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#1 Karen B

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Posted 06 June 2012 - 10:41 AM

A couple of years ago I rolled out white stoneware and made 3 large platters (19 inches) with extensive surface design.
They all broke into 3 or 4 large pieces.
Now I have a Bailey slab roller and made a couple of rounded rectangle tiles (18 x 10). I ran them once through the slab
roller (5/8"). They both cracked into large warped pieces in the bisque.
I tried again with the platters and rolled them down gradually in different directions until they were at 5/8 inch. I put them
on the wheel to cut them round (20 inches) and did a little compression with a rib, as well as pulled up a rim. They are drying
now and I hate to lose them in the firing.
My questions are, should I buy single shelves to fire them on? Could it be the fault of the overlap on 2 half shelves? Should I try putting,
maybe, grog underneath them to create a movable surface? Or, could the way I rolled them out do the trick?
I'm firing to cone 6 electric with an extended cool down program.
Attached is a picture of one of the platters, rolled by hand, that I glued back together. You can see the cracks.
Thank you.

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#2 Mark C.

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Posted 06 June 2012 - 12:52 PM

What clay body are you using-???That is a key question
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#3 Karen B

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Posted 06 June 2012 - 12:55 PM

[quote name='Mark C.' date='06 June 2012 - 12:52 PM' timestamp='1339005146' post='17835']
What clay body are you using-???That is a key question
Mark


It is Jeff's White, a cone 6 stoneware clay body developed by jeff zamek and sold commercially.

#4 Marcia Selsor

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Posted 06 June 2012 - 02:29 PM

if you had both problems because the pieces were across two shelves, the. Consider using coils under the platters. I extrude 1/4 inch coils and arrange them in a wheel spoke pattern under large pieces. this helps the heat circulate around the pice evenly. And will avoid any stress of using two shelves.

Just my opinion.
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#5 clayshapes

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Posted 06 June 2012 - 02:54 PM

Beautiful platter! What a heart breaker! I've never had that happen -- I fire to cone 6 on single full shelves. And I roll mine out with a rolling pin too. I do have a bit of warping now and again but I really have no expertise in this matter ...I was just posting to say your platter is beautiful, and that I'm sad for your cracks!! (I don't know why my avatar says I'm "advanced" by the way. I'm the opposite)

#6 Mark C.

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Posted 06 June 2012 - 03:49 PM

A couple of years ago I rolled out white stoneware and made 3 large platters (19 inches) with extensive surface design.
They all broke into 3 or 4 large pieces.
Now I have a Bailey slab roller and made a couple of rounded rectangle tiles (18 x 10). I ran them once through the slab
roller (5/8"). They both cracked into large warped pieces in the bisque.
I tried again with the platters and rolled them down gradually in different directions until they were at 5/8 inch. I put them
on the wheel to cut them round (20 inches) and did a little compression with a rib, as well as pulled up a rim. They are drying
now and I hate to lose them in the firing.
My questions are, should I buy single shelves to fire them on? Could it be the fault of the overlap on 2 half shelves? Should I try putting,
maybe, grog underneath them to create a movable surface? Or, could the way I rolled them out do the trick?
I'm firing to cone 6 electric with an extended cool down program.
Attached is a picture of one of the platters, rolled by hand, that I glued back together. You can see the cracks.
Thank you.


I am not familiar with Jeffs White but it sounds like a tight body (lack of sand or grog??) I work with cone 10 porcelain and it is a tight body as well. For larger forms I use a body called 1/2 and 1/2
its 1/2 porcelain and 1/2 wso from Laguna Clay Co. I would get a body that is less tight
I feel it may be the body first and the firing 2nd.
If you use a body that can handle large forms but still is a little rough coat it with smooth slip.
Also use the coils as Marcia suggested or grog or better still put a THIN slab of same clay under piece that does the dragging and bisque and glaze fire it under the piece for booth fires and it will shrink and take the slide better-you can put grog or silica balls (round fired ball bearings from silica-these are hard to find now) A thrown form is stronger than a slab form as well
I have fired a fish that was 3 feet long and spanned 3 shelves with the slab under it and all was well more than once.
Mark

PS work out the body issues and test a large piece before doing all that work on it. I have fired large (18") platters in my electric on edge as well before
Mark Cortright
www.liscomhillpottery.com

#7 neilestrick

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Posted 06 June 2012 - 04:20 PM

Put some silica sand on the shelves, it will allow the platters to slide. The split shelf does not matter. Some clay bodies just aren't good for large slab work like that....
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#8 Karen B

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Posted 06 June 2012 - 05:04 PM

Beautiful platter! What a heart breaker! I've never had that happen -- I fire to cone 6 on single full shelves. And I roll mine out with a rolling pin too. I do have a bit of warping now and again but I really have no expertise in this matter ...I was just posting to say your platter is beautiful, and that I'm sad for your cracks!! (I don't know why my avatar says I'm "advanced" by the way. I'm the opposite)


Thank you so much for your kind words Clayshapes.

#9 Karen B

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Posted 06 June 2012 - 05:09 PM


A couple of years ago I rolled out white stoneware and made 3 large platters (19 inches) with extensive surface design.
They all broke into 3 or 4 large pieces.
Now I have a Bailey slab roller and made a couple of rounded rectangle tiles (18 x 10). I ran them once through the slab
roller (5/8"). They both cracked into large warped pieces in the bisque.
I tried again with the platters and rolled them down gradually in different directions until they were at 5/8 inch. I put them
on the wheel to cut them round (20 inches) and did a little compression with a rib, as well as pulled up a rim. They are drying
now and I hate to lose them in the firing.
My questions are, should I buy single shelves to fire them on? Could it be the fault of the overlap on 2 half shelves? Should I try putting,
maybe, grog underneath them to create a movable surface? Or, could the way I rolled them out do the trick?
I'm firing to cone 6 electric with an extended cool down program.
Attached is a picture of one of the platters, rolled by hand, that I glued back together. You can see the cracks.
Thank you.


I am not familiar with Jeffs White but it sounds like a tight body (lack of sand or grog??) I work with cone 10 porcelain and it is a tight body as well. For larger forms I use a body called 1/2 and 1/2
its 1/2 porcelain and 1/2 wso from Laguna Clay Co. I would get a body that is less tight
I feel it may be the body first and the firing 2nd.
If you use a body that can handle large forms but still is a little rough coat it with smooth slip.
Also use the coils as Marcia suggested or grog or better still put a THIN slab of same clay under piece that does the dragging and bisque and glaze fire it under the piece for booth fires and it will shrink and take the slide better-you can put grog or silica balls (round fired ball bearings from silica-these are hard to find now) A thrown form is stronger than a slab form as well
I have fired a fish that was 3 feet long and spanned 3 shelves with the slab under it and all was well more than once.
Mark

PS work out the body issues and test a large piece before doing all that work on it. I have fired large (18") platters in my electric on edge as well before



Thank you Mark. I will investigate other white stoneware options, and, or, wedge in some grog for future works. I appreciate your suggestions and will try them, and, am in awe of the platters fired successfully on edge. I will not be trying that. : )

#10 Karen B

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Posted 06 June 2012 - 05:10 PM

Put some silica sand on the shelves, it will allow the platters to slide. The split shelf does not matter. Some clay bodies just aren't good for large slab work like that....




A relief to hear your feedback on the split shelves. Thanks!

#11 Karen B

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Posted 06 June 2012 - 05:14 PM

if you had both problems because the pieces were across two shelves, the. Consider using coils under the platters. I extrude 1/4 inch coils and arrange them in a wheel spoke pattern under large pieces. this helps the heat circulate around the pice evenly. And will avoid any stress of using two shelves.

Just my opinion.
Marcia


Thank you, Marcia, for your opinion. I will test the coil method.

#12 Matt Oz

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Posted 06 June 2012 - 05:59 PM

Very nice platter. Not that this helps, but the cracks kinda look cool.

#13 Karen B

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Posted 06 June 2012 - 07:39 PM

Very nice platter. Not that this helps, but the cracks kinda look cool.


It helps my moral, thanks.

#14 Mark C.

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Posted 06 June 2012 - 10:25 PM

I only bisque fire them on edge in the electric.
Mark
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#15 Lucille Oka

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Posted 06 June 2012 - 11:41 PM

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?

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#16 clay lover

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Posted 07 June 2012 - 07:50 AM

I only bisque fire them on edge in the electric.
Mark



Mark ,would you explain this some? Do you mean the large plates are not sitting flat on the shelf? I can't imagaine how this works, if so. I would LOVE to know more about this, I struggle to have every large piece flat on the shelf in the bisque, fearing warpage if not. I really pack a bisque and sometimes that is a problem.
I would think the edge would be to fragile to support the weight of the rest of the platter as greenware.
Eagerly awaiting more info, need to bisque a big load of large plates and sweating fitting them in flat and getting everything else in.

I make square plates that are sized to fit 2 on a glaze shelf, but won't fit 2 on a bisque before shrinkage. Eats up alot of shelf space in the bisque. Tried stacking them once and got sort of reduction ?? marks where the spacers between,( slices of kiln posts in each corner of center) . sat.
Love a better idea. Thanks

#17 Ben

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Posted 08 June 2012 - 09:15 AM

Cracking in the bisque lets us eliminate glaze fit issues so lets concentrate on making and firing issues for pieces with large, flat surfaces.

1. Are you wedging the clay before making the slabs? If not try wedging. If so, describe your wedging technique.
2. Have you tried cutting a "pre-slab" from a much larger block of clay? I use pairs of dowel rods and a wire stretched in a frame to cut slices of even thickness.
3. What type of bats are you using?
4. After throwing the rim are you cutting them off the bat immediately or do you wait a while?
5. Do you flip them at some point and take them off the bat (so that the bottom can catch up to the rim)?
6. Do you have pictures of the broken pieces of the slabs that you bisqued? I wonder what pattern the breaks formed.

and finally (for now)

7. Describe your firing schedule including kiln dimensions.

Hopefully we can find a solution.

Mark,
Please do elaborate about firing things on edge. I'd love to learn about that too.

#18 Karen B

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Posted 08 June 2012 - 04:14 PM

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.

#19 Mark C.

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Posted 08 June 2012 - 05:55 PM

Ben place platter in bisque kiln(electric) on edge on outside wall leaning almost straight up so foot contacts side wall at a brick point (not on an element). This would be near the top not bottom of kiln. Bottom is to uneven heat (cooler on bottom always) fill rest of kiln with ware. Fire as you normally would a bisque.
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#20 clay lover

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Posted 08 June 2012 - 08:40 PM

Mark, very interesting. If it is "almost verticle, that the bottom of the platter is almost covering a large area of the elemants? Does this mean that al the effort I have been making to keep large flat pieces sitting flat on klin shelves is wasted effort? Can I tumble stack plates like it do other small er things in the bisque?
Holy Goose bumps, Kiln Man, that would be freeing!Posted Image




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