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A forum friend and I have been discussing the cause/s of s cracks. I would like to poll the forum for some additional information.

 

1 do you experience this problem more with stoneware or porcelain?

2. If stoneware, is the body white, tan, grey, or dark?

3. If porcelain, do you know if sodium was used as the primary flux?

4. Does it occur more during hot weather?

5. Have you noticed any differences in outcome if the base is thick or thin?

 

Feel free to share thoughts, opinions, or other information. Thanks for the input.

 

Tom

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Tom
 
I have some observations and conclusions but am currently traveling with only intermittent internet access or opportunity to put thoughts 'on paper'; I will respond more later next week. 
 
Remember cracks develop when the stresses imposed exceed the local strength. 
 
LT

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S crack happen for a multitude of reasons, but I don't the primary flux or the color of the body has anything to do with it. In my experience, fine grained bodies, like white stoneware bodies that are high in ball clays, are more prone to S cracks than other bodies, including porcelain. Thick bases will be more likely to crack, but it's all about how evenly the piece dries. You can leave a bottom 3/4" thick with 1/8" walls and it won't crack if you dry it slowly enough. If you leave water in the bottom of your piece it's more likely to crack. Stresses caused during the throwing process will also contribute, especially in wide bottomed pieces like plates and platters. Good compression of the base is important. Ultimately, evenness is key. If you have a bowl that is rounded on the inside and you trim it flat on the outside, it's more likely to S crack because it's thinner in the center of the foot and thicker at the edge. Again, you can make that work but you have to dry it very slowly. If you want to be able to dry things at a 'natural' speed without keeping it covered for days and days then everything must be even. When I make large deep bowls, I do a rough trimming of the foot while the piece is still really wet, like wet enough you can still mold the clay by hand, and try to get the bulk of the material out that needs to be trimmed out. Then I let it dry to leather hard and do a final trimming. Otherwise the inside bottom is days behind the rim in drying and It takes forever to get it to dry evenly, and is more likely to S crack or crack around the outside just above the foot.

curt likes this

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Thanks for the input Neil. Mgo and sodium interact, but very low on the list of probables. Flux however is very high on the list. In the months ahead I will posting some results of my sodium study. The clay industry is moving more and more to sodium fluxes because they are 1/3 the costs. Read an interesting output report: sodium is produced at 50,000 times the rate of potassium. No, that is not a typo. Thanks for weighing in.

 

Nerd

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I am following this, thanks for posting.

I have a ongoing joke that I should change my studio name to "The Cracked Plate", because I can't seem to make a successful plate! S cracks are the main culprit.

I have started a Cracked Plate Gallery Wall, mounting broken plates on the outside of my studio. It helps me feel better about the items that come out of the kiln... "Oooh, another one for the wall!"... I'm actually really starting to cherish my cracked plate gallery, hahah :)

 

I do use a fine grog stoneware (Plainsman M340 & M325) so I will try something with more grog and see if that helps!

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1 - neither

2 - white

3 - don’t know

4 - no

5 - no

 

Can't remember the last time I had an S crack. I use porcelain and a smooth white "porcelaineous" stoneware for the most part. Coning the clay is something I automatically do, this article stresses the importance of that in preventing them. I've always thought that whenever a clay cracks, in drying, bisquing, cooling, or glaze firing it's because of a stress put on the clay. 

 

@Roxy, neither of those clays should be giving you S cracks, they are super forgiving bodies. Have you tried coning the clay?  As Neil said really keep the slip and water out of the bottom of the pot while throwing. Also, when you finish the underside of the pot, even if you are not trimming it, compress it with a rib all over the base, particularly in the center.  

Joy pots likes this

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I am following this, thanks for posting.

 

I have a ongoing joke that I should change my studio name to "The Cracked Plate", because I can't seem to make a successful plate! S cracks are the main culprit.

I have started a Cracked Plate Gallery Wall, mounting broken plates on the outside of my studio. It helps me feel better about the items that come out of the kiln... "Oooh, another one for the wall!"... I'm actually really starting to cherish my cracked plate gallery, hahah :)

 

I do use a fine grog stoneware (Plainsman M340 & M325) so I will try something with more grog and see if that helps!

 

So far I have not had any of my plates S-crack which I found very surprising. I definitely would try a more robust clay and also I spend a long, long time with the flat side of a wooden rib, smoothing and compressing the bottom. The next day I turn it upside down so it can even out. 

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Thanks for the input so far, much appreciated. Min I had read that article, along with a few others. So far, mechanical stresses and drying seem to be topping the list.

Nerd

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1. I only use one claybody, which is a stoneware.

2. dark reddish brown

3. n/a, not porcelain

4. occurs more in DRY weather, which in my location means winter

5. this is the only difference in outcome that I can detect. I rarely get them anymore, but when it happens it's due to uneven thickness, ie thicker around the edge of the floor, thinner in the middle. Uneven thickness leads to uneven drying leads to stress leads to cracks.

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Neil said is all.

 

I rarely ever see "S" cracks in my bottoms. For large bottle forms and very large pieces I place newspaper on a small amount of sand under the form to wick moisture away. This also allows for movement for shrinkage. I will usually change the newspaper out once or twice. I would have to say in my cases water is the contributing factor. Nothing like collaring and finishing a piece and think "did I sponge that out good. For the beginner I would say water, compression, and design are big players.

 

Staying tuned for another wonderful article from Glazenerd.

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I've worked with lots of stoneware clay bodies both in the US and in Japan.  I throw off the hump frequently.... one of the most well-known "S crack" issue producer for a lot of people.  I have almost no issue with S cracks off the hump.  None at all ever with wheelhead thrown stuff.

 

(No porcelain to speak of... so no comments relating that that icky white pure stuff ;) .)

 

I've generally seen more fine particle clays be more prone to this issue.  I am assuming this is relating to the drying issue I mention below.

 

In my observation it has a LOT to do with the WAY that the pieces are formed, which is then combined with drying issues.

 

I think the reason the S cracks are S shaped instead of straight relates to the torque forces induced on the clay as it is worked on the wheel.  (You don't get an S crack in a rolled slab plate...... you get a straight crack.) 

 

As the clay is moved away from the precise geometric axis of the wheelhead, the friction from the contact with the hands wants to retard the movement of the clay particles.  The more friction and the more time that it takes to get this clay opened up and thinned out, the more the "twist" induced in the clay.  I am guessing (need serious electron micrograph study to prove it) that very tiny micro-stress-cracks/voids are forming within the clay (and other materials) as this happens.  These micro-cracks/tiny voids set up the "seeds" for some later destruction.  The less well aligned the clay particles are in this area... the more these stress cracks / voids remain.

 

Those that throw very directly and quickly seem to have less issues with S cracks.  There is less "drag" from the fingers happening on the clay to introduce torque stresses.  Effective and efficient movement of the clay seems to be key in this.  Get the particles well aligned and the shape formed with the minimum revolutions of the wheel and minimum of skin/clay contact.

 

Then you have the issue of drying.  The thicker areas dry slowest.  More shrinkage in dryer areas and less in wetter areas.  Rims tend to dry faster than interiors.  This increases the stresses applied to the above mentioned micro-cracks.  Those that tend to form things quickly and with little water pooling in the bottoms for any period of time seem to have less s cracks.  Water pooling for any time in the lower area is absorbed into the clay there... making it wetter.  This part of the clay form will shrink more than the rest of the form.

 

Then there is the potential issue of thick and thin uneven bottoms.  Which exacerbates the drying stresses.

 

Like most things.... I feel that no ONE thing causes it.  It is a concatenation of events that all pile up.

 

All theory........ until proven by serious and well structured study.

 

best,

 

....................john

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I find evenness and speed of drying as the most important thing. I was going to write something about that, but John and Neil sum up my feelings.

 

My process for science: 

 

I throw a lot of pots off the hump. 100% of my work right now is off the hump. I get zero s-cracks. I pull up the amount I want to use, then I flatten it out into a plate shape, align the plate, then pull the walls of my cup with that amount of clay. I dry them out until I can pick them up without warping them. Then I put them in a greenhouse tent upside down for the rest of the day/night. I then trim the following day and dry naturally in the studio upside down until bone dry. I think the most important part of my process is the plate alignment and then the remaining day of greenhouse tent. The day in the tent makes the entire body of clay pretty much evenly dry, instead of the rims drying and the foot being too wet. This really allows me to shape and disfigure the outside of my cups when I am done trimming as they are not to dry to crack them when I push them around.

 

I haven't had an s-crack throwing off the hump in almost a year now. I have done this process with every clay I have used and I don't find it makes a difference. The worst clay I have ever used for s-cracks was Bmix. But I was in my first year using that. I dislike that clay still now. 

 

Can't wait to see your results Tom.

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I just hammered a pot that had developed an s-crack during the bisque firing. (had been saving it for glaze testing)

 

The floor is pretty even all the way across, but there is extra thickness at the edge of the floor. And the entire floor is thicker than the walls. And this happened during the dry months of winter.

 

post-1612-0-92594100-1496419130_thumb.jpg

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Here is what I learned from my short experience. When we first started to throw my wife and I had a lot of s-cracks. We learned to compress the bottoms which helped a lot. I then noticed that when I turned the pots upside down or placed them on a open rack to dry we would still get s-cracks. Sometimes it even felt like if we looked to hard at the pots the bottoms would crack.

 

I started covering them with a cardboard box or placing them in a brown paper grocery bag to slow down the drying, so far no s-cracks but now that summer is coming I'm guessing things will change. 

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Only time I got S cracks was when testing a white, fine-grained stoneware. Out of 45 test cups, 8 had S cracks to show only after glaze fire. The S cracks were because of very thin bottoms and sloppy throwing in not mopping water out of the bottom after each pull. My everyday tan fine-grained clay seems very forgiving. Never an S crack.

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 I pull up the amount I want to use, then I flatten it out into a plate shape, align the plate, then pull the walls of my cup with that amount of clay.

 

That is the standard Japanese technique for throwing off the hump.  It is what I do also since being taught that in Japan.  It is said to alleviate some of the torque stresses.  With the same clay body... I find if I throw this way....no S cracks.... if I don't.... some will get S cracks.

 

best,

 

.......................john

Joseph F likes this

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 I pull up the amount I want to use, then I flatten it out into a plate shape, align the plate, then pull the walls of my cup with that amount of clay.

 

That is the standard Japanese technique for throwing off the hump.  It is what I do also since being taught that in Japan.  It is said to alleviate some of the torque stresses.  With the same clay body... I find if I throw this way....no S cracks.... if I don't.... some will get S cracks.

 

best,

 

.......................john

 

 

This explains a lot. I learned everything from watching stuff and reading stuff. I probably saw a Japanese potter throwing off the hump using this method, just tried it... and never went back.

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Only ever had one S crack (fine white stoneware) which was thrown off the hump. I tend to compress my bases carefully and throw fairly dry anyway. (Would like to know more about this Japanese technique but that's off topic...)

 

Once had a mug with parallel cracks but that was due to accidentally flexing the bottom after turning it too thin.

Joe

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The references to " fine grained" ball clay intrigues me. In general terms, the finer the particle: the less alumina and higher plasticity. Conversely, the finer the grain; the more water clay platelets hold. So the assumption being made about " wicking" water does hold a fundamental truth. An 0.25 micron bentone can hold 15 times it's weight in water. So I do believe the science is there to support that premise. What needs to be proven is the reactions of sodium.

 

Nerd

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A four month old test block of sodium based porcelain:

 

All perty and pleasant until you apply pressure to the ends:

 

The sodium has crystallized, causing the block the cleave open when pressure is applied. Although it is four months old, the chemical reaction (ionic bonding) started the day it was mixed.

 

Nerd

 

Copy and pasted from another thread:

 

 

Everything you are describing here, are all symptoms of sodium based clay. Sodium is hydrophobic ( sheds water rapidly), which accelerates drying times. I have been conducting a study on the effects of sodium in clay: sodium fluxes accelerate drying by 50% plus. Cracking is just one of the symptoms of drying, but is compounded by the strong positive bond polarity of sodium. " memory" is caused by a positive electrostatic charge of the clay body; sodium is the highest positively charged fluxes, which increases the properties of memory. So two of the biggest mechanical issues associated with pottery; are compounded by using sodium as the primary flux.

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A four month old test block of sodium based porcelain:

 

All perty and pleasant until you apply pressure to the ends:

 

The sodium has crystallized, causing the block the cleave open when pressure is applied. Although it is four months old, the chemical reaction (ionic bonding) started the day it was mixed.

 

Nerd

 

Copy and pasted from another thread:

 

Everything you are describing here, are all symptoms of sodium based clay. Sodium is hydrophobic ( sheds water rapidly), which accelerates drying times. I have been conducting a study on the effects of sodium in clay: sodium fluxes accelerate drying by 50% plus. Cracking is just one of the symptoms of drying, but is compounded by the strong positive bond polarity of sodium. " memory" is caused by a positive electrostatic charge of the clay body; sodium is the highest positively charged fluxes, which increases the properties of memory. So two of the biggest mechanical issues associated with pottery; are compounded by using sodium as the primary flux.

 

Link to conversation.

 

Standard's 630 is a stoneware that contains mullite and fire clay, are they hydrophobic?

Here's a link to the MSDS in PDF format for this clay

 

FWIW, I threw a 12" plate and a 12" lidded tortilla warmer with this clay and I didn't have any s-cracks. It was the first time I threw something this flat and wide (maybe I got lucky).

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A four month old test block of sodium based porcelain:

 

 

All perty and pleasant until you apply pressure to the ends:

 

 

The sodium has crystallized, causing the block the cleave open when pressure is applied. Although it is four months old, the chemical reaction (ionic bonding) started the day it was mixed.

 

Nerd

 

Copy and pasted from another thread:

 

 

Everything you are describing here, are all symptoms of sodium based clay. Sodium is hydrophobic ( sheds water rapidly), which accelerates drying times. I have been conducting a study on the effects of sodium in clay: sodium fluxes accelerate drying by 50% plus. Cracking is just one of the symptoms of drying, but is compounded by the strong positive bond polarity of sodium. " memory" is caused by a positive electrostatic charge of the clay body; sodium is the highest positively charged fluxes, which increases the properties of memory. So two of the biggest mechanical issues associated with pottery; are compounded by using sodium as the primary flux.

 

 

Okay, but our pots don't sit wet for months. And we typically wedge before throwing or at least cone on the wheel, so this layering happens in the bag it gets mixed up before it becomes a pot. And why doesn't it happen to the walls, too?

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