cut at the top.  you can also compress the sides of a flat slab by smacking it with a stick.  the easiest way is to cut the slab, put it on a flat surface like a table and slide a yardstick (that is at least as thick as the slab) smartly against it all the way around the slab.  you will notice a distinct difference in the size of the finished heart because you are compressing it so much.  the dip at the top of the heart should be rounded and compressed with a thumb in all directions.  oops, the cat is back.  she knows it is hug your cat day.

 

glad you know about sand under the pots.  do not use stilts, use sand.  if you think there are cracks in bisque, a quick dip in water will show the crack as it dries.  look at stress points.

hi to your cat   :D   I have made the 'v' of the heart less sharp and more rounded to avoid sharp angles but still had cracking, i will try compressing the edges more.  Is it special sand that you put on the kiln shelf??

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jojess, what is the size of your wall hangings?  The cracking may be due to stress on the clay from the size of the hanging and the slab thickness -- 1 1/2 cm is about 1/2 inch.  Try getting the hanging above the shelf -- using sand or any other options listed in this thread.  That should allow the hanging to cool more evenly.  With a large, thick slab flat on the kiln shelf, the heat from the kiln shelf is likely keeping the hanging from cooling evenly.  When firing, you items both expand and shrink at various times of the heating cycle.  So, any weak points are really stressed. 

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as far as warping goes.. it took me a little bit to learn how to throw a platter or something similar... When I started throwing them, they either cracked or warped.. When they warped they also had a twist to them, very weird looking ...

 

I fixed both issues by throwing them thinner 1/4'' in the middle and slowly going thicker towards the outside rim.. That extra thickness helps them hold shape and makes them stronger.. something else I noticed is that if my kiln shelves had a gap in them, I got more cracks/warping so, I closed it...

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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?

Hi Min   the crack has no glaze going down into it  it just looks like a clean break.  I use commercial underglazes and commercial clear glaze over the top   am too new to start mixing my own yet!!!

 

 

A lot of good advise given to you already but I'm wondering about cooling dunts starting at the weak spot of the pieces. How fast does your kiln cool down and how big are these pieces? Plus, what temp do you crack open the lid?

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Use a round cutter (drinking straw/metal tube etc) to make the v of the heart, then use a knife to cut from the hole already created.  This should help to stop the cracks from propagating from the sharp corner.

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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?

Hi Min   the crack has no glaze going down into it  it just looks like a clean break.  I use commercial underglazes and commercial clear glaze over the top   am too new to start mixing my own yet!!!

 

 

A lot of good advise given to you already but I'm wondering about cooling dunts starting at the weak spot of the pieces. How fast does your kiln cool down and how big are these pieces? Plus, what temp do you crack open the lid?

 

Hi Min,  i have the electronic controller set to skip for the cooling.  I open the kiln at approx 30-50 degs C

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Use a round cutter (drinking straw/metal tube etc) to make the v of the heart, then use a knife to cut from the hole already created.  This should help to stop the cracks from propagating from the sharp corner.

 

thanks Benzine, i will try that with a cutter   I did make the centre point a very soft curve, still cut with a knife tho and it still cracked.

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Yeh, sharp edges to cracks would suggest too fast cooling, or too soon opening of kiln.

Hi Babs,  i had softened the curve, my kiln is set to skip a cool cycle.  I open the kiln 30-50degs C

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jojess, what is the size of your wall hangings?  The cracking may be due to stress on the clay from the size of the hanging and the slab thickness -- 1 1/2 cm is about 1/2 inch.  Try getting the hanging above the shelf -- using sand or any other options listed in this thread.  That should allow the hanging to cool more evenly.  With a large, thick slab flat on the kiln shelf, the heat from the kiln shelf is likely keeping the hanging from cooling evenly.  When firing, you items both expand and shrink at various times of the heating cycle.  So, any weak points are really stressed. 

Hi,  the hearts vary in size with the largest being 22cm in length 18cm wide and approx 1-11/2 cm thick.  i will try sand next time.  thanks for all the suggestions.

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We have had success with large flat pieces by either placing them on sand (this needs to be done carefully to avoid getting the sand on other pieces, but it can be done) or firing the piece on a "cookie". This cookie goes through the bisque and glaze firings. The cookie and the piece will shrink at the same rate. Other things to consider: try not to overwork the piece during construction, don't make it too thin, use a clay that is better suited to this form, and make sure that there is absolutely NO glaze on he bottom. Even the smallest bit of glaze will stick the piece to the shelf, causing it to pull itself apart.

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Hello,

I have a similar problem and it involves a lot of pieces made by a lot of people so I hope someone can help.

 

I like to have clay parties to share the joy of working with clay, so I got together a bunch of co-workers and we made about 25 suns/flowers. I rolled out the slabs ahead of time to make the process faster. The ides was to cut and embellish three sizes of circles, and bisque fire them unattached. Then we would glaze them in three colors, and I would stack them to adhere during firing. All went well with the bisque firing.

 

I started to worry about whether this stacking of glazed pieces would actually work so I glazed some I’d made first and got big cracks on the bottom, largest circle.

 

I then assumed it was because of the different glazes reacting to each other. I told everyone we could not stack the pieces and they’d have to use glue after the firing to attach their flowers or suns together. Everyone glazed their work and the plan was to not stack them.

 

I loaded the kiln and fired about half of them, unstacked. I peeked in this morning and saw one of the bigger pieces on the top shelf was cracked badly, with glaze having gone down into the crack (unlike another post here)

 

So - I now think that instead of blaming it on the stacking method it may be that I need to use sand or coils beneath the pieces.

 

I hate the idea of ruining everyone’s careful work. Please help if you can.

 

PS - Can images be attached here? If so, I could upload a photo.

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cracks

 

Here are the two I made with severe cracking on bottom layer. They are about 8" across.

 

Also, getting this figured out is crucial because I had planned to do the same project with the public at the library where I work. Thanks!

 

 

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From the photos, it appears the cracks are only occurring, at the inward points, is this the case?  If so, they could be stress cracks.  Try rounding the points more, to avoid the buildup of stress, when firing.

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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.

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.

I reuse the coils over and over. So it isn't a lot of coils. I would place them like sun rays under the foot of your 20" piece. I use them on architectural pieces which can get quite large.

They let the clay move as it shrinks and let's the heat circulate. I extrude the through a perforated steel plate full of 1/4 " holes. I keep them in a zip lock bag near the kiln posts.

 

I agree with benzine about the stress points. If you can reduce the amount of clay you are appliquéing, it would also help reduce the stress. Also punch a lot of pinholes on the back side where the clay is thicker. It will help with firing.

Marcia

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Thanks so much! I will definitely try the coils on the next batch. Or sand (since I am not going to be doing a bisque firing anytime soon) - is it really okay to just use play sand???

 

I don't think it is only stress points though. I went out and looked at the finished pieces and three of the worst cracked ones have no points cut into them as you can see here. 

DSCN1682

 
Only one of these is around 8-9" wide. The other two are smaller. Other pieces in the load came out fine. Could it somehow be the kiln?
 
By the way, the clay we used was Sea Mix w/ sand from Seattle Pottery Supply and I fired to cone 6.

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It is not the kiln. These are stress fractures -- could be the result of uneven heating/cooling or the forms getting snagged on an uneven kiln shelf surface during expansion/contraction during firing. They can also result from construction of the ware.

 

Getting them up off the kiln shelf -- using sand, grog, cookies, coils, slats, etc. -- is a good step. That will give the form a floating surface for the expansion/contraction that occurs during firing and more even cooling. Should this happen again, notice if the cracks are starting from the edge closest to the elements or to the center of the shelf. When you have a firing issue like cracks, take a second and photograph the shelf before unloading so you can figure out what happened. How densely is the kiln loaded? Are the shelves close to each other or do you have some spacing for air circulation -- the shelf above will also heat the items below. And, are there rough spots on your kiln shelves where a flat item could get snagged as it expands/contracts during firing?

 

And, as Benzine suggested, work on reducing/smoothening sharp notches . . . a place where most cracks start because folks tend to overlap their cuts and the deep part of the notch is not a strong. A good method is to use a needle tool or nail to make a hold at the point where the cuts join, then use your fettling knife or x-acto knife to cut to the hole. The round of the hole will make that a stronger notch.

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Thank you - I will definitely get them off the shelf. I will also use the cutting method you described next time. Holes made from drinking straws may work nicely as the starting point.

I really appreciate this forum. Although I took some classes  years ago in making pieces I learned nothing about firing, and am on my own when troubles arise!

 

Oh - one more question - do you think the stacking and adhering (someone said applique?) technique is doable, or bad practice?

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