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Large Platters Cracking


Karen B

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I agree it is a beautiful platter. The cracks make it look like an instant antique.

Can you show the back of the platter? Do you have any other shapes that are successful with this clay body?

 

 

Thank you. The back is white. will take picture when I have more time. i have thrown lots of pieces with this clay. nice plasticity. normally I use S112 from Laguna, used to be from Miller. But I need white clay in order to get this yellow glaze to be effective. i have a porcelain I use for throwing but it is a waste to use it for slab work which needs to be thicker, thus negating the beautiful translucency.

 

 

 

I was interested in the structure of the platter. Is there a foot? Also it appears that the cracks are actually dunted. Was the kiln opened while the ware was cooling down? The edges of the cracks appear to be rounded which can imply thermal shock of some sort.

 

 

Thank you for your help. No foot. As I said, I used an extended cool down program. I do not open kiln till below 100 degrees or usually room temp. I was under the impression that rounded edges on cracks indicates melt, which happens while firing. Sharp edges happen when it cracks while cooling as the melting is finished.

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Yes I thought that there possibly wasn't a foot. Try making a foot on the bottoms of your platters and raised 'keys' on your tiles.

A rounded edge crack or 'dunting' indicates that the cracks happened while the glaze was molten and had the chance to roll over the edge. Dunting can also occur while the kiln is firing not just in cool down.

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Also it appears that the cracks are actually dunted.

 

 

I thought it looked like dunting too. In my experience a drying crack will open on the surface on which it began and taper away from that surface. The vast majority of cracks either begin on the bat or at trimming (when the compacted clay is removed from a thick bottom and the non-compacted clay is exposed--giving different rates of expansion and evaporation, or tears from a bad/dull tool). Any cracking I've seen like that was from dunting. But, I don't do slab building so I hesitated to comment.

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Also it appears that the cracks are actually dunted.

 

 

I thought it looked like dunting too. In my experience a drying crack will open on the surface on which it began and taper away from that surface. The vast majority of cracks either begin on the bat or at trimming (when the compacted clay is removed from a thick bottom and the non-compacted clay is exposed--giving different rates of expansion and evaporation, or tears from a bad/dull tool). Any cracking I've seen like that was from dunting. But, I don't do slab building so I hesitated to comment.

 

 

No trimming involved. Dunting is cracking when heated too fast or cooled too fast. Neither occurred. The issues in question are the clay used and the drag on the shelves or the overlap on the split shelves. When the current work is dry I will be trying the very good suggestions for firing. If there are still problems, I will investigate other clays, or wedge in silica sand to the current clay body. Thank you.

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Also it appears that the cracks are actually dunted.

 

 

I thought it looked like dunting too. In my experience a drying crack will open on the surface on which it began and taper away from that surface. The vast majority of cracks either begin on the bat or at trimming (when the compacted clay is removed from a thick bottom and the non-compacted clay is exposed--giving different rates of expansion and evaporation, or tears from a bad/dull tool). Any cracking I've seen like that was from dunting. But, I don't do slab building so I hesitated to comment.

 

 

 

I do large platters in my kiln to cone 6. The first thing I found with platters is to use the right clay body. Some clay bodies will not do well with large scale projects. I have tried a number of things to prevent warping from weights to partial drying before putting them on the mold, using a rib to move the clay in different directions to ensure good compression etc.). I have found in general, you have some that work and some that don't get to even the bisque firing. When I have one that comes out of the mold okay I am very, very slow in the drying process. Sometimes it can take a few weeks or more to dry. Then after drying, I let it sit on the shelf a few more weeks. When it is finally ready to fire I do my best to make sure it is as flat as possible. THEN it goes into the kiln and I do a very slow bisque. If it comes out of the bisque okay and it is glazed, I always fire on sand. It is extremely rare that I have one break at this point. The sand helps for some reason??? Your work is absolutely beautiful. I am wondering if it has to do with uneveness is some places when firing (i.e., I think some of your pieces have areas that are raised and some less so). Not sure. My guess is that is has to do with the clay body and drying time. Slow everything down and see if that and some sand in your final glaze firing helps. Just my suggestions. For me, I have lost more large platters before they enter the kiln in their showing cracked areas then after. One other thing is that I have very cautious how much water I put on my piece after drying. This too can add to cracks. For example, I try to make sure the piece is as finished as possible when drying. If I want to smooth out any surfaces I do so with a green sanding pad and then use a barely damp sponge on the surface. I hope this helps.

 

Nelly

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No trimming involved. Dunting is cracking when heated too fast or cooled too fast. Neither occurred. The issues in question are the clay used and the drag on the shelves or the overlap on the split shelves. When the current work is dry I will be trying the very good suggestions for firing. If there are still problems, I will investigate other clays, or wedge in silica sand to the current clay body. Thank you.

 

 

 

Dunting can also occur if there is a burst of cool air entering the kiln chamber either through the peep holes or if using a ventilation system that is attached to the kiln where the top peep hole is left open. Sometimes a back draft can occur with opening doors and windows or turning on the vent in the middle of firing.

There are a lot ways that can cause this to happen. Because we do not know the conditions in which you fire you are the one to make the observations and determine what actions to take. We can only suggest what may have gone wrong to help you correct it.

I hope this helped.

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I was referring to glaze dunting. However, given a closer look, it appears as if the glaze from one side of the crack has fused with glaze from the other in places. When I first looked I thought I saw sharp lines in the crack. I've never actually seen a quartz inversion dunt that occurred during heating, but then I don't do hand building. I do throw large platters though and they tend to have more problems with cracking than any other piece. But, those cracks generally arise from drying issues or exposure of the inner clay mass from trimming. Smarter people than I suggested trying another body, that seems like a good idea to me. Good luck fixing your problem, no one likes cracks, especially on pieces that took that much work.

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It does look like the cracking occurred before the glaze had solidified on cooling. Is that correct? Does the glaze show sharp edges at the cracks or does it show signs of being fluid when the cracks occurred?

 

If the cracking happened before the glaze froze it points to forming or firing or the claybody as the source of the cracking.

It may be that this clay body simply cannot make flat surfaces this large. Look at this page for some ways to test your clay body's drying performance.

http://digitalfire.com/4sight/pictures/tests.html

 

Another thing to ponder is the kiln size vs the platter size in electric kilns. Heat radiates from the elements. It is going to be hotter sooner close to the elements and the ware at the center will catch up when the load gets past read heat and as the temperature climb slows. This is fine for a shelf full of coffee mugs. This is not great for any item that spans too much of this temperature difference. It could be that the kiln is not big enough for your platters to be fired horizontal.

 

 

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I had a wise old experienced potter tell me once, "If it came out of the glaze with a crack, it went in to the glaze with a crack, You just didn'y know it."

Any thoughts?

 

I agree

 

Slow dry .........if u dry for four days double it. Bags box what ever it takes

 

When u glaze dry it well or do a long preheat at 180 f

 

If that does not work then try to glaze both sides ur glaze might shrink the top an the bottom is freeing out.

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Another thing to ponder is the kiln size vs the platter size in electric kilns. Heat radiates from the elements. It is going to be hotter sooner close to the elements and the ware at the center will catch up when the load gets past read heat and as the temperature climb slows. This is fine for a shelf full of coffee mugs. This is not great for any item that spans too much of this temperature difference. It could be that the kiln is not big enough for your platters to be fired horizontal.

 

That is a very good point.

 

It seems that I've read that if you want to dry your pieces at a reasonable rate, just put them in a warmed, somewhat sealed container. The water vapor surrounding the ware will act as an equalizing resistance and the heat will expedite drying. But I must say that Thursday I threw 30 mugs at home, Friday I put handles on, Saturday I bisqued, and Sunday I glaze fired them. My perception of a 'reasonable rate' is likely quite different from others.

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Rim to Rim works well or stack one right onto the next so the feet line up as you would in a cupboard when they are stored-no more than two high

As rim to rim takes up more space I like the one on top of another. This only works if they are made to stack..

I'm a little unclear why you had use stilts to stabilize them. If your shelve is flat the plates will be flat.-I have zero experience at cone 6 but at cone 10 the plates will lay as flat as the shelve is-that is if the shelve is curved so will be the plate in a glaze fire and at that high temp porcelain will slump a tad. If the shelve is flat so will the plates. I do not worry about the bisque fires as the glaze is where this slumping can occur.I have stacked plates on top of mugs in a bisque with no problems-so maybe you are worrying to much.

MarkMark

 

Thanks for the encouragement. I did stack them ,wavy rim to rim, they came out perfectly from the glaze. I was taught by an uber-cautious man with 40 years of experience, and he almost never lost a thing, so I tend to be also very cautious. It is good to learn when it is OK to not worry so much. Thanks for your advice.l

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  • 1 month later...

I wanted to let you all know that I finally fired my platters. I had 2 successful platters fire nice and flat with no cracks. They had about a quarter inch of grog smoothed out under them.

 

A third platter did crack, and I had only put a very thin layer of grog underneath. I believe that the issue was the heat circulation on the bottom and top. I had read that the shelves heat at a different rate from the clay.

 

I had put a thin layer of grog under the one tray that cracked, based on the theory that the friction from shrinking on the shelves was the only issue, but obviously that wasn't correct.

 

I used grog because that is what I had, but I intend to try Marcia Selsor's suggestion of the coils under the pots. Working with grog on shelves is not easy. It wants to fall onto everything and in spite of the utmost care, it did find it's way onto other pieces.

 

Thanks.

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  • 2 weeks later...
  • 3 weeks later...

Arcata Mark:

 

You made a comment in one of your posts early in this thread something like this - " if the clay is a little rough, just use some smooth slip." Are you suggesting that after trimming a foot on a large platter with a clay that is rough you could make it considerably smoother by coating the foot with a neutral slip?

 

I hope that is what you are saying because there is a clay that I no longer throw functional pieces with because the finished foot is so rough and I would love to start using it again.

 

Can you please fill in the blanks on this process? Many thanks for this and all your very informative posts.

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Arcata Mark:

 

You made a comment in one of your posts early in this thread something like this - " if the clay is a little rough, just use some smooth slip." Are you suggesting that after trimming a foot on a large platter with a clay that is rough you could make it considerably smoother by coating the foot with a neutral slip?

 

I hope that is what you are saying because there is a clay that I no longer throw functional pieces with because the finished foot is so rough and I would love to start using it again.

 

Can you please fill in the blanks on this process? Many thanks for this and all your very informative posts.

 

 

I have coated many a pot with slip-mostly on my salt ware but a lot more back some years-I used to coat stoneware with smooth porcelain slip on a regular basis. This works if the shrinkage rates are similar so so testing is needed. Coating the pot is what i used to do-The foot could be coated as well I suppose -If the test proves a close fit I can see no worries. I;m guess when you sponge the foot the grog is what's left and is rough? Do this when pot is damp. If the slip flakes off after bisque then its not a good fit.

I actually slip bisque ware than salt fire but the salt really fuzes it on.

Mark

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Arcata Mark:

 

You made a comment in one of your posts early in this thread something like this - " if the clay is a little rough, just use some smooth slip." Are you suggesting that after trimming a foot on a large platter with a clay that is rough you could make it considerably smoother by coating the foot with a neutral slip?

 

I hope that is what you are saying because there is a clay that I no longer throw functional pieces with because the finished foot is so rough and I would love to start using it again.

 

Can you please fill in the blanks on this process? Many thanks for this and all your very informative posts.

 

 

I have coated many a pot with slip-mostly on my salt ware but a lot more back some years-I used to coat stoneware with smooth porcelain slip on a regular basis. This works if the shrinkage rates are similar so so testing is needed. Coating the pot is what i used to do-The foot could be coated as well I suppose -If the test proves a close fit I can see no worries. I;m guess when you sponge the foot the grog is what's left and is rough? Do this when pot is damp. If the slip flakes off after bisque then its not a good fit.

I actually slip bisque ware than salt fire but the salt really fuzes it on.

Mark

 

 

 

Thanks Mark...I will definitely experiment with this and feedback results.

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You could make a smooth slip out of your clay body by running it through an 80 mesh screen. Then it will match and fit perfectly.

 

 

This is the best Idea as it will fit the body perfectly.

Mark

 

 

I agree Mark - great idea Neil. Will try it next week.

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