cracks in large platters even with grogged stoneware

[Mudfish]

I have tried all the suggestions I have read on here about this topic. coils, cookies, sand, grog, even a "heat sink" idea that I read about on this forum. I still get cracks during the cone 9-10  reduction glaze firing. Sometimes cracks happen during the bisque firing also.  With the last firing, I programmed in a slow cool - 100f per hour to 1500f, however, the controller apparently errored after that and shut the pilot  off for the rest of the slow cool program. When I caught the error at 700F, I closed the damper hoping to retain enough of a slow cool so cracks would not happen. I am guessing the cracks are happening during the cooling since the glaze is not melted down into the cracks. What is a good slow cool program? Thank you all

[Pres]

I'm not certain of anything with ^10 reduction, but have some experience with platters, as I throw communion plates for those using whole loaf bread. I have found out a few things over the years about my own pieces.

• sharp curves from a large base to a sharp perpendicular are problematic. Often a crack will start in the corner spreading in both directions. . maybe something about the thicker area?
• Too many foot rings open up more possibilities for cracking, usually only two foot rings are needed, one on outside diameter where form leaves the base, and one 2-3 inches from center. 

I would try forms with a slight rise from the base, or a rounded  rise to the perpendicular you have.  cut fewer foot rings, and smooth the bottom between them keeping it level.

All of this IMHO, take it as suggestion, not criticism.

 

best,

Pres

[Mudfish]

thanks, Pres. I have one of these forms on the wheel right now, and its at a stage where I can lift the outer base to a rounded rise.  I will give it try. I have made smaller diameter platters with a more rounded rise and had no problems with cracking, so maybe that is the remedy. Appreciate your help ! 

[Jeff Longtin]

If we can take the firing issue off the table we're left with process. Two thoughts: 1) how much grog have you used? You may have too much in the clay, and 2) water- do you throw with lots of water? Perhaps the clay is getting too wet, in the throwing process, and its causing this shrinkage/cracking?

The other thought: to pick un on the Pres point, are you spending a lot of time trimming the pot? Making three rings must take awhile? Perhaps too much contact with the trimming tool. Have you tried not trimming the pots with a tool? (Leave them unfooted in other words.) Maybe use a trimming stick on the wheel and leave it at that.

[Mudfish]

Jeff. I will pay attention to the grog additions, as I really dont measure, just kind of sprinkle. As for the water usage, I see where that would cause shrinkage/cracks also. If I ever get this problem resolved, I will post pics of my work without the hideous cracks! Thanks everyone

[Russ]

Have you tried using a rib and compressing the heck out of the bottom while on the wheel?

[LinR]

I used to make large flat trays.  I didn't put a foot on them and fired them on a 1/8" plus bed of silica sand.  But I only fired to cone 6.  I didn't grog the clay.  Lin

[Bill Kielb]

Just some things that strike me
For cracks happening at bisque, I would look for production techniques and handling prior to. Grog has never really helped me develop much extra strength. It generally Does open up the body.

So cracking at bisque when the body is just being sintered maybe very telling
For production I would always start with compression, real compression not just pushing down though. For bottoms, out to inward ribbing, not so much in to out  …….. rims - ok, push down ……… walls - ribs are great. I would also not over work during the trimming stage, hard to do with lots of grog, I get it. Are we adding grog for appearance and is this creating weak areas for cracks to originate? Well mixed - homogeneous? Gently ring your bisque, often hairline cracks can be discovered at this point.

For firing, I would definitely make sure it was constructed well and for sure, make sure it can slide easily on the kiln shelf which means silica or alumina or something to let this grow and shrink in all directions. In its present form it looks fairly rough maybe due to the grog so getting this right might take some doing. Your last picture almost looks like there is a shelf pluck mark, top right.

it would be great to examine the bottoms of all these to see if the rough multiple foot rings likely contributed to some of the failures.

I would start with the cracked bisque though, this may be telling you something.

Edited March 31, 2023 by Bill Kielb

[Denice]

I don't do a lot of throwing but I have learned that keeping the  thickness of the clay the same is the key.   I even try to keep the turns in sculptures  the same,   If I can't I will make the area angled to ease into a area with varied thickness.  My throwing teacher in college would walk behind our wheels hollering (compression, compression, compression)  especially on a flat platter.   Denice

[Mark C.]

You said you are firing down-so I assume you have a gas controller on your kiln. Talk about what type of kiln you have downdraft (slow cooling) or updraft -whats size kiln?

I have fired cone 10 for 50 years and never fired down (no controller on any of my gas kilns yet) I always pack them tight and close them up as soon as the kiln tops out with my cone.

Looking at your form-I think more compression is needed-also what clay body is it and how tight is it. Unglazed on the bottoms can also add to the cracking issues .Same thinckness as others noted is key. Less water when forming also helps. More compression and the right clay body . Grogged body is good but that now for me points to lack of compression and fast cooling (damper open???)

If you are firing down I may ask why are you doing that???

Those all look to be cooling cracks to me. To fast cooling (open damper will do this) where is the cold air coming from? 

Tell us more details please 

 

Edited April 1, 2023 by Mark C.

[Retxy]

I'm a newbie here with glaze formulation constantly on my mind. I noticed some shivering of your glaze where the base rises into the sides of your platter... Are you sure this is not from a glaze fit issue? Does it happen with all your Glazes or just this one? I'm SO sorry, I know your pain. 

[Babs]

Another thing to do is to "ping" your biscqued pots. Often unnoticed cracks which will not get better when going through glaze firing.

Try to get a hold of Hamer and Hamer,  your cracks are textbook examples.

"A POTTER'S dictionary of materials and techniques."

Grog can cause cracking as as the clsy body drIes, the clay shrinks but, surprize, surprize, the grog doesn't.  

Not saying that your images reflect that.

Are you supporting the belly of your large dishes as you turn?

What do your unturned/ trimmed platters look like?

Are you flipping the platter as soon as it can hold itself after throwing? 

[Mudfish]

5 hours ago, Retxy said:

I'm a newbie here with glaze formulation constantly on my mind. I noticed some shivering of your glaze where the base rises into the sides of your platter... Are you sure this is not from a glaze fit issue? Does it happen with all your Glazes or just this one? I'm SO sorry, I know your pain. 

yes, the one platter does have some crawling - probably too much glaze was sprayed on. 

[Mudfish]

Marc C. I do fire down in hopes the wares will cool off slowly and avoid these cracks. Also, I use a slow cool to avoid pinholes and to assure the glazes stay matte. My kiln is a downdraft, 9cubic ft. I really feel like I compress enough both ways - outward to inward, and vise versa.

[Mudfish]

Thanks, Babs. Yes, I do support these forms when I flip them over to dry. I use sponges, soft cloths..,etc. to support the center so they won't sag. I turn the forms often, and keep them loosely covered so that they dry evenly. I think I will omit using grog as I don't think it is helping. I will look for the book you suggest - I'm sure its on Amazon. I have made several more of these forms, but with a more rounded bottom edge, as Pres suggested. I'm hoping that is a solution to the problem. I will probably be bisque firing them with my next kilnload in about a week and I will let you all know if they survive the bisque fire

[Mudfish]

Also, I have considered that it is an issue of the inside bottom is glazed, while the outside bottom is not. In that case, I will need to throw the base thicker, so that I can trim on the bottom to have enough space for glaze on the bottom also

[Min]

I'm not sure but to me it looks like the glaze has rounded over slightly on the crack in the second, third, and part of the 4th image, perhaps the crack in that pot was there either before glazing or on the temp rise up. 

A totally off the wall suggestion here but maybe something to consider. I've made dozens of oval shaped baking dishes (used ^5 clay with sand) fired in ox. which I realize you aren't doing but perhaps the forming method might be an alternative way of making them that could work for you. Instead of throwing one large baking dish for the type in the image below I roll out the base, compress both sides, and let it stiffen up to cheddar cheese leather hard. While the base is drying I throw a bottomless ring the height of the walls with a thin tapered flange on the inside. I cut the wall from the batt and when soft leather hard make the oval shape. When the wall ring is as firm as the slab base I attach the two, slipping and scoring the edges that join.  Outer bottom wall edge is trimmed where it meets the base and I use a pony roller to really compress the seam. On the inside the tapered flange is ribbed onto the base. Rolled rim on the top edge of the wall to make a built in handle. I haven't made these as round forms like yours but I don't see why it wouldn't work as would adding a footring. I agree that glazing the bottom might help although I don't glaze the bottom of these forms. 

 

 

 

 

[Mudfish]

I will try this idea also. A couple of years ago, I did try a version of your idea, here. I don't remember flipping the slab over to rib both sides, though. Anyway, the form ended up cracking all around the join as it was drying. I abandoned the idea but maybe I should have kept trying.

[Mudfish]

On 3/30/2023 at 8:23 PM, Bill Kielb said:

Just some things that strike me
For cracks happening at bisque, I would look for production techniques and handling prior to. Grog has never really helped me develop much extra strength. It generally Does open up the body.

So cracking at bisque when the body is just being sintered maybe very telling
For production I would always start with compression, real compression not just pushing down though. For bottoms, out to inward ribbing, not so much in to out  …….. rims - ok, push down ……… walls - ribs are great. I would also not over work during the trimming stage, hard to do with lots of grog, I get it. Are we adding grog for appearance and is this creating weak areas for cracks to originate? Well mixed - homogeneous? Gently ring your bisque, often hairline cracks can be discovered at this point.

For firing, I would definitely make sure it was constructed well and for sure, make sure it can slide easily on the kiln shelf which means silica or alumina or something to let this grow and shrink in all directions. In its present form it looks fairly rough maybe due to the grog so getting this right might take some doing. Your last picture almost looks like there is a shelf pluck mark, top right.

it would be great to examine the bottoms of all these to see if the rough multiple foot rings likely contributed to some of the failures.

I would start with the cracked bisque though, this may be telling you something.

Thanks, Bill I will pay more attentiion to trimming and making the bottom smooth. I will abandon the grog idea. If cracks are happening during the bisque, they are too fine to notice, and they expand in the glaze fire. Cracks have happened during the bisque firing, but it is hairline cracks that are visible, so I don't continue on with glazing. Maybe all of these cracks are happening during the bisque firing and they just are not visible until the cone 9-10 temps 

[Mark C.]

I think that glaze looks to be a matt or satin Matt which can put lots of stress on the pot as well unless it has glaze on bottom. Does this happen with shiny glazes or just your matt glazes? I cannot see any need to fire down with these glazes or pots. Try closing the damper as soon as the cone gets to where you like and CLOSE the Damper all the way to cool. 99.99% of all potters do this .

[Min]

4 hours ago, Mudfish said:

I will try this idea also. A couple of years ago, I did try a version of your idea, here. I don't remember flipping the slab over to rib both sides, though. Anyway, the form ended up cracking all around the join as it was drying. I abandoned the idea but maybe I should have kept trying.

One thing I forgot to add was it's a good idea to place some weight in the middle of the base as the pot is drying. (I use a rectangular kiln brick and leave it there until the pot is dry) I use a very thick slip made with magic water and my claybody for the joining slip.

[Babs]

What kind of temp rise do you have on the bisque firing from ambient to say 600degC

Does any of the pot jut out from shelf into direct flame path?

Closing  All air inlets at top temp? Glaze and bisque.

Shelves clean?

Flick all your bisque pots . LIsten  for the ping or pung. Like Min I think some of those cracks are happenning at or before bisque stage.

 

 

[Mudfish]

Just unloaded the kiln today and to my great relief these platters have NO cracks ! I'm so happy to be batting a thousand this go-around. I hope these results continue. What I did was to lift the bottom edge while throwing these forms to round out that area, omitted the grog, paid more attention to closing up damper for the cool down, and I fired on a fairly thick bed of silica. Thank you all so much

Edited April 11, 2023 by Mudfish

[Roberta12]

Beautiful @Mudfish ! Nice work!

 

[Mudfish]

Thank you, Roberta